Figure 4.1Pentagon of Power
Any machine you make, inherit, or maintain needs a source of mechanical power. Something has to spin or push or pull to make the machine do the work of moving scenery on stage. In the simplest form, you could be the source of mechanical power turning a crank on a manual winch or pulling a rope wound around a turntable. But this is a book on automation. Manual machines, though useful in some productions, aren’t interesting in automated shows. So each of our machines must have a prime mover that creates the desired motion of the machine whether it’s spinning a winch drum, turning a friction wheel, rotating a sprocket, or pushing and pulling a piston.
Figure 4.2Motor is primer mover on a hoist
Source: Courtesy of Creative Conners, Inc.
The most common mechanical power source is an electric motor. You are undoubtedly familiar with motors. From the tiny motor in your phone vibrating to alert you of a notification to the motor in your car that pushes your windshield wipers rhythmically across the windshield to the motor in your cordless drill to the motor on your air compressor. We are surrounded by motors in our everyday lives because they are supremely useful at taking electric potential and converting it into a physical spinning motion.
The vast array of motors confused me the first time I set out to build a machine for the stage. When I was a 19-year-old Junior at Ithaca College, I was excited to have my second opportunity to both fill the role of Technical Director and take a crack at mechanizing a scene change. My first foray into mechanization had been a year earlier using a pneumatic cable cylinder to move a platform from the wing to center stage. That rig was designed and installed on a whim, and later scrapped when my faculty supervisor presented irrefutable safety concerns. With my second attempt to work a machine into a production, I was determined to build an electric winch that would pull a pallet around a curved track.
The small snag in my plan to produce an electric winch was that I had only a vague understanding of what an “electric winch” was and how to build one. Undaunted, I pored through catalog pages in McMaster-Carr and Grainger to find the central component: an electric motor. If you have ever found yourself in a similar situation, you can sympathize with the confusing variety of motors and my paralysis at an overwhelming choice without enough knowledge to narrow the options. If you are embarking on your first motorized adventure, hopefully this chapter will save you some valuable time as well as some heartache.
There are a handful of characteristics that must be considered when picking out a motor to use on stage.
Speed Control
In our business, we (almost) never just turn a motor on and let it spin. Our machines need to be programmed so that the motion can be choreographed to work with the artistic intent of the production. Whatever motor we choose, it needs to be capable of variable speed.
Torque
The movements we demand of machinery often require a combination of very slow and very fast movements with heavy loads. The motor must be capable of producing near-constant torque throughout its speed range. We may require that the motor produce full-torque at zero speed, which is not always an easy task for a motor.
Noise
Unlike heavy industry, our machines are expected to perform as quietly as possible with the ultimate goal of being silent. In practice, we just need to be quieter than the whine of the wiggle lights so that noise complaints go to the lighting technicians instead (I kid, I kid).
Flexible Mounting
The motor will need to connect with a mechanical speed reducer, a spring-set safety brake, and possibly a rotary encoder for programmable positioning. It may also be cost-effective to pull a motor from one machine and move it to another for a different effect in another show.
Electrical Requirements
You should consider what power is needed on stage to run the machine. The electrical service will need to be the proper voltage and have sufficient current to match your machine.
Price
I have worked on productions that range in budget from dozens of dollars to millions. Regardless of the budget, the cost of equipment is always a concern. When the only good, safe option is expensive, the money will be found (or the effect will be cut), but if the same performance value can be achieved by two options, cheap always wins.
Considering those factors, we can focus on the following motor options for stage equipment:
1.AC 3-phase induction motors
2.DC permanent magnet (or field wound) motors
3.Brushless DC servo motors (also known by various pseudonyms)
4.Stepper motors (for light-duty effects).
Each type of motor has distinct characteristics that will influence your ultimate selection, but before we explore each in detail it is worth covering some basic concepts of motor operation. This is a cursory glance at motor principles, not an in-depth exploration. There are many excellent texts to satisfy your curiosity of how motors work and the intricate details of the construction and the math used to model motor behavior. Two of my favorites are Electric Motors and Drives by Hughes and Drury, and Electric Motors and Control Techniques by Gottlieb. At the risk of academic heresy, I think it’s important to realize that you can utilize motors without deep theoretical understanding. Knowing how to interpret manufacturer specifications will suffice for most applications within technical theatre.
Motors have two parts: the rotor and the stator. The rotor is the bit that rotates. The stator is the stationary housing that surrounds the rotor. The electric motor harnesses the powerful phenomenon of magnetism and the force produced when magnetic flux crosses paths with a current carrying electrical conductor.
Figure 4.3Motor rotor and stator
Magnetic flux is the radiating lines of force that flow from the north pole of a magnet to its south pole. Though an invisible force, you have undoubtedly acquired an instinctive understanding of how magnetic flux works. Like poles on magnets repel and unlike poles attract. Ferrous metals are drawn to magnets and will adopt the polar charges of their clingy overlords, which is easily seen when stringing together paper clips with a refrigerator magnet.
Less obvious than the force produced by a permanent magnet is the magnetic force produced when an electric current is run through a conductor. Electrons bumping down a length of copper wire produce an axial magnetic force. A small chunk of wire connected to a 9 V battery produces discouragingly little magnetic force, but if you have ever played with electromagnets you know that winding a lot of wire around an iron bar will produce strong magnetic flux as soon as you pass an electric current through it. In fact, just winding some copper wire in a tight helix and passing a current through the wire will produce good magnetic force but an iron core will intensify the magnetic flux produced by the charged copper wire. Equally useful, the iron will not stay magnetized if the current is turned off so you can make a switchable magnet that can be turned on and off, or easily switch its magnetic polarity by reversing the electric current flow.
Figure 4.4Electromagnet
Motors exploit these principles in ingenious ways to produce a magnetic force in the stator of a motor that repels the rotor so that the magnetic force chases the rotor round and round. How the rotation of these forces is achieved varies for each type of motor and each method has strengths and weaknesses for our use on stage. However, we need to delay that discussion just a little bit longer to cover some electrical concepts. Much like the preceding motor theory, my coverage of electrical theory in this chapter is quite basic and covers just enough to understand motor operation. If you’re a seasoned electrician, skim ahead to the motor descriptions.
