Effect of lower that specd. voltage on elect. motor 4

[flaeng]

A 35 HP. electric motor is rated to operate at 230v. What effect will operating the motor at 208v. have upon the motor?

 

[Skogsgurra]

There will be several effects. Some negative and some positive.

The positive first: The idle current will be lower. The power factor will be higher. The starting current will be lower.

Positive/negative: The starting torque will be about 18 percent lower, which either means that your machine will not start or that it will start smoother - if it starts.

Negative: The slip will be higher. This means that the losses go up and the efficiency goes down. The maximum available torque will be about 18 percent lower, meaning that the motor is more prone to stalling if the load goes up momentarily.

Conclusion: The motor will run very well if not fully loaded. It can not deliver rated power if you really need that.

 

[UKpete]

Nothing to add to skogs' post except I have repeated below a table from the Brook Crompton "little red book" that I've posted before:

VOLTAGE 6% UP VOLTAGE 6% DOWN
Full load speed up to 0.5% down 0.75%
Starting torque up 12% down 11%
Starting current up to 6% down 5%
Full load current down 4% up 5%
Efficiency 1/2 FL down 1.5% up 2%
Efficiency 3/4 FL down 1.0% no change
Efficiency 1/1 FL no change down 1%
Power factor 1/2 FL down 4% up to 4%
Power factor 3/4 FL down 3% up to 2%
Power factor 1/1 FL down 2% up 1%
Temperature rise down 4% up 6%

In your case of course the voltage is nearly 10% down hence you will need to adjust these figures accordingly.

 

[aolalde]

I apologize for contradicting Skosgurra’s conclusion.
According to the most important international standards, which regulate actual General-Purpose motors manufacturing, alternating-current motor shall operate successfully under running conditions at rated Load with a variation +/- 10% of rated Voltage, +/- 5% of rated frequency and a combined variation in voltage and frequency of 10% (sum of absolute values).
23 Volts is 10% of 230 Volts, Then your motor could work full loaded between 253 and 207 Volts.
Whoever, I agree that the motor performance parameters will be different of nominal and low voltage condition could be detrimental for the life expectancies of that motor.

 

[Skogsgurra]

aolalde,

I did not know about that standard. I have a mostly European background and since the 230 V mentioned is a typical European voltage, I assumed that the European limits apply. They state that a three-phase induction motor shall perform well under +/- 5 % variation in mains voltage. This contradicts in a way the accepted +/- 10 % variation (IEC 38) in mains voltage that is allowed (but not recommended) in European 400/230 V grids.

Or is it the "broad voltage" motors you refer to? They have the nameplate stamped with 220 - 240 V and can work with 220 -5% and 240 +5% 220 - 5% is "just on the edge" to 208 V and - as you say - should work without any problems.

I am interested in the new standard saying that motors shall tolerate +/-10% voltage deviations and would like to have a reference to it.

 

[aolalde]

Skosgurra.

The standard that I verified was NEMA MG1 part 12, but now that I am checking VDE 0530 and it allows only +/- 5% voltage variation as you stated.
I do not have on hand IEC 34 but you should be right for European standards, I am most familiar with American standards. I apologize again; we should know the origin of such a motor since the design criteria’s change from manufacturer to manufacturer and from country to country. Your approach is more conservative but secure. Thanks for your reply.

 

[Skogsgurra]

And thanks for your reply, aolalde. We both learn from this. I like that.