For my next project, I want to do something with motors, but I don’t know much about them so I decided to research the topic. This post will summarize what I’ve learned. My research is from Motors for Makers: A Guide to Steppers, Servos and other Electrical Machines by Matthew Scarpino; I highly recommend it for those interested in further reading. It talks about motors, gears, linear actuators, as well as projects you can build using motors, like a quad-copter. You can even learn about generators! (I’m not sponsored)

Overview

All motors work based on the same principle: use magnetic fields and electricity to cause motion. However, there are a lot of nuances to motors:

1. What kind of magnets do you use? You can have a permanent magnet, which is simple but can’t be controlled, or an electromagnet, which can be controlled but requires control circuitry and power. If the electromagnet is on the moving part of the motor, how do you get it power?
2. Magnets interact differently with different materials:
   1. Another magnet will can be attracted or repelled, depending on how the poles are aligned
   2. a hunk of iron will be attracted and get as close to the magnet as possible
   3. A piece of wire that is carrying current will act like a magnet; which direction its pole points can be changed by reversing the direction of current flow
   4. A conductor that is moving near the magnet will have current induced in it, also making it act like a magnet. In this case, current through the conductor cannot be directly controlled.
3. What type of power does the motor use? DC? AC? How many phases?
4. How do you control the motor? How do you know when and where to provide power?

The list above shows that different types of motors implement the same basic theory in different ways.

For this post, I’ve organized motors based on how they’re powered since that will probably be the first thing anyone looks at to see if a motor is right for their application. Below are DC, polyphase, single phase and universal motors.

DC Motors

DC motors are probably what you’re going to use most in hobby projects since they work very well with batteries and voltage regulators. There are 4 types: brushed, brushless, steppers and servomotors.

Brushed Motors

A brushed motor is as simple as you’re going to get: you have permanent magnets arranged in a circle, and a loop of wire in the middle attached to the shaft. By forcing current through the wires, they become electromagnets and the loop (and shaft) orients itself to the magnets. After the loop has oriented itself, the direction of current flow is reversed, causing the loop to rotate and orient itself again. This process is repeated, resulting in the shaft turning as the loop rotates.

First, lets talk about the commutator. The commutator is an electro-mechanical component that acts like a switch; it feeds the loop of wire current. As the wire rotates, the commutator continues to feed the wire current. When the loop of wire rotates beyond a certain point, the direction of current is automatically reversed by the commutator, keeping the loop energized and the shaft turning. Once the loop has rotated beyond another point, the direction of current is reversed again. The commutator is what gives the brushed motor its name; in order to keep electrical contact between a stationary component (stator) and the a moving one (rotor), the commutator uses brushes of copper (or spring loaded pieces of graphite). Thanks to the commutator, the circuit powering the motor doesn’t need to worry about reversing current when you only want to spin the motor in one direction; just provide power and you’re good to go.

In order to control the motor, use this rule of thumb: the rotation rate of the shaft depends on the voltage across the motor, while the torque output depends on the current through the motor. Now, you could do some calibration where you see how much voltage and current you need for a certain load and then design a circuit to provide exactly that, but the more common way to control a motor is use an H bridge.

An H bridge is 4 switches (realized by transistors) that connect as shown above. The H bridge allows a microcontroller to let current flow one way or another, depending on which direction you want the motor to turn. The H bridge also allows no current to flow, which will disable the motor.

H bridges, besides letting you control direction, also allow you to control speed. The switches are turned on and off by a digital signal. If the digital signal is pulse wave modulated (PWM), then that allows the switches to be on (say) 10% of the time; then the motor will turn at approximately 10% of the maximum rate. The motor speed can be adjusted as desired by changing the duty cycle of the PWM signal.

There are three types of brushed motors: permanent magnet DC motors (PMDC), Series-Wound DC Motors (SWDC) and Shunt-Wound DC Motors (SHWDC). PMDC is what we’ve talked about so far. In an SWDC, the stator’s permanent magnets are replaced by electromagnets that are in series with the rotor windings. In SHWDC, the electromagnets are in parallel with the rotor windings. PMDC are simple but get weaker over time, SWDC have a lot of torque but are difficult to accurately control the rotation speed, and SHWDC have a relatively flat torque-speed curve, making speed control easier over various loads.

Brushless Motors

Brushes were necessary because you needed to provide electrical power to a loop of wire that moved; is there a way to avoid that? Instead of stationary permanent magnets and a moving loop of wire, brushless DC (BLDC) motors have permanent magnets embedded in the rotor and windings of wires that act as electromagnets on the stator. When one winding is energized, the permanent magnets on the rotor align (rotate) to be close to the electromagnet. By de-energizing one winding and then energizing the winding right next to it, you cause the rotor to rotate again. By constantly changing which winding is energized, you can get the rotor to constantly rotate.