A Little Electrical Background
Without knowing the concept of electricity, understanding how electric motors work can be difficult. We should take a moment to cover the basic vocabulary used to describe electricity and review the typical electrical service you are likely to encounter backstage. This is by no means a replacement for the National Electric Code (NEC) or the knowledge and experience of a licensed electrician. Automated machines are often powered by large electrical currents that should be treated with respect. If you intend to wire your own motors, or construct your own motor control cabinets, I highly recommend picking up a copy of the NEC Handbook and consulting with a licensed electrician. The NEC Handbook is a remarkably accessible guide for safe wiring practices and has been written to help practicing electricians avoid fire and safety hazards. Since it’s written with the purpose of guiding practicing electricians, any licensed electrician will be intimately familiar with the NEC and can provide priceless advice and assistance to insure safety.
With that firmly stated, let’s review electricity.
Electricity was described to me once as a waterfall, where the potential energy is the height of the waterfall and the current is the volume of water passing over the waterfall. In this analogy, we would measure the height of the waterfall in volts and the volume of water in amps. Personally, I have an easier time imagining electricity as the classic 1970s board game Mouse Trap. A marble (electron) is starting up high and rolling downhill (potential or voltage) and we stick a Rube Goldberg contraption in the way of the marble to get interesting work done. The bigger the marble, the more stuff it can push around (current).
Figure 4.5Mouse Trap game as analogy for electricity
The power of electricity is expressed in watts and is the product of voltage and current (amps).
The formula for power is easily remembered by the mnemonic West VirginiA. The relationship between power (W), potential (V), and current (A) is important to comprehend. You can achieve the same power by increasing voltage and thereby dropping current. For instance, a motor that draws 10 amps at 460 VAC will draw 20 amps at 230 VAC but produce the same output power. Conversely, lowering the voltage will require more current to maintain the same power level. Amps, the common abbreviation of Amperes, measure current and translate physically to the number of electrons that are pushing through a conductor, so more current requires bigger wire. Volts measure the potential energy or desire of electrons to get to ground. Higher voltage indicates a stronger desire to jump to ground and therefore will require wires with stronger insulation to keep the electrical charge from arcing. The same machine running at higher voltage uses less copper, but requires better insulation.
Electrical current is produced either as direct current (DC) or alternating current (AC). Direct current (DC) flows in one direction, the electrons shuffle from high potential to lower potential in a single direction. While there are DC generators, it is more common to get DC power from a battery or a DC power supply that rectifies alternating current (AC). Modern electronic devices are generally DC as witnessed by all of the little black or white wall warts plugged into wall outlets to power computers, phones, and mobile chargers. Those compact devices convert an AC input to a DC output, as well as lowering the voltage.
DC voltage is potential energy measured from a reference ground or common. In the Mouse Trap analogy, the voltage would be the initial height of the platform where the marble is launched.
Alternating current (AC) changes polarity in relationship to ground in cyclical, sinusoidal pattern. AC is the power that comes from the utility service and is available at your wall outlet and company switch. At the risk of taking the Mouse Trap analogy too far, to simulate alternating current the game would be played on a ship in a turbulent storm so that the marble rolled back and forth across the board. The challenge of creating machines that can turn that oscillating energy into useful motion is somewhat analogous to the challenge presented in AC motors.
AC voltage is measured between the Root Mean Square (RMS) of the waveforms that are producing power. The math used to calculate the RMS isn’t terribly important, but you should know that the voltage measured is not the peak of the power source but rather a calculated value that approximates the useful equivalent DC voltage. The frequency of the alternating current varies depending on your geographical location. In the US, AC power completes a full cycle 60 times per second, or 60 Hz. European power operates at 50 Hz. The frequency of the mains power provided by the utility service doesn’t usually affect decisions in motor selection since we will be running our motors from speed controllers that drastically manipulate the incoming power to produce the correct power for the motor, but the voltage, current, and number of phases is hugely important when selecting equipment and hooking it up.
Figure 4.6Root Mean Square measurement of AC voltage
In the United States, 120 VAC single-phase is common wall power. The standard Edison socket has three pins: ground, neutral, and hot. In this arrangement, ground and neutral are both at 0V. The hot leg is at a potential of 120 VAC when referenced from ground. Commonly, these outlets are limited to 15 amps, 20 amps, or 30 amps.
The next step up in voltage is single-phase 220 VAC. In the United States, a 220 V receptacle has two hot legs and a ground. When you measure the voltage with your multimeter, there is 120 VAC potential between ground and either hot leg, but between the two hot legs there is 220 VAC. Abroad power may have 220 VAC on one leg.
The higher voltage is used for machines that require more power than can be practically delivered at 120 VAC. As discussed earlier, by raising the operating voltage a machine can produce the same power with lower current requirements. Lower current equates to small wire, which is less costly and easier to handle.
The voltage values vary substantially depending on the service supplying the building. The voltage will range between 208 and 240 VAC depending on the transformer feeding the building. This often causes concern when hooking up an automation system, but typically that concern is unfounded. Most amplifiers used in motor speed control are built to accept a range of input voltages from 200 VAC–240 VAC, which can be easily checked in the manufacturer’s documentation.
The most efficient power source for AC motors is three-phase service. Typically, the company switch on stage will be a “low” voltage 208/230 VAC supply. A three-phase outlet will have three hot legs and a ground. Much like single-phase 220 VAC, the voltage between ground and any single hot leg will be 120 VAC, but the voltage between any two hot legs will be ~230 VAC. Again, the actual voltage will range depending on the building service.
Figure 4.7230 VAC three-phase waveform
Often, three-phase receptacles will have five wires. In that case, a neutral is included in the receptacle as the fifth wire so that an appliance can use that neutral to provide a 120 V source to any internal component that needs to operate at the lower voltage. Because of this ambiguity a three-phase service is often described as either “four-wire” or “five-wire.” A four-wire receptacle has three hot wires and a ground wire. A five-wire receptacle has three hot wires, a neutral wire, and a ground wire.
Figure 4.8Four-wire receptacle connected to three phases and ground
Figure 4.9Five-wire receptacle connected to three phases, ground, and neutral
For more details on the practical aspects of wiring and connectors, see Chapter 14.
The electrical service enters the building and is then distributed to outlets and equipment throughout the facility. Like branches on a tree, the electrical feed is split at junctions and fans out into smaller circuits. Each junction is protected by either a fuse or circuit breaker to prevent too much current from flowing downstream of the junction. Excessive current poses a serious fire hazard, so every conductor needs protection from drawing too much current. The circuit conductor after the last fuse or circuit breaker is known as a “branch circuit.” Most of the time, the branch circuit is the conduit and wire run from a circuit breaker panel, or panelboard, to an outlet mounted on the wall.
