208V motor on 240 not good- but why?

I have a 1950's 5 hp motor for a shaper- and have run it on a 240 volt RPC- though not for hours on end. ( I am refurbishng the shaper)

From recent threads I know that 208-220v motors are considered OK on 240V supply- and that 208V and 200 V are not considered OK as it is over the 10% "rule".

208 V is 13.3% lower.

I have read one advantage of motors with higher voltage (440 for example) is that the wiring can be smaller and the motors smaller for the same HP output.

Based on the above I would assume a motor designed for 208 volts would have heavier guage wiring than one designed for 240V. This should give it greater amp carrying capacity- which it would need if it is loaded to the full 5 hp rated capacity.

I do not know what the difference would be in the windings between a motor designed for 208 V vs 240 volt.

I know the insulation needs to be appropriate for the voltage- but we are only looking at an additional 3.3% deviation from the 10" "rule".

Can someone explain just what makes the 208V motor on 240V unacceptable?

It seems to me that this would be OK if the insulation holds up. I would be more concerned with a 240V motor being fed 208V due to the potentially smaller wire in the windings, and increased heat build up.

JRT

The guys who designed the motor reckoned that it will have a good life when running at 208V. When you run it at 240V, the current through it should increase by 240/208 ~ 16%, but as the iron used in the cores saturates, the current increase will be greater then this, though by how much depends on how much iron the designer put in it.
In general older motors are built with more iron in then modern motors but the insulation will not cope with the high temperatures that modern ones do.
I have just remembered a method of measuring the temperature of copper, such as transformers and motors. The resistance of copper increases by 5.6 ^ 10-4 per degree C. So if you measure one of the motors windings at start (20 degrees C) and its say 4.2 ohms, at 100 degrees C it will be 4.2 + 80 X 5.6 ^10-4 or 4.2 + .0448 or 4.2448 ohms. So beg, borrow or steal some ohmmeter or resistance bridge that can read to 4 figures. Just measure the resistance at cold, then every 1/4 Hr or so . Now you can figure out the winding temperature and if you plot your results you can get a feeling for the time the motors temperature is likely to stabilise at and at what temperature.
FWIW I would not exceed 80 degrees C for an old motor, others may think even this is too high.
Frank