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Video Transcript

Welcome back, this is Joe and I’m Janette with Groschopp.  In this fourth video in our series “How to Choose an Electric Motor,” we’ll begin discussing our four motor types —Universal, Permanent Magnet DC, AC Induction and Brushless DC. Throughout these videos, we’ll go over their construction, characteristics, advantages and disadvantages, as well as teach you how to interpret the motor performance curves for speed, torque and efficiency.

There will be trade-offs between the different motor types and sometimes the best choice isn’t obvious. That’s why we’ll give you some aids to help you understand the pros and cons to make the selection process easier.

Janette, do you want to start us off with universal motors?

Sure Joe. Universal motors are great for AC or DC applications that need a simple, high speed, low-cost motor with intermittent duty cycles.  They’re durable and suited for rough, high vibration applications where low noise is not needed, such as commercial concrete drills or floor sanders.

As you can see, we have the basic components of a Universal Motor laid out here.  Both the armature and field in this type of a motor are wound with magnet wire, and the electrical connection to the armature is made through the commutator and brushes.

There are two characteristics that set the Universal Motor apart from most other motor types. First, it runs off of both AC and DC power, giving it the name Universal Motor. Second, a Universal motor is capable of running at extremely high speeds, from 8,000 to more than 20,000 rpm. These two characteristics make the universal motor fairly unique and ideally suited to specialized applications.

Several other characteristics of the universal motor to consider are the high starting torque; the 55 to 70 percent efficiency rate; and the average life expectancy of 500-2000 hours.

One main advantage of the Universal motor is its high power density.  It’s not uncommon for a Universal Motor to have over twice the continuous output power as an equally-sized AC motor.  Additionally, its low cost, portability, and ability to run without a control make it worth looking at.

A few downsides to universal motors are that they are noisy, inefficient at low voltages, burnout quickly in stall conditions, require high maintenance due to the brushes and the open frame design. They also have poor speed regulation, and the high speeds prevent gearbox usage.

That’s right, Janette. Before we look at universal motor performance curves, I’d like to quickly comment on low voltage and Universal Motors.  If you operate a Universal Motor under 100 volts, the efficiency drops drastically. With a battery input source, the universal motor could quickly drain it. As a rule, other motor options should be considered for low voltage situations.

Here we have the typical speed-torque curve of the Universal Motor. This is the cold motor curve, and you can see that it’s fairly linear with good starting torque. The sharp upward curve as the torque decreases is a unique characteristic of the universal motor.  

Now we’ve added the hot motor curve.  Here you can see the effect that heat has on motor performance.  For a given torque point, the motor will run a little slower as it heats up.

Alright…now we’ve added the motor’s efficiency curve, the black dashed line.  Under ideal circumstances, the motor’s peak efficiency will occur very near the motor’s operating torque.  However, in most cases the actual peak efficiency will occur at a higher torque than the rated torque.

Janette, I’d just like to make a comment about speed torque curves. On our website, you’ll find that our motors have speed-torque curves associated with them. This allows you to see exactly how any given Groschopp motor will perform.

Up next in our series on “How to Choose an Electric Motor,” we’ll look at Permanent Magnet DC Motors. For more information about Groschopp or any of our Universal Motor products, check out our website at www.groschopp.com.