Variable Speed Drive Output


Thread Starter

Robert Machetto

I'm an electrical designer interviewing for a job through a staffing company. The hiring company says I have to answer seven control questions before I can interview. Six of the questions were a snap - the seventh was "What output voltage would you expect on a 460VAC VFD running in a linear mode at half speed" -- (also voltage in a squared mode). Any experts out there?
460 volts. (Except at the instant of startup). Motor speed is a function of frequency, not voltage.
I've been a drives technician for nine years and have some experties here. A 460v ac drive in a linear mode will give you 7.66v per Hz if you do the math at half speed "30hz" your voltage will be 230v. Now If you use a digital meter even a true RMS model you'll read about 310v. If you use a analog meter like a simpson you'll get a better reading. The digital reads the peaks.
With the drive in a squared mode the voltage will be consideably less. It's function is to produce less voltage when the motor does not need it. Usually in the pump or fan curve where it does'nt produce enough flow and the load is reduced.

Hakan Ozevin

In linear mode 230 V, in quadratic mode 460/4=115 V. Although the speed of the motor is dependent on the frequency, not voltage, VFD's decrease the voltage as well. The reason can be explained as follows:

For a constant torque load, the power demand of the load decreases by the speed. So if you decrease the frequency, but not the voltage linearly, the rest of the power will be dissipated as heat in the motor- we call this the motor is saturated (which we dont want).

For a quadratic torque load (e.g. a fan), the power demand of the load decreases by the *cube* of the speed. Therefore the VFD should decrease the voltage by the *square* of frequency. Of course these are theoretical figures. For vector control devices the VFD will automatically adjust the voltage, but it is still near 230 V at half speed. Also, the user can add some voltage manually depending on the application.

Pierre Desrochers

It all depends on the load. I've seen less than 220 volts but then again I've seen 440 Volts. The current needed will dictate the Voltage supply... in an open loop anyway.


James Bouchard

If your VFD is a constant torque model you would have 230 volts since torque is Voltage/Frequency and to get half speed you would be at about half the maximum speed so to maintain the same torque level as at full speed you would need only half the voltage.
If you have a drive designed for fans or pumps the torque required is not linear and some drive makers designed for this so they would provide even less torque so the voltage could be lower than half.

On drives that use pulse width modulation the voltage is constant and the time it is present varies to provide the correct response. If you put a meter on the motor your reading will depend a lot on the type of meter as many meters are not designed to provide an rms value when presented with waveforms other that a pure sine wave.

I realise this probably does not help you a lot but maybe it puts you on the right track.

James Bouchard

Vitor Finkel

In order to keep motor current and torque at nominal values, and also to avoid saturating the magnetic nucleus, you should keep an squirel cage
(AC) motor Volts/Hz constant. So: half speed --> half Voltage, or 230 Vac.

A running motor seldom can be considered as an open loop, since counter emf (Voltage) is generated in function of it's speed, and in consequence, drawn current will always be a function of applied emf minus self-generated counter emf, so there is a lot of feedback involved there.

Starting current in a squirel cage motor can be from 3 to 6 times larger than rated current, because there is no counter emf while motor is stopped ( starting)


Vitor Finkel [email protected]
P.O. Box 16061 Tel (+55) 21 2285-5641
22221.971 Rio de Janeiro Brazil Fax (+55) 21 2205-3339
I believe that it would be 230VAC. You need a constant volts/Hz ratio.

It is difficult to measure without a good true RMS multimeter. A cheapo meter will read the peaks and display a very high value.

Bill Sturm
Well it looks like you have some good answers here already, but i'm bored. Most of these answer are correct. If your base freqency is set at 60hz and the drive is linear you will have 240 volts at 30hz. If you set the base freqency at 120hz you will have 120 volts at 30 hz. Good luck.