Motor selection for 25% voltage variation 2

[bacon4life]

I am purchasing a new 115/15 kV mobile substation and would like to check on the best way to provide reliable pump motor operation over a wide voltage range. Due to various different nominal distribution voltages, the 120/208 station service transformer voltages will range from 100V to 125V.

Can I just oversize the pump and fan motors by (115/100)^2 = 33% to account for the reduced torque at the lower voltage? Would that also give an acceptable margin for over excitation cooling at 125V? Each three phase motor will be in the 0.25 to 5 hp range.

I could also put in a boost transformer with a selector switch for each nominal voltage. However, I would like to Keep It Simple since installing the mobile is sometimes a rush job in the middle of the night.

Any other options I should consider?

Thanks,
Mark

 

[davidbeach]

DC motors?

 

[waross]

Dual rated motors. These may not be available in the size you need, but you may find motors rated for 230/208 Volts.
As you know, over voltage saturates the magnetic circuit. Under voltage causes a loss of torque. There are some motors designed to have adequate torque on 208 Volt systems, but with enough iron to run on 240 Volt systems (230 Volt rated).
Failing that, you may consider over sizing the motors so that the torque will be adequate on the lowest anticipated voltage.
Re cooling. Once you pass the knee of the saturation curve I doubt that you will be able to save the motor with any amount of cooling.
If over sizing starts to get ridiculous you may consider a combination of a boost transformer and a voltage monitor relay to switch between ranges automatically. You may even consider three ranges.
95 Volts to 110 Volts, 110 Volts to 120 volts and 115 Volts to 130 Volts.
Select a transformer with a 16 Volt secondary winding. For 110 to 120 Volts go direct. When the voltage relay detects low voltages use a contactor to boost the voltage. When the relay detects high voltages use a contactor to buck the voltage.
The voltages I have suggested are for illustration only. When you start researching available motors and buck boost rated transformers you will probably find your own optimum voltage settings.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Assuming 3 phase motors, another approach is to use motors rated for the lowest possible voltage, i.e. 208V or even 200V if you want more margin of safety, and use a VFD for them even if you don't need variable speed. The reason is, a VFD can be programmed to ALWAYS put out a voltage that is equal to or less than the incoming line. So if you program the VFD to ALWAYS put out 200V 3 phase, and the line is 200V, no problem, but if the next time the line is 240V, the VFD still sends 200V to the motor. The other beauty to that is if you size the VFD correctly, you can even power the 3 phase motors from a single phase source. I've done this for several portable equipment OEMs over the years, works like a charm especially if the motors are 3HP or less (because up to 3HP most VFDs do not need de-rating for a single phase supply). The VFDs add money, but because they also soft start, the equipment lasts longer as well.

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[waross]

I like your solution also jraef. lps
If this problem was on my desk I would probably price out both solutions and possibly pay more for the VFD solution due to greater dependability. In my solution a faulty relay or transformer may take out all motors. The failure of a VFD will affect only one motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter