exploding capacitors
exploding capacitors
(OP)
This thread will be similar to a recent one regarding condensing fan motors, but since that one hasn't received a conclusive answer, I'm reluctant to hijack his thread.
Application is a two pump skid. Pumps are in parallel with a common inlet header and a common outlet header. Only one pump will operate at any given time in lead-lag configuration. There are check valves on the discharge side of each pump to prevent back flow through the pump that is down. Tech (knowledgeable tech) switched pumps and reported watching the de-energized pump motor spin slowly in reverse direction and suspects that the check valve is leaking by causing the pump to turbine backwards. As he was watching, the cap violently exploded. The contactor was open. The motor is an inductive single phase motor with start and run caps. AC caps aren't supposed to have a polarity. I have to investigate next week. Anybody seen this before?
Application is a two pump skid. Pumps are in parallel with a common inlet header and a common outlet header. Only one pump will operate at any given time in lead-lag configuration. There are check valves on the discharge side of each pump to prevent back flow through the pump that is down. Tech (knowledgeable tech) switched pumps and reported watching the de-energized pump motor spin slowly in reverse direction and suspects that the check valve is leaking by causing the pump to turbine backwards. As he was watching, the cap violently exploded. The contactor was open. The motor is an inductive single phase motor with start and run caps. AC caps aren't supposed to have a polarity. I have to investigate next week. Anybody seen this before?





RE: exploding capacitors
I have seen centrifugal pumps being backdriven at impressive speeds. The reported symptoms included reduced bearing life, on a succession of brand new replacement motors. I have not seen exploded capacitors in similar circumstances, but I surely haven't yet seen all the ways in which a capacitor can do something spectacular.
Consider multiple check valves (of different types that will hopefully fail in different ways at different times), or power operated switching valves driven by the skid controller.
... Or reverse rotation sprags/brakes.
Mike Halloran
Pembroke Pines, FL, USA
RE: exploding capacitors
Keith Cress
kcress - http://www.flaminsystems.com
RE: exploding capacitors
Was it first time this configuration and componentes were used by your organization?
Or is it a standard design that you have used before?
In short - is this a possible component weakness? Or is it some kind of design oversight?
I ask because there have been exploding AC capacitors in my life and they could be traced back to batches that didn't quite meet expectations. Some manufacturers do not test all their products 100%
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: exploding capacitors
is at resonance the voltage on the capacitor and inductor (motor) is many times higher than the supply. I
choose a vale that is off resonance that runs the motor at about 180V for quieter operation. On RE sites
I see pump motors being used as poor boy hydro generators. This leads to two theories for an electrically
isolated motor.
If the motor can reach an over speed condition, the frequency may be close enough to LC resonance that the
run capacitor sees over voltage and fails.
Start capacitors are not designed for continuous operation. If the motor stays in an under speed condition
and the centrifugal switch never opens the start capacitor overheats after a period and explodes.
I have another theory but it involves aliens.
RE: exploding capacitors
No, several people corroborate the spinning slowly version. It was during commissioning of the other pump being replaced due to smoke. I designed the panel and have used it at a half dozen sites or so over the years.
I initially considered the motor generating concept (still my best guess) but an AC cap is able to change polarity 60 times a second. My next thought was voodo, but aliens sound more reasonable. Perhaps the cap was bad, but it exploded with AC power removed. What I plan to do is explore the possibility of a short causing errant current somewhere. Incidently, I misspoke when I stated that the pumps employed a lead-lag strategy. It's actually a 100% redundant system using one pump as a spare.
Thanks again guys. Generally it's the fundamental things that are overlooked.
RE: exploding capacitors
Regardless of the actual speed, the evidence strongly suggests that the speed was sufficient for the combination of the frequency and the magnitude of the back EMF to destroy the capacitor.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: exploding capacitors
Now it comes down to WHY a motor that is disconnected from the line was regenerating at all, because in theory, there was no grid excitation of the windings and without excitation, no generation. Two possibilities on that:
1) Residual magnetism in the motor was sufficient to make it BEGIN to regenerate, then the Start cap kept it going until such time as it popped the cap. There are several website devoted to the "home power gen" market wherein people seek out and find single phase motors with high levels of residual magnetism, then set up a capacitor circuit to resonate with that and make a self-excited island generator out of an AC motor. Not all motors will have sufficient RM to accomplish this, but clearly some do.
2) The operator "selectively removed" the part of the story wherein he energized that pump by mistake, immediately turned it off, then energized the other one. That scenario might explain WHY there was back flow because if the check valve was dampened, it had not yet closed when pump 2 was started. The on top of that if he performed that changeover too quickly, the centrifugal switch in Pump 1 had not even engaged yet, so the Start cap was ALREADY charged and in the circuit supplying excitation, then when Pump 2 was energized and created back flow through the still open valve on Pump 1, it didn't take a lot of regen to exceed the capacitor value and pop the cap.
I vote for #2. Leaving out parts of a story is a common way of avoiding blame. He may not have understood WHY that happened, but he likely knew that he SHOULDN'T have done that.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: exploding capacitors
RE: exploding capacitors
RE: exploding capacitors