Author Topic: IGBT Dimmer - PWM AC Power Control for an Immersion Heater  (Read 16564 times)

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Offline willz1200Topic starter

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IGBT Dimmer - PWM AC Power Control for an Immersion Heater
« on: August 25, 2018, 01:43:32 pm »
Hello,

I'm currently in the process of building a solar power diverted to route surplus electricity to my 3kW immersion heater. So far I've made a triac phase cut dimmer which is wirelessly connected to CT clamps and AC voltage reference. It works okay but I'd like to implement a PWM IGBT dimmer to reduce EMI, acoustic noise and put a more uniform load on my inverter. There are two topologies I'm currently looking into, if you know of any other good ones please say.  :-+ I found some schematics online which show a rough idea of what I mean. I also read that PWM dimming can be used in conjunction with zero crossing detection to slightly improve efficiency, I don't understand why this is true? Surely the constant switching will cause all the losses, what difference does the phase make?

Topology 1) Rectified AC applied to the heater through one IGBT:
It appears that this topology is more popular, but I'm concerned about the high power dissipation in the bridge rectifier.
The schematic that I found shows a MOSFET instead of an IGBT, is there any significant benefits of using the IGBT besides its lower switching losses?


Topology 2) Two anti series IGBTs applying the AC directly to the heater where each body diode allows the opposite cycle to pass:
Will this topology result in an overall lower power dissipation?
Will a separate rectified power source be needed to driver the gates and is a driving ic necessary?


I'd also like to put more thought into the EMI filtering, is a single common mode choke sufficient?
I came across this image of the 4-Noks Power Reducer which appears to be topology 1, looking at the top filtering PCB I see several common mode chokes and capacitors, why are so many different sized chokes used? How do you go about selecting them?
« Last Edit: August 25, 2018, 02:11:41 pm by willz1200 »
 

Offline T3sl4co1l

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Re: IGBT Dimmer - PWM AC Power Control for an Immersion Heater
« Reply #1 on: August 25, 2018, 02:39:41 pm »
First of all, did you try a zero crossing controller?

Heaters don't need to turn on and off very fast.  Skipping a line cycle every now and then is more than good enough for the purpose.

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Offline willz1200Topic starter

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Re: IGBT Dimmer - PWM AC Power Control for an Immersion Heater
« Reply #2 on: August 25, 2018, 03:10:49 pm »
Yes I'm currently using a MOC3021, AC Optocoupler and interrupts on an ATTINY44 to perform zero crossing detection which fires a triac. It works fairly well but its not ideal for controlling heavy loads. The idea is to finely tune the power consumption of the immersion heater to match the solar power surplus in real time. Which is why I'd quite like to build a PWM driver :)
 

Online David Hess

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Re: IGBT Dimmer - PWM AC Power Control for an Immersion Heater
« Reply #3 on: August 25, 2018, 03:19:00 pm »
The schematic that I found shows a MOSFET instead of an IGBT, is there any significant benefits of using the IGBT besides its lower switching losses?

IGBT switching losses are not lower than MOSFET switching losses except maybe at very high voltages where large MOSFET die size results in high capacitance.  IGBTs like bipolar transistors have an advantage in cost at high voltages because MOSFETs have a die size which is proportional to the square of the voltage.

Quote
Will this topology result in an overall lower power dissipation?

You will have one diode voltage drop instead of two from the bridge rectifier so losses will be slightly lower unless IGBTs can conduct in reverse?  I have no  idea; bipolar transistors can.  One of a pair of MOSFETs would conduct in reverse so there would not even be the forward voltage drop of the diode making losses even lower.

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Will a separate rectified power source be needed to driver the gates and is a driving ic necessary?

The gate drive has to come from somewhere.

Quote
I'd also like to put more thought into the EMI filtering, is a single common mode choke sufficient?

Maybe.

Quote
I came across this image of the 4-Noks Power Reducer which appears to be topology 1, looking at the top filtering PCB I see several common mode chokes and capacitors, why are so many different sized chokes used? How do you go about selecting them?

Sometimes two or more common and differential mode suppression networks are used in series to provide enough suppression.
 

