Improper Voltage and Motor Operation

[jmbelectrical]

An engineer is proposing to make a 240V, single-phase connection to a 277V, single-phase fan motor. For me, this immediately raises a few questions:

1. It's my understanding that most motors are rated to tolerate a +/- 10% in voltage, but how is a motor affected by such a large deviation in voltage?

2. 277V is obtained from phase to ground in a 480/277V, Y-connected system. 240V is obtained from phase-to-phase in a single-phase transformer or a delta-connected system. Low voltage issues aside, how does the fact that both conductors would be energized in a 240V connection affect the motor?

3. Similarly, 208V, single-phase motors are intended to be connected to 208/120V, Y-connected systems. But what if you were to connect it from hi-leg to ground (208V) in a 240/120V, delta hi-leg system? I understand that most electrical codes would prohibit such a connection, but this is purely hypothetical.

Thanks in advance,

 

[jraef]

1) You are right about the tolerance of +- 10% being the "norm", but it is by no means absolute. Only the motor mfr. can answer that question for sure. The chances are though that if they went to the trouble of making a non-standard motor voltage of 277V, it's not likely to have a different tolerance, and maybe not even that much. (Yes I know many motor mfrs make 277V motors, but it is not really "standard" in that it is done primarily for OEMs, which means they typically build them to OEM specs, not necessarily NEMA specs).

So what happens when you starve the motor of it's designed voltage is that the motor peak torque is reduced by the square of the voltage reduction. So 240V is 87% of the design voltage, so your motor torque will be reduced to 75% of what the motor is capable of. This may have the effect of increasing slip, which means the motor will pull more current for the same task. If it was over sized to begin with you may never notice an effect, but if not, you will increase the current drawn for the load and risk overloading the motor. Read this as reference.

http://www.motorsanddrives.com/cowern/motorterms12.html

2) Volts is volts is volts; phase to phase or phase to neutral, makes no difference.

3) Using the high leg to ground on a 120/240V red leg delta is inadvisable. Delta voltages to ground are inconsistent.

2a and 3a) Also, you can NEVER use ground as a current carrying conductor, you would use Neutral, which although referenced to ground, is not the same thing.

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