240/460 3 ph motor

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mtnelectrical said:
What would be the problem running this motor on a 208 v system??
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Depends on what you are running with it.

By the way, most likely it's a 230/460V motor, that would be the standard "utilization voltage" rating for NEMA motors. If it truly says 240/460, it's an oddball import and that may be relevant to this.

NEMA motors are designed to accept +-10% voltage and run within spec. So a 230V design is good down to 207V without compromise. Unfortunately, when we say 208V is the "distribution voltage", it's acceptable for that to be down to 95% of that, so it might be as low as 198V. A motor designed to truly work on a 208V system then is designed as a 200V utilization voltage. At the acceptable lower end of your 208V distribution voltage, your 230V motor will be starved for voltage.

What does that mean? It means a loss of torque. Less torque means more slip, more slip means higher current draw. Higher current draw means more heat. More heat means shorted life. In short, "stuff" flows down hill and your machine, whatever it is, will be at the bottom. HOWEVER, if your motor is over sized, as they often are, then the net effect may mean only the difference between lasting 25 years vs lasting 20 years; and you may not be around to worry about that. So that's why I say it depends on what it is connected to.

If its a centrifugal pump for example, chances are about 99% there is some spare capacity in the motor size, because it's nearly impossible to find a pump curve that exactly matches a standard motor size, and since the pump won't work if it can't produce the torque, pump designers go with the next size up. So it may be that your pump motor will not care that it is only getting 197V because the commensurate loss of torque only increases the current a little, but it is still below FLA.

Compressors however tend to be exactly the opposite. Compressor OEMs tend to cheat a little, often stating the peak torque output of a motor, known as the "breakdown torque" as the rating of the motor, because a compressor often is thought of as intermittent duty (although they are almost never used that way). Unfortunately when you lose voltage to a motor, the breakdown torque, the very thing they are using in their cheat, varies by the SQUARE of the voltage reduction. So even if you are at 207V when that motor turns on, your peak torque drops to 81% of its maximum value. That then means if you had a compressor that said it "develops 15HP" on the nameplate, that becomes 13.5HP. If the compressor relied on that full 15HP peak to start it, it will stall. But even if it's ok at 207V, what happens at 198V is that you are now cut down to the equivalent of 74% of the peak output, so now your motor only spits out 11HP at best on startup. I can't think of any compressor OEM that oversized their cheat motors by that much.

If you do have some sort of oddball import motor that says "240/460V" on the nameplate, this only makes it worse all around.
 
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just one side note, I have found, is that a motor that is rated say 230 volt, industrial mind you, is only rated that way in anticipation the motor is a long distance from the feeder, thus allowing for voltage drop from the MCC or OCD. So will a motor that is nameplated 230v actually operate at 208v given the 230 v nameplate rating. Im not disagreeing with anything but only asking a question. But it seems the motor is already being derated.
 
As jraef stated, it depends on the motor and the application.
Plus there is no room for voltage drop or POCO voltage being on the low end of tolerance.

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