Since you no longer need to provide power to a rotating spool of wires, you can throw out the commutator, which wears out over time. However, the commutator also provided the useful function of automatically switching current in the rotor, which resulted in an electromagnet that changed polarity as it rotated. Because that feature is no longer taken care of in the motor, the circuit designer must do that. Most BLDC motors have 3 phases, A, B and C; in order to provide the correct sequence of energized windings, you must apply power across the three phases in the correct order. An example sequence is shown below:

	1	2	3	4	5	6
A	Power	Power	None	Return	Return	None
B	Return	None	Power	Power	None	Return
C	None	Return	Return	None	Power	Power

Reversing this sequence will allow you to turn the motor in the opposite direction, which means no H bridge is necessary.

There is another source of complication with the sequence; you have to know when to go from one step to another. Going too quickly will prevent the motor from spinning, while going too slowly will cause the motor to spin in a jerking motion. To know the correct timing, you must know the orientation of the motor while it spins. This can be found out in two ways: you either use a sensor (expensive and bulky but good performance) or use back-EMF (cheaper and smaller).

Designing a motor control system is outside the scope of this post, but I’ll just mention that the two common ways to use back-EMF to control the motor are zero-crossing detection (ZCD) and extended Kalman filter (EKF). But the point is that controlling a BLDC is more complicated than a brushed motor. Fortunately, there are modules you can buy that handle all the details of the sequence and timing for you, called electronic speed control (ESC) systems. All you have to do is provide it with power and an interface (usually PWM) that tell it how quickly you want the motor to spin. ESCs can be programmed to behave differently, so you can adjust them for your application.

There are two types of BLDC motors: inrunner and outrunner. Inrunner have rotor inside the stator, hence the name. They are generally low torque, high RPM motors. Meanwhile, outrunners have the stator inside the rotor. This design provides ample room on the rotor to have lots of permanent magnets, which means outrunners are high torque, low RPM motors.

Stepper Motors

Stepper motors have the unique ability to rotate at specific increments; for example, they can rotate in 6° steps. They are great for applications that require precise controls, like the movement of a robotic arm or moving a platform into position. Stepper motors are very similar to inrunner BLDC motors in construction; they have a rotor with permanent magnets on it surrounded by a ring of electromagnets on the stator. By energizing one coil, you rotate the rotor; energizing another coil will cause the rotor to rotate again. The primary difference is the control: BLDCs are meant to rotate smoothly and continuously, while steppers move in discrete jerks. Additionally, while BLDCs only use windings and magnets as necessary to get rotation and torque, stepper motors require a lot of windings and magnets to get good angular resolution.

A stepper motor typically has two phases, A and B. There is also the opposite of these phases, A’ and B’, which are actually the same phase but the winding is done in reverse; this means that when A is a north pole, A’ becomes a south pole. The poles winding are arranged so you get the following repeating order: A, B, A’, B’, A, B, A’, B’… Now there are two ways you can make a stepper motor move: full step and microstep.

Full step means you rotate the motor by its angular resolution, so a 6° stepper motor will rotate 6°. You can do this by powering one phase at a time (power A, then power B while un-powering A) or you can do two phases at a time for more torque (power A and B, then power A’ and B’ while un-powering A and B). Note that when A and B are powered, the rotor will align itself to be at its half way point; as an example, a rotor will rotate 3° for a 6° stepper motor if the rotor was aligned with A, and then both A and B were powered.

The example above shows that you can move by smaller increments than the angular resolution, and that’s what microstepping is. By having multiple phases on at a time, and powering some phases more or less than others, you can cause the rotor to rotate by less than one full step. This will allow you to take an 8th of a step, for example.

There are three types of stepper motors: permanent magnet (PM) steppers, which are what we’ve been talking about, variable reluctance (VR) steppers, and hybrid (HY) steppers. PM steppers have good torque, but have poor angular resolution. VR steppers replace the permanent magnets with a hunk of metal; since the metal is easy to machine and shape, VR steppers have much higher angular resolution than PM magnets. However, since VR steppers rely on the force attraction between metal and electromagnet, as opposed to permanent magnet and electromagnet, they have much lower torque. Hybrid steppers have the best of both by providing good torque and angular resolution, and are what you see in most applications like 3D printers. They have teeth on their permanent magnets like VR steppers do, as well as on the iron cores on the stator, which increase angular resolution.