Offline Richard Crowley

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Re: IGBT Dimmer - PWM AC Power Control for an Immersion Heater
« Reply #4 on: August 25, 2018, 03:26:32 pm »
The thermal mass of the water is like a humongous capacitor that is integrating your nice PWM back into a running average measured in seconds (or minutes).  Using PWM for any kind of large-scale heating process is massive overcomplication.
 

Offline willz1200Topic starter

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Re: IGBT Dimmer - PWM AC Power Control for an Immersion Heater
« Reply #5 on: August 25, 2018, 03:46:43 pm »
Quote
IGBTs like bipolar transistors have an advantage in cost at high voltages because MOSFETs have a die size which is proportional to the square of the voltage.
When you say higher voltages do you mean in the kV range? Because briefly looking around 400V appear to be a similar price point.

Quote
One of a pair of MOSFETs would conduct in reverse so there would not even be the forward voltage drop of the diode making losses even lower.
So do you think MOSFETs are the way to go? I was thinking of trying the FDA24N40F which is 400V, 23A maybe a slightly higher voltage rating would be better? I'll pull some ideas together in LTspice.

Quote
The gate drive has to come from somewhere.
Of course, but what I mean is one of the schematics uses a zener diode to clamp the gate voltage and the other uses a drive IC.

Quote
Sometimes two or more common and differential mode suppression networks are used in series to provide enough suppression.


Okay, so is each individual suppression network tuned for different harmonics that can be expected?
 

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Re: IGBT Dimmer - PWM AC Power Control for an Immersion Heater
« Reply #6 on: August 25, 2018, 03:54:32 pm »
A TRIAC is probably the most efficient option.

A rectifier would drop too much voltage.

Two IGBTs would drop more voltage than a TRIAC.

Two MOSFETs connected back-to-back could theoretically drop less voltage, than a TRIAC, but it would work out very expensive. A TRIAC typically drops about 1.5V, so the MOSFET would need to drop under 0.75V in order to compete with that. The FDA24N40F has far too higher on resistance and will drop about 3.5V, at 13A with two devices back-to-back. The MOSFET needs to have an on resistance of under 0.75/13 = 58mOhms, to drop the same voltage as a TRIAC, which would not be cheap for a device rated to 400V.

As mentioned above, there's no point in chopping the mains voltage up by PWM and powering a heater, because the thermal time constant will be enormous. It would also produce significantly more EMI, than your current configuration, a zero crossing TRIAC, which is already the most optimum solution.
 

Offline willz1200Topic starter

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Re: IGBT Dimmer - PWM AC Power Control for an Immersion Heater
« Reply #7 on: August 25, 2018, 03:54:47 pm »
Quote
The thermal mass of the water is like a humongous capacitor that is integrating your nice PWM back into a running average.
But won't this result in a running average of a lower voltage which will cause less power to be dissipated in the heater?

 
Quote
Using PWM for any kind of large-scale heating process is massive overcomplication.
Yes it is a less traditional method, but in some ways it is less complicated, because in theory the driver doesn't need to perform zero crossing detection. Hence no interrupts or monostable circuit are needed.
 

Online Zero999

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Re: IGBT Dimmer - PWM AC Power Control for an Immersion Heater
« Reply #8 on: August 25, 2018, 04:06:12 pm »
Quote
The thermal mass of the water is like a humongous capacitor that is integrating your nice PWM back into a running average.
But won't this result in a running average of a lower voltage which will cause less power to be dissipated in the heater?
I don't understand the question. The heater doesn't have any significant capacitance. It just stores heat, so the effect would be the same as measuring the power dissipation and passing the result through as low pass filter, i.e. RC circuit.

The resistance of a water heater doesn't change much over its working temperature range and given the same duty cycle, the power dissipation should be the same whether the 230VAC is chopped up and PWMed at several kHz or PWMed with zero crossing, at a much lower frequency than the mains.

