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Efficiency in DC motor

Minder

Apr 24, 2015
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A bit of a loaded question.
A typical quality DC motor has a fairly flat torque curve from 0rpm up, where it is at maximum, to a very small drop at max rated rpm.
So It is a fairly efficient motor right across the rpm range where a given load can be maintained right across the whole rpm range
M.
 

duke37

Jan 9, 2011
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The efficiency will be zero at zero speed and zero with no load. There will be a maximum with an intermediate load. Losses are due to windage and resistive losses. The optimum load will have the motor running close to its maximum speed so windage will not change much.

I would think that maxixmum efficiency would be with a load giving a high current, probably more than the maximum rating of the motor.
 

Minder

Apr 24, 2015
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I would equate maximum efficiency occurring when at rated continuous current and exerting maximum torque which is at zero rpm.
If a motor is operating outside of its rated continuous current, that is hardly efficiency IMO.
M.
 

duke37

Jan 9, 2011
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Efficiency is power output/power input.
At zero speed there is no power output so the efficiency is zero.
Operating a motor outside its rating is not advised but this is always done on switch on when speed is zero or very low. The motor will not last long if held in this state.
DC motors are normally run on a constant voltage supply and the current will be roughly proportional to demanded torque. A bigger motor with a lower resistance is likely to have a higher efficiency.
 
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