Does PWM Turn Off in a VFD after Reaching Desired Speed?

[gtallens1979]

Hello All:

We have a customer that is using a VFD basically as a starter on a motor due to in-rush limitations not allowing them to use a soft-starter only. Once the motor gets up to speed, they no longer utilize the speed variability in the VFD. The motor runs at full load speed.

My question is if that is the case, does the PWM coming from the VFD discontinue or is it there because the voltage and current are still coming through and from the VFD to the motor?

Thanks in advance.

 

[xnuke]

It's still there. That's how it makes the output.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[waross]

No PWM equals no output.

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

 

[BrianPetersen]

I suspect the original poster doesn't know how a VFD works. It takes the incoming AC power, regardless of exact voltage and frequency and waveform, and rectifies it to charge its internal DC bus. Then it chops up the DC (via PWM) to produce a simulation of an AC waveform of the desired frequency and voltage. There is never a direct connection between the incoming AC and the outgoing (chopped/simulated) AC. The fact that the output AC frequency might be the same as the incoming AC frequency is of no significance or concern or relevance. The output of a VFD is always chopped-up DC.

 

[Compositepro]

A VFD will consume about 4% of the input power at full speed. When it is used only for starting, is it is not usually bypassed by a contactor after starting has completed? Or are there problems involved with switching the VFD output?

 

[itsmoked]

Mucho problems with trying to switch-out a VFD. You would have to tell the drive to stop via coasting then close around it, while positively physically disconnecting its output, onto the now unsynchronized motor. Bad juju. Live with the losses.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

You cannot have the VFD output and the Bypass connected at the same time, it could (would) blow the transistors in the VFD. So what Keith is referring to is that it must be an "open transition" from VFD to Across-the-Line power to the motor. What can happen though is that during that open transition, the motor is not getting power so it begins to decelerate. Then when the bypass connects, that can mean a significant current and torque spike when it does, of if the transition is too fast and the magnetic fields in the motor are still active, it's like connecting two generators out of synch. I've seen that shear off a 500HP motor shaft.

Some more sophisticated VFDs offer what's called a "synchronous bypass" capability wherein the VFD compensates for the speed drop when transferring to Across-the-Line by slightly over speeding the motor immediately before transfer. But the concept can mean spending significant extra money on a bypass system, so it's generally not used until you get into large drive systems, i.e. hundreds of HP, where the energy savings start to outweigh the added cost. I have heard of people doing t for LV motors, but I have never seen it done because of the costs involved. The few times I have used it were for MV motors and drives, where we have had multiple motors but only one at a time needs to have speed control and the cost of additional VFDs is so extreme that it makes sense economically.


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

 

[TugboatEng]

You might be thinking of a soft starter which has a bypass relay once desired speed is reached.