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  • David Peterson

Why are there SO MANY Different Kinds of Motors?

When it comes to providing motion in a system, there actually aren't a whole lot of options.

We usually turn to either traditional fueled engines, fluid (air/hydraulic) power, or electric motors.


Electric motors are an ideal choice in many scenarios for a lot of reasons:

  1. They are clean and efficient, not wasting a lot of energy.

  2. The control is completely electric which may have some safety concerns, but is not toxic or dirty like exhaust and oil leaks.

  3. Control circuits can provide incredible motion precision and network control

There are certainly some good reasons for using motors.


But WHY are there SO MANY different kinds of motors out there?

Here's a brief list:

  • DC with permanent magnet stator

  • DC with stator windings

  • Stepper

  • Brushless DC

  • DC servo

  • AC 1-ph with capacitor start

  • AC 1-ph with capacitor phase shift

  • AC 3-ph synchronous

  • AC servo

This is not even CLOSE to a complete list, it just gives you an idea of what's out there.


I could imagine if different companies developed different motors, they might each have their own uses, but then after time, if one was better, wouldn't that one become the main motor in use?


Here's a brief breakdown of the benefits of several of those motors and why you might expect to see one preferred over the other.


DC Brushed Motor:

These small, compact motors can run at extremely high RPM with very low voltage. This makes them ideal for battery-powered DC applications where conversion to AC would be an unnecessary step.

There are also large DC motors that can supply output power on the order of many 10s or 100s of HP.


DC motor control circuits are quite simple - a pulsed control with varying width "PWM" can control the average current to the coils resulting in varying speeds. The circuits are effective and very simple.


DC Brushless Motor:

Many of the same benefits as the DC brushed motor, except that these contain no carbon brushes to alternate the polarity of the magnets in the rotating center (hence the name 'brushless').


Benefit: low-friction, extremely high speed with much greater efficiency and longer life. If supplied with high-current batteries, they can move heavier loads at high speeds, like in a drone propellor.


Problem: the control circuit is much more difficult since the coils on the outside of the motor must be alternated att he proper rate the get the proper speed. Max speed isn't always the goal. Usually, brushless motors include Electronic Speed Control (ESC) modules to pair along with the controller being used.


DC Servo Motor

Don't let the terminology fool you - these are a lot simpler than it might seem... If you have a sensor to read the position or RPM of a motor, that's called 'feedback'. If the control circuit reads the feedback and adjusts the voltage to get just the right position or RMP, you have a very well-controlled system.


If a normal brushed DC motor and the sensor are packaged together, this is a servo motor. The 'Servo' part of the servo motor doesn't indicate a fancy kind of motor, but rather the integrated sensing ability.


Capacitor Start AC Motor

This capacitor start (almost the same as the split-phase capacitor motor) only applied to 1-phase motors, you won't see a 3-phase motor with a capacitor.

They are recognizable because one side of the metal casing will have a large capacitor-shaped bulge that houses the capacitor inside.


The purpose of the capacitor is to slightly advance the time at which the current goes through a special start winding. This means that two different magnetic fields that are in slightly different locations around the rotor will be magnetized at a slightly different time.


The reason for this delay is to provide a consistent direction when starting. Once the proper start has been accomplished, the start winding can be turned off (capacitor-start) or it can be left on permanently (split-phase cap).


These motor are useful when you have a simple appliance that needs a moderate amount of power (up to a few HP) but it needs run from a simple 120-volt outlet. Washing machines, air compressors, and many small machine tools like drill presses use these motors. The power is easy to find.


Synchronous 3-Phase Motor

This motor is the ultimate simple to run and easy to understand motor in existence.


Three phases of voltage enter the building through a 3-phase service panel (many shops and all industrial facilities have easy access to power). The three phases are distributed in windings around the outside of the motor.

As the voltage of each phase rises and falls, it drives current first through one winding, then the next, then the last, then back to the first again and repeats. The direction of spin is then very predictable with zero extra circuits or devices required.


To reverse the direction, any of these three phases wires can be swapped so rather than progressing around the circle by phase 1-2-3, it might be 1-3-2 for example which would be the reverse direction.


These are great motors for many reasons, but the main one is that they are supplied with service that can provide up to 1000's of amps, leading to HP on the magnitude of hundreds sometimes.

In addition, these motors are quieter, more efficient, and much simpler to control than any other type. The speed is usually controlled by use of digital Variable Frequency Drives (VFDs). These are staple components inside nearly every facility.



Simply put, I find motors of any kind to be fascinating, but certainly learning the benefits and risks of different types can lead to the right choices. The last mistake we want to make is choosing the wrong part for the job, simply due to a lack of understanding.


Our educational programs teach not only the motors, but just as importantly the control circuits and devices. Contact us to ask about our training.

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