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Driving a Zero Crossing 3 Phase thyristor with PWM ?

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picbits

Well-Known Member
Came across this at work .......

There are a bunch of Siemens 3 phase thyristor modules being driven by a 200Hz PWM @ 24V.

The thyristor modules seem pretty simple - put 24v on and it will turn on the three phases when they zero cross. Remove the 24v and they turn off.

Now ........ surely trying to drive them with an unsynchronised 200Hz PWM is going to cause phasing issues ? The PWM is run through a PID loop to control heater temperatures but say for example we have a 10% duty perfectly synchronised with our 50Hz mains, the start of the PWM pulse will be at the zero crossing point of the 50Hz mains therefore will never turn off.

If the PWM frequency was (for example) not exactly 200Hz, i.e. 200.0001Hz, we could be in a situation where you'd get a phasing effect and the output would turn on and off only when both the PWM and the mains frequency were within a certain part of the phase.

Your thoughts ?
 

MaxHeadRoom78

Well-Known Member
It sounds to me that as they are turned on at zero cross there is no phase angle control as the triac would stay conducting for each half cycle.
So the only way the PWM would seem to have effect is for varying burst cycle firing, as is done in situations controlling heating elements etc.
The PWM freq. would need to be longer than the supply frequency.
Max.
 

tcmtech

Banned
Most Helpful Member
I use PID controllers with solid state outputs to run SSRs that have zero cross triggering for the oil preheaters on my used oil burners and they work well but the PWM base frequency has to be at least a magnitude of order (10x lower) less than the line frequency of the power the SSR is switching to do any good.

If you need higher than 10 step PWM duty cycle incrementation that needs to be taken to a proportionally lower frequency than the AC line works at. 100x lower for 1% incremental stepping.

On my boiler oil preheaters PID units I use something like a 20 second base frequency with a 5% minimum duty cycle stepping so the SSR's batch fire in 1 second incremental steps and I have no real issues keeping the oil preheaters within ~+- 3F on a 300 F setting once the PID loops stabilize.

The reality is if you are heating something the thermal mass and its normal rate of temperature change it can have given a fixed power input will determine the realistic minimum PWM frequency so there is no point in having a cycle frequency that is magnitudes of order faster than the natural thermal response time curves of the material for a given temperature change let alone one that is faster than the power sources own line frequency and a zero crossing circuit design dictates.

Basically, don't use a micrometer to measure distances where counting steps as you walk is more than accurate enough.
 

fourtytwo

Member
Came across this at work .......

There are a bunch of Siemens 3 phase thyristor modules being driven by a 200Hz PWM @ 24V.

The thyristor modules seem pretty simple - put 24v on and it will turn on the three phases when they zero cross. Remove the 24v and they turn off.

Now ........ surely trying to drive them with an unsynchronised 200Hz PWM is going to cause phasing issues ? The PWM is run through a PID loop to control heater temperatures but say for example we have a 10% duty perfectly synchronised with our 50Hz mains, the start of the PWM pulse will be at the zero crossing point of the 50Hz mains therefore will never turn off.

If the PWM frequency was (for example) not exactly 200Hz, i.e. 200.0001Hz, we could be in a situation where you'd get a phasing effect and the output would turn on and off only when both the PWM and the mains frequency were within a certain part of the phase.

Your thoughts ?
I wonder if this is actually just a pwm gate drive as is often used to both save gate power and ensure the device stays on if the load is disruptive, for example a brush motor, such loads often cause thyristors/triacs to turn off unexpectedly if the gate drive has already been removed yet at the same time these devices can be thirsty to drive with DC. It also opens the possobility of using small pulse trafo's for isolation though 200hz is much to low for that. Anyway have a look and see if this reletevly high frequency drive comes on on different half cycles or perhaps if phase controlled is not there for the entire half cycle. Just a technique I am used to.
 
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tcmtech

Banned
Most Helpful Member
Now that I have though this out a bit more here how I see this sort of control system needing to be made.

At a 60 Hz line frequency the three phase power has 360 zero crossing points per second and thusly for 1/2 of one cycle of any phase to equate to say 1% of the duty cycle the primary PWM base frequency would need to be 100 X that or .36 seconds or ~ 2.77 Hz.

Now that said, in theory that would be optimizing the electrical limits the SSR's can work with but in more realistic views anything that requires a three phase power source to heat it likely has a lot of thermal mass in play and thusly is going to have rather slow thermal response time that would make the 2.77 Hz base frequency with a 1/2 cycle minimum duty cycle way past OCD level overkill on control resolution.

So much so that the the PID loop may very well not see the anticipated temperature changes its wanting in that .36 second PID loop cycle (or even 100+ of them) and will just be shooting from full off to full 100% output in bulk batch fire mode operation most of the time because of it.

I'm no expert on PID control but I do know that the PID loop cycle time setting has to be a reasonable match to the thermal response times of whatever its monitoring to work properly let alone with repeatable and consistent accuracy.
 

picbits

Well-Known Member
I wonder if this is actually just a pwm gate drive as is often used to both save gate power and ensure the device stays on if the load is disruptive, for example a brush motor, such loads often cause thyristors/triacs to turn off unexpectedly if the gate drive has already been removed yet at the same time these devices can be thirsty to drive with DC. It also opens the possobility of using small pulse trafo's for isolation though 200hz is much to low for that. Anyway have a look and see if this reletevly high frequency drive comes on on different half cycles or perhaps if phase controlled is not there for the entire half cycle. Just a technique I am used to.
Pure resistive 6kw three phase load. Not isolation trafo's etc - just a straight zero crossing Siemens module for either on or off. Siemens actually do a proper clip on module for this which will convert PWM / 4-20mA / 0-10V into a slow "PWM" phase control i.e. a 0.2Hz frequency with a 0-100% mark space ratio.
 
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