a blind spot on my old eye about dc motors! could anybody kindly remove it?

Thread Starter


Joined Jul 29, 2020
hi everybody ready to help with a probably elementary misunderstanding.
I have only knowledge about building diy brushed motors, no theoretical background,

I have read that dc motor maximal mechanical output energy happens when Eb,back emf, is iniput voltage /2 and max output energy is thus V/2 x armature current.

( this V/2 got derivating P in respect of I (P= IV- IaxIaxRa and setting V=2xRa=0)

The question is :how proportion mechanical energy out/ electric energy in (efficiency) is often said to be even 90-98 % , should be 50%?!

(also why not number of ampere turns effect on mechanical output energy, these effect of course Eb but....)

thanks a lot


Joined Jun 5, 2013
I think you are confusing max power output and max efficiency. They are not going to be at the same current. When running at max efficiency, the back EMF is going to be more than half of the applied voltage. Yes, the motor will be running at less than the maximum power output, but it will be running at much higher efficiency. In fact, I suspect that most motors would overheat and be damaged if run at max power output continuously.
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Joined Jan 23, 2018
Maximum power output and maximum power efficiency will NEVER be at the same operating point for an electric motor. AND the Maximum output point will not be for constant operation because of heating. That is part of the definition.
The theory and operation of electric motors is very well covered in a whole lot of publications, and is easily available, but ignore what you see on youtube and similar media.


Joined Jul 18, 2013
@wbs If you consult the manufacturers spec sheet, it will show the characteristics of the motor, a typical performance curve will show that for a typical high performance DC motor, the Continuous torque/current rating curve is maximum at zero RPM and fairly flat up to the max. rated RPM.
The Peak torque curve can only be used for a momentary period without damage to the motor occurring.
The manuf. spec sheet will show these figures.