VFD configured to run at a constant speed? 2

[CuriousElectron]

Greetings,
Can a VFD be used to run a motor at a constant speed(disabling the variable frequency option)?
Thanks,
EE

 

[davidbeach]

As long as it’s not a forbidden speed. But why? There’s almost certainly another, more efficient, lower cost, way of achieving a different set speed.

 

[jraef]

Or a different set output of whatever the motor is doing. VFDs are good for VARYING the speed, but if used to simply permanently change speed, they are kind of a waste, not only due to the cost, but energy too. VFDs are at best, 97% efficient, so there is a 3% "penalty" of energy consumption for fixed speed change. If you need VARIABLE speed, they are far more efficient than other means of doing that, but to just CHANGE a fixed speed, you are better off using something else.

But yes, you CAN do that.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[che12345]

Dear Mr.CuriousElectron

Q. " Can a VFD be used to run a motor at a constant speed(disabling the variable frequency option)? "
A1. VFD operation principle is based on [generates a varying frequency and voltage internally] to suit the running speed of the motor. The frequency generated by the VFD can be higher/lower than the utility power source , which is [fixed at 50/60 Hz]. Therefore a VFD can be adjusted to operate the motor to run at a higher/lower than the (name-plate rated speed). The voltage generated by the VFD can be lower but (usually) not higher than the utility power source.
A2. It is very convenient to set the VFD to run at N1-rpm and for future to N2-rpm. BTW. There are [fixed two-speed] motors with Dahlander windings on the market. They are [fixed] N1 and N2 , but with different P1/P2 ratios, depending on the characteristic of the load. Caution: select [N1 and N2] and [P1 and P2] that suit your load.
A3. If your application requires a [fixed speed] say (slightly low/higher than the motor rated speed), there are other more economical/efficient ways, instead of using a VFD to achieve the end result.
A4. FYI: VFDs are expensive, require regular attention/maintenance (require manufacturer's representative to attend to any faults), short life (usually? not more than six year!).
Che Kuan Yau (Singapore)

 

[waross]

That was pretty well covered last week:

720 RPM motor on a VFD
thread237-469957


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

 

[dkjfnvfd]

What does "constant speed" mean in this context?

A VFD can definitely drive a motor at some fixed speed. E.g. you could run the motor at a fixed 100% or 50%. Slowing a high duty machine will provide a nice RoI due to the cube relationship between reduced rotor speed and reduced power consumption. You will also see benefits in reduced transmission system operation and maintenance costs, since you can get rid of it, and a VSD usually requires fewer, less frequent maintenance activities.

If you want to use the VFD to run the motor at 100% of nameplate speed, then it is a not a good idea because energy consumption will increase by about 5%. It's hard to suggest an alternative without understanding the specific application you have in mind, however usual alternatives when running at 100% include soft-starters, or "right-sizing" the machine so that the load is as close to the 75% sweet spot as possible.

HTH

 

[LionelHutz]

Slowing a high duty machine will provide a nice RoI due to the cube relationship between reduced rotor speed and reduced power consumption.

There are MANY loads that don't follow the cube relationship. Even loads claimed to have the cube relationship often don't follow it nearly as well as the sales literature claims. Lots of times, the slower speed just means you are doing the same work, but doing it less efficiently over a longer period of time.

Slapping a VFD onto a fixed speed system to operate it at a different fixed speed and claiming a ROI is taking the lazy way out. Properly engineering the system in the first place always yields a better energy efficiency.

The place for a VFD is in a system that requires the varying speed. For example, a system which requires a source of filtered air at a constant flow. The VFD can vary the fan speed to maintain the same air flow as the filters go from clean to dirty.

 

[dkjfnvfd]

That's pretty interesting, I wasn't aware of that.

We don't meter anything where I work so I've never seen a real before/after picture. Do you have anything like that?

Not sure what you mean about doing the "same work" though. If a motor's running slower, then surely it's performing less work? E.g. a vent system will push less air.

 

[LittleInch]

Curious electron - you seem to have a habit of starting threads and not responding. You've started 63 threads and made only 31 replies.

That's not really how it's supposed to work here, you ask a question then respond to questions and have a bit of to and fro to get your question answered as well as possible and it provides feedback to those spending their time for free in providing you with data.

And that means more than "Thank you for your answers" - it means actually engaging with the posters.

Please.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BrianPetersen]

Fans operating in the atmosphere (not against a fixed downstream pressure) follow the cubed relationship well.

A variable-displacement swash plate pump in a hydraulic power unit (very common application) won't follow the cubed relationship, assuming the demand is set based on a fixed pressure setpoint (which is the normal case). Slowing it down while remaining within system capacity will just make the displacement-modulating mechanism go to a bigger displacement thus demanding the same (ish) power, i.e. more torque at lower RPM. Depending on where you are on the motor's torque curve, the motor might not be too happy about that, and it won't save power. Of course, if the pump displacement mechanism hits max pump displacement, the pump will no longer keep up with demand ... you'll use less power, by operating the load slower. Probably someone isn't going to be happy about that, either.

A centrifugal water pump operating against a fixed head won't follow the cubed relationship, either. Slow it down too much, and it simply stops pumping, acting dead-headed. That's not good.

Certainly there are cases where such applications can be slowed down (e.g. because it was oversized in the first place), but be prepared for the results.

 

[dkjfnvfd]

Thanks for the explanation!

I mainly work on vent systems ATM as an electrical engineer, so I don't know so much about the mechanical implications. I usually get designers to tell me or work out lowest "safe" flow rate and we set motor speed to reflect that.

We have VSDs on the diesel fuel lines at work which I understand work well? But I haven't had any experience with those.

 

[LionelHutz]

Not sure what you mean about doing the "same work" though.

First off, use the whole sentence and don't cherry pick a little bit of it out to make your point.

Secondly, work is moving something. Say it's a water pump and you're filling a tank. At full speed you might move 100 gallons per minute. At 75% speed you might move 25 gallons per minute. So, you just take four times as long to do the same work. BUT, the system with the VFD running at 75% speed is less efficient so the energy input to move 100 gallons of water is higher. No savings. Once again, it's a process control thing. You use the VFD to run at 25 gallons per minute because the process requires it, not because it saves energy.

The failure when claiming VFD power savings occurs by only looking at the input power instead of looking at the energy input required for the work done.

 

[dkjfnvfd]

AH yep that makes sense.

I usually think about flow rates based on safety case rather than batch or process so it didn't immediately click.

Thanks for explaining.

 

[Compositepro]

The point is that work (energy) and power (the rate of doing work) are two different things. Reducing power can cause the total amount energy consumed to accomplish a task to increase.