Figure 4.10Typical service layout
Identifying the circuit protection that is responsible for a particular outlet is crucial when you need to connect a machine to a power source. You must either make certain that the branch circuit protection is sufficient for safe operation, or install a supplementary fuse or circuit breaker that is sized correctly for the electrical demands of the machine. For instance, you want to plug in a winch and motor controller in your shop to test it before load-in. The outlet on your shop wall is rated for a maximum current draw of 20 amps. The motor controller bears a label that reads, “Supply proper branch circuit protection.” The circuit on the wall has a 30 amp breaker. It is possible that the winch could get jammed up, or malfunction, and draw substantially more than 20 amps. Since the branch circuit will only trip above 30 amps, the winch motor could run indefinitely at a dangerous current level until it either breaks down or starts a fire. In such a situation, you need to take care to have an electrician install a fuse, or circuit breaker, in between the wall outlet and the motor controller or replace the circuit breaker in the panel with one that is rated for a lower current. In the reverse situation, where you have a winch rated for 30 amps, and a circuit protected by a 20 amp breaker, there is no fire risk because the breaker will trip before the winch draws too much current, but you have artificially limited the capacity of the machine.
Continuing in our example of shop testing a winch before load-in, you need to confirm that the power circuit is of the correct type and voltage, as well as identifying the proper circuit protection. Every NEMA (National Electrical Manufacturers Association) outlet has a specified voltage and conductor arrangement. For example, a NEMA L15–30 outlet is to be used for a four-wire, three-phase, 30-amp, 250 V (max) power circuit. It is a twist-lock, designated by the “L,” and has a ground pin plus three hot legs that are placed in a circular pattern so that the plug can be inserted into the outlet and then turned a smidge to lock it and prevent it from being kicked out of the wall. However, at a glance it looks awfully similar to a NEMA L14–30. That receptacle is also a twist-lock with four pins, but is instead intended for a 125/250 V 30-amp circuit so it has a ground, neutral, and two hot legs. If your winch controller has an L15–30 plug expecting a three-phase 30 A 208/230 V input, and the wall outlet was an L14–30, you might be tempted to rummage through the bin of connectors in the electrics shop and grab “the one that fits.” That isn’t a good idea. Instead, use a multimeter to measure the voltage at the outlet to make sure the circuit is appropriate for the winch. Following the earlier description of three-phase power, you know that the appropriate circuit should measure ~120 VAC from ground to each of the hot legs, and 208–230 VAC between any two hot legs. Using the probes of your multimeter, confirm that those expectations are met.
Figure 4.11L15–30 meter reading between ground and third phase
Figure 4.12L14–30 meter reading between ground and neutral
Confirming the voltage of the power source with your multimeter is an excellent habit to form. Any time you encounter a new outlet, meter the power before you plug in your equipment. Even though the NEMA standards should be followed and outlet configurations all have a specific purpose, I routinely bump into outlets in theatres and scene shops that were wired for the incorrect power configuration out of ignorance or convenience. Taking a minute to check the power source will save your equipment, avoid unnecessary troubleshooting, and build your confidence.
To gain a clear picture of what is feeding the outlet mounted to the wall, take a look at the panelboard, or circuit breaker panel, feeding the outlet. As automation technicians, it is important to feel comfortable with the power source and be able to identify which circuits are powering your machinery. The panelboard is mounted to the wall in permanent installations, or may be mounted to a rolling case for touring shows or events in lieu of a power distribution rack. Looking at the panelboard gives you a clear picture of what service is available and how the power is being distributed.
Figure 4.13Anatomy of a breaker panel
The panel should be labeled with the service voltage and may have a Main Breaker at the top of the panel. Looking inside the panel (with it de-energized; never open a live panel), the conductors feeding the panel come in the top and either connect through the Main Breaker or fasten to terminals directly on the bus bars that power the branch circuit breakers. In a three-phase panel, three wires will feed the power bus bars running down the center of the panel, a ground conductor will connect to the ground bar on the side of the panel, and a neutral wire will connect to the neutral bar, which is usually on the side of the panel opposing the ground bar. In a single-phase panel the construction is similar, but the circuit breaker bus bar is fed by two hot conductors instead of three. Looking at the construction of the panel reveals a few interesting characteristics.
Figure 4.14Breaker panel internal layout
First, the hot phases alternate in a pattern vertically: slot #1 is phase #1, slot #2 is phase 2, slot #3 is phase 3, slot #4 is phase 1, etc. A single-phase 120 V breaker occupies just a single slot. The circuit wiring would have the hot wire (black) connected to the breaker, the neutral wire (white) connected to the neutral bar, and the ground (green or bare copper) connected to the ground bar.
A single-phase 220 VAC circuit would have a breaker that occupies two vertical slots. Depending on the construction of the breaker, it will either have an actuator (the bit that looks like a switch) or two actuators that are mechanically tied together so that the actuators always move in unison. The circuit wiring has for a single-phase 220 VAC circuit could be either three-wire or four-wire. A single-phase, three-wire, 220 VAC circuit would have a two hot conductors (typically black and red, but the colors can vary) connected to the breaker and a ground wire connected to the ground bar. A single-phase, four-wire, 220 VAC circuit would have the same wiring with the addition of a white wire connected to the neutral bar. The neutral wire in the four-wire, single-phase circuit allows for equipment that needs both 220 V and 120 V power to operate.
A three-phase circuit would have a breaker that occupies three vertical slots. The wiring for a four-wire, three-phase, 208/230 VAC circuit would have three hot conductors (black, red, blue is common) connected to the breaker and a ground conductor connected to the ground bar. The wiring for a five-wire, three-phase, 208/230 VAC circuit would add a neutral conductor for equipment that requires 120 VAC power.
Second, the ground and neutral bars are bonded together and therefore have the exact same electrical potential. This clearly shows that if you meter between the neutral and ground terminals of a circuit and get any voltage, something is definitely wrong with the power wiring.
Armed with a little basic electrical knowledge, we’re ready to figure out how we can put electricity to good use making motors spin.
