Motor Types

Motor Types (Video 2/4 in Motors Series)

Highlights

  •    All electric motors rely on electromagnetic fields to help them spin.
  •    Electric motors can be categorized based on how they change their electromagnetic field direction.
  •     DC motors should only be connected to DC power sources, and AC motors should only use AC power.
  •    Sending too much voltage or current into a motor can damage it and even cause fires.


Notes

There are a number of different types of electric motors. More general information about electric motor basics was covered in the previous section, Motor Intro. Read through that section first to gain a basic understanding of the motor terminology and construction. In this section, we will discuss a few of the most common types of electric motors: brushed DC motors, brushless DC motors, synchronous AC motors, and asynchronous (induction) AC motors.

In general, all types electric motors have an electromagnetic section that generates a magnetic field as well as a permanent magnet that follows the moving field, causing the motor to rotate. The various types of motors are different in their methods of “commutation.” Commutation simply refers to how the current is sent to the electromagnet to generate the magnetic field in a particular, changing direction. Each type of motor has different hardware and different methods to bring current from the power source (generally the stationary part of the motor) to different parts of the electromagnet (generally the rotating part).

Brushed DC Motors

The “brushed” DC motor gets part of its name from pieces of hardware (originally wire brushes) that touch the electromagnet commutator in the center to deliver current. To keep the motor spinning, the commutator repeatedly flips the flow of current in the electromagnet. Modern versions of these motors typically have graphite or metal plates instead of wire brushes for better commutation and to deliver more controlled, gradual changes in magnetic field direction. The other part of the name (DC) refers to the type of electrical current (direct current) that is required in order to run the motor properly. You can learn more about DC power from the AC and DC power page.

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Brushless DC Motors

Brushless DC motors are very common types of motors used by hobbiests and in devices such as multi-rotor UAVs. A separate page has been written just for brushless DC motors.

Synchronous AC Motors

“Synchronous” AC (alternating current) motors have electromagnetic coils on the outside stationary section of the motor, and the inside rotating section contains either a permanent magnet or electromagnet. Sets of wires on the outer electromagnet are connected in pairs, and these pairs are energized in a specific sequence to generate a magnetic field in changing directions. This continuous rotation of the magnetic field keeps the motor spinning. The motor output shaft and rotor turn at the same speed that the different wires are energized (hence, “synchronous”). Synchronous motors are most useful when a constant shaft speed is desired.

It’s important to note that this type of motor uses alternating current (AC) power instead of DC power. Typical AC motors have three phases of current that each come in as very fast, continuous sine wave inputs. Each phase is connected to one pair of electromagnet wires on the outer stationary part of the motor. The magnetic field direction changes based on where the strongest voltage is located at that time, which will vary based on the amount of current coming from each phase. Read through the AC and DC power page for more information about how AC power works in general.

Motors like the synchronous AC motor can be more difficult to start than DC motors. At low power levels (when the motor is first starting), the AC input sine waves and resulting magnetic field do not always produce enough movement in the right direction to keep the motor spinning. Due to motor starting resistances that must be overcome, sometimes additional power input from another source (such as a DC motor) is needed to get the AC motor spinning. “Squirrel cages” may also be added to the rotor to help the motor turn at low speeds and voltages. This additional piece of hardware is comprised of a steel cylinder and copper or alumnimum conductors. More information about the squirrel cage can be found here.

Asynchronous AC Motors

Asynchronous AC motors are similar to synchronous motors, except that they do not spin at the same rate that the electromagnetic windings are energized (in fact they must spin slower), and they do not require mechanical contact via commutators to produce a magnetic field. Instead, they use electromagnetic induction. Induction relies on transient electrical currents that generate changing magnetic fields over time.


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Additional Information

To learn more, visit one of these recommended website below, or simply search the internet for the terms introduced in this lesson!
Edison Tech Center — The Electric Motor
Wikipedia — Squirrel-cage rotor