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Motors under load..

Prototype

Member
One thing that has been bugging me is why a motor under load would draw more current. I get WHY it has to draw more current to do more work, but I cant get the mechanism of how. I tried to think of the motor like a transformer with the rotor being the secondary, but then my idea falls apart because although there is a magnetic link between the two, that is where the similarity ends...the load is mechanical, not electrical.

Then that got me thinking about generators, now my guess is that the extra current draw on a generator creates a greater magnetic field in the windings which the engine needs to work harder to force the rotor through, am I on the right lines?

Thanks
 

rjenkinsgb

Well-Known Member
Most Helpful Member
It's related to the motor "Back EMF".

A motor generates a voltage proportional to its speed; the current drawn is in turn proportional to the difference between the supply voltage and the generated back emf voltage (and motor etc. resistance)..

Running light, the back emf is almost the same as the supply and the current is low. As the load increases and the speed starts to drop, the back emf reduces and so the current increases.
 

Prototype

Member
It's related to the motor "Back EMF".

A motor generates a voltage proportional to its speed; the current drawn is in turn proportional to the difference between the supply voltage and the generated back emf voltage (and motor etc. resistance)..

Running light, the back emf is almost the same as the supply and the current is low. As the load increases and the speed starts to drop, the back emf reduces and so the current increases.

Thanks rjenkinsgb, that makes sense.
 

dr pepper

Well-Known Member
Most Helpful Member
You could day the off load current was the losses of the motor, both electrical and mechanical.
 

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