Motors: Minimum Permissible Frequency

I've just been told that motors run through a VFD shouldn't be run under 20 Hz... No reason provided, just the advice.

I can't think of any good reason except that the cooling drops with the speed. In my case, I'm typically drawing less than 50% of FLA (though in a continuous-duty application with a moderately variable load) and the motor casing doesn't get too warm.

Can anybody validate or debunk that advice, along with justification either way?

At this point, I'm not at liberty to start fiddling with the drive components to change shaft-to-output ratio - I've just got to go with the VFD.

I just don't want to do something stupid and destroy my drive or motor.

1. Torque is limited by the function of speed. The lower speed means lower torque.
2. Fan cooled motors don't fan cool very well at less then half speed. Flow by a fan is a quadratic function if I remember correctly, so that means and 1/2 speed you make 1/4 cooling.

One thing you can do to help the cooling is use a motor with an auxilary cooling blower operated by a second smaller motor. You could also use a motor with more poles, one rated for 1150 RPM instead of 1750 RPM.

I've used motors at all speeds.
Generally, If you want to run a motor at the slower speeds for an extended period of time, use a separate DOL cooling fan.
Ideally, you should choose the correct motor and components so that the motor runs at its rated speed (or close to it) most of the time. But that is not always possible.

Beggar
If this is a small (ie cheap) 4 pole motor then you could try changing it to a 6 pole with the same frame size. This can often resolve the problem at very little cost.

It also depends on the design of the motor. Standard AC motors probably shouldn't be run heavily loaded at low speeds due to heating, but an infinity:1 designed motor can sit forever at full load and 0RPM.

There is no simple answer to this question. But, regarding the above posts, it is not true that available torque is a function of speed. Any induction motor, as far as its electrical and magnetic performance is concerned, is a CONSTANT torque producer from motor nameplate speed down to essentially zero speed.

The hook in the above statement is the motor's thermal performance which, depending on a lot of things but mostly its enclosure design (TEFC, TENV, aux blower, etc), will limit the torque to less than the constant torque actually available.

Bottom line---there is no good reason to say that an induction motor is categorically limited to 20Hz. On a fan which unloads rapidly as it slows down, an ordinary TEFC motor can easily run down to 5Hz without thermal problems. Of course, the fan isn't moving any air at that speed but that is another subject.

On a machine tool or winding application where the load torque actually increases as the speed goes down, the motor may not be able to go slower that 30Hz depending on how it was sized, even tho it is an inverter-duty aux blower motor.

So, beware of such simple rules-of-thumb. The load requirements have to be within the electrical, mechanical, and thermal limits of the motor. That's a rule you can trust.

For those with solid drive application backgrounds, you will note that the above discussion refers only to continuous conditions. For short term overload conditions, the constant torque capabilities of any motor including types like TEFC can be fully utilized since the motor has heat sinking time which can be used to your advantage. But that's another subject too!!!

I once had a customer who didn't believe me when I told him his motor on a constant torque loas should not be run below 30 Hz. After he paid for a rewind (I had it in writing) he became a believer.

As Dick indicated, you have to look at the load, the motor, and the drive as a system. Looking at indivicual components without reference to the interactions between them will lead to problems.

One other item I feel compelled to mention is that Andybr's suggestion about changing motors cannot be implemented without some study. A 6-pole motor has a nominal speed of 1200 rpm, and a 4-pole motor has a nominal speed of 1800 rpm. As stated above, you have to look at the implications of speed and torque changes on the load before doing this.

The old SCR chopper VFD's had a big problem with harmonics around 12 and 7 Hz (5th and 7th sub harmonics), and could have problems due to it.

If this is not a modern PWM VFD, the advice could well be accurate. In addition, mechanical harmonics (critical frequencies) or torsional stresses on the shaft may indicate a higher than zero minimum operating speed. I've seen all of these at times on different systems.

As a general rule, though, there is no reason not to run a motor at zero Hz and full torque, with the warnings given in previous posts.

You can avoid the damaging of a motor by overheating caused by it running at off nominal speeds in two ways:

Let the VFD calculate the heating based on setting correct motordata in the VFD parameters. The VFD can then make a much better approximation of the heating of the motor than a regular thermorelay can.
If the VFD believes that the motor is overloaded it will typically reduce speed and/or stop completely.
Many VFDs has this functionality allready.

Even better is to monitor the actual temperature in the motor windings. For motors running continously at off nominal speed this is compulsory to my opinion.
Many VFDs allready have inputs for monitoring PTC sensors.

Of course this doesnt allow a motor to run at low speeds if the problem is that there isnt enough cooling at the low speed. But at least you wont burn a motor in the proces of finding that out.

DickDV said:

On a machine tool or winding application where the load torque actually increases as the speed goes down, the motor may not be able to go slower that 30Hz depending on how it was sized, even tho it is an inverter-duty aux blower motor.

Click to expand...

Please, explain, why on winding application load torque increases as speed goes down. Do you know any links to white papers, application notes on this topic?
Thank you.

PaulB.

I am assuming that both DickDV and paulB are referring to center driven winders.

A center driven winder has two items that affect the spindle speed: line speed and roll diameter. For a given roll diameter there isn't a significant change in the spindle torque requirement with changes in line speed.

For a given line speed there IS a significant change in torque requirement with diameter. For a given line speed the spindle speed is inversely proportional to roll diameter. So for a given line speed, as diameter goes up, spindle motor speed goes down and torque requirement goes up. This has the appearance of a system where torque requirements increase as motor speed decreases. You just need to know where the torque increase comes from.

Keith

There you have it, paulB. And not from DickDV either!! Thanks Keith.

Thank you. Now it is clear.

PaulB.

On the older 6 step inverters, due to the switching action, they were limited to 20 hz on large motors. These motors were NON inverter duty motors. On motors made for inverter duty, usually 6 hz was the bottom limit. The motor could not produce sufficient torque to actually rotate. That is why many motors were not allowed to run under 20 hz.

Todays PWM VFD's typically can run a motor at 1.5 hz in V/F mode. Flux vector contols have min speed of 0.1 hz. The drives will allow the motors to get FLA at the lower speeds. However, continued running at the lower speeds can also have the built in motor thermal protection tripping the drive off to prevent burning the motor.