Undervoltage Motor Performance

In an AC squirrel cage motor, torque varies by the square of the applied voltage. So if you have 90% voltage, you have 81% torque, but if voltage drops just another 10% to 80%, torque drops to 64%. Lower torque at the same load will result in lower speed and thus, increased slip. Increased slip will increase current as the motor attempts to return to normal speed, usually to the point of over load.

So if you have 2 identical pumps side by side in a parallel operation, one with less voltage than the other, the lower voltage pump will pull more current than the one powered with the correct voltage.

There is an additional problem in this application if a non return valve is not fitted on the output side of each pump. The pump with the lower voltage motor will be operating at a reduced pressure to the pump with the higher voltage motor. Hence you can get potentially get back flow and the slow running pump is not doing any useful work.......

My experience is that they generally overheat when running under voltage (due to the extra heat) and fail far earlier than they should.

Naturally the amount of extra current/low voltage plays a role in the heating, just a few volts too few should not be a problem....but 115volt motors running on 95 volts or leass will get really hot. Seen it on US computer systems when the main transformer was incorrectly plugged for the UK.....always measure what the input voltage REALLY is before setting up, guesses are simply not good enough.....

These type of motors actually handle a slight over voltage far better than under....

I got a good reputation as a "Finder" and "Fixer" in the 70's due to such problems.....

In India a requirement is motor should be able to accelerate even at 80% voltage.In the case of a pump the torque reqirement at full load will be less by 10-15% depending upon how u select the motor.All LT motors have a pull out torque which is about 200% for LT Motors and hence stalling due to low torque may not be the probelem.When the voltage drops the current increases and this could result in tripping.Check over load settings and also look into the selction .Again check when 2 pumps are operating in parallel one could be near run out condition requring more power.a deatled study of both electrical that is motor and pump (Mechanical) aspects.