 
Quote
Using PWM for any kind of large-scale heating process is massive overcomplication.
Yes it is a less traditional method, but in some ways it is less complicated, because in theory the driver doesn't need to perform zero crossing detection. Hence no interrupts or monostable circuit are needed.[/quote]
The additional complexity can be avoided by using an opto-coupler TRIAC with a built-in zero crossing circuit, such as the NTE3097 or MOC3042.

http://exa.unne.edu.ar/ingenieria/sysistemas/public_html/Archi_pdf/HojaDatos/Optoelectronica/moc3041.pdf
http://www.nteinc.com/specs/3000to3099/pdf/nte3097.pdf
 

Offline drussell

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Re: IGBT Dimmer - PWM AC Power Control for an Immersion Heater
« Reply #9 on: August 25, 2018, 04:16:55 pm »
The thermal mass of the water is like a humongous capacitor that is integrating your nice PWM back into a running average measured in seconds (or minutes).  Using PWM for any kind of large-scale heating process is massive overcomplication.

Well, from the controlling the heating side of things, yes, of course it is over-complicated to have such "fine-grained control", but that isn't what the OP trying to accomplish.  He is trying to (as precisely as possible) use up his extra solar capacity by heating the water.  The fine-grained control is needed for the supply side, not the load, to be able to smoothly use only the excess generated power.

 

Offline Richard Crowley

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Re: IGBT Dimmer - PWM AC Power Control for an Immersion Heater
« Reply #10 on: August 25, 2018, 04:21:38 pm »
I don't understand the question.
Look at the ENTIRE system (from solar energy to the temperature of the water).  The water is a THERMAL "capacitor". It doesn't care whether you are powering the heater from a varying DC current, or from a PWM AC current, or from a slow on-off power (whether AC or DC).  The "slow on-off" is the method used by the overwhelming majority of temperature control systems.  Because of the massive integration of the energy sink (whether the water in your hot water tank, or the air temp in your house.)
 

Offline drussell

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Re: IGBT Dimmer - PWM AC Power Control for an Immersion Heater
« Reply #11 on: August 25, 2018, 04:23:22 pm »
The resistance of a water heater doesn't change much over its working temperature range and given the same duty cycle, the power dissipation should be the same whether the 230VAC is chopped up and PWMed at several kHz or PWMed with zero crossing, at a much lower frequency than the mains.

It is true that it doesn't change the total amount of power drawn, but it does affect how the power is drawn from his inverter.

My understanding is that he is trying to make this load be a more consistent load to his inverter, so that it looks more like a varying resistive load instead of hacked-up pulse-here, pulse-there load.  This makes perfect sense to me and seems that it could well be desirable in some cases like this so that the inverter is isn't having to supply random spikes of 3kW extra.  Not that it shouldn't be able to handle it but I think I understand the logic....
 

Offline Richard Crowley

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Re: IGBT Dimmer - PWM AC Power Control for an Immersion Heater
« Reply #12 on: August 25, 2018, 04:24:30 pm »
Well, from the controlling the heating side of things, yes, of course it is over-complicated to have such "fine-grained control", but that isn't what the OP trying to accomplish.  He is trying to (as precisely as possible) use up his extra solar capacity by heating the water.  The fine-grained control is needed for the supply side, not the load, to be able to smoothly use only the excess generated power.
Point taken.  From that perspective, then PWM seems justified. 

But then what difference does the efficiency of the switching methodology make when the ultimate point to the exercise is to simply waste power as heat energy?  What difference does it make whether the energy is dissipated as heat in the switch vs in the load?
 

Offline willz1200Topic starter

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Re: IGBT Dimmer - PWM AC Power Control for an Immersion Heater
« Reply #13 on: August 25, 2018, 04:28:18 pm »
Quote
A TRIAC is probably the most efficient option.
Thanks for clearing that up, doesn't the unsymmetric nature of triac phase cutting, generate more noise than PWM?

Quote
The MOSFET needs to have an on resistance of under 0.75/13 = 58mOhms
That is very low, I think I'd struggle to even find that.

Quote
The additional complexity can be avoided by using an opto-coupler TRIAC with a built-in zero crossing circuit, such as the NTE3097 or MOC3042.
Aren't these drivers only suitable for burst mode control? Which would give me a small dimming window? Meaning less dimming levels.
 

Offline willz1200Topic starter

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Re: IGBT Dimmer - PWM AC Power Control for an Immersion Heater
« Reply #14 on: August 25, 2018, 04:41:55 pm »
Quote
The fine-grained control is needed for the supply side, not the load, to be able to smoothly use only the excess generated power.
Yes that's my main aim, slowly increase power using a pid controller until the grid tie is on the edge of import/export.

