Freq drives/motors

[shawn73]

I was asked recently at work if it is hard on a motor to run it at a low frequency constantly. (say, 2 hz.) I wouldn't think that it would be, but I thought I'd check. Can ANY motor be safely driven off of a frequency drive? What characteristics should I be looking for for compataility? Thanks again.

 

[petersonra]

Re: Freq drives/motors

Originally posted by shawn73:
I was asked recently at work if it is hard on a motor to run it at a low frequency constantly. (say, 2 hz.) I wouldn't think that it would be, but I thought I'd check. Can ANY motor be safely driven off of a frequency drive? What characteristics should I be looking for for compataility? Thanks again.

There is not a simple answer to this question.

Many motors will not turn at all at such a low frequency, since the torque being generated is so low if it is a standard VFD. A vector drive will produce full torque at any speed, including zero speed.

Many motors will overheat if run at low frequencies since they depend on internal fans to cool the coils. The fan brings in less air at lower speeds, so the motor heats up.

Standard motors also have standard insulation that can be damaged by the higher peak voltage of the non-sinusoidal VFD output.

The fix for this is to buy only motors rated for invertor duty, and if the motor speed requires it, buy an external fan for cooling. Most motor manufacturers can give you an external fan that mounts right on the motor.

 

[tony_psuee]

Re: Freq drives/motors

Shawn,

I can't answer

Can ANY motor be safely driven off of a frequency drive?

My experience has been with three phase TEAO and TEFC fractional and integral HP motors on VFD's. We have manufacture's recommendations on the minimum speed and that is typically 10%, 6Hz. This is for a minimum air over for heat dissipation. Again, that is from the manufacturer and what you require for the application could be higher. Our's is typically 20Hz. Motor lead lengths are a common issue and in some cases motor and VFD compatibility. IMO, you get what you pay for.

Hope this has been helpful.

Tony

 

[charlie tuna]

Re: Freq drives/motors

73,
you should consider what type of task you are trying to accomplish. there are different drives designed to function at high and low speeds. a "static drive" is capable of running at low speeds or even at stop for long periods of time without any problems. in a large process plant a motor was coupled up to a gear box about the size of a volkswagon. the function was to turn a shaft ten turns in one direction and then reverse itself and turn ten turns in the reverse direction. we removed the motor and gear box. connected a static motor directly to the shaft. programmed the static inverter and energized the system. it would run all day ---ten turns one way then ten turns in the opposite direction -- same torque as the gear box and you couldn't feel any heat on the motor frame. it had fans on the motor ends to carry off any heat and they ran on temperature stats. the old system had a motor running at 1750 rpm all day.

we have used these static inverters in printing presses with good results --built in breaks ---no brushes or speed control problems.... we have had standard motors fail when connected to drives --- they should be "drive rated motors" --- the voltages produced by the drive is usually twice the normal voltage of an across the line operation. they are more efficient and will pay for themselves in short order.

we will see cable less elevator cars ---static drives will be geared directly to the elevator shaft rails - they will be more accurate than cable cars and of course more efficient....