DC Motor Voltage Effects

[JTProE]

What are the effects of decreasing the terminal voltage on a DC motor (with brushes). I cannot find the manufacturer's data sheet on this specific motor. Is there a certain voltage the motor will no longer work at (assuming it is higher than the back EMF)? Can I say that for every volt it is dropped, the life of the motor will be decreased by x% ?

Thanks,
JT.

 

[Skogsgurra]

The DC motor is voltage controlled. The back EMF is proportional to speed. So, when you reduce armature voltage, the speed is reduced. Life can increase, but it can also decrease due to less cooling.

All of the above is true if the excitation is kept constant or if it is a PM motor.

If there is a separate parallel field winding, there are several other things to take into account. Or if it has a series field winding. None of which has been stated.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[telmatrix]

Yes... Skogsgurra is correct. There are many types of DC motors, so you'll need to clarify what you have and what is the typical load scenario/application if you wish to gather answers which are useful and relevant. Sounds like you have a DCPM machine however.

Other effects you will have to consider is "brush drop" and environmental conditions such as temperature and humidity.
Brush drop is a non-linear voltage drop which is similar to a silicon diode drop in an electronic circuit. The appearance of brush drop is a consequence of the thin layer of cuprous oxide, an insulator that develops on the surface of the commutator. This is a result of a reaction between atmospheric oxygen and the commutator material facilitated by the heat generated by motor operation and arcing. Note conduction through the cuprous oxide layer occurs due to localized dielectric breakdown when current is passed through the brushes. The pros of this cuprous oxide film are that the voltage drop tends to reduce arcing effects, and serves to extend brush life. The con is that the voltage drop reduces the efficiency of lower voltage motors. Moreover the structural integrity of the film can change in very low humidity environments; consequently brush wear may be accelerated under these conditions.

Hope this helps...