Quote
But then what difference does the efficiency of the switching methodology make when the ultimate point to the exercise is to simply waste power as heat energy?
Point taken maybe I should bolt the FETs to the hot water tank :-DD I was just wondering how the use of IGBTs and MOSFETs compared and each PWM topology. But yes the main idea was to put a more uniform load on my inverter. The triac method causes some odd audible noises from my inverter, compared to boiling the kettle which doesn't produce the same noises.
 

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Re: IGBT Dimmer - PWM AC Power Control for an Immersion Heater
« Reply #15 on: August 25, 2018, 04:59:56 pm »
Quote
IGBTs like bipolar transistors have an advantage in cost at high voltages because MOSFETs have a die size which is proportional to the square of the voltage.

When you say higher voltages do you mean in the kV range? Because briefly looking around 400V appear to be a similar price point.

Very few IGBTs are made for use below 600 volts because MOSFETs will generally be more economical.  600 volts and higher is where IGBTs become competitive.

I actually did not know that they made any IGBTs below 600 volts until I just looked but there are a few for special applications.

Quote
So do you think MOSFETs are the way to go? I was thinking of trying the FDA24N40F which is 400V, 23A maybe a slightly higher voltage rating would be better? I'll pull some ideas together in LTspice.

Pick a IGBT and then compare the price to that of a MOSFET with an on resistance which produces the same voltage drop.  For greater cost the MOSFET will always deliver lower static losses which will dominate in a low frequency switching application.

Quote
Quote
The gate drive has to come from somewhere.

Of course, but what I mean is one of the schematics uses a zener diode to clamp the gate voltage and the other uses a drive IC.

AC applications like this would usually use a TRIAC or MOSFET based solid state relay which do not support PWM faster than the line frequency.  Higher frequency PWM requires more gate drive power so a different circuit is used to provide it.

Quote
Quote
Sometimes two or more common and differential mode suppression networks are used in series to provide enough suppression.


Okay, so is each individual suppression network tuned for different harmonics that can be expected?

Something like that.  The lower frequency suppression network will not perform well at high frequencies so a separate high frequency suppression network is also used.
 

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Re: IGBT Dimmer - PWM AC Power Control for an Immersion Heater
« Reply #16 on: August 25, 2018, 06:19:53 pm »
I don't understand the question.
Look at the ENTIRE system (from solar energy to the temperature of the water).  The water is a THERMAL "capacitor". It doesn't care whether you are powering the heater from a varying DC current, or from a PWM AC current, or from a slow on-off power (whether AC or DC).  The "slow on-off" is the method used by the overwhelming majority of temperature control systems.  Because of the massive integration of the energy sink (whether the water in your hot water tank, or the air temp in your house.)
I understood your point. What I didn't get was the original poster's question.

Quote
The thermal mass of the water is like a humongous capacitor that is integrating your nice PWM back into a running average.
But won't this result in a running average of a lower voltage which will cause less power to be dissipated in the heater?

Quote
A TRIAC is probably the most efficient option.
Thanks for clearing that up, doesn't the unsymmetric nature of triac phase cutting, generate more noise than PWM?
If the PWM is at a much higher frequency, than the mains, then it will generate much more RFI, than a TRIAC with phase control.


Quote
Quote
The additional complexity can be avoided by using an opto-coupler TRIAC with a built-in zero crossing circuit, such as the NTE3097 or MOC3042.
Aren't these drivers only suitable for burst mode control? Which would give me a small dimming window? Meaning less dimming levels.
The range of power control is the same with burst control as with phase control, the difference is it occurs over a much longer timescale.
 

Offline willz1200Topic starter

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Re: IGBT Dimmer - PWM AC Power Control for an Immersion Heater
« Reply #17 on: August 25, 2018, 06:21:08 pm »
For greater cost the MOSFET will always deliver lower static losses which will dominate in a low frequency switching application.
Yes after looking around I found these:
FMH30N60S1 - 106 mOhms (£1 each on aliexpress)
FDL100N50F  - 43 mOhms  (£2.50 each on aliexpress, £13.19 each on RS)
The only issue is that they could be counterfeit parts on aliexpress but probably worth a try. It might be best to just get a big heat sink and cheap out. After all it won't be running at high current for a great deal of the day.

