Varistor Failure

It was presumably trying to protect the quipment from overvoltage or surges in the mains supply.
That's what it's there for.
Del

Mr. Del,

I understood the concept but inspite of the UPS which is giving constant voltage, the failure has happend. Is there any other possibility for the failure.

• Old age
• Ambient temperature too high
• Short-circuit downstream
• Mechanical impact
• Water ingress
• Etc., etc.

Right. BOOPATHY's UPS is probably NOT a constant-output-voltage device. If the AC line voltage is high enough it may simply pass it on along, and only if it fades or drops out does it replace the output with its own. That's the uninterruptible aspect. UPS with this architecture can pass along AC-line spikes and surges.

This is true, but, the effect is limited to transients within the bandwidth of the UPS's operating frequencies, and whatever is possible through the maximim switching speed of it's amplifiers. That said, there is ample opportunity within that band to produce the spikes, since they are sourced by the motor, not the amp. The motor's coils are cut off from driving EMF by the amp, and the field of the motor coils drive back into the curcuit like a flywheel. That is what pops the varistor. The motor itself may be producing bouncing transients just from the action of it's commutators. In any case all of this happens on the load side of the amp. If that is where the varistor is, it's doomed.

You need to clarify where this dead varistor lies. Is the varistor part of the input or output circuitry of the UPS? From your question the PCB with the dead varistor could also be part of the paint mixing machine or a separate controller for the mixing machine.

It is still most likely that this varistor died from an over voltage condition. I suspect that the over voltage source was the back EMF created by the motors of this mixing machine as the motors were turned OFF. There could be other parts of the circuitry that have less obviously failed that were supposed to reduce and spread out over time the total number of Joules the varistor was supposed to dissipate during a stop.

It could also just be simple age fatigue from the repeated jolts this varisitor has seen over time.

I highly doubt you have a large voltage issue you're chasing a ghost if you're looking at voltage. your varistor saw more current than it could handle so it blew

It may be a TOV (temporary overvoltage) problem. Varistors are not designed to protect against power frequency overvoltages but they do have a TOV rating. Check the ac overvoltage/time characteristic to ensure that it exceeds the energy rating of your application.

Dear Sir,

The Paint Mixing Machine Promix Which has got solenoid valves and some geared flow meters and actuators actuated by solenoid valves operated by 12 vdc.

This varistor is in the input of the dual power supply board (PCB) to this machine.

The fuse rating on this board is 2A.

The UPS supply is divided for three of such machine. Other machines or ok.

Only in one power supply board i faced this problem.

three years back we faced this varistor failure after which we ahve installed UPS.

Please suggest,

Not all UPS's produce a nice sinusoidal AC output. It may be synthesized using a switching technique that approximates the sine wave. Motors expect a nice fundamental frequency source of energy.

Since you seem to think the UPS is necessary, perhaps you would consider installing an inductor (sometimes called a choke) to limit the high frequency noise. The name for this product is "Line Reactor". You generally pick them based on how much current will be going through them. Because a Line Reactor in installed series with the load (your mixer) it becomes the part of the circuit that limits the power that gets past your fuse.

You could also use a 1 to 1 isolation transformer instead. Either way, the spikes that seem to have eaten up your varistor, will be absorbed as heat in the iron core of the Line Reactor or Isolation Transformer.

Generally Line Reactors are much less expensive than Transformers.

I was looking up tin whiskers as a possible cause when I came across this....

http://en.wikipedia.org/wiki/Failure_modes_of_electronics#Metal_oxide_varistors

"Metal oxide varistors typically have lower resistance as they heat up; if connected directly across a power bus, for protection against electrical transients, a varistor with a lowered trigger voltage can slide into catastrophic thermal runaway and sometimes a small explosion or fire.^[21] To prevent this, the fault current is typically limited by a thermal fuse, circuit breaker, or other current limiting device."

Sounds like it did what its supposed to do if it is installed correctly....

http://www.wisegeek.org/what-is-a-varistor.htm