Proportional control - How fast can a SSR turn on-off

[salenetus]

I have an element rated at 3kw at 240v. I use a SSR rated at 40 amps and controlled via a 3-32 volt dc source.

Now I hope to be able to pulse this source and turn the ssr on and off several times a second to achieve proportional control over the element and thus reduce its capacity and load on the mains.

Can anyone tell me how fast on average can I pulse this type of arangement to achieve a resonable stable load? The element is powered by a generator and I don't want to have the genny speeding up and slowing down so I need to achieve the above process (PWM) but would still like to use SSR's as they are cheap and seem solid.

 

[itsmoked]

3kW/240V = 12.5A
The 40A has it covered.

That SSR will be dumping about 15W and so will go up in smoke if you do not have it properly heatsinked somehow.

A standard zero crossing SSR cannot be turned of more rapidly than 16ms in 60HZ land and 20ms in 50Hz land.

You should probably not bank on more than about 20 ON/OFF per second to prevent beat frequency problems.

This may not work worth a darn with a too small generator since when the SSR is ON you are overloading the generator. I don't think the generator's voltage regulator is going to be happy about this. Don't try testing with anything sensitive being powered as you may actually get some major voltage spikes.

Why not just change the element or reconfigure it. Or use a VARIAC which would be superior.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[OperaHouse]

If you don't need all that power, put parallel elements on series or operate off 120V. Half voltage would be quarter power. Dropping th voltage 48V with a buck transformer may be enough. This would be preferable to pulsing.

 

[jraef]

Just a point of order that may explain why you cannot "turn an SSR off" any faster than what itsmoked said:

You cannot technically turn a standard SSR off! You can stop telling it to stay on, but it only turns off when the current sine wave passes through the zero cross which occurs every 8msec on each direction. Then since you are switching AC, there will be conduction in each direction. So, at 60Hz, 1 second dived by 60 = 16.7msec, or 1 div. 50 = 20msec. The amount of ON time it has can be controlled within that time domain by where on the sine wave to gate it on, but the OFF point is fixed by frequency. You would also need to contend with the fact that even the best SSRs will have approx. a 2ms turn on time (Gate pulse to full conduction).

http://www.crydom.com/support/tips/story.htm

There are types of thyristors called GTOs (Gate Turn Off) which require both an ON pulse and an OFF pulse to two gates, but they are not typically found in off-the-shelf SSRs.

 

[sreid]

I'd like to point out that turn off (zero current crossing) occurs every 10 ms (50 Hz) or every 8.3 ms (60 Hz).

For a constant load on the generator, I would use phase control where the on time is some percentage of each half cycle (like light dimmers).

 

[itsmoked]

I bet an overloaded generator is not going to appreciate that any either.

You want maximum heating out of the system, then you need to get it from smoothly using what's available.

That's with some sort of transformer, or resistance increase of the heater circuit.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[roydm]

I have built several large skids doing just that. Pulse Width Modulation is the most common way of controlling elements. I assume you are using one of the small oblong SSRs with the aluminum base The nice thing about those is you can bolt straight onto the back panel. Use some heatsink grease to get better contact or one of the ready made finned heatsinks available. You shouldn't need to cycle so often as your element has a lot of thermal inertia, something like a 10 second cycle time should do. The generator may protest about the constant cycling (depending on the percentage of it's full load you are switching. If you have the option of splitting your load you could possibly have a base load e.g. 1500 W on steady with 1500 under PWM control.
Good Luck
Roy