Thanks for the information :)
 

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Re: IGBT Dimmer - PWM AC Power Control for an Immersion Heater
« Reply #18 on: August 25, 2018, 06:22:17 pm »
Quote
The fine-grained control is needed for the supply side, not the load, to be able to smoothly use only the excess generated power.
Yes that's my main aim, slowly increase power using a pid controller until the grid tie is on the edge of import/export.

Quote
But then what difference does the efficiency of the switching methodology make when the ultimate point to the exercise is to simply waste power as heat energy?
Point taken maybe I should bolt the FETs to the hot water tank :-DD I was just wondering how the use of IGBTs and MOSFETs compared and each PWM topology. But yes the main idea was to put a more uniform load on my inverter. The triac method causes some odd audible noises from my inverter, compared to boiling the kettle which doesn't produce the same noises.
Oh I see what you're trying to do now: make a variable resistor. Yes PWM can probably be used to to that, but you'll nee a fairly large filter to get rid of the EMI.
 

Offline willz1200Topic starter

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Re: IGBT Dimmer - PWM AC Power Control for an Immersion Heater
« Reply #19 on: August 25, 2018, 06:37:09 pm »
If the PWM is at a much higher frequency, than the mains, then it will generate much more RFI, than a TRIAC with phase control.
Okay, Would the PWM noise be easier to suppress due to the uniform change in current?

Quote
The range of power control is the same with burst control as with phase control, the difference is it occurs over a much longer timescale.
Wouldn't that result in more difficulty in modulating the instantaneous power?

Oh I see what you're trying to do now: make a variable resistor. Yes PWM can probably be used to to that, but you'll nee a fairly large filter to get rid of the EMI.
Super, Do you have any recommendations on what suppression network I should use?  :) 
 

Offline drussell

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Re: IGBT Dimmer - PWM AC Power Control for an Immersion Heater
« Reply #20 on: August 25, 2018, 08:36:49 pm »
It just occurred to me that since the heating element doesn't care whether it is fed AC or DC, another possibility, in theory, is to use a "power factor correction" type circuit with a regulator controller designed for PFC, some of which have a way to vary the output voltage via a control signal.

It is basically the same idea as the OP already has but it might be more easily implemented using components typically intended for PFC rather than rolling your own.  PFC is designed to basically just make your load look resistive, even when it isn't, which is essentially what the OP is trying to do, in a general sense.

Just a thought.  :)
 

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Re: IGBT Dimmer - PWM AC Power Control for an Immersion Heater
« Reply #21 on: August 25, 2018, 09:35:31 pm »
I’m curious about what might be best for this...

From the perspective of ‘don’t be mean to the inverter,’ a TRIAC is out (generates lots of harmonics in phase control or ‘lumpy’ waveforms in full wave mode).

So it’s MOSFETs / IGBTs. For a given rating they behave in somewhat similar ways.

For maximum efficiency, I would look at back-to-back connection (large MOSFETs would have lower losses that IGBTs and will make cooling easier and cheaper). An incoming line filter (e.g. 2x Y caps, common mode choke, filter inductor and X cap) will be required to keep the PWM out of your house wiring (and away from the inverter). A snubber will be required to eat voltage spikes across the power devices - and any inductance in the heater circuit will push more and more energy into the snubber.

Is there a thermostat in the heater wiring? Or an over-temperature cutoff? These devices may not be able to switch off properly (I.e. arc over and catch fire) in some circuits.

I don’t suppose you can get high voltage DC straight out of your inverter?
 

Offline Richard Crowley

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Re: IGBT Dimmer - PWM AC Power Control for an Immersion Heater
« Reply #22 on: August 25, 2018, 10:35:25 pm »
I have an old reel-to-reel tape machine (Sony TC-850) that can operate at the user's selection of 3.75 or 7.5 or 15 inches per second (IPS)  It uses a servo to maintain the selected speed. (Toothed gear and magnetic reluctance pickup to sense motor speed.) And a split-phase AC motor with the capacitor to generate the alternate "phase".  Technology from the 1960s.

