keeping thyristors off
keeping thyristors off
(OP)
I'm trying to control a universal motor with triacs (actually "Alternistors" from Teccor). The armature is connected to 4 of them in a bridge configuration. I'm having trouble keeping the two that are suppose to be off during dynamic braking from turning on. The dynamic brake event is "dynamic". There is a sudden increase in circuit current through the conducting triacs (with corresponding sudden voltage increases across the non-conducting triacs) as well as voltage spikes from the commutation. I've tried various snubber circuits. I've tried heavily filtering across the armature (reducing the commutation voltage spikes). The problem gets worse as the triacs get hot. A scope shows that the voltage does not get near the rated blocking voltage for the triacs and the dv/dt does not seem to be any where near the limits for the triacs (or the optos). The offending triacs do manage to stay off for anywhere from 5-15 ms after the start of dynamic braking. Maybe a combination of volts and dv/dt and temperature is turning them on? I read in another thread that thyristors need time in the off state to build up to their rated blocking values. I hadn't heard of this. Are there any technical articals out there on this?





RE: keeping thyristors off
My experience is that triacs are never fully off after removing the gate signal. In the case of incandescent light dimmers a small amount of leakage does not matter too much. In the case of a blinking light sign the leakage current does slightly illuminate the bulbs in the "off" state but that actually reduces thermal stress on the bulb filaments. From a reasonable distance the moving light illusion works just a well if the bulbs glow a little when "off".
RE: keeping thyristors off
I think that your problem is either dv/dt firing (transients) or a rating problem. Also, keeping the gate-cathode (MT2) resistance low helps.
The turn-off time is normally in the 50 - 100 microseconds range (for power devices) but there are faster animals than that. And slower as well. But 50 - 100 is typical.
h
Gunnar Englund
www.gke.org
RE: keeping thyristors off
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If we learn from our mistakes,
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RE: keeping thyristors off
Triacs can be very difficult beasts to control, particularly in nasty inductive circuits. The problem being that they conduct in both directions, and sometimes will just refuse to turn off at the zero crossing.
As others have already said, beware of susceptibility to dv/dt switching, and make sure the gate driver has a very low shunt impedance when it is supposed to be holding the triac gate at zero volts.
I am in agreement with mc5w that for difficult loads, SCRs are far better behaved.
I have had all sorts of dramas with using Triacs to control laboratory centrifuges fitted with brush motors, and Triacs driving neon sign transformers. In both cases a redesign of the circuit using SCRs completely eliminated the smoke and flames.
RE: keeping thyristors off
SCRs also have an advantage which is that you can feed a small amount of reverse bias into the gate during the off period which will help the SCR resist any voltage spikes that could cause the SCR to accidentally fire. If a spike is applied across the cathode and anode of an SCR the capacitive displacement current can act as a false gate signal. This is similar to excessive dV/dT.
RE: keeping thyristors off
The other writers have already pointed out what I think your problem is. The voltage surge is causing false activation. At warmer temperatures, the problem is accellerated.
RE: keeping thyristors off
RE: keeping thyristors off
Repetitive fast load generated transients can be far more troublesome and usually need to be snubbed. That can be done either across each SCR or perhaps across the load itself.
For EMI reasons placing some heavy snubbing directly across the motor may not be a bad start, and it may be all you need to do.
RE: keeping thyristors off