You will run into many types of motors. People have been goofi ng around with different ways to make a motor nearly as long as they have been messing with electricity. Here is a bit of overview on some various types of DC motors.
Brushless DC Motors
The brushless DC motors shown in Figure 4.19 are cousins to the DC PM motor we discussed earlier, but instead of using brushes for commutation, they usu- ally use some type of electronic control. To accomplish this, usually the inside of the motor has the permanent magnets (where the armature is in the DC PM motor). This is known as the rotor. The windings are on the outside and are usually referred to as thestator, or fi eld windings. 16 There is no requirement for
FIGURE 4.19
Cool little brushless DC motor out of my RC airplane.
16 You can fl ip-fl op the magnets and the windings. You might see a motor with the magnets on the outside and the windings in the center. The windings are still stationary and the magnets still move. The RC world calls them outrunners.
the magnets to be on the inside and the windings on the outside, but the wind- ings are stationary and the magnets rotate. The rotor is turned by switching the stator windings on and off in a sequence that creates torque on the rotor. This is known as electronic commutation, as opposed to brush commutation, which we already learned about.
Often you will be told how a DC brushless motor is so much more effi cient than a DC PM brush motor. There is some hype to dig through here. Though this claim can be true, it is not entirely due to the fact that it is a brushless motor, as the brushless motor guys would have you believe. You will see num- bers showing improved effi ciency, but that is generally due to the choice of magnets. Most brushless motors are using rare earth magnets that have a much higher fl ux density than the more common ceramic kind. What this results in is fewer turns of wire for the same torque and speed. Fewer turns of wire means shorter wire, which means lower resistance. Since the resistance of the windings is the largest loss in the motor, this makes the motor more effi cient.
DC PM motors commonly use the ceramic magnets, resulting in more turns of wire. To make them more effi cient, you need to increase the wire diameter to lower its resistance. It is possible to use the stronger magnets in a DC brush motor.
The most common place I have seen this is in hobby stores. There are some pretty cool motors like this for RC airplanes. When built with these “ super ” magnets, the DC PM motor is pretty close to the same effi ciency as the DC brushless motor.
Assuming good bearings, the next point of loss in a motor is in commutation.
In the DC PM motor the brushes and brush contacts are the method of com- mutation. This interface is not perfect and creates a resistive loss. In the DC brushless motor, commutation is done with some type of silicon switch such as an FET, for example. Typically it takes at least six of these parts to commu- tate a DC brushless motor. These FETs have a resistive component (RDS on) that causes loss in the form of heat.
The biggest advantage to a brushless motor is right there in its name. It has no brushes. The brush in the DC PM motor will nearly always be the fi rst thing to wear out. Brushes by their nature are designed to wear out, but don’t let that stop you. There are many types of brush motors available, and often they will be just fi ne for the application.
One thing to note about brushless motors is that the controllers are more com- plex, requiring three to six times the power devices that brush motors use. But once you have them under control, you have already spent most of the money
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needed to make them go in both directions. So if that is a feature needed, it could make a brushless motor more of a candidate.
Stepper Motors
Stepper motors are a type of DC motor in which the output moves a specifi c distance each time you energize a winding. They are a cousin to the brushless motor and a weird animal called the switched reluctance motor. 17 The abil- ity to move a specifi c step makes these devices commonly used in positioning mechanisms. Printers use them by the bucket load.
Positioning is relatively easy since you can energize the windings and count the number of steps you have made to determine where the motor shaft is.
Stepper motors are characterized by their moving torque and holding torque.
This is important to know because if you exceed either, your motor could slip, and that would cause your count to be off.
17 Somewhere between an AC motor and a brushless DC PM motor lies the switched reluc- tance design. It is rare enough that the reader is left to his or her own resources to fi nd out how this unique design works.
Thumb Rules
In a motor, you put electrical power in and get mechanical power out.
Voltage * current ⴝ power; speed * torque ⴝ power.
Linear controls cause less EMI.
Linear controls are simple and cheap.
Linear controls are less effi cient due to heat loss.
Switching controls are more effi cient.
Switching controls cause more EMI.
Switching controls are generally more complex and expensive.
Constant voltage makes for constant speed with a DC PM motor.
Constant current makes for constant torque with a DC PM motor.
Don’t forget the freewheel diode in a switcher.
Replace the freewheel diode with an FET and you have a brake.
Use an H bridge to change directions.
Brushless motor controls are inherently bidirectional.
Stepper motors move in small steps or increments.
AC and Universal Motors
As we mentioned earlier, long ago a smart guy by the name of Tesla helped us all by convincing the powers that be that we should have an AC means of power distribution (vs. the local DC generators that Edison wanted). One key factor that helped with this debate was Tesla’s invention of the AC motor.
There are many types of AC motors. One of the most common and the one we are going to review here is theAC induction motor .
An AC induction motor induces a current in the armature by varying the magnetic fi eld in the stator. This induced current in turn creates a magnetic fi eld that causes the rotor to turn, pushing against the fi rst magnetic fi eld. When I fi rst learned this, it seemed to me that an AC motor can pick itself up by its bootstraps, so to speak. One result is that the motor tends to have a “sweet spot ” where the rota- tional speed is just right, generating maximum speed and torque. At lower speeds the torque drops off pretty fast. This leads to the fact that AC motors are not known for low-speed torque (unlike the DC versions we just discussed). For this reason and the fact that AC motors run off a sinusoidal alternating signal, a huge percentage of AC motors are fi xed-speed outputs where the speed depends on the frequency of the AC signal. There are variable frequency drives or controls, simi- lar in architecture to DC brushless drives. They can vary the frequency into an AC motor, creating a variable speed AC drive. Since AC motors do not have such a simple torque speed curve, these controls can be fairly complex, often using DSP chips to handle all the math needed to get what you want out of one of them.
