I have seen this question regarding DC motors: Which type of dc motor take constant power from supply even when load various?
One has answered it like this:
No motor takes constant power from the supply even when the load varies unless the load varies in a manner that requires constant power. When such loads are driven by DC motors they usually have electronic controls that separately control the armature voltage and field current to meet the operating requirements.
This question may be a poorly constructed or misinterpreted exercise in the study of the classical DC motor configurations, series, shunt and compound motors. In that case, the expected answer may be "series." The speed vs. torque capability curve of a series motor is essentially a constant power curve.
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Do you agree with the one who believes that thinking of a DC series motor as a constant power DC machine is a "misinterpretation"?
What is the problem considering that a DC series motor is constant power?
I think, constant power motor cannot exist. Because that would violate the law of conservation of energy. If more output is desired, more input will obviously be absorbed to fulfill the need. So if a load needs more mechanical power, obviously more electrical power will be extracted from source. And this may also vary according to load.
Dear Dr. Kunal Sen, Thanks a lot for your prompt response. The point you are mentioning seems correct but if you look at the torque-speed curve of a DC series motor, it shows a constant power characteristic, as the torque decreases as the speed increases. How could you justify this constant-power curve?
Dear Mr. Mohammad Naser Hashemnia,
You may also consider the various losses (mechanical & electrical) occurring during the operation of the motor.
Also you said about the variation of the load on the motor, please refer the following:
We can see from below that the power curves for a D.C. motor with respect to both speed and torque are quadratics.
1) P(w) = -(Ts/Wn)*W^2+Ts*W
2) P(T) = -(Wn.Ts)*T^2+Wn*T
From these equations, we again find that maximum output power occurs at [T] = ½Ts, and [W] = ½Wn respectively.
I think this will prove helpful for you!
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