3 phase AC motor derating calculation

[ssstern]

Hi,
I am trying to figure out when I am varying the voltage and cycle of a three phase AC motor, what kind of derating would I need to do. I know if I am switching from 60Hz to 50Hz, the HP(final)=HP(initial)*50/60. Is there anyway to calculate change of RPM and other parameters? How do I derate my motor HP if my voltage changed.

Thanks

 

[dpc]

Speed is directly proportional to frequency.

Otherwise, check here first and come back with any specific questions: faq237-1224

 

[waross]

As a rule of thumb, the full load torque and the maximum allowable current stay the same. Voltage, HP and speed change in the ratio of 50/60 or 60/50.
But see the excellent FAQ FAQ237-1224: Motors: Changing between a 50 and 60Hz supply

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

 

[ssstern]

Thanks for the tip. Another question I have is that how would HP of the motor be derated if the input voltage got varied from 460V to 415V for instance? If it's using the same frequency. Would that be harmful to the motor if the input voltage was varied from the motor rated voltage?

 

[ssstern]

I just realize from the faq section:
"If the load remains the same and the voltage is lowered the motor will draw more current to continue meeting the load's hp requirement. Remember the motor will still be running at the same speed since the frequency wasn't changed." Would I run into any heating problem?

 

[itsmoked]

Very possibly in that particular case!

But it depends on several things.

For instance:
Does the motor run continuously or for more than, say, 20 minutes at a time.

Is the motor running at its rated output?

Is the load constant?

If that motor is running at its rated hp and you shave off 10% of its voltage the current will now be over name plate and its early demise is assured.

If the motor's constant load is 10% less than its rated hp it may be fine as the current won't be exceeding name plate.

Or if it's only cyclically loaded and is idling most the time it will cool between the otherwise short over-currenting events.

Not an easy call from here.

Main thing is if you have a motor going into that situation, you leave space for several boost transformers and then measure the motor current while the motor is working. If it's below name plate it's OK if it's over name plate put in the transformers.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ssstern]

How about voltage unbalance issue? Can I just use the %unbalance equation (max deviation from average)/(Average voltage) and replace the voltage on the denominator to find out the change of %voltage unbalance? Can I make the assumption that max deviation from average voltage doesn't change? If I can, can I use the NEMA derating chart to find out what is the motor HP should be rated at if I drive at the lower voltage?

 

[dpc]

Voltage unbalance relates to voltage differences between phases, not operating at an off-nominal voltage (on all three phases)

 

[ssstern]

So is there anyway to derate the motor if the input voltage got changed. and why does current increase as the voltage drop? I am assuming I can't use the regular V=IR relationship since motor doesn't really behave linearly. Could I make a rough assumption that if my input voltage drop 10%, my rated HP should also drop 10% to ensure the motor would operate under a safe region?

 

[waross]

Yes. If your currents are at or below nameplate current your motor should be safe. If your currents are above rated current your motor will probably overheat. Use this as your final hands on field check.


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

 

[itsmoked]

The number you want to use is P=VxI.

P=>hp

That motor, like a faithful dog, is going to want to turn at its rated speed and deliver the needed hp. The hp comes from the power it draws. If the voltage drops the only way for it to still get the power is for the current to rise. You are right this is not linear but for the first 10% of abuse it's fairly linear.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Or if it's only cyclically loaded and is idling most the time it will cool between the otherwise short over-currenting events.

You may know this already Keith, but Mr. Cowern has quantified this very well. Look under RMS loading in the Cowern papers on the Baldor Motor site.
Cowern has a knack for keeping his explanations simple and easy to read, understand and implement.
You probably know this but others may be interested.

http://www.baldor.com/support/literature_load.asp?LitNumber=PR2525

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