How Electric Motors Work

December 1, 2002  for  QuietFlyer Magazine

Much has been written about choosing the right motor, estimating performance, installing the motor in your plane, and so on. This month, I’ve decided to go back to basics and describe how the motor actually works. Do you need to know this to fly electric models? Probably not, but a good understanding of the functioning of a motor can help you diagnose problems. And some people, myself included, like to know how everything works. So, if you’re interested, read on!

I’m going to start with the very basics, so if you already know some of it, feel free to skip ahead. I won’t be offended.

Magnets

The fundamental driving force behind all electric motors, whether brushed or brushless, AC or DC, is magnetism. We’ve probably all played with magnets at some time or other, and have learned about them in science class in elementary school.

Recall that any magnet has a north pole and a south pole (it just so happens that the earth is a magnet whose poles happen to correspond very roughly to the geographical poles, hence the names for the magnet’s poles). If you take two bar shaped magnets and line them up, they will be attracted to one another if one’s north pole is next to the other’s south pole. If you line them up north to north or south to south, they will repel each other. Opposites attract.

Consider an assembly of three magnets, as shown in Figure 1. The left and right hand magnets are fixed to some surface, and the center magnet is free to rotate about its center.

Because of the attraction of opposite poles, the center magnet will rotate until it is aligned as in Figure 2.

Because the magnet has weight, and thus momentum, it would actually overshoot slightly, and then come back, overshoot again, and so on a few times before settling down.

Now, imagine we could work some magnetic magic and swap the center magnet’s north and south poles just as it overshoots the first time, as shown in Figure 3.

Instead of coming back, it would now be repelled by the fixed magnets, and keep turning so it can align itself in the other direction. Eventually, it would reach the state in Figure 4, which looks suspiciously like Figure 1.

If we perform this pole-swapping every time the center magnet just finishes overshooting the aligned position, it would keep turning forever.

The problem is how to perform this feat of magnetic motion.

Electromagnets

The magnets we play with are called permanent magnets. These objects have a fixed magnetic field that’s always there. The poles are fixed relative to one another and relative to the physical magnet.

Another kind of magnet is the electromagnet. In its simplest form, this consists of an iron bar, wrapped in a coil of wire, as in Figure 5.

By itself it does nothing. However, if you pass an electric current through the wire, a magnetic field is formed in the iron bar, and it becomes a magnet, as in Figure 6.

If you turn off the current, it stops being a magnet (that’s a bit of a simplification, since in reality, it ends up remaining a weak magnet, but we needn’t concern ourselves with that for the moment).

So far, the electromagnet already seems quite useful, since we can use it to pick up iron, steel, or nickel objects, carry them somewhere, and then drop them by just turning off the power (wrecking yard cranes do this with entire automobiles).

The really interesting thing about an electromagnet is that its polarity (the location of the north and south poles) depends on the direction of current flow. If we pass the current through in the opposite direction, the electromagnet’s poles will be reversed, as shown in Figure 7.

Eureka!

If we replace the central magnet in our set of three magnets with an electromagnet, as in Figure 8, we have the beginnings of an electric motor.

Now we have two problems to solve: feeding the current to the rotating electromagnet without the wires getting twisted, and changing the direction of the current at the appropriate time.

Both of these problems are solved using two devices: a split-ring commutator, and a pair of brushes. Figure 9 illustrates these.

The two semicircles are the commutator, and the two arrows are the brushes. The current is applied to the brushes, indicated by the "+" and "-" signs.

With the current as shown, the electromagnet will be repelled by the two permanent magnets, and it will turn clockwise. After it has turned almost half way around, it will be in the state shown in Figure 10.

Then, just as the magnet reaches the aligned state, the split in the commutator passes under the brushes, and then the current through the electromagnet reverses, which takes us back to the condition in Figure 9. As a result, the magnet keeps turning. We have a motor!

Some Terminology

The discussion above has culminated in the design of a simple two-pole, two-slot, permanent magnet, brushed, direct-current (DC) motor.

The term two-pole refers to the fact that there are two permanent magnet poles involved in the operation of the motor, the south pole of the left hand magnet and the north pole of the right hand magnet. The motor would actually work with only one fixed magnet (for example, only the left hand magnet), but would be less powerful and efficient.

The rotating electromagnet is known as the armature. Two-slot means that the armature consists of a single coil of wire around a single bar with only two ends (the term "slot" refers to the gap between the armature ends, since the armature is not typically bar shaped, but has a wider end).

Real Motors

In a real two-pole motor, the two poles are often the two ends of the same magnet. Although the motor may appear to contain two separate magnets, the steel motor case ties them together to act as a single magnet. It’s really as if our motor were built like in Figure 11, with the rotating electromagnet inside a hole in the permanent magnet.

Practical real motors usually have at least a three-slot armature, and a commutator with three segments. There are however still only two brushes. Higher voltage and higher efficiency motors have even more slots (an odd number) and more segments on the commutator (the same as the number of slots), and more brushes (always an even number). Photos 1 and 2 show the armature, commutator, and brushes from a typical low-cost three-slot motor.

Figure 12 illustrates a three-slot motor in conceptual form. Notice that the brush is now wider, contacting the commutator segments over a wider area, and actually spanning two segments sometimes.