But for the control element for the motor, I was amused that they just put an AC "resistor" in series with the motor. The "AC resistor" consisted of a bridge rectifier with a TO-3 transistor (2N3055 or equivalent?) on the DC side.  Rather clever, I thought. With the pass transistor bolted to the center of a beefy aluminum plate around 100mm square.  Certainly much quieter (electrically and acoustically) than some other alternatives.  Of course, that configuration was only possible because the big power transformer had a dedicated secondary winding to power the capstan motor.
 

Offline willz1200Topic starter

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Re: IGBT Dimmer - PWM AC Power Control for an Immersion Heater
« Reply #23 on: August 25, 2018, 10:59:58 pm »
It just occurred to me that since the heating element doesn't care whether it is fed AC or DC, another possibility, in theory, is to use a "power factor correction" type circuit with a regulator controller designed for PFC, some of which have a way to vary the output voltage via a control signal.
I'm not very familiar with power factor correction controllers and haven't had much luck in finding a controller that accepts a control signal, can you point me in the right direction? Its worth considering if it will also allow for fine-grained control :)

I’m curious about what might be best for this...

From the perspective of ‘don’t be mean to the inverter,’ a TRIAC is out (generates lots of harmonics in phase control or ‘lumpy’ waveforms in full wave mode).

So it’s MOSFETs / IGBTs. For a given rating they behave in somewhat similar ways.

For maximum efficiency, I would look at back-to-back connection (large MOSFETs would have lower losses that IGBTs and will make cooling easier and cheaper). An incoming line filter (e.g. 2x Y caps, common mode choke, filter inductor and X cap) will be required to keep the PWM out of your house wiring (and away from the inverter). A snubber will be required to eat voltage spikes across the power devices - and any inductance in the heater circuit will push more and more energy into the snubber.

Is there a thermostat in the heater wiring? Or an over-temperature cutoff? These devices may not be able to switch off properly (I.e. arc over and catch fire) in some circuits.

I don’t suppose you can get high voltage DC straight out of your inverter?

Yes I think the extra money I spend on the MOSFET will save money on the heatsink, So there's probably not a huge difference in the price point... Worth going for MOSFETs. Excellent and just RC snubbers will be sufficient? In some cases I've seen snubbers dedicated to the optocoupler is this necessary? I think the ball park recommendation is 100nF 100ohm but I've got an app note somewhere to find the proper values. Yes the immersion heater has a builtin thermostat. High voltage DC would be ideal my inverter is an ABB PVI-3.6-TL-OUTD-S unfortunately it looks like theres only AC out.

I have an old reel-to-reel tape machine (Sony TC-850) that can operate at the user's selection of 3.75 or 7.5 or 15 inches per second (IPS)  It uses a servo to maintain the selected speed. (Toothed gear and magnetic reluctance pickup to sense motor speed.) And a split-phase AC motor with the capacitor to generate the alternate "phase".  Technology from the 1960s.

But for the control element for the motor, I was amused that they just put an AC "resistor" in series with the motor. The "AC resistor" consisted of a bridge rectifier with a TO-3 transistor (2N3055 or equivalent?) on the DC side.  Rather clever, I thought. With the pass transistor bolted to the center of a beefy aluminum plate around 100mm square.  Certainly much quieter (electrically and acoustically) than some other alternatives.  Of course, that configuration was only possible because the big power transformer had a dedicated secondary winding to power the capstan motor.
So was the transistor just operating in its linear region to limit the speed? I bet the block of aluminium kept it cool, finding nice big PCB mounted heatsinks (For a decent price) has proven to be quite tricky, cpu heatsinks always work but they're not very nice to mount.   
 

Offline Richard Crowley

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Re: IGBT Dimmer - PWM AC Power Control for an Immersion Heater
« Reply #24 on: August 25, 2018, 11:12:12 pm »
So was the transistor just operating in its linear region to limit the speed?
Yes. It seems rather crude in the 21st century, but it had some elegance to the basic concept.

Quote
I bet the block of aluminium kept it cool, 
Yeah, the heat-sink was larger than the motor.
 


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