AC Induction
We mentioned already that electric current moving through a conductor will produce magnetic force. The reverse phenomenon also holds true: if a magnetic field is moved over a conductor, an electric current is induced in the conductor. You probably studied induction in a physics class and spent hours with Faraday’s Law and Lenz’s Law. We needn’t revisit the calculations here. The key point to remember from those earlier studies is that induction relies on a changing magnetic field. AC power is constantly changing, reversing its electron flow cyclically, and therefore makes it easy to take advantage of induction. If a wire is wound around a ring of iron and connected to an AC power source, it will create a moving magnetic field through the iron ring. A second wire can be wound around the iron ring and the changing magnetic field traveling through that coil of wire will produce an electric current. What I’ve just described is a transformer, which is a useful electric component for isolating an electric source from a load and can be used to change AC voltage by changing the ratio of windings between primary and secondary coils. The same principle can be used to make an elegantly efficient motor.
Figure 4.15AC induction
Figure 4.16AC induction motor
The AC induction motor has a stator that is constructed of wire wound around iron bars. Those windings are grouped into the same three phases of the electric supply and symmetrically distributed around the stator. When an alternating three-phase current is applied to the windings, a constantly changing magnetic field is produced. The arrangement of the windings, coupled with the changing current, creates a rotating magnetic field.
The rotor of the induction motor is constructed as a cylindrical cage of copper rods held in place by a series of laminated steel shims. This motor is sometimes referred to as a “squirrel cage” motor, though for me the vision of a hamster wheel makes more sense since I’ve seen more hamster wheels than squirrel cages. There is no physical connection between the rotor and stator. The cylinder formed by the rotor bars almost touch the interior walls of the cylinder formed by the stator with only a small air gap between.
Figure 4.17AC squirrel cage rotor
As the rotating magnetic field of the stator sweeps around the hamster wheel rotor, the laws of induction correctly predict that an electric current will be induced on the bars of the rotor. The induced current in the rotor will in turn create a magnetic field that opposes the stator field. The opposing magnetic forces cause the rotor to turn, just as any opposing magnetic forces will push away. As long as there is a change in magnetic field between the stator and the rotor, a current will be induced on the rotor, the current will produce an opposing magnetic force, and the rotor will spin. The key point here is that the magnetic field of the stator must keep moving faster than the rotor, or to phrase it another way, the stator field must be slipping past the rotor field. If the rotor and stator ever run at the same speed, in synchronicity, there will be no difference in velocity between the magnetic field of the stator and the copper bars of the rotor. No current would be induced, no opposing magnetic field, and the rotor would cease to have any torque. The stator field is always dragging the rotor along, running circles around the rotor, but not in sync which is why AC induction motors are known as asynchronous motors.
The operation of an AC induction motor is not intuitive for most people at first (at least it wasn’t for me the first time I opened one up). But as you form the mental image of its operation, the elegance of the design is astounding. Using the benefits inherent in alternating current, induction, and magnetism a motor can be simply constructed out of iron and copper which produces the energy required on both the stator and rotor (with nothing physically connected to the rotor) creating a magnetic force. Brilliant, isn’t it?
Now that we understand the basic operation, how does the three-phase AC induction motor score on our checklist of priorities for theatrical use?
To vary the speed of the motor, we have to slow down the magnetic field of the stator. That field is being produced by the three-phase circuit powering the motor, so it stands to reason that the frequency of the AC waveform must be slowed down. Varying the frequency of the AC waveform powering the stator is easy to say and envision, but much harder to do in practice. Up until the last few decades, varying the speed of an AC induction motor was quite expensive but, as we’ll see, the price of the required electronics has reduced greatly. The electronic speed control for an AC induction motor is called a Variable Frequency Drive (VFD). To be effectively controlled by a VFD, an AC induction motor must be made to a higher standard than a motor that operates at a single speed. When purchasing an AC induction motor, make sure you purchase an “Inverter Duty” motor that is built to produce consistent torque at varying frequencies.
To change direction of rotation, we only need to change the order of the phases supplying power. Swapping any two of the three power leads will cause the motor rotation to reverse. This is good to know if you wire up a winch, or any three-phase motor, and need to invert the “forward” direction from clockwise to counter-clockwise or vice versa.
This is a good spot to emphasize that the only AC motor you want to use in your stage machines for automation work is the three-phase motor. If you look in a Grainger catalog, you will see many motors listed that work on 120 VAC or single-phase 220 VAC and have names like “split-capacitor start” or “shaded pole” motor. They employ extra components to kick-start an AC motor with single-phase service power but cannot be well controlled with good torque through a range of speeds. A three-phase induction motor with a good variable frequency drive (VFD) is required for good torque at variable speeds.
Compared to their DC counterparts, AC induction motors don’t have great torque at lower speeds. However, when paired with a good VFD, they can produce very respectable torque at reasonable low speeds. To obtain excellent low-speed torque, or the harder, zero-speed torque load holding, a VFD will need the help of a sensor on the motor’s rotor to produce the magnetic gymnastics on the stator to produce strong torque on the rotor.
AC motors are very quiet. Usually some other part of the machine or the movement of the scenery will drown out the little noise produced by the motor. Their minimal noise level is probably the second biggest contributing factor to the AC motor’s popularity in stage automation.
When selecting an AC motor you usually want to pick a NEMA C-Face motor (or IEC frame in Europe) so you can easily attach it to a speed reducer, brake, and possibly a C-Face encoder. NEMA C-Face is US standard sizing that specifies mounting dimensions to make motors and accessories interchangeable. There is a section later in this chapter dedicated to C-Face with a handy chart of the various sizes. Outside of the US, IEC has a similar system for motor dimensions but of course the systems are not cross-compatible.
Figure 4.18C-Face motor
The other common option when purchasing an AC motor for mounting into a machine is to purchase a pre-assembled gearmotor from a manufacturer like SEW or Nord. A gearmotor mates the motor with a speed reducer and, optionally, a brake and encoder to your specification. You can select the horsepower, speed reduction, and various accessories from the manufacturer and it will arrive as a packaged unit. This is often the best way to purchase a motor since it saves you the hassle of assembling the various components, but you do lose some flexibility since you can’t change the speed reducer later on to accommodate a different machine.
Because of their simple construction, AC induction motors are inexpensive when compared to DC permanent magnet motors and brushless servomotors. Also, there isn’t a severe price increase when increasing horsepower, which makes it reasonable to purchase a bigger motor than you may need to insure a little headroom in a machine. For example, at the time of this writing the upgrade from a 3 HP motor to a 5 HP motor only costs 20% more. So you can get roughly 60% more power for 20% more money. That’s a pretty good bargain.
Figure 4.20AC motor nameplate
Looking at the nameplate of an AC motor gives you the specs of the motor at a glance. The interesting points are:
•Horsepower – It is what it says on the tin: horsepower rating.
