A Useful Introduction to Electric Motors

One of the fundamental elements of an electrical engineer is knowing about the different types of electric motors. Without electric motors, we would not have power tools, household appliances, or even cars. Those who are looking to enter an online or traditional electrical engineering program, or currently enrolled students, can use this post as a handy reference guide for studying the basics of DC and AC electric motors.

BDC Motor

DC Motors

DC stands for “direct current,” and they were the first type of motors that were commonly used because they could be powered from a pre-existing power distribution system. The speed in a DC motor can be adjusted through the intensity of its current.

The current is carried through an armature or stator. The armature contains the coiled (star-formation for a stator) copper or aluminum wiring (commonly referred to as “windings”), and they both provide points for the wire to connect to the rest of the motor so that the electricity can continue to flow properly. There are a few different types of DC motors that are briefly discussed in the following list:

  • Brushed (BDC) Motors (pictured to the right) have a brush on the inside that alternates the electrical current (commutates) through the armature at the same speed as the rotation of the motor.

    There are three main variations of BDC motors, and the difference between them lies in how speed and voltage are controlled and pass through the motor’s windings:

    1. Shunt Wound
    2. Series Wound
    3. Compound Wound
  • Brushless (BLDC) Motors, as the name implies, do not have internal brushes. Instead, the current in the motor is commutated electronically through the stator and the magnetic field. A rotor (a circular piece of the motor designed to rotate on its axis to create torque) rotates the stator and the magnets (producing the magnetic field) at the same frequency to continue the current’s flow.

AC Motors

A motor that reverses the direction of an electrical current at regular times is called an alternating current motor, or more commonly known as an AC motor.

This type of motor uses opposing and attracting magnetic forces to propel the AC through the motor. There are magnets attached to both an external stationary stator, and an internal rotor. The rotor is able to move because the current is reacting to the magnetic poles, creating a rotating magnetic field. There are many different types of AC motors, and the most common ones are mentioned below:

  • Induction Motors, also known as asynchronous motors, rely solely on AC as a power source. An induced current occurs in the rotor conductors and is caused by the interaction between the rotor and the magnetic field rotating around the stator. This interaction creates a relative speed which is less than a synchronous speed (defined below). Synchronous AC Motor

    There are two primary types of rotors used in induction motors that are listed below:

    1. Squirrel Cage Rotor – The copper or aluminum wires/coils (windings) are embedded into the cylindrical steel rotor. This type of rotor got its name based on its appearance.
    2. Wound Rotor – The windings are attached to external resistances through several slip rings.
  • Synchronous Motors (pictured to the right) need both AC and DC as power sources. These motors require an additional starting mechanism that allows for the poles attached to the rotor to begin moving almost as fast as what is needed, the synchronous speed. The synchronous speed is when the rotor is moving at the same speed as the currents that are being supplied to the motor. The magnetic field will then affect both the rotor poles and the rotating poles attached to the stator, allowing them to reach the synchronous speed.

There are many more exciting facets to motors, with this list as just the beginning. In an electrical engineering program, you will have the opportunity to expand your knowledge about the inner workings of electric motors and so much more!