Servomotors

Servomotors aren’t technically their own type of motor; they’re closer to packaged systems or modules rather than individual components. Servomotors, usually called servos, are a motor of some type hooked up to a sensor (usually a potentiometer or rotary encoder) and a control circuit. They operate based on the principle of closed loop; an external controller tells the servo what angle to rotate to, and the control circuitry inside the servo will rotate the motor to that position.

Servos are very easy to use because the hard part of motor control has already been done for you; usually you just have to send a PWM signal to tell the motor what position you want, and you’re done. Be sure the servo you want to use can give you the range of motion you need for your application; a lot of the ones I’ve seen are close to 180° or 360° of motion, though I’m sure there ones that can do as many rotations as you want.

Polyphase Motors

Polyphase motors are, as their name suggests, motors that require more than one phase of AC power, usually three. Polyphase motors are used primarily for industrial and heavy-duty applications, and you probably won’t use them for hobby projects.

All 3 phase motors have the same stator; like BLDC motors, the stator has a bunch of electromagnets. What’s important, though, is that the motor has 3 phases that are all 120° apart electrically. When wired correctly, the three phases produce a magnetic field that rotates over time. The equation for the magnetic field’s rotation speed is

n_s = 120 * f / p

n_s is the rotation speed in RPM of the magnetic field, called synchronous speed, f is the frequency of the mains voltage, and p is the number of poles per phase. For example, a three phase motor with 2 poles per phase will have a magnetic field that rotates 3600 times per minute, or 60 times per second.

Asynchronous (Induction) Motors

Faraday’s law of induction states that a changing magnetic field causes current to flow in a conductor. If a close loop conductor, embedded in the rotor, is placed in the stator’s rotating magnetic field, then the rotating field will induce current to flow around the loop. Since a current carrying conductor in a magnetic field will have force applied to it, the magnetic field applies force to the rotor and cause it to start spinning.

Asynchronous motors get their name from the fact that they cannot rotate at the same rate as the magnetic field. If they somehow did, then the conductors would rotate at the same rate as the magnetic field, which means there is no change in magnetic field from the loop’s perspective; this would cause the current in the conductors to drop and the rotor to start slowing down. For this reason, asynchronous motors always rotate at a lower speed than the synchronous speed.

A more complicated type of induction motor is the wound-rotor motor (WRIM) or slip-ring motor. Since the rotor is powered by induced current, the outside circuitry has no way to adjust the rotor’s behavior, which would be necessary if you wanted to adjust the motor’s power curve. These types of motors have slip rings that place the adjustable resistor in the path of the conductor loop. Squirrel cage rotors (used in regular induction motors) have high inductance, so there is a phase difference between the induced voltage and induced current, reducing the amount of torque the motor can produce on startup. The resistors, which increase the loops resistance, provide power factor correction, improving the motors starting torque while reducing startup current. Since these resistors limit the maximum current (and therefore maximum torque) in the rotor, and the phase difference between induced current and induced voltage decreases as the rotor gets faster, the resistors become a problem rather than a solution, so the resistance is reduced after startup.

Another way to reduce the startup current is to use a star configuration initially, then switch to delta in the stator windings. In a star configuration, when voltage is applied (say across 1 and 2), the voltage is split between two coils, which reduces the amount of current necessary. However, since current is reduced, the maximum torque output of the motor is also limited. That’s why motor control systems can use contactors (similar to relays) to switch from star configuration, at start up, to delta for regular operation. Delta configuration, since the full voltage appears across each winding, has a greater maximum torque output.

Induction motors, as are all AC motors, are controlled using either eddy-current drives or variable-frequency drives (VFD). Eddy-current drives sit between the motor’s shaft and the output shaft. A controller monitors the output shaft, and if the output is rotating below the desired rate, then it is connected to the motor’s shaft through a clutch. If the output shaft starts to rotate too quickly, then the clutch is disengaged. This closed loop system allows the output shaft to spin at whatever rate the user desires.

VFDs are similar to PWM controlled H bridges used for brushed DC motors. A VFD takes in mains voltage (AC), and rectifies it to a DC voltage. Then, using transistors, the VFD outputs a sinusoidal pulse width modulated (SPWM) power to the motor, one for each phase. This way, by changing the frequency of the sinusoid, the motor effectively gets power that’s at a different frequency from the one provided by the power grid, changing the rotation rate of the magnetic field.

Synchronous Motors

Induction motors require the rotor to rotate at a slower rate than the magnetic field since this is a necessary requirement to induce current in the conductors. Synchronous motors, meanwhile, do not need induced current to provide power to the rotor, so they can rotate at the same rate as the magnetic field.

There are three types of synchronous motors: doubly excited synchronous motors, permanent magnet synchronous motors and synchronous reluctance motors.

