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• Motor Curve

Video Transcript

Hi, this is Janette and I’m Joe with Groschopp. As we continue our series “How to Choose an Electric Motor,” we’re discussing our four motor types.  Previously, we looked at Universal Motors, now we’ll be diving into Permanent Magnet DC Motors.

DC motors are one of the most well-known motor types. They’re ideal for low power applications and those needing high starting torque, and are often seen in the automotive and agriculture industries.

A DC Motor consists of a wound armature and a commutator with brushes that interact with the magnets in the housing.  DC motors typically have a totally-enclosed construction.

They have a straight speed-torque curve, with high starting torque and low no-load speeds. They can run on DC power or AC line voltage with a rectifier. Operating speeds of 1,000 to 5,000 rpm and enclosed construction make DC motors suitable to use with gear reducers.

DC motors have the second best efficiency rating of our four motor types at 60 to 75 percent. The brushes must be inspected regularly and changed every 2,000 hours to maximize the life of the motor.

There are three main advantages to a DC motor. One, it works well with gearboxes. Two, it operates on DC power without a control. But, if speed regulation is needed, a control can be used and is inexpensive when compared with other control types. Third, most DC motors are a great option for a price-conscious application.

Drawbacks to consider with a DC motor are the brushes because they are high maintenance and can create some noise.  Cogging can occur at speeds of less than 300 rpm, and there’s potential for significant power loss on full wave rectified voltage. If you’re using a gear motor, be cautioned that the high starting torque can damage the reducer.

Now we’ll look at the typical speed-torque curve of the DC motor.  You can see the linear curve, we spoke of earlier.

If you look closely at the hot motor curve, you’ll see an interesting phenomenon.

As the motor temperature is elevated, the no-load speed is increased.  This is due to the effect that heat has on the magnets.  As the motor cools down, the speed will return to normal.  Note the other end of the curve where stall torque is reduced for a “hot” motor.  

Here we’ve added the black-dashed efficiency curve. Under ideal circumstances, the motor’s peak efficiency will occur near the motor’s operating torque.

Stay with us as we continue our series “How to Choose An Electric Motor” with a look at AC Induction motors. For more information about Groschopp or any of our DC Motor products, check out our website at www.groschopp.com.