Suitable replacment for this big fella?

[c_hegge]

This cap is on an AC electric motor which won't usually start. You can hear it trying to run, but it just won't turn. I have 3 questions:

1. Is it likely that the pictured part is the problem?
2. If it is likely to be the problem, what would be a good replacment for it (Preferably from RS components, since I get free freight with them)?
3. Will it explode in fire?

[Toasty]

Re: Suitable replacment for this big fella?

- Check if it has a start/run relay - may be stuck in run position or burned out.
- If no relay, perhaps a centrifugal mechanism that opens the switch. Look for 2 hinged plates with some springs on one of the ends of the rotor.
- Could be the cap.
- If this uses a contactor (heavy duty, multi-pole relay) to start, then see if it uses heaters and check them for open.

Can you turn the motor by hand to be sure the bearings aren't dry/seized.

Replace with close to the same uF rating and at least match the voltage. An air conditioning or electrical supply house would probably have them in stock.

Good Hunting!
Toast

[Toasty]

Re: Suitable replacment for this big fella?

See these for some examples:

http://www.tpub.com/content/construc.../14259_294.htm
http://www.tpub.com/content/construc.../14259_295.htm

[c_hegge]

Re: Suitable replacment for this big fella?

I can't seem to find the relay anywhere. There are actually two caps. I'm guessing one's the start cap and the other's the run cap. I already found a similar replacment for the other one from RS. The motor can be turned, so the bearings shouldn't be a problem.

Maybe I should also add that the bahavior can be a bit random. 90% of the time it doesn't spin at all. It tries for about 30 secs before the circuit breaker on it trips out. 5% of the time it turns slowly for about 30 secs before it trips the breaker and the other 5% it works fine. It's been getting progressively worse since it first started happening.

In case it helps, here's a pic of the other cap.

EDIT: What actually is the capacitance on the first cap? 108uF or 130uF? Or does that mean that it could be anywhere in that range? (Assuming MFD=uF)

[PlainBill]

Re: Suitable replacment for this big fella?

The intermittent characteristic sounds like the switch or a bad connection, not the cap. That cap is rated for 108 - 130 uF. This guy has a method for measuring the capacitance under load using a light bulb and a DMM. You'll have to extrapolate a bit, but he has the formula to use.

PlainBill

[Toasty]

Re: Suitable replacment for this big fella?

Second (white) cap is the start cap. First (black) is the run and is as you suspected, ranges from 108 to 130 uF.

With 2 caps now showing, you're looking at the figure 14-27 in those first links. A cap-start, cap-run type of motor.

You say it turns slowly... how slow?

Can I see a pic of the whole motor, motor rating plate, and a view of the wiring between both caps and the motor?

Not finding an external relay does indicate the possibility this uses the centrifugal switch type for start/run.

Toast

[PlainBill]

Re: Suitable replacment for this big fella?

Second (white) cap is the start cap. First (black) is the run and is as you suspected, ranges from 108 to 130 uF.

With 2 caps now showing, you're looking at the figure 14-27 in those first links. A cap-start, cap-run type of motor.

You say it turns slowly... how slow?

Can I see a pic of the whole motor, motor rating plate, and a view of the wiring between both caps and the motor?

Not finding an external relay does indicate the possibility this uses the centrifugal switch type for start/run.

Toast

I don't know how they do it in Australia, but a centrifugal switch is the standard arrangement for small electric motors here in the US. By the way, Toasty, you've got it backwards. The smaller value, higher voltage cap is the run capacitor. The black cap is the start cap.

PlainBill

[Toasty]

Re: Suitable replacment for this big fella?

Yup, I certainly did. Thanks!

Start high, run low.

Only been 12 years since I had to fix one of these in a window AC unit.

[PlainBill]

Re: Suitable replacment for this big fella?

Yup, I certainly did. Thanks!

Start high, run low.

Only been 12 years since I had to fix one of these in a window AC unit.

Three years for me, in a pool pump, in August.

PlainBill

[c_hegge]

Re: Suitable replacment for this big fella?

This one is from an air compressor. It's a bit of a birds nest of wires in there. I'll get a pic soon.

[c_hegge]

Re: Suitable replacment for this big fella?

Here are some pictures. I pulled the back off the motor to see the switch. I can't find any evidence that it's stuck. It seems to move freely.

[Toasty]

Re: Suitable replacment for this big fella?

Is the centrifugal mechanism free to move also? If the switch moves, but the centrifugal mechanism is stuck, that would explain why it's not coming up to speed. Weight and spring mechanism should move in and out on the shaft.

Do you have a DMM that will test capacitance? Disconnect it & make sure it's discharged (measure volts across it) and check it that way. Or, use the guide PlainBill pointed to, then do so. If you are uncomfortable messing about with it that way, then buy the replacement.

I tracked the Icar (white) cap to this local distributors page:
http://www.commchem.com.au/products/capacitors/

They also have the start cap in a matching capacitance.

Toast

[c_hegge]

Re: Suitable replacment for this big fella?

Sorry when I said the switch, I meant the mechanism. all parts of it seem free to move. My DMM doesn't measure capacitance. I found this for the white (run) cap http://australia.rs-online.com/web/s...duct&R=1964703. And this for the black (start) cap http://australia.rs-online.com/web/s...duct&R=0117221. would these be suitable replacments?

EDIT: Another option for the run cap :http://australia.rs-online.com/web/s...duct&R=3778813

[Toasty]

Re: Suitable replacment for this big fella?

They should be fine.

[PlainBill]

Re: Suitable replacment for this big fella?

I repeat, the symptoms do not match a bad start cap, and certainly not a bad run cap. 'Won't start' problems in single phase electric motors can be traced to only a few causes: Excessive load, open start cap, open or shorted winding, bad start switch or relay. The usual cause of an intermittent problem would be pitted contacts on the centrifugal switch, or a sticky centrifugal switch.

PlainBill

[delaware74b]

Re: Suitable replacment for this big fella?

*2 on the centrifugal switch. My company has a customer who owns an 5-hp reduced frame (can't remember the frame size) 1-phase motor on a restaurant exhaust hood fan. About once or twice a year, we go on the roof to pull it and adjust that switch. We figure it gets hung up with all the chinese food grease that blows past it while the fan is running. Symptoms are the breaker trips before the heaters in the starter pop. The first time it happened, it took out the start caps.

[c_hegge]

Re: Suitable replacment for this big fella?

I've tried polishing the contacts on the switch with sandpaper, as there was a bit of corrosion. It doesn't seem to help. It still tries to spin for 30 secs, but the breaker trips. I did actually get it to spin once, but it was slow and I could hear a lot of arcing going on inside and it tripped the breaker.

[Toasty]

Re: Suitable replacment for this big fella?

Find the arcing. Follow your nose...

You hear the noise from inside the motor? Is it with the cap cover off? Is it coming from inside those caps?

Might have to pull that motor apart. Especially if it's the windings. If so, it's toast.

[Krankshaft]

Re: Suitable replacment for this big fella?

Are you sure the centrifugal switch isn't holding the start contacts apart? When the motor is at rest the contacts should be together connecting the start cap to the start winding.

I had to repair a 50s Rockwell capacitor start motor a few months back after I disassembled it to replace the bearings I accidentally bent the copper ring contact the plastic centrifugal switch disc pressed against to disconnect the motor cap when the motor was at speed.

This caused the start cap to remain disconnected and the motor wouldn't start I just had to bend it back ever so slightly just a few degrees.

[c_hegge]

Re: Suitable replacment for this big fella?

You hear the noise from inside the motor?

Yes, the noise comes from inside the motor, not the caps.