1. Doubly excited synchronous motors have windings on the rotor that receive DC power through slip rings. This turns the windings into electromagnets which align themselves to the rotating magnetic field, causing the rotor to spin at the same rate as the rotating magnetic field. Interestingly, the electromagnets do not provide enough torque to get the motor started, so the rotor also has a squirrel cage. At startup, the squirrel cage, through induction, provide the torque to get started; then the electromagnets provide the extra torque necessary to rotate at the same rate as the magnetic field.
2. Permanent magnet synchronous motors (PMSM) have permanent magnets on their rotors, which means they operate just like brushless DC motors. The permanent magnets rotate to align themselves with the rotating magnetic field.
3. Synchronous reluctance motors are just like the variable reluctance stepper motors; the rotor is a large hunk of metal. The metal will rotate to be as close to the energized winding on the stator, and you can rotate the metal by constantly changing which winding is energized. Due to their low torque output, they’re pretty rare.

Synchronous motors are also controlled using eddy-current drives or VFDs; see asynchronous motors section for details.

Single Phase Motors

The discussion for single phase motors will be focused on the stator since that is what distinguishes single phase motors from polyphase ones.

Polyphase power is really useful because a three phase motor can easily create a rotating magnetic field; the phases are conveniently out of phase with each other, and all you need to do is make sure the windings of the stator are also out of phase. However, with single phase motors, you don’t have that luxury; a single phase can only create an oscillating, not rotating, magnetic field. That’s why stators in single phase motors are designed to create a second phase using the first. Note that the second phase isn’t as strong as the first, and they’re both pretty close in phase, but for most applications that is good enough. There are three ways to create this second phase:

1. Split phase motors: these motors have an auxiliary winding and a switch in parallel with the main winding. The switch is closed when the motor is at low RPM, and disconnects as the motor starts spinning faster. The main winding has low resistance and high inductance, while the auxiliary winding has high resistance and low inductance. This means that even though both winding are receiving the same voltage, their currents are out of phase with each other (typically about 30°). This small phase difference is enough to produce a rotating field which gets the rotor to start turning. Once the rotor starts spinning, the switch disconnects, killing auxiliary winding. After startup, the inertia of the rotor and the oscillating magnetic field from the primary winding is enough to keep the motor going.
2. Capacitor-start motors: similar to split-phase motors, but it adds a capacitor in-between the auxiliary winding and the switch. The capacitor further increases the phase difference between the main and auxiliary winding, increasing startup torque while decreasing startup current. The switch also removes the auxiliary winding and capacitor from the circuit once the rotor is fast enough. There are two variations of this design:
   1. Permanent split capacitor (PSC) motor removes the switch, so the auxiliary winding and capacitor are always in parallel with the main winding. It has lower starting torque but is more efficient and reliable.
   2. Capacitor-run motor is a PSC motor with a capacitor and switch in parallel with the permanent capacitor. The capacitor and switch increase the capacitance (and therefore phase difference) at startup, and then get removed from the circuit, leaving only the permanent capacitor. They have the high starting torque of capacitor-start motors with the efficiency of PSC motors, but these motors are more expensive.
3. Shaded-pole motors: a conductive ring, called a shading coil, that is electrically isolated from the stator’s windings is placed on a part of each iron core. As the winding creates an oscillating magnetic field, current is induced in the conductive ring which also generates a magnetic field that is out of phase with the original magnetic field. This will produce a very weak rotating field and is only used for very low torque applications, like turning the hands of a clock. This motor design is very economical.

Universal Motors

Universal motors can work off of DC or single phase AC, and have similar construction to SWDC motors. This means the windings of the stator and the windings of the rotor are in series with each other through a commutator. The reason universal motors can work off of DC or AC is because they only care that the current through the stator and the rotor go the same direction; reversing currents going through both simultaneously will just flip all the poles in the stator and rotor, so not much has changed in terms of producing torque. Since the stator and rotor windings are in series, they will always have the same current, therefore universal motors can work on DC or AC.

The key difference between universal motors and SWDC motors is that universal motors are designed to have a low phase shift between voltage and current. Since SWDC motors only work on DC, the design is not optimized for low inductance. Meanwhile, in a universal motor, the inductances from the stator and rotor windings cause enough phase shift to prevent the motor from working when it is running off of AC. To address this, universal motors have as few windings as possible on the stator’s windings, as well as a compensating winding in series with the stator winding to reduce phase shifts.

Like SWDC motors, universal motors have good staring torque, since increasing current through the stator also increases current through the rotor. Universal motors also share the same drawbacks as SWDC motors, though; the commutator wears out over time, which reduces motor reliability and life-span.