Also notice that both ends of electromagnet number 2 are contacting the "-" brush at the particular point in time captured by Figure 12. This means that no current is flowing through electromagnet 2, and only number 1 and 3 are on.

Effectively, the armature is now a pair of electromagnets; number 3 is being attracted by the north pole of the right hand permanent magnet, and number 1 is being repelled.

One twelfth of a turn later, as in Figure 13, all three electromagnets have current flowing through them.

Now, electromagnet number 1 is being both repelled by the right hand permanent magnet, and attracted by the left hand one. Number 2 is being repelled by the left magnet, and number 3 is still being attracted by the right magnet.

Another twelfth of a turn later, in Figure 14, electromagnet 1 is being attracted to the left hand magnet, and number 2 is still being repelled.

Electromagnet 3 is turned off. This progression of electromagnets switching on and off continues as the motor turns, eventually returning to the state of Figure 12.

The Brushless Motor

There are a number of drawbacks to the brush and commutator mechanism used in a brushed motor: the brushes cause friction, there is some electrical resistance in the brush-to-commutator interface, and the mechanical switching of the armature current results in sparking, which can cause radio interference. Brushless motors do away with the brushes and commutator to get around these problems. The result is greater efficiency (more output power for a given amount of input power), and less electrical interference.

The basic principles by which a brushless motor operates are exactly the same as those of a brushed motor. Figures 15 and 16 show two stages in the operation of a simple brushless motor.

Notice that Figure 15 is almost identical to Figure 9, except that there are no brushes and no commutator, and the types of the magnets have been exchanged. The permanent magnets have become electromagnets, and vice versa. The rotating permanent magnet is being repelled by the two electromagnets.

In Figure 16, almost a full turn later, the polarity of the left and right hand magnets has changed. The rotating magnet is now being pulled into alignment.

The problem to be solved here is how to cause the electromagnets to reverse their polarity at the right time. One could devise some sort of mechanical scheme controlled by the rotating permanent magnet, but this would nullify the main benefits of brushless motors.

Instead, the electromagnets are controlled by external circuitry. This circuitry monitors the current position of the rotating magnet, and energizes the external magnets appropriately to keep the motor turning. This circuitry is part of the brushless electronic speed control (ESC).

There are two ways for a brushless ESC to monitor the position of the rotating magnet. One is by way of magnetic sensors (based on the Hall-effect). These sensors report back to the ESC through a separate set of wires. The other method is known as "sensorless". Roughly, in this method the ESC monitors the three motor power wires for fluctuations caused by the spinning magnets.

Brushless Terminology

Since the electromagnet assembly in a brushless motor remains stationary, it is called a stator instead of an armature. The rotating magnet assembly is called the rotor.

Real Brushless Motors

Just as a real brushed motor rarely has only two poles and a two-slot armature, a real brushless motor rarely has only a two-pole rotor and a two-slot stator. Most commercially available brushless motors have at least four poles, and a nine or more slot stator. However, for purposes of comparison, Figure 17 illustrates a hypothetical two-pole three-slot brushless motor, corresponding to our two-pole three-slot brushed motor.

Notice there are three connection points to receive power from the brushless ESC (a motor with more than three stators has them wired in three groups, so there are still only three power leads).

In the state represented by Figure 17, power is being applied to the two leads labeled "+" and "-", which energizes the electromagnets as shown. The upper left electromagnet is attracting the rotor’s north pole, the lower left one is repelling it, and the right hand electromagnet is repelling the rotor’s south pole. As the rotor turns, the ESC will change which leads have power applied to them. Sometimes only two leads will, as in Figure 17, and at other times all three leads will (just like in Figure 13 for a brushed motor).

Real World Issues

The theory of motor operation described here is correct, but somewhat simplified. If you examine the diagrams closely, you’ll notice situations where the polarity might reverse too soon, apparently causing the motor to stop. Because of a number of factors, such as the time it takes for the magnetic field to collapse, and the momentum of the armature, a real motor won’t necessarily stop in this situation.

The relationship between the position of the armature (or rotor) and magnets (or stator), and the time that the electromagnets change their polarity, is known as "timing". In a brushed motor, it is adjusted by repositioning the brushes relative to the permanent magnets. In a Hall-effect sensored brushless motor, it is the sensors that are repositioned. In a sensorless motor, the ESC adjusts the timing automatically based on the feedback it is getting from the motor.

The optimal timing depends on motor speed and current, and for maximum efficiency, should be adjusted for the particular operating condition of the motor.

If you are familiar with internal combustion engines, this is similar to setting optimal spark plug timing. Theoretically, the plug should fire when the piston reaches the top of the cylinder (top dead center), but due to engine momentum and the time it takes for the fuel to actually burn, the plug must fire sooner. Modern car engines adjust this electronically to precisely suit the conditions; older car engines used a vacuum driven advance mechanism to adjust it according to engine load.

Other Motors

There are many other types of electric motors, such as AC induction motors, AC synchronous motors, stepper motors (really a specialized form of brushless motor), and so on. All of these motors operate on variations of the principles we’ve looked at. They differ only in how they perform the job of the commutator. Currently, none of these other types of motors are used in electric flight.

                    

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• Electromagnetic Interference Reduction