DC Motor efficiency factors

[Glenn G]

Hi forum,
I've been thinking about efficiency factors, I've read that they are more efficient at higher loads? I was wondering why this is...

On the one hand at higher loads (maybe lifting a heavier mass) the rotation rate is reduced so this reduces energy losses due to friction? (so goes with the increased efficiency idea)
but to lift heavier loads wouldn't you need a higher current? (so more I^2R heat losses in the winding (so this reduces efficency?)
also at low rotation speed less back emf? (less energy losses due to eddy currents?)

Have I got the basic ideas correct here? Would appreciate input.

This also got me thinking about efficiency of a motor if you were lifting a constant load but reducing voltage (assuming the voltage was enough to lift the load), I would predict that efficiency would drop with increasing voltage due to ...
1) increased rotation speed and therefore friction losses
2) increase back emf/eddy current losses

Is this correct also? am I missing anything meaningful?
Regards,
Glenn.

 

[anorlunda]

Engineers always like to think of the limiting cases. For any motor, the efficiency at zero load is zero by definition. We also expect efficiency versus load or efficiency versus RPM to be a continuous curve. Put the two together and you have a continuous curve that must go through the point (0,0).

It is also reasonable to expect that the efficiency curve will have a local maximum at some nonzero load, because of some of the factors that you cited.

What is not reasonable to assume is that the maximum efficiency and the maximum load are necessarily the same point.

Forgive me for being pedantic. All the above is not dependent on the kind of DC motor. There are many. If you want a better answer, be more specific about the kind of DC motor you are asking about.

 

[CWatters]

Modern DC brushless motors can be very efficient, perhaps as high as 95-96%.