# How will a motor respond as a generator?

Consider a permanent magnet motor which consumes maximum 12V @ 1.2A and turns at 1000 RPM.

If I run this motor from a different motor at 1000 RPM then will it produce 12V @ 1.2A?

What will happen if I run it at 2000RPM?

Also wanted to understand how the same motor acts when used as a generator.

• No it won't because that would imply 100% power transfer efficiency. More speed = more output voltage. There are plenty of internet sites on this subject. Nov 16, 2020 at 11:48
• Can you suggest me some sites? Also what about current Nov 16, 2020 at 11:51

You need to know your motor's efficiency, or its winding resistance, or its back EMF at 1000RPM. Winding resistance is probably easiest to measure so I'll start there.

Best is to find all of these in the motor's datasheet and verify that they are (approximately) consistent as below : if they aren't, that indicates roughly how much you can trust the datasheet.

If its winding resistance was 2 ohms, at 1.2A it would drop 2.4V, corresponding to a back EMF of 9.6V at 1000 rpm (or, 104rpm/V). Wasted power would be 3.6W (or 25% of 12*1.2 = 14.4W) or 75% efficiency (slightly on the high side for such a small DC motor).

So you could expect 9.6V into no load, or 7.2V generating 1.2A (8.6W) or at 1000 rpm.

Running it as a generator at 2000rpm this would correspond to 19.2V into no load, or 16.8V generating 1.2A.

This should let you work out likely performance once you know the characteristics of your motor.

• I don't think the wasted power sentence brings anything to the party here because it's not that clear if the motor is on load or not - i.e. it could all be wasted? Maybe I'm missing something Brian? Nov 16, 2020 at 15:01
• @Andyaka "consumes maximum 12V 1.2A" pretty clearly suggests on-load power consumption (at least that's the way I'm taking it in my answer).
– user16324
Nov 16, 2020 at 15:16
• I just wasn't too sure that it did suggest that. I mean it could be a big lump of a motor that takes 100 amp on full-load. Just saying. Nov 16, 2020 at 15:19
• @Andyaka Possible. If the OP is asking a question he didn't mean to ask, he may be disappointed with the answer (or how it pans out in practice) But that's life. If it DOES draw 100A stall current, the winding resistance certainly won't be 2 ohms though.
– user16324
Nov 16, 2020 at 15:21
• @BrianDrummond you are correct about the fact that motor when on load almost near to maximum weight it can carry then it consumes 12V 1.2A and your above answer helped a lot thankyou I was looking for answer exactly like this. Nov 17, 2020 at 4:35

An ideal permanent magnet brushed motor would produce 12 V at 1000 rpm. A motor that size will be far from ideal, so you would expect something rather less than 12 V. At 2000 rpm, you would expect about double the voltage. With a suitable load, you would be able to draw 1 2.A, as long as you had sufficient power from the driving motor.