AC motors have been around for years, making them relatively inexpensive, and their lack of brushes makes them last a long time. They can be built syn- chronously, like a stepper motor, so that you know they have moved a set distance every cycle of the AC wave. You will see them in all sorts of places:
running compressors in a refrigerator to timing the icemaker circuit in the same fridge. Back before the “ day of the diode, ” they were used in millions of clocks.
Universal motors are like PM motors without the permanent magnet. They use windings with current fl owing through them in the outer fi eld instead of said magnets. What makes them universal is the ability to wire them to work with an AC or DC source. I shocked myself more than once rewiring the motor down in the old milking barn trying to fi gure this out.
Motors of all shapes, sizes, types, and voltage preferences are out there.
Hopefully I have provided enough background so you at least sound smart when you’re asked about this topic.
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Solenoids
Thesolenoid is an electromagnetic device that typically moves to only two posi- tions. Akin to the stepper motor, solenoids are rated by holding force and mov- ing force.
Take a coil of wire and an iron rod that just fi ts inside the coil, as shown in Figure 4.20 .
Energize the coil. The rod will center itself in the coil due to the magnetic fl ux running through it. It is in fact reluctant to leave the warm home of its cozy little coil. This tendency for ferrous material to align itself with magnetic fi elds is known as reluctance .
Shut the magnetic fi eld down and the rod moves easily. Usually a spring is used to push the rod out of its cozy coil shell until power is switched back on and the rod returns to showing its reluctance yet again.
Solenoids are great if you need a short linear motion that is controlled by some- thing electronic. Also, this concept is the basis for an electromagnetic cannon.
We don’t have time to cover that topic in this book, though. Too bad; cannons are fun.
I
Magnetic Field
Iron Rod
FIGURE 4.20 Iron rod in coil of wire.
Relays
Relays don’t actually make anything move except the part inside them that closes a switch, so they might seem a little out of place in this discussion.
However, they are defi nitely electromechanical in nature and I couldn’t think of a better place to talk about them. Relays are very tough; they predate the transistor by a long time and are still in use. That should say something. They are basically the combination of a solenoid and a switch. A magnetic force pulls the switch shut or open, depending on the particular device. Markings on a relay usually indicate the coil that operates the relay and the labels NO, NC, and C. These abbreviations, also sometimes seen on switches, mean nor- mally open, normally closed, and common, respectively. NO and NC refer to the state of the switch when the coil isn’t energized. C is a connection to both these switches.
There are two important specs on a relay: the coil voltage and the contact rat- ings. If you under-drive the coil, you might get the switch to close, but there are no guarantees. Contacts are often rated at a minimum as well as a maximum current. Most engineers are diligent about paying attention to the max current, but they often ignore the minimum current. Many relays used in a power set- ting (which is very common these days) rely on a certain amount of current to be present when the switch opens. This current creates an arc that cleans the contacts and keeps them from corroding. Do you have a relay that simply stops working after a while? Chances are you are not meeting this spec. Use a relay in your design and you get to hear that satisfying click, letting you know that something is really working in that magic box.
Catching Flies
One thing all these motors, solenoids, and relays have in common is a coil of wire that is switching current at some point. A coil of wire is an inductor, and an inductor doesn’t like current changes. 18 So what happens when you shut off the current in an inductor? As the magnetic fi eld collapses when you cut off the current, a large voltage spike is generated (because it wants to keep current fl owing). This spike is sometimes called the fl yback. To keep this spike from damaging components and to use the energy in it, most applications employ a fl yback or, as it is also called, a freewheel diode that shunts this spike back to its source, as shown in Figure 4.21 .
18 By now this phrase should feel natural and intuitive. If it doesn’t, go back and study an inductor and how it relates to current till this concept makes sense.
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If the response of the LR in this circuit is slower than the switching frequency, the diode acts as part of a fi lter keeping current moving through the motor.
It smoothes out current changes, which in turn smoothes the torque changes.
(Remember how torque is proportional to current?)
In other cases, this diode might simply be capturing a transient signal to pre- vent circuit damage. Figure 4.22 shows an example using a diode to protect a relay circuit.
You can see that the voltage spike, inductive kick, or back EMF, as it is called, never gets over –0.7 V because once it does, it forward-biases the diode and FIGURE 4.21
An example of a fl yback diode around a motor.
Vcc
Relay MCU
FIGURE 4.22
Flyback diode on a relay coil.
current fl ows back into the other end of the inductor. Now you know how to make a fl y catcher out of a diode.
More Thumb Rules
When the thumb rules go on and on, break them into smaller, more digestible pieces.
AC induction motors induce a current in the core, which in turn creates a magnetic fi eld that turns the shaft.
Universal motors can be wired for AC or DC.
Solenoids are reluctant to leave the cozy coil cave when current is on.
Pay attention to the minimum switching current on relays.
Catch your fl ies with diodes to keep voltage spikes out of your circuits unless you are trying to make a shock box to surprise your buddy.