# Running 230V motor on 208 Inverter

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#### Loren Schreiber

I have a question about running a 230/430V 3phase Baldor vector motor on 208V 3phase through a Mitsubishi V200E vector drive. If my understanding of inverters is correct, the motor will never see even a full 208 volts except at maximum frequency from the inverter. Do I really need to worry about a buck/boost transformer if I can live with the reduced output to the motor at 208V? Should I worry about running the motor at the lower voltage?

Thank you,

Loren Schreiber
Systems Integrator, MCued http://www.mcued.com/

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#### Melvin Caldwell

Kind of a two part question.

First, the VFD (variable frequency drive) varies the frequency not the voltage. The formula to determine the speed of the motor is RPM = (120 * F) / N Where F is Frequency in cycles per second and N is number of motor poles

Second, a good rule of thumb is never operate a motor at more than 10% above or below the rated voltage. This is for normal conditions and I would not try it on a variable speed application. Have you thought about just replacing the motor with a 208 motor?

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#### Mark E. Ludlow

Beg to differ to this response. A VFD keeps a more-or-less constant voltage-to-frequency ratio. Within this general relationship it's possible to increase the voltage at lower frequencies to provide more starting torque.

There's no problem running a 230V (or even a 208V) motor with a 480V inverter as long as the maximum frequency of the inverter is set to 120
Hz. (Most 230V inverters will even allow for a software adjustment specifically for 208V). What changes of course is the horsepower/torque curve. A 460V motor coupled to a 460V inverter will provide constant torque from 0 to 60Hz and constant horsepower beyond that, until, say, 120 Hz. A 5 hp, 230V motor operated with a 460V inverter will require a 10 hp-rated inverter in this application.

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#### Steve Bailey

As long as the VFD will be happy with 208 volts, you should be OK. Set up the drive's volts/hertz ratio for 230/60, and limit the max output frequency to 54 Hz.

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#### Hakan Ozevin

> Kind of a two part question.
> First, the VFD (variable frequency drive)
> varies the frequency not the voltage.
> The formula to determine the speed of the motor
> is RPM = (120 * F) / N Where F is Frequency
> in cycles per second and N is number of motor > poles

This is not correct. A VFD changes the voltage as well as the frequency. The formula for the speed of the motor is correct, but as speed decreases, magnetic flux need of the motor decreases as well. Therefore, as frequency is lowered, voltage should be lowered too, in order to prevent the motor's magnetic flux to be saturated (and energy to be wasted).

> Second, a good rule of thumb is never operate a
> motor at more than 10% above or below the rated
> voltage. This is for normal conditions and I
> would not try it on a variable speed
> application. Have you thought about just
> replacing the motor with a 208 motor?

This is not correct either. You can run the motor at lower voltages, provided that the torque supplied at that voltage is enough to run the load.

The result is, you can use this VFD if you need 90% power from the motor.

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#### JGrumby

I do it all the time. You didn't state your motor size so I cant say how much more current you'll draw, but you will draw just a little more current. If you size with a healty margin, rather than shaving it close, which is never a good idea anyways, you'll be just fine.

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#### JGrumby

I do it all the time. You didn't state your motor size so I cant say how much more current you'll draw, but you will draw just a little more current. If you size with a healty margin, rather than shaving it close, which is never a good idea anyways, you'll be just fine.

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#### Bob Peterson

the volts/hertz ratio is typically fixed based on the voltage and Hz rating of the motor. so if you have a 230 V - 60 Hz motor, you would typically get 3.83 V/Hz. The 208V - 60 Hz invertor probably will put out 208/60 = 3.46 V/Hz.

It might be possible to change a setting in your drive so it puts out 3.83 V/Hz. Your max frequency would then be about 54 Hz with the full V/Hz range.
If you are operating at less then full load at the higher speeds it may not even notice.

Bob Peterson

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#### Tom Gianni

Melvin,

Sorry. You are incorrect. A VFD (motor drive) certainly does vary the voltage as well as frequency in order to maintain a constant V/Hz ratio. There are many textbooks available, as well as supplier documents, where you could learn more.

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#### suseka

If you can set the motor name plate voltage, kw,rpm,current into the VFD (most VFD are capable of this), then the VFD will take care of every thing. The VFD software will create a motor model and you can run a 208 or 230 v motor from even a vfd connected to 480 v ac supply. So far your motor name plate voltage do not far exceed the maximum vfd output voltage you can run the motor to full speed.

The motor can be run to maximum frequency of the motor without any problem if the full load of the motor is only around 80-85% of the motor kw.
From the data you specify is it a 50 Hz motor?