•Frame – If it’s a NEMA C-Face motor, the frame designation will be printed so you can match the motor to other components.
•Poles – The synchronous speed of a motor at a given frequency is determined by the number of poles on the stator winding. The formula for determining synchronous speed of an induction motor is:
•Synchronous Speed in RPM = (120 × Frequency) / Number of poles
A common four-pole motor would have a synchronous speed of 1800 rpm at 60Hz (120 * 60 / 4).
•Volts – AC motors that you’ll use on stage are typically 230/460 V motors, meaning that they can be wired for either voltage depending on how the stator windings are connected to the power. Either on the motor nameplate, or inside the junction box of the motor there is usually a diagram that shows how the motor can be wired for either voltage. For production, most of the time we wire motors for 230 VAC in the US. Abroad, however, or in permanent installations, 460 V is common. High voltage operation requires less current, and therefore less copper, making it cheaper.
•Full-load amps (FLA) – The maximum safe operating current of the motor. Given as two values for low-voltage operation and high-voltage operation. For instance, a ¼ HP motor might have an FLA of 1.16/.58 and a voltage rating of 230/460. This means at 230 V the full-load current draw is 1.16 A, but when wired for 460 V the full-load current drops in half to .58 A.
•Max Safe RPM – AC motors can be spun well beyond 60 Hz when using a Variable Frequency Drive. If you feed in AC power faster than 60 Hz, the motor will oblige by spinning faster. The Max Safe RPM gives the upper limit. Once you exceed the utility supply frequency of 60 Hz (or 50 Hz outside the US), the torque of the motor drops off proportionally. A motor that produces 3 ft*lbs of torque at 1750 RPM will produce 1.5 ft*lbs of torque at 3500 RPM.
•Torque – Though you can calculate the torque based on horsepower and speed, some motors will also stamp the output torque of the motor at a given frequency on the nameplate. The torque of the AC motor is not constant throughout its speed range, and those specifics of the torque performance are only shown on the manufacturer’s specification sheet.
Because of the price, quiet operation, and easy availability, AC induction motors are the most common motor used in scenic automation. If you are starting a new project, or upgrading some older machinery, and need to buy a motor, you should use an AC induction motor unless there is a compelling reason not to. It is the current default choice.
DC Permanent Magnet
The DC permanent magnet motor has a stator constructed of permanent magnets of opposing poles. One side of the stator is magnetic north, and the opposing side is magnetic south. In a large horsepower motor, the magnets are quite large and have a strong magnetic pull, strong enough that you can stick your wrench to the side of the motor housing while working on a DC machine. The rotor is made of copper windings. When DC voltage is run through the rotor winding, or armature, the winding becomes an electromagnet with a fixed magnetic polarity and will twist either clockwise or counter-clockwise, depending on how the magnetic polarity of the rotor interacts with the magnetic flux created by the stator.
Figure 4.21DC motor anatomy
To conduct DC voltage to the rotor, carbon brushes contact a conductive ring that is fastened to the rotor. The brushes are placed 180 degrees apart from each other. One brush is connected to the positive charge of the DC supply, the other to the negative. The ring, known as a commutator, is sliced into segments with each segment attached to a different winding on the rotor. As the rotor spins, the carbon brushes break contact with one segment of the commutator and reconnect with the next segment thus energizing the next set of coils on the armature. This mechanical action of continually adjusting which set of coils on the armature are energized maintains constant opposing magnetic forces between the stator and rotor. To change speed, the DC voltage is raised or lowered. Higher voltage generates faster speed, lower voltage produces slower speed. To change direction, swap the polarity of the voltage feeding the rotor. It is a very simple arrangement and more intuitive than the AC induction motor.
How does the DC permanent magnet motor stack up on our feature list?
DC speed control is much simpler than AC induction motors. Rather than having to sculpt AC waveforms at varying frequencies, a DC drive merely needs to adjust voltage to regulate speed. Traditionally, this meant that electronic speed controls for DC motors were a lot cheaper than VFDs for AC motors. However, the volume of VFDs produced to satisfy industrial needs has reduced the price so that now the gap between speed control technologies is not so great. DC speed controls are still cheaper in the 2 HP and lower range, but not by enough to make a difference in your purchasing decision.
DC motors have excellent low-speed torque, and they have a much easier time producing full-torque at zero speed than AC induction motors. Rather than having to precisely rotate a revolving magnetic field to induce a current on the rotor and generate a magnetic field on the stator, DC motor controls just need to apply more or less current to alter the strength of the electromagnets on the rotor while the stator magnetic field is fixed by its permanent magnets. Holding a load steady is just an exercise in finding the precise magnetic strength required keep the rotor suspended against the stator.
DC motors are noisy. The carbon brushes continually making and braking contact with the commutator ring creates a constant hum. That hum grows louder as the motor works harder to move a load since it is then drawing more current, which creates a bigger arc at the commutator. The noise is often enough reason to banish DC motors from the stage and instead hide them in the trap room, or shroud them in sound baffling to reduce the annoyance.
DC motors in ¼ horsepower and greater are available in C-Face configurations. These NEMA C-Face sizes are identical to AC motors and so they can easily be swapped out. In the past ten years I have seen a lot of theatres pulling DC motors off of winches and turntable machines and replacing them with AC motors. Because the C-Face mounting is identical, the machine can be used without modification regardless of what type of motor powers it.
In fractional horsepower sizes, DC motors come in a wide variety of shapes and sizes. In this small range, they also come in pre-packaged gearmotors, like miniature versions of the large AC induction gearmotors. For small effects, these small motors can be quite handy since they are easy to automate with precise speed and position control while fitting into spaces much too small for three-phase AC motors.
Because of the large magnets used in the stator, and their waning popularity in industrial applications, DC motors are substantially more expensive than AC motors once you get above 2 HP. In smaller sizes, below ¼ HP, DC motors are quite economical but situations are rare where tiny motors are useful.
The nameplate stuck to the case of a DC permanent magnet has a lot in common with an AC induction motor, but there are a couple of differences.
•Horsepower (HP) – It is what it says on the tin, horsepower rating. The horsepower rating is the same for DC and AC motors.
•Frame – If it’s a NEMA C-Face motor, the frame designation will be printed so you can match the motor to other components. The frame size is the same for DC and AC motors.
•Armature voltage – The maximum DC voltage that can be applied to the armature. At the rated armature voltage the motor will spin at the rated RPM. Unlike AC motors which can be run at higher frequencies to achieve higher RPMs, DC motors have no safe headroom above maximum voltage. Applying higher voltage risks damaging the motor.
