Drive heating elements with a VFD

[DesignerMike]

Long story, but I am working on a little R&D project that requires a immersion heater total draw of about 50 amp.

The heater will be installed where there is normally a large extrusion pump. There is a VFD wired to the pump via plug/recepticle.

Question is can I drive the immersion heater with the VFD that normally controls the pump?? I have asked three people and no-one has a definative answer. My gut tells me no, but I also don't want to spend a big chunk of money to wire something for a one-time test.
If it would work, how would the frequency from the VFD affect the resistant heater elements?

Obviously I'm not an EE

 

[Skogsgurra]

As long as there is enough voltage and current and if you can set the VFD to a suitable voltage (use V/f mode) then I see no problems. I have used heaters to test load VFDs without even thinking about any problem. And I had no problems either.

Another solution is to by-pass the VFD and use the existing cabling to feed the heater.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[OperaHouse]

That would be an interesting concept, using a VFD for a proportional temperature controller. Certainly would be an expensive way to do it. The real problem running off the motor lines is how do you control the temperature of the heater. It would have to be balanced and even just a relay for control would have to tolerate a wide voltage.

 

[ozmosis]

DesignerMike
Yes, it is possible. We have supplied a few thousand drives to an OEM who wanted to use the vfd to control heating elements. As Gunnar points out, V/F (scalar) control is necessary not 'vector' control. Higher PWM switching frequencies are typically required so this will sometimes put additional stresses on the VFD so be aware of any derating the VFD manufacturer stipulates on the current rating.

 

[dpc]

sed2,

Just curious as to why higher switching frequencies are required?

 

[DesignerMike]

Thanks to all for the great information.

I'll post the results if we decide to go this way. It is probably more likely that we will pull the VFD temporarily and install a controller that is more predictable and matched to the heating elements. (but still use the existing wiring).

 

[ozmosis]

dpc
They were in this particular case due to the design of the heating elements. I'll be honest, I can't recall why (it's about 6yrs ago now) but I do know that <6kHz and we seemed to have a few problems with the drive we used. I think 12kHz was the optimum switching frequency and this put other considerations into the cooling of the drive.

 

[waross]

If you have a temperature transmitter with a 4-20 ma. or other analogue output, and the VFD has a 4-20 input, you may be able to adapt and connect them for quite good temperature control with the VFD. Just a thought.
Any comments Gunnar, is this reasonable or am I about to learn something new?
respectfully

 

[jraef]

It's overkill, but I see no reason why it would not work. We used to do preliminary VFD load tests with heater elements in a 55gal drum of water before hooking up a very expensive motor to them. Never had any issues. We were not testing the heater's performance however.

JRaef.com
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