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| How to choose TVS diodes to dispel flyback surges from relay coils? |
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| calzap:
Tim, Thanks for your comments. 70 VA is the in-rush of the contactor coil (24 VAC). Contactor specs meet those of the motor for HP, FLA and LRA. In 11 years, the contactor failed once (after a couple of years). That's when I noticed the 24 VAC transformer had been spec'd for the sealed VA of the contactor rather than in-rush VA. Put in a bigger transformer, and no contactor problems since. I'll never say who specified the too small transformer. :-[ The voltage monitor is an ICM Controls ICM491. It monitors the 240 VAC supply from the power company for high/low. If supply is out of range, the monitor allows no 24 VAC to reach the contactor coil. This component was failing every other year. Finally, I put a couple of MOVs and a couple of RC snubbers across its 240 V terminals and increased ventilation through controller cabinet. Seems to have worked ... no recent failures ... knock on wood! Right now there are no real problems with the system. But the controller is being upgraded to have additional safety features for the motor and pump. These include flow switch on the intake pipe, float switch on the intake pipe (in a side-arm), temperature sensors on the motor and the pump, and water detector under the pipes. Maybe an airflow sensor for the motor. These sensors will interact with the 24 VAC supply to the contactor via relays. Hence, my interest in TVS diodes for flyback. Everything will be on DIN rail with indicator LEDs and easy means of bypassing individual sensors for diagnostic purposes. Mike |
| DBecker:
--- Quote from: T3sl4co1l on February 07, 2020, 02:12:36 pm --- Conversely, a TVS or MOV is inflexible with respect to voltage. In an automotive environment, you might be fine with a 18V TVS, in the average case, but what about load dump? At mains voltage, you might be fine with a 360V MOV, but what about surge? If it's acceptable for the load to turn on only momentarily, you can place the limiter across the contacts. If not, you may need transient limiting at the source as well as contacts (a trio of MOVs between source, load and contacts, perhaps), or something more complicated to address the situation. --- End quote --- Automotive inductive outputs have changed from snubber networks, which are large, expensive and (slightly) unreliable, to clamped protected drivers. These have a "clamp" that turns on the transistor slightly at 40V-70V e.g. a zener diode to the gate to limit the shut-off voltage excursion. Typically relay drivers are clamped at 40V, and injectors/actuators at 70V. The result is a clean, fast turn-off of the injector or relay, with little risk of the driver being damaged by over-voltage if the inductance is higher than expected or from a snubber capacitor degrading. This clamped driver approach works best with low voltage DC. Where it does apply, it's far superior to snubber networks and much easier to design with. |
| trobbins:
Good stuff Mike. Yes some level of protection/filtering of the 240Vac coming in to the control devices (but not the motor feed itself) would seem very appropriate, to help constrain stress on local control devices. Whether that also supresses any local disturbances such as from the control relay for the contactor coil I guess depends on the type of protection/filtering deployed. Did you have something in mind that may add some transient impedance between the main AC feed and the control AC supply, and protect the control AC side (such as MOV or TVS device) ? Yes, nothing like some extra process protection devices to remove system and pump/motor stresses for the known about faults/blockages that can occur over years/decades. Those stresses could well be a major cause of past failures. The motor itself may be inducing transients, both from it's own inductive energy, and from line inductance as motor current starts/stops, so some form of MOV module with health indicator and even fault contacts, would appear reasonable when connected across main incoming connection (live/neutral/PE), although such protective devices would also bear the brunt of mains feed transients being drawn in to your location. With respect to 24VAC control, are other devices being powered from that other than the contactor coil? I guess your protection contacts are. A TVS has a much more controlled operating range, so a better way to go than a MOV if coil energy is to be constrained right at the AC coil, but maybe even a TVS needs to be softened so as to avoid TVS damage if mains supply tolerance and surges etc get through to the 24VAC level. |
| Circlotron:
Have you considered using an opto coupled triac for switching the contactor coil? They switch off at the zero current crossing so the energy stored in the coil is essentially zero at that point. Snubbing it should be very easy. |
| calzap:
The SquareD transformer that supplies the 24 VAC is pretty hefty and robust for its VA. It's fused on both primary and secondary sides. Short of a lightning strike on the lines, I don't think the power company can hurt it. It probably acts at a pretty good choke on spikes from the input side as well. But it would be a good idea to put transient protection on its output as well. So, I'll do that. Circlotron: I know essentially nothing about practical use of opto-coupled triacs. I'll have to educate myself. Thanks for the suggestion. Mike in California |
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