•Full-load amps (FLA) – The maximum safe operating current of the motor.
•RPM – The speed of the motor when supplied with the rated DC armature voltage.
Today, DC permanent magnet motors mostly feel like technology from a bygone era. The noise and expense of the motors make them inappropriate for many machines. However, there are some situations where DC motors still should be considered. As I’ve mentioned periodically, small effects are best produced with DC motors. Battery powered machines are better done with DC motors since the battery produces a constant DC voltage. Varying that voltage to regulate speed is simpler than inverting the DC power to AC power. In other circumstances, the number of conductors that can be run to a motor is limited and in that case the difference between two power conductors needed for a DC motor versus three for the AC induction motor can be a benefit.
DC Brushless Servo
The most exciting motor technology recently is the brushless servomotor. While not new to industrial applications or high-end stage automation systems which can afford their traditionally high price tag, brushless servomotors and their controls are becoming increasingly affordable. Similar to the eventual commoditization of AC motors with Variable Frequency Drives, the price of brushless servomotors is rapidly entering a range that makes it worthy of consideration in many applications.
Figure 4.22Servomotor
The key features of the brushless servomotor are its small size, powerful torque, and low weight. A 2HP AC induction motor weighs roughly 40 lb. A brushless servomotor rated for the same continuous torque weighs about 15lb and is roughly ¼ of the size. Since we are often designing machinery that has to be loaded-in and loaded-out, the lower weight is a significant advantage. The reduction in size allows more horsepower to be crammed under platforms, inside show decks, or packed onto the flyrail.
How does the brushless servomotor achieve these advantages? Through a clever inversion of the DC permanent magnet motor construction. As we discussed, a DC permanent magnet motor has a stator made of magnets and a rotor made of windings. Placing the windings on the rotor has two drawbacks. First, the windings get hot when current runs through them. To keep the motor from melting its core, air flow has to be engineered, which requires space and increases the size of the motor. Secondly, placing the windings on the rotor mandates the use of the commutator to conduct power to the windings and flip the polarity of the armature as the rotor spins.
To eliminate the drawbacks inherent in the design of the brush-type DC motor, the brushless servomotor places the windings for the motor on the stator and embeds permanent magnets in the rotor. Because the windings are part of the stator and physically attached to the case of the motor, the case can be designed as a heatsink to dissipate heat generated from the current flowing through the windings. By eliminating the need for fan cooling and airflow, the size of the motor can be reduced. Also, since the rotor now has permanent magnets, the magnetic polarity is fixed and doesn’t require any electricity to be conducted to the rotor, thereby eliminating the commutator, which is a source of noise. Additionally, brushes and their maintenance are obviously eliminated.
Figure 4.23Brushless servo construction
With the windings now on the stator, a rotating magnetic field must be generated to create motion on the rotor. This is similar to the AC induction motor stator; however, in this case, the rotating electrical field is not slipping past the rotor to induce a charge on the rotor, rather the rotor’s embedded magnets already provide a polarity that will react to the rotating field of the stator. In the brushless servomotor, the rotor spins synchronously with the stator field. This has the effect of combining the high-torque characteristics of a DC motor with the quiet operation of an AC motor.
But how is the stator field generated? The servomotor must be paired with a drive that can precisely time DC pulses into the stator windings in alternating polarity at high speed. This is not terribly different than the role of a VFD with an induction motor. In fact, you will see brushless motors described as AC Brushless Servomotors, DC Brushless Servomotors, and Permanent Magnet Brushless Synchronous Motors. These are all synonyms for the same type of motor.
The brushless motor amplifier must have a sensor on the motor rotor to know when to send the correct polarity current to the stator windings. Requiring that sensor adds an expense not needed to spin a DC or AC motor. However, since we need to use motors for positioning scenery on stage, an encoder is always required for control purposes. That same encoder can be used for both speed regulation and positioning when connected to a control system.
Let’s score the brushless servomotor by our standard theatrical criteria:
The brushless servo must be paired with a drive for speed regulation, but with the correct electronics it is an incredibly precise machine. It is capable of very fast accelerations, spinning both at low speed and at very high speed with excellent torque output. Depending on the motor, speeds as high as or higher than 7500 RPM can be attained. Obviously, that’s too fast to use directly so a mechanical speed reducer is required, but it means that you can use much larger reduction ratios to get great torque.
Brushless servomotors have good low-speed torque, and good torque through a wide speed range. A motor will be rated for continuous duty torque and separately for peak torque that can be sustained only temporarily. Peak torque is several times greater than continuous torque, so it is possible to overcome large forces for short bursts with a brushless servo.
These motors are virtually silent.
The motors do not come in C-Face mountings and instead follow a different sizing standard. Additionally, because of their high output speeds they cannot be directly coupled to traditional speed reducers. High-efficiency speed reducers must be used that can handle the higher speeds without falling apart.
SEW-Eurodrive makes servo gearmotors that come pre-packaged, just like their AC induction gearmotors. On the one hand, this makes it easy and convenient to purchase, but on the other hand it creates a bulky, heavy package that defeats some of the motor’s advantage.
Though the price is coming down, you will still pay around a 25% premium for brushless servomotor systems. If budget is the main consideration, AC induction motors are still a more economical route.
I expect that soon we will see more and more brushless servomotors being employed in scenic automation. With prices dropping, the clear advantages of weight, size, and capacity for bursts of high torque fit very well with the demands of our industry. There is a complexity cost with the electronics, and some limitations of cabling that we’ll dig into in subsequent chapters, but I think those tradeoffs will be worth it for most applications.
Stepper
Most of the time in scenic automation we are dealing with large loads, moving at high speeds, requiring precision and constant feedback to insure that everything on stage is in exactly the right place. However, that capacity, precision, and feedback come with complexity and a financial cost. Sometimes there is an effect on stage that needs a motor and needs some programmability but does not warrant the use of any of the previously mentioned motors. For instance, to automate the clock face in A Christmas Carol (by Charles Dickens) using two AC induction motors, VFDs, and a closed-loop motion controller is bulky, expensive, and overly complex. That could be a good time to look at Stepper Motors.
Figure 4.24Stepper motor
Stepper motors are nifty little compromises packaged to look like a motor. They are cheap, easily programmable, have bizarre torque/speed curves, and go exactly where they are commanded except when they don’t and then you have no way of knowing that the stepper motor isn’t in the correct position. Obviously not a motor for every occasion, but occasionally they are a great solution.
