Low Voltage on a Motor

Motor torque, I believe, is proportional to the voltage squared, everything else being equal.

http://www.gohz.com/relationship-between-induction-motor-voltage-and-current

But that's not the whole story. If the voltage drops and the motor doesn't produce enough torque to drive the load, it will slow down. A characteristic of induction motors is that the torque increases as the motor speed slows down (within reason). It's termed a higher slip rate.

So, as long as the voltage does not drop too low, the motor will slow down until the motor torque equals the torque demanded by the load.

Good Answer.

thanks that was what i was looking for.

An electric motor connected to a generator that runs a pump? I am no PE, as all of you know, but is that the best way to power a 7.5 HP max pump? Why not just power the pump with the motor?

Good point, but it's likely a larger generator used for the entire facility, not just this one pump. So when the generator gets loaded up by OTHER things in the facility, it drops the voltage to this pump. Not too uncommon of an issue.

are you not concerned as to why the gen set voltage is dropping?

We had a similar issue with voltage when an electrical engineer replaced our mains incomer transformers, down sizing for more efficiency and reducing the voltage from 240V single phase to 220V without telling us, over the first couple of months all sorts of odd motor failures occurred. Eventually we traced most to pump motors running under voltage and drawing higher current leading to higher working temperatures in the windings resulting in insulation breakdown failures. Once found we retapped all surviving motors to run on the reduced voltage with no further failures thus far, though we know the thermal stresses are likely to have reduced the remaining life of the windings.

The motor interprets the voltage drop as an increase in load,and the rotor lags,or slips

as if it was loaded.

This will result in overheating of the motor and premature failure.

Heat is the number one enemy of electrical motors,and is accumulative...it cannot be

reversed.

Check your motor amps and you will see that they have increased beyond the name plate rating.

The motor and pump will operate on the pump characteristic curve. For any given rpm, the pump will require a certain hp to deliver the flow rate from the characteristic curve.

The motor will draw the wattage required to deliver the required hp to the pump.

To deliver the required power, the motor will draw more amps to maintain the wattage. If the voltage drops from 440 to 380, then current will go from 5.3amps (50% load) to 6.1 amps to maintain the wattage, provided the frequency of the power is maintained.

I can't see that a 6.1 amp draw on a 10.6 amp rated motor should necessarily be a problem. Induction motors draw power based on the load that is applied to the motor. The only issue might be heating in the rotor of the motor due to the higher current, but I would expect the greatest issue would be on startups when you have higher load due to inrush and bringing the motor up to speed under load.

Back to the "Why is the voltage dropping" issue, another concern that should be explored is if the extreme loading that is causing the voltage drop is balanced or not. I would suspect not, because the fact that it happens at all indicates a general lack of attention to details.

So if the voltage is not only low, but also UNBALANCED, there might be significant "negative sequence current" that will cause extra heating of the motor regardless of the fact that the running amps are still below the nameplate rating. If so, most cheap simplistic OL relays will not pick that up in time to protect the motor windings. You may want to invest in a good solid state OL relay with phase current imbalance protection or a separate voltage imbalance relay that will drop the motor starter off line of that happens. If using current imbalance, set to no more than 30%, less is better. If using voltage imbalance, set it to 5%.