Que On Overvoltage

..." An assumption people often make is that since low voltage increases the amperage draw on motors, then high voltage must reduce the amperage draw and heating of the motor. This is not the case. High voltage on a motor tends to push the magnetic portion of the motor into saturation. This causes the motor to draw excessive current in an effort to magnetize the iron beyond the point where magnetizing is practical."...

Motors generally have a tolerance voltage range in which they will operate, the further you go from the optimum voltage level the more stress the motor endures and the shorter the service life....

http://ecmweb.com/design/highs-and-lows-motor-voltage

https://en.wikipedia.org/wiki/Voltage_spike

That's right, the higher voltage doesn't really make the motor work harder, because that is determined by the load on the motor. So what happens is that the motor saturates and just heats up, consuming more energy for no useful gain. Here is the classic chart explaining it.

The link by Solar Eagle #3 to motor voltage theory was interesting (a trip down memory lane) but oddly enough there was no mention of 'back-emf' and 'speed' and the effect these have on the 'actual' current.

It seems intuitive to say the current increases when the voltage increases which makes sense when everything else stays the same. But with a motor there are other things happening as well and it is the change in these things that have an effect on the current that can cause an increase when the voltage decreases.

The OP might begin to think the motor doesn't know Ohm's Law. Well of course, the motor does know Ohm's Law, because it takes into account the back-emf it generates within itself. And it is the back-emf (a sort of unknown value) that explains the increase in current when the supply voltage reduces.

Assume the motor is running within the parameters of the nameplate, then for any given steady load (whatever is connected to the output shaft) the motor will run at a steady speed. A drop in voltage will mean less torque and that causes the motor to slow down.

This is where the back-emf comes into the equation. A magnetised armature spinning inside a magnetised field as in a motor, will also act like a generator and will produce a voltage in the field within itself in opposition to the supply voltage - this is why it is called a back-emf.

The back-emf is proportional to the speed. So when the motor slows down it will produce less back-emf to oppose the supply voltage, hence allowing more current to flow.

That is to say, the supply voltage meets less 'resistance' - so the current goes up.

But again, it must be said that the motor operates within it's nameplate parameters.