The stator of a basic stepper is made of a series of windings that are oriented radially instead of axially, so that it looks like a bunch of electromagnets pointing towards the center of the motor. The rotor is a chunk of iron shaped like a star. Depending on which windings on the stator are energized, the rotor will hop over to be in line. The rotor does not generate a magnetic field itself, rather it swings around more like a compass needle searching for true north. Every time the stator changes to the next set of windings, the rotor indexes forward or reverse, one step at a time.
Figure 4.25Stepper motor construction
The drive for a stepper is called an indexer, and it is responsible for sending out the correct pulse to the stator. Because the rotor moves a precise amount every time the stator switches windings, the indexer can precisely position the rotor by sending an exact number of pulses. At high speed, the stream of pulses looks like continuous rotation, but the indexer is counting pulses the entire time and keeping track of where the rotor is, or rather where it should be. There is no sensor on the rotor, so if the rotor stalls out under load, the indexer will have no idea and will happily continue sending out pulses assuming the rotor is keeping pace with the stator. This is OK for a clock face, but clearly unacceptable for an elevator, hoist, or winch.
Indexers often have a basic programming interface that can be accessed from a computer over a serial link. In a matter of an hour or less you can wire up a stepper, power supply, indexer, and a computer and start programming it to move to a precise position, at a certain speed, with acceleration and deceleration. That is pretty powerful stuff if you can live with the stepper motor’s limitations. These motors are what you might find driving the axes on a low-cost CNC machine.
Figure 4.26Stepper indexer
To be fair to the stepper, let’s compare it with our standard criteria.
The stepper speed is easily controlled by the indexer and can be programmed for positioning as well.
The torque range of steppers is pretty low, which limits steppers to small effects. Within their range, steppers do have good low-speed torque, but at higher step speeds the torque decreases dramatically. Every stepper manufacturer has a slightly different speed/torque graph for its motors, so carefully study the datasheets to make sure you’ll have enough torque at the speed you need.
Steppers make a distinct whine that changes pitch as the speed varies. Steppers are quieter than DC permanent magnet motors, but can make some awful sounds at certain speeds.
Steppers follow the same NEMA sizes as brushless servomotors (see Fig. 4.27). You can attach them to speed reducers, but it isn’t common since they have good torque at low speed and bad torque at high speed. However, just like other motors the rotor of a stepper is easily rotated when de-energized, so you may need a holding brake.
When you consider the ability to program position, speed and acceleration, steppers are great value. They are very cheap in comparison to the typical closed loop motion control system with any of the other motor types we’ve investigated.
NEMA C-Face
In the US, NEMA devised a standard for modular mounting to make it possible for motors to be easily installed and replaced. Motors that adhere to this standard are called C-Face motors. NEMA C-Face motors have a prescribed shaft size for their rotors, a hole pattern for mounting the motor, and the thread size of the mounting holes. There is a series of frame sizes to accommodate motors small and large. If you have a motor with a 56 C frame, you are guaranteed that another motor from another manufacturer with a 56 C frame will fit in the same mounting regardless of the internal motor construction. Typically, if you are constructing a machine and want the motor to be a modular part of the assembly, you should choose a NEMA C-Face motor for both flexibility and ease of maintenance and repair.
Figure 4.27NEMA frame sizes
| NEMA Frame size | Motor shaft length |
| 56C, 56HC | 1 7/8 in |
| 56J | 2 7/16 in |
| 143TC, 145TC, 146ATC, 1412ATC, L182ACY, 186ACY, 186ATC, 189ATC | 2 ¼ in |
| 182TC, 184TC, 219ATC, 2110ATC | 2 ¾ in |
| 213TC, 215TC | 3 3/8 in |
| 254TC, 256TC | 4 in |
Outside the US our funny little system of measurement isn’t used, so NEMA C-Face doesn’t apply. For metric machinery, IEC (International Electrotechnical Commission) standards apply and fill a similar role as the NEMA C-Face. When selecting a motor, be wary of crossing the streams. Regardless of how much more sense the metric system makes, if you are building a machine in the US your life will be less painful if you stick with NEMA sizes and imperial hardware. If building a machine abroad, or destined to be used abroad, it makes sense to use IEC standards.
Electric motors are fantastic for most automated machines. They are easy to control, affordable, come in a wide variety of types and sizes, and are well understood by many technicians, which makes them easy to service and maintain. As great as they are, electric motors aren’t always the right prime mover for a machine. Sometimes an effect requires immense force in a small space, or the actuator needs to be near-silent when it moves. In those cases, fluid power is good to consider.
At a minimum, a hydraulic system consists of a reservoir of oil, a pump, a valve, and an actuator. The combination of pump, reservoir, often a filter, and possibly some valves in a packaged unit is called a Hydraulic Power Unit, or HPU. The pump pushes oil out of the reservoir and into a cylinder or hydraulic motor, causing the cylinder to extend or the hydraulic motor to spin. The hydraulic system is closed. Oil exiting the actuator is returned to the reservoir where it can be sucked up again and pressurized by the pump. Any oil poured into the reservoir from a bucket, or barrel, is recirculated through the system until the oil is replaced, or unintentionally leaks out all over the floor of your trap room.
Figure 4.28Simple hydraulic system
The HPU is rated for flow rate in GPM (gallons per minute) and pressure in PSI (pounds per square inch). A small HPU might push 1 GPM at a maximum pressure of 1500 PSI. A medium-sized HPU might push 10 GPM at a maximum pressure of 2000 PSI. A large HPU might be capable producing 70 GPM at 2000 PSI, or even more.
Since the hydraulic oil compresses very little, most of the energy used to push the oil with the pump is transferred through the oil and into the actuator to move the load. The pump is typically spun with an electric motor, though any mechanical power source can be used. The pump and actuator can be separated by a flexible hose or rigid steel tubing.
Hydraulic systems are appropriate in machines that require a lot of force in a small space. Hydraulic actuators are simple devices that transmit the force of oil pressure into either linear or rotary motion. The oil pressure is generated by a remote pump that doesn’t have to be nearby. I like to think of a hydraulic system as a big liquid speed reducer. The fast spinning motion of the pump motor is reduced into a flow of oil. That oil can be routed through a flexible hose to an actuator where the force is ultimately transmitted. Being able to split the primary power source from the output force into two flexibly coupled devices (pump and actuator) enables you not only to fit a lot of force into a small footprint but also lets you keep the big, noisy pump far away from the stage to eliminate noise. Hydraulic actuators are practically silent when filling with oil to push or pull scenery.
