Types of Motors

You will come across many types of motors. People have been trying different ways to make a motor nearly as long as they have been messing with electricity. Here is a brief overview on some types of DC motors.

Brushless DC Motors

The brushless DC motors are cousins to the DC PM motor but instead of using brushes for commutation, they usually use some type of electronic control. To accomplish this, usually the inner side of the motor contains permanent magnets (where the armature is in the DC PM motor). This is known as rotor. The windings are on the outside and are referred to as the stator, or field windings.
There is no need for the magnets to be inside and the windings on the outside, but the thing is the windings 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 called electronic commutation, the idea opposite to brush commutation.

The biggest advantage to a brushless motor is right there in its name, it has no brushes in it. The brush in the DC PM motor will nearly always be the first thing to wear out.  There are many types of brush motors available and often they will be just fine for the application.

Stepper Motors

The Stepper motors are a type of DC motor in which the output moves a specific distance each time when we energize a winding. They are a cousin to the brushless motor and a weird animal called the switched reluctance motor. The capability to move a specific step has made these motors commonly used in positioning mechanisms. These are used in printers 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 distinguished by their ability of moving torque and holding torque. This is very important to understand because if you exceed either, your motor could slip and that would cause your count to be off.

AC and Universal Motors

Long time ago a guy by the name Tesla helped us all by convincing the powers that we should have an AC means of power distribution (vs. the local DC generators that Edison wanted). One main 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 the AC induction motor.

An AC induction motor induces a current in the armature by causing variation in the magnetic field in the stator. This induced current in turn creates a magnetic field that causes the rotor to turn, pushing against the first magnetic field.

. One result is that the motor tends to have a “sweet spot” where the rotational speed is almost right generating maximum speed and torque. At lower speed levels the torque drops off a bit fast. This directs to the fact that AC motors are not known for low-speed torque. A huge percentage of AC motors are of fixed speed outputs where the speed relies on the frequency of the AC signal. There are variable frequency drives or controls which are similar in architecture to DC brushless drives. They have the ability to vary the frequency into an AC motor creating a variable-speed AC drive. As AC motors do not have as that simple torque speed curve, these controls can be complex to some extent, often using DSP chips to handle all the math required to get what we want out of one of them.