Starting cap failure?

There are many ways to check the capacitors. You can find capacitor "meters" and testers from about $ 15 up. You may be able to find a shop that works on air conditioners and take the capacitors to them and have them checked. Usually they will have a voltmeter device that just hooks across the capacitor. Takes longer to find the meter than to check the capacitor.

After 15 years I reckon I would just get another cap and see if that fixed it. What sort of device takes out the cap, centifugal ?

Or as a quick test clip some additional C across it.

NT

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** That reminds me of what was the common practice with suspect electros BEFORE we all got ESR meters. How quick we do forget.

.... Phil

There is an electronic device that's got its innards potted. It senses the starting current and when the current drops to the set level it drops out the contactor that connects the starting caps. There 10 starting caps connected in parallel so I don't think it's such a good idea to replace them all just in case that's the problem. They are all stuck in place with some pretty good double sided foam tape so removing them would be a hassle too. I cannot see how many mfd the caps are rated for but I can see on one cap that they are made for a phase converter. I just remembered that years ago the device mentioned above failed. When I tried to start the converter it would just hum. There is a button inside that I can press that bypasses the starting device and energizes the contactor coil. When the starting device failed the first time I was told by the phase converter maker to press this button and see if it starts OK. It did which is how I diagnosed the problem. The company that made the converter has gone out of business so I can't call them for advice. But I'm gonna try the button thing again and if it starts fine then I guess I know the problem. What I don't understand is why the converter sometimes starts to spin up slowly and at other times just sits there and hums loudly. And then starts just fine on the next attempt. Could it be that when it starts to spin up slowly that the rotor was in just the right position that the balancing caps in the converter provide enough phase shift to start spinning the rotor, albiet slowly? Thanks, Eric

Whoops! I was wrong about the starting device. I just assumed it was some sort of potential relay like I had seen in other motors. It's a bad thing to assume sometimes. In reality the starting device in an adjustable delay solid state relay. The on time, once energized, is from .25 to 5 seconds. It resets when power is removed. Supossedly good for 100,00,000 operations. But the original timer failed after only approximately 600 operations. Eric

Knowing little about converters, it sounds like it could be mechanical? Is it anything like a starter motor with a 'bad' spot? It's hard for me to see how a cap would need two tries to do it's job right.

George H.

Yeah, I'm starting to think the contactor contacts are the problem. Just so you know, a Rotary Phase Converter (RPC) is just a 3 phase AC induction motor that is run from single phase power. Once the motor is spinning it generates power in the third winding, AKA leg, that would normally be connected to a three phase supply. They are started the way many single phase motors are started. A typical type of single phase induction motor has a starting winding that is powered only while the motor is starting. This winding is shifted in phase about 90 degrees from the main winding. Both physically as well as electrically. The electrical phase shifting can be done using resistance, capacitance, or a combination of the two. There are other schemes as well but the capacitance method is common on motors that need lots of torque when starting, like a motor on an air compressor. A simple RPC can be made using just a 3 phase motor. A rope can be wrapped around the motor shaft and then pulled fast to get the motor spinning. Once it is spinning single phase power is supplied and the motor will spin up to operating speed if the shaft was spinning fast enough from the rope pull. Another motor can be used to spin up the RPC motor as well. This motor is then either disconnected mechanically or just turned off once the RPC motor is running. Capacitors can also be used to supply phase shifted current just like a single phase motor. Again, once the motor is up to speed the starting capacitance is disconnected. Even though the RPCs described above will supply 3 phase power the generated power will not be perfectly in phase with the other two phases. And depending on the load the phase shift will vary as will the voltage. So more sophisticated RPCs will use capacitors across the windings to balance the power so that the phases are very close to 120 degrees apart and the voltage in the generated leg is close to the voltage supplied to the motor by the single phase line. That's my simple explanation of RPCs. Eric