Starter capacitor question

[®smpr_fi_mac®]

I think I know the answer to this, but I'm asking just to be on the safe side.

A while back I replaced the started capacitor in my Delta DC. It's a 120V DC. I bought a 53-64 MFD cap. I installed it two days ago and the DC spun up nicely. Tonight, it wouldn't start unless I spun it by hand. After a few moments it went to "half speed" and then tripped the 15amp breaker.

Pulling it open, I noticed (dunno why I didn't when I first saw it) that it's marked as 250VAC.

I'm thinking that this is what's wrong, and that if I replace it with the proper voltage cap, it should work, right?

[blackhat]

The voltage rating number on a capacitor is the insulation rating of it's guts. It has to be higher than the voltage applied to it. A 250 volt capacitor would be marginal on a 120 volt motor because the back EMF generated by the windings could be higher than that. I suspect more likely it's a problem with the centrifugal switch in the motor if it has one or the start winding or relay.

[®smpr_fi_mac®]

blackhat said:

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The voltage rating number on a capacitor is the insulation rating of it's guts. It has to be higher than the voltage applied to it. A 250 volt capacitor would be marginal on a 120 volt motor because the back EMF generated by the windings could be higher than that. I suspect more likely it's a problem with the centrifugal switch in the motor if it has one or the start winding or relay.

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Thanks. Do you know how I would go about testing those other possible issues? The relay I know how to test, if it's mechanical--apply voltage and listen for the "click". The other two, well, I don't know.

[blackhat]

You will be best off pulling the motor off the machine. If you can get some pics of the wiring diagram and the bottom end of the motor, there might be enough info there to help you further. Or drop it at a motor shop and ask them to diagnose it.

[MichaelMouse]

The capacitance suggests a "run" capacitor. There's an obvious difference physically as well. http://www.capacitorformotor.com/startin...citor.html

As they state, voltage rating should be at least 1.5 times the line voltage. So you have the proper rating if the one you have is 250. With the differences so obvious, you probably have the proper run capacitor. There should be no centrifugal switch, as a run capacitor is constantly in the circuit.

Clean the dust out to make sure you're not shorting or leaking to the frame of the motor. Sometimes that's where the problem lies. You can meter the windings to check for shorts.

[®smpr_fi_mac®]

Here's what I have--no centrifugal switch, and yes, it's a run capacitor.

The black wires are tagged 1-4. One and two are going to white; three and four to black. The green is grounded to the motor's frame. The two red go to the capacitor's contacts.










I may just need to find a small motor shop in my area.

[blackhat]

What was the original capacitor?

[thewizzard]

1. If it worked initially but then failed it is possible the new cap was "bad" and has failed again. Do you have access to a cap tester ? That would answer the question quickly.

2. At least on my air-conditioner unit, the fan and compressor have a dual run cap but neither the fan or compressor would start with a failed run cap. The fan could be push started. The compressor is sealed so who knows. Simple point is that a run cap can cause a start failure.

3. Voltage: as indicated, you usually want at least 1.5x the line voltage but you can always go higher without any issue.

4. Capacitance: this is important. This is the Farad rating on the cap. It should be the same as the motor originally came with. Too high or too low is going to cause problems.

5. Wiring: make sure all the connections are actually tight. Its pretty easy to twist one of those little wire nuts on a short motor lead and actually pull the connection loose.

6. Other

[®smpr_fi_mac®]

I've checked the connections; they're good. I have a meter but haven't tested the new capacitor yet. Don't remember the original cap's rating, but I bought a replacement that matched it based on advice I found here.

It'll run without the capacitor; I have to spin it up by hand, though, AND it only reaches about half speed. I spoke with a motor repair tech. He said with these symptoms he's sure it's a bad winding in the motor.

Time for a new DC.

[Robert Adams]

I have never seen a dust collector with that style motor. That's a cheap low efficiency motor especially for a dust collector which needs lots of power.