[Behind the Lines] Ep. 3 with Ken Stafford on Motors and Gearboxes!

[Richard Wallace]

Originally Posted by Joe Johnson

My criticisms should have been more temperate.

Joe J.

Dr. Joe, I think your criticisms were, as they have always been here on CD, directly on point. I hope if you find the opportunity to offer criticism of my contributions you will be just as direct and on point. Please keep calling 'em as you see 'em. BTW, I recall your presentation of similar material (at the first Novi remote kickoff, IIRC) being a great inspiration to me.

My only minor addition to Prof. Stafford's excellent presentation would be a reminder that motor power is also limited by the motor's internal temperature rise, specifically the temperature of its armature windings and commutator. As copper's temperature rises, so also does its electrical resistivity. An armature winding that is 100 Celsius degrees above the temperature at which the motor's performance characterstics were measured will reduce that motor's peak output by about 30%. I posted some test results for CIM motors a while ago that confirm this.

Rep points to the first poster who can provide a link to that data, and to the first poster who can provide a calculation based on theory that explains the 30% reduction mentioned above. [Thank Ether for inspiring the rep reward.]

To support Prof. Stafford's comments on fan cooled motors, it has been my observation that such motors exhibit much less armature temperature rise than enclosed motors, provided that they are operated on the correct (high speed) side of their power curve peaks. However, as Dr. Joe pointed out, a stalled motor gets no benefit from its fan, so in applications such as drive trains where stalling is a foreseeable situation, a larger enclosed motor (e.g., CIM) should be preferred over a smaller fan cooled one.

[Jared]

Originally Posted by Richard Wallace

Rep points to the first poster who can provide a link to that data, and to the first poster who can provide a calculation based on theory that explains the 30% reduction mentioned above. [Thank Ether for inspiring the rep reward.]

It's funny you should bring this up today, as I was just looking at this last week.

Here's the data:
http://www.chiefdelphi.com/forums/sh...13#post1215913

[Jared]

For the calculation:

First, we must find the motor's current peak performance.
This is 41% efficient (given by datasheet). Assuming all losses are due to resistance of the motor (this isn't true, but I'm just trying to see if I can get anywhere close to the measured 30% difference), we can calculate the resistance R = v/I = 12/68 = 0.17647.

Copper has a resistivity constant of around 0.0039 per *C. So, the new resistance at a temperature 100 *C higher would be 0.17647*(1+0.0039*100) = 0.245 ohms. Our loss of power is proportional to resistance, and the ratio of the two resistances is 1.38, meaning there is a 38% loss of power at the higher temperature. This is higher than the actual value, because my approximation of the original resistance is too high. It could be more accurately measured if I had a motor in front of me.