Resistive Heater - VFD
Resistive Heater - VFD
(OP)
Has anyone used a VFD to control power output to a resistive heating system? The system is currently ON/OFF control (with a contactor), approx. 50 KW, 480 VAC.
A client has purchased a VFD to allow varying control to the resistive heating element. I am more familiar with SCR control in this application, have never seen/used a VFD for heater control. It would seem that on paper the VFD could vary power to the heaters, but would present issues with EMI/RFI that would be less with an SCR.
A client has purchased a VFD to allow varying control to the resistive heating element. I am more familiar with SCR control in this application, have never seen/used a VFD for heater control. It would seem that on paper the VFD could vary power to the heaters, but would present issues with EMI/RFI that would be less with an SCR.





RE: Resistive Heater - VFD
An SCR controller varies the duty cycle of power fed to the heater, by varying either the cycle frequency or the on-time. You can definitely buy them to control 50 kW (and larger) heaters, but if the heater is already divided up into separate circuits, you can use a number of smaller SCRs or solid state relays on each circuit.
RE: Resistive Heater - VFD
And yes, you might have radiated EMI/RFI problems. Also possibly noise problems due to the heater elements vibrating at the carrier or PWM frequency.
RE: Resistive Heater - VFD
RE: Resistive Heater - VFD
I understand how the VFD works. I understand frequency won't change the heater output. The VFD controls to a V/Hz curve, and in theory as the frequency is varied the voltage is varied, so I would think the setup would inherently allow variation of voltage to the heater.
I also agree it is the wrong tool for the job and have suggested they procure an SCR controller. But was curious if anyone had rigged up something like this before using the VFD.
RE: Resistive Heater - VFD
I don't see the (output) harmonics being any less with a VFD, as opposed to a SCR controller.
The VFD relies on the motor's inductance to smooth out the current waveform. Without that inductance the heater's current waveform will be as choppy as the voltage waveform. This is not good wrt radiated EMF.
Also, if the VFD is a typical 6-pulse type of front-end, I don't see there being a significant reduction in harmonics on the incoming either ( although a 6-pulse VFD will have some inductance/capacitance on the front end that will knock out some of the higher-order harmonics).
Why anyone would use a VFD in this application is a mystery to me. Perhaps, it was kicking around the plant as a spare?
Other than the harmonic issue, I believe that a VFD will work in this application. This assumes that V/f can be kept near constant.
"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)
RE: Resistive Heater - VFD
Just to confirm - the VFD is not being proposed to alleviate any sort of harmonics or noise issues. I believe it is being proposed because someone specified the wrong solution and now that is what they have on site. I agree (or assume) the EMI/RFI will be worse with the VFD than with an SCR, for the reasons mentioned above.
I have recommended they not use the VFD and that they purchase the right controller. But checking with others to get their experiences. Sounds like others are in agreement with what I was thinking.
RE: Resistive Heater - VFD
RE: Resistive Heater - VFD
That said, if it is an INDUCTION heater, that's a different story, PWM is used for that all the time. I have also wondered if you used a transformer between the drive and resistive heater, if that would give you more control. Probably would (because of the inductance of the transformer) but there are potential issues with using transformers with variable frequency, so you would have to try to modify the VFD output to change voltage only, and few VFDs are going to allow that. It's not what VFDs are designed to do...
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Resistive Heater - VFD
On the other hand a hundred dollar PID controller and some 70Amp solid state relays are probably cheaper than both a phase controller or a VFD and will actually do a good job of temperature control.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Resistive Heater - VFD
I have seen transformers used between VFDs and motors.
They were driving submersible pumps in a SAGD (Steam Assisted Gravity Drainage) application. There were a number of wells on the pad. Some had filters after the VFDs, some had transformers between the VFD and the motor.
The transformer would probably work for a heater as well.
Bill
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"Why not the best?"
Jimmy Carter
RE: Resistive Heater - VFD
RE: Resistive Heater - VFD
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Resistive Heater - VFD
Some VSD manufactures offer VSDs with an adjustable voltage operating mode designed for such applications. It enables frequency to remain fixed while allowing variable voltage output to control the heater. Works well for varying the field strength on an electromagnet too.
Powerflex 700VC and the more recent Powerflex 753 are two candidates.
I'm sure other manufacturers may also have this functionality.
RE: Resistive Heater - VFD
Varying the voltage without varying the frequency may not address the issue of low or no inductance in the heater.
Why change one VFD for another VFD when the optimum solution is a controller at about 20% of the cost?
Bill
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"Why not the best?"
Jimmy Carter