400Hz operation
400Hz operation
(OP)
I have a problem with an SSR when driving a three phase fan at 115V, 400Hz, max 440Hz. The powerfactor is not very high typically 0.3 . The problem is that occasionally two of the three SSR's feeding the fan do not switch off when commanded to do so via the inbuilt opto. The problem is worse as the supply frequency is increased. Looking at voltages across the SSR the dV/dt is well within limits so something else presumably must be happening. Does anyone ahave any insight into what might be happening ?
Cheers, Nick
Cheers, Nick





RE: 400Hz operation
This should determine whether the problem is related to the poor power factor and provide a cure if it is. If there is no improvement with increased power factor, you will have eliminated one possible cause.
respectfully
RE: 400Hz operation
Inductive loads are tricky and current remains even after voltage has gone through zero. It is a problem at 50/60 Hz - and even worse at 400 Hz. Make sure your specs are right before trying anything else.
Gunnar Englund
www.gke.org
RE: 400Hz operation
RE: 400Hz operation
Gunnar Englund
www.gke.org
RE: 400Hz operation
Are you using these SSRs to do speed control (PWM) or to just turn the fan on and off?
Are the SSRs being "failed" or do they seem to be happy even after not turning off the fan?
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: 400Hz operation
RE: 400Hz operation
I finally found another vendor with the same pinout and am investigating that route also.
RE: 400Hz operation
However, some SSRs use inverse SCRs and these have much better zero crossing commutation. With problem loads, try fitting the inverse SCR type of SSR, it will almost certainly solve your problems.
RE: 400Hz operation
If the problem is solely dV/dt then the rates I can measure using a scope of approx 1V/us is way down on the manufacturers spec sheets of 1000V/us. I wonder if the 1000V figure is under ideal conditions at say 50Hz and unity power factor and maybe at 400Hz and PF of 0.3 the critical dV/dt rating is much less ? Certainly if I drop the frequency to say 300Hz the incidence of not turning off is much reduced, If I raise the frequency to 500Hz I can induce the fault every time. Obviously it is probably a combination of actors malking the difference, eg power factor changing with frequency, available turn off time reducing at higher frequency, higher di/dt etc. Snubbers are probably the answer and 0.1u + 100ohms has been suggested. Just wish I could do the math to prove it !
RE: 400Hz operation
You just need the PF/snubber correction that barely fixes the problem then improve it by 50% or some other value to assure production success.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: 400Hz operation
Is there any possibility that the input control to the SSR is picking up noise or has some leakage current in the "off" condition ? Try placing a dead short across the control terminals and see if that helps.
RE: 400Hz operation
Doing the math is fairly simple. The problem is what data you shall plug into the equations.
You need to know a lot of things and most of those things can be found by using a dual (at least) channel scope, a differential probe for voltage across SSR and a DC currnet clamp to observe load current. It may also be useful to have two more channels to observe on/off signals and the 400 Hz mains voltage.
When you have this, you will probably see what is going on - even without the math. If you are really careful, you can use a shunt resistor between load and ground to measure current and a 10:1 or 100:1 probe to measure voltage.
Gunnar Englund
www.gke.org