how can I tell the design voltage of an unlabeled 3 phase motor? - Page 2

# Thread: how can I tell the design voltage of an unlabeled 3 phase motor?

1. Hot Rolled
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Run the motor unloaded at 230V. Run it again at 460V.

Whichever voltage results in the lowest current draw is the correct voltage. (or, run the motor near full load and look for the lowest current draw per volt)

This is based on the fact that overvolted motors are generally beyond the point of saturation, resulting in a decrease in inductance, which causes a significant increase in current draw. A motor that is run at too low of a voltage draws more amps due to higher slip caused by the lower voltage.

If the motor sounds angry while idling then it probably isn't happy.

----Relevance of following technical details is arguable----

The voltage that a motor wants is dependent on mechanical load, too. I've seen charts for this for certain motors and tested it myself. I applied various loads to motors connected to a variac and found at idle the motors were most efficient at much lower voltages than they were rated for. Voltage required to reach peak efficiency went up with load up to the rated voltage at FLA. I think a major factor of this is the DC resistance of the motor. If so, it can be easily calculated what voltage the motor should be given for a given load and programmed into a VFD to get a few percent savings on power drawn by motors already on VFDs.

The voltage a transformer can handle can be found by applying power with no secondary load and ramping it up until there is a knee point where current draw climbs faster than the voltage input does. That is where the transformer saturates and it won't work well at all above that. Be aware the most transformers are not run all of the way to saturation for efficiency reasons, and their rated voltage is derived from the point where at full load (or average expected load) resistive losses match inductive losses, which ends up being peak efficiency, allowing for the smallest and cheapest transformer that satisfies that load rating.