Devices (similar to a VFD) that pulse width modulate or time proportion AC power are commonly known as "SCR's", rather than VFDs.
SCRs, like VFDs take a continuous control: 4-20mA or 0-5Vdc)signal.
In answer to your question, yes, there can be a compelling reason to use an SCR for a single heater element: the available contol signal is continuous, not binary like a relay or open collector output.
Another reason is that the load might require a current capacity that exceeds stand-alone solid state relays, or that a stand alone solid state relay requires well engineered heat sinking, which is, presumably, part and parcel of a commercial SCR package.
Strictly speaking a vfd would not vary the heat at all. DC to Light, it's all the same to a resistive load. VFDs can vary the voltage to
protect the motor but that is a secondary effect. So, it's extremely unlikely a compelling case can be made for using a VFD to control heating elements. The associated blowers, yes, the heaters, no. Unless there's something I'm missing.
Dear Sir,
As such VFD can not be used for heater control application. However Thryistor based control devices are available, which control the firing angle of thyristor unit to control the supply / power to the heater. For this you have to use a PID controller along with a temperature sensor (RTD/Thermocouple. Such type of arrangement very good control performance, means practically no deviation between SP and PV. For further details you can write to me directly.
I wouldn't do that. The VFD is designed for an inductive load with the electrical characteristics of a motor. The drive will get confused. If you want analog control of a resistance heater, there are SCR power controllers designed for that kind of load (or you can time-proportion them with your own solid state relay if the inertia of the heating element and heated load allow for it).
I can't think of a good reason for doing it, but if the drive is set up in Volts/Hertz mode with no speed feedback (and 'boost' turned off, and accel/decel turned all the way up, and all similar parameters specific to motor control are defeated or modified appropriately) then it will change heater output more or less proportionately to the 'speed' reference command. A long time ago I once used resistive loads when doing preliminary post-repair tests of 6 step AC drives, but have no experience with PWM drives pushing resistive loads. I would suspect, however, that PWM drives (especially when run at higher carrier frequencies) would tend to stress heater insulation systems.
There are better and cheaper ways to provide continuous control to resistive heaters - phase angle controlled SCRs, or zero-fired proportional controllers such as the Payne 18EZ series. Phase angle controllers operate by turning on a set of SCRs (one for each half cycle) anywhere from very early in each cycle (for 100% output) to very late in the cycle (for 0% output), but tend to create a fair amount of RF noise since they aren't switched as the AC wave crosses zero volts.
Zero fired proportional controllers turn on a variable number of full half cycles at their zero firing point, and hence generate little RFI. For instance, if one is set up for 60 cycles = full output then calling for 50% output turns on 30 cycles, and doesn't trigger the next 30 cycles, 25% output turns on 15 cycles (and doesn't trigger the next 45 cycles), and so on.
Good point... when responding to the original post I conveniently forgot most modern drives have such functions, and which may not be defeatable via
parameterization. Guess my head was firmly back to the bad old days thinking about the drives I once repaired (Parametrics Parajust series), which barely had a functional current limiting circuit, let alone the features we've come to expect.
VFD can be conveniently used for Heater control in V/F mode. Though SCR can be used we need to use application specific control boards. How ever with VFDs we can use a readily available PLCs to program the heater application.
> VFD can be conveniently used for Heater control in V/F mode. Though SCR can be
> used we need to use application specific control boards. How ever with VFDs we
> can use a readily available PLCs to program the heater application.
In the V/f mode, the waveform synthesized by the VSD is an approximation to a sin wave. The higher the frequency of the sin wave, the higher the voltage. This means that as your frequency setpoint goes up, the voltage goes up. It should work just fine with a resistive load like a heater. The higher the frequency setpoint, the hotter the heater.
The V/f mode is used because the inductive load of a motor would make the current go up as the frequency was reduced. By reducing the voltage at the same time that frequency is reduced, current remains constant.
To keep the phase balance constant, you could use three heater elements wired in star or delta.
> Is there a compelling reason to use a VFD for heater control?
Sounds like a waste of a money to me when there are SCR devices available specifically designed for that application. I guess if you're stuck at the North Pole with a bunch of VFD's on hand...then go for it !
For heater control, Its better to use SCR based power pack. This power pack will work on 3 phase supply, Output of power pack will be controlled through SCR. You can use temp sensor signal to PID controller and O/P (ma / Volt) of PID controller to power pack to get the desired output in terms of heater firing angle.
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