capacitor on motor smashed!!

[Zigeuner]

[quote author=Tony Wells link=topic=1587.msg9429#msg9429 date=1302314790]
No Zig, he never said. That is an important consideration, and one reason I recommended a 250 or 370 volt cap.
[/quote]

Extra capacitance and voltage rating is not harmful so long as the physical size is acceptable. My RPC needed 500 + µf to start a 5 hp three-phase motor. I use about 100 µf per hp as a general rule but if he's running on 120VAC, probably 75-80 µf would be OK.

 

[Starlight Tools]

The start winding on that motor is only 120 volts, it will be in parallel with one of the run windings on 220V and both of them on 120V so the Cap will be fine being a 120 V one. There are very few of those motors that need the 250V cap.

Walter
The Motor Doctor.

 

[Tony Wells]

True, Walter. I guess I was thinking about an A/C condensing unit I worked on last month. Cap there was 370 V.

 

[Starlight Tools]

Tony

I use 250V caps when I want a bit more start time. Usually the larger voltage capacity will give a hard start motor a bit of grace, but not much, in the start windings.

those 370V caps were probably run caps, not start ones. they are generally higher value.

Walter

 

[Tony Wells]

Walter, correct me if I am wrong, but I am thinking that the value of the cap rather than the operating or rated voltage determines the start coil time. It's a function of charge time. The voltage rating is determined by the dielectric material and strength, and the terminal spacing, etc. I can't see how that changes the charge time.

Of course, I'm leaning back on general electronics knowledge, not specific to motor caps.

And, perhaps you're right on the run cap. I just checked for ESR, leakage, and value. All was good, so I buckled it back up. Just spring prep for the shop air system.

 

[Starlight Tools]

Tony

The higher voltage rating means the dielectric is a bit stronger and takes more to punch it with a voltage spike as it overheats.

When we have a submersible water pump that goes through capacitors too often, we will up the voltage and often that stops the mayhem. It takes a lot of energy to get a pump going when it has a head of water on it.

Upping the voltage will not change the start time, it will only protect the Cap for a few more milliseconds against blowing up when we are in a hard start situation where the motor takes longer to get up to speed. Overall this is not recommended as everything is being taxed by a slow starting motor, not just the cap.

Fan and AC motors, are very commonly Capacitor Run motors. It is amazing how much energy is required to move air! On these motors the start circuit is never removed from the circuit. Here the Capacitance is low, often only a few uF, somewhere often between 5 to 25. As the capacitance decreases, the motor has a hard time starting, but as the capacitance increases, the running current rises. Even a few uF can change the current. These capacitors are often all metal cans, but can also be sealed plastic as well.

Walter

 

[Tony Wells]

OK, pretty much same thing, just different words. The additional voltage rating "beefs up" the cap allowing a little more abuse before failure (punch through the dielectric). Agreed. I thought I was reading that you gained start time by using a higher voltage rating cap. Gain is in life of the cap time. Agreed.

I was talking about the cap for the compressor, so I believe it is a start cap. Last year, I had one fail, open, and of course, no start. I have also seen one that simply drops it's value and will not start consistently. An A/C buddy sometimes parallels another cap to the "weak" one to get by in a pinch. That should only be until a proper cap is available.

 

[ChuckB]

I am using the same Westinghouse motor as you. The cap it uses is a Dayton P/N: 4X066 available at Grainger. (324-388 MFD @ 110-125VDC) You are using 110VAC, right?

Some of the automatic doors I use to work on had AC motors that used starting capacitors.. on a couple of occasions, I would get to the job to find the motor humming and the thermal overload kicking on and off..motor red hot.. a capacitor failure.. not having one in the truck, I would pry the lock ring off the top of the capacitor.. slide the main body out..unroll it..find where it shorted.. tear a piece of mylar from the end.. slightly shorten the length of the plates.. patch the shorted area of the plates.. oil the body.. slide it back in..got me by until I could get back with a new part..

 

[Tony Wells]

Hahaha! Great, Chuck! Not many people would trouble themselves even to attempt that.

 

[pjf134]

Chuckb,
That 1 HP motor came off my big fan, the only problem was the motor only went cw and I needed ccw, so what I did was figure 8 the belt so it went the right way. I did check years ago on the net and the motor was not reverseable, maybe it's time to check again. The fan was a 1 1/2 HP 3 phase that came on it, but no setup in shop for it yet. I only put the motor on my lathe to check it out, but now since I got the pulley on my 1/2 HP fixed I will take it off and use the 1 HP for the big fan again. If you have the same motor, can it go ccw? If you need more info I can take a pic of it.
Thanks,
Paul