In addition to little noise and a dense power package, hydraulics are useful in effects where electricity can’t be used safely, like underwater. In a water effect, an electric motor isn’t practical for both shock risk and obvious damage to the machine from being submerged. Hydraulic actuators have neither of those limitations.
Controlling Pressure and Flow
From an automation perspective, since hydraulic performance is governed by oil flow we need to understand how to control the flow. The force of an actuator is determined by oil pressure and the area of the actuator exposed to the oil pressure. Hydraulic cylinders are sized by their bore, piston rod, and stroke length. A 3 in bore cylinder with 12 in stroke will have a contact area exposed to the oil pressure equal to the area of the bore. To determine the force produced by the oil hitting the cylinder, we multiply the bore area by the oil pressure.
To calculate the area of the circular piston:
Figure 4.29Circle with radius
where r is the radius of the cylinder bore, which is half the diameter.
Plugging these values into the formula for the area of a circle:
To calculate the resulting force of oil pressure across the area of the piston:
The pressure produced by the pump directly affects the amount of force an actuator can produce. If 7070 pounds of force is inadequate for the load, the options are to get a cylinder with a larger diameter bore or increase the oil pressure in the system. If you get a bigger cylinder the force increases, but also increased is the amount of oil required to move the cylinder. To calculate the amount of oil a cylinder uses when extending the rod, we compute the volume of the cylinder of oil created by the bore and stroke length.
Figure 4.30Cylinder
To figure out the volume of the cylinder:
With a 3 in bore and 12 in stroke, the volume of the cylinder is:
Pump flow is sized in gallons per minute. To convert the volume of the cylinder from cubic inches to gallons:
If our pump could push 1 gallon per minute, we can calculate how many seconds it will take to fully extend the cylinder by converting the flow to gallons per second.
At that flow rate, we can calculate how many seconds it will take to fill the cylinder fully with oil.
Increasing the bore will increase the volume of oil required to fill the cylinder. Assuming the pump stays fixed at 1 GPM, it will take longer to extend the cylinder. When picking a pump, we must consider both the pressure and the flow rate to generate the correct force and speed respectively. To determine the horsepower needed to spin a pump at a given flow rate and pressure, you can use the formula:
The larger the flow, the bigger the pump, and the more expensive.
With the pump sized correctly, how can we control the speed and direction of the oil flow? In our motor discussion we saw how to manipulate magnetic fields to alter speed and direction, but hydraulics take a more physically direct approach.
Bang-Bang Effects (Fixed Speed)
If all you need to do is flip the oil direction to extend and retract, an electrically actuated solenoid valve is perfect. The valve is built to open or close when energized. Directing the flow from the pump to the cylinder extends the cylinder. Conversely, directing the flow from the cylinder to the reservoir allows the cylinder to retract. In this arrangement, the cylinder will extend and retract either at full speed, or at a speed set by manual flow control valves to reduce the oil flow.
Variable Speed Pumps
Fixed speed probably isn’t good enough for an automated effect. You need to control the rate of oil flow to vary the speed. One way to vary the rate of oil flow is to attach a motor to the pump and control the speed of the motor. Speeding the motor up will speed up the pump causing the oil to flow quicker. Slowing the motor down, slows down the oil flow.
Electrically Controlled Valves
Rather than spinning the pump with a variable speed motor, you could spin the pump constantly and then use an electrically controlled flow control valve to vary speed. This requires some planning for the pump so that it can maintain consistent pressure regardless of the flow valve condition.
We’ll dig deeper into the different valve and pump control options in subsequent chapters, but for now we’ll just leave it that to control the speed of the oil flow in a hydraulic system we can either control the pump speed or valve speed.
Pneumatics are the little brother of hydraulics. Many of the same concepts apply with the notable difference that air is being used to transmit mechanical energy rather than oil. Instead of a hydraulic pump, pneumatic actuators are powered by an air compressor. The compressor pushes surrounding air into a closed vessel. To prevent the air compressor from running constantly to produce a stream of compressed air, a storage tank is used to collect compressed air. The compressor stockpiles air in the tank and when the pressure drops below a settable threshold, a pressure-switch fires up the compressor to recharge the tank. Air compressors are rated by pressure and flow. Pressure is rated in PSI, just like a hydraulic pump, and flow is rated in CFM (cubic feet per minute) instead of GPM (gallons per minute). A cubic foot is equal to 7.48 gallons. A typical, permanently installed air compressor in a shop is rated for 14 CFM at 175 PSI. If we convert CFM to GPM, that air compressor can move 104 GPM, which is a huge volume of air when compared to an HPU, roughly 10x larger than a medium-sized HPU. However, note that the pressure is roughly 10% that of an HPU.
Pneumatic actuators look similar to their hydraulic big brothers, but pneumatic actuators are rated for lower pressure operation. Hydraulic cylinders and motors are rated for 1000 PSI and higher, while pneumatic cylinders and motors are typically rated lower than 300 PSI. Because of the lessened pressure demands, pneumatic actuators are often made of aluminum and thus substantially lighter than hydraulic actuators.
Figure 4.31Hydraulic vs. pneumatic cylinders
The method for calculating the force produced by a pneumatic cylinder is identical to hydraulic systems since both are governed by the physics of fluid power. Given the lower pressure rating of air compressors and actuators, the force produced is low when compared to a similarly sized hydraulic actuator. However, the large flow rate allows for very fast movements. Light, quick movements are easier and cheaper to achieve with a pneumatic actuator.
Unlike hydraulic systems, in a pneumatic circuit air that is squeezed out of an actuator is not recaptured. The exhaust is released into the atmosphere. There is noise created from the exhaust of each actuator. On stage, this hissing noise must be managed either by using mufflers at the actuator, or by running an exhaust house far enough away to attenuate the noise.
Oil is a moderately rigid medium, but air is highly compressible. This compression causes pneumatic effects to be spongy and imprecise. As a result, pneumatic actuators are typically relegated to roles that don’t require high precision, such as setting locks on trap doors, pulling pins on Kabuki drops and activating brakes on turntables.
We’ve covered a lot of ground in this chapter and delved deep into the Machinery portion of the Pentagon of Power. This gives you a great understanding of how the prime mover in a machine works. Next we look at how to provide controllable power sources to motors and valves.