How to Choose an Electric Motor: AC Motors

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Video Transcript

Thanks for joining us, this is Joe and I’m Jannette with Groschopp. We’re back with the sixth installment in our video series: “How to Choose an Electric Motor.” In this video we’re discussing AC Induction Motors. AC motors are one of the most common types of motors used in a variety of applications because they run off the AC voltage found in wall outlets.

The typical construction of an AC Induction Motor has a wound stator with a rotor, consisting of laminations, and cast aluminum, and a shaft.

The stator winding determines most of the performance characteristics of the motor.  AC and DC motor commutation are similar, however, the winding of an AC motor takes advantage of the alternating AC power to do the commutation without requiring brushes. The field is created by the currents induced in the rotor. Think of it like an “inside out” DC motor.

An AC motor, depending on its winding, is built to run off either single phase or three phase AC input power.

Can I quickly jump in?  There are multiple types of single phase motors: capacitor start, split phase, permanent split capacitor, and many others.  We manufacture permanent split capacitor motors, or PSC, so we will be focusing on these, as well as three phase motors throughout the video.

Thanks for clarifying Joe. Let’s get back to the motor specs. The output speed of the motor is relatively fixed depending on the number of poles with which the stator is wound and the frequency of the input voltage. This is typically limited to 3400 rpm.  Unlike a DC or Universal motor, the value of the input voltage doesn’t affect the speed.

The primary advantage of an AC motor is the lack of common wear parts, such as brushes and commutators, giving them a long life with low maintenance.  In fact, if properly applied, AC motor life is limited only by the bearings. Other strengths are their ability to maintain speed over a wide range of torque, the low starting current, gearbox compatibility and speed control compatibility for the three phase motors. Typical AC motor efficiency is around 40 to 70 percent, which is lower than the DC motor.

Some weaknesses to consider with single phase motors are poor speed regulation and that they require low starting torque for the capacitor to start. AC motors, in general, also have low power levels comparative to size, a lack of portability and high cost per horsepower when compared to other motor types.

Be aware when considering an AC motor that many single phase PSC motors will actually have less starting torque than their rated torque. This is important because if your application needs to start with a load, a three phase motor is likely the better choice.

There are several reasons to choose a three phase over a single phase. Three phase motors have good speed regulation when a control is used and are cheaper than  brushless DC motors. They have a higher starting torque than single phase and have the potential for higher or lower speeds than a single phase when using a control.

Shown here is a typical 3 phase AC motor performance curve.  This curve is fairly linear and flat until the breakdown torque is reached.  After breakdown, the torque drops off sharply and even folds back a bit.  In a single phase PSC motor, the fold back is even more pronounced.

As you can see, under ideal circumstances, the motor’s peak efficiency, the black dashed line, will be near the motor’s operating torque.

Stay with us as wrap up our look into our four motor types with Brushless DC Motors.  As always, for more information about Groschopp or any of our AC Induction Motor products, check out our website at www.groschopp.com.

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