What Capacitor.

Billy Bunting

A Mechanic mate of mine has an electric motor driving a Compresor, the Motor occasionally sticks and just humms as if the Capacitor isn't giving the required kick start, problem is that there are no details on either the Motor or Capacitor so not sure of what size Capacitor to fit, anyone any idea how i can figure out whats required ?

indie armada

its goin to be a 400-440 volt cap for a start, without the details of the motor or the cap in my limited experience id go by size. after that its just trial and error, but usually the physical size of the cap is indicative of the microfarrad rating. if he could bring the motor down to the lads in electric motors in dublin ind est, they would probably know just by looking at the motor, they are the most experienced lads ive come across when it comes to these things.

maxamillius

most capacitors have the size printed on them,others use a colour coding system

check out this link
http://www.csgnetwork.com/capcccalc.html

other then that do as indie armada says above!

2011

Billy Bunting wrote: »

A Mechanic mate of mine has an electric motor driving a Compresor, the Motor occasionally sticks and just humms as if the Capacitor isn't giving the required kick start, problem is that there are no details on either the Motor or Capacitor so not sure of what size Capacitor to fit, anyone any idea how i can figure out whats required ?

Hi Billy.
First of all a number of things could cause the motor not to start. From my experience capacitors in single phase motors generally don't fail. If the capacitor was indeed kaput the motor would never start.

Also if your fired is a mechanic there is a reasonable chance that the compressor is 3 phase as most workshops would have a 3 phase power supply and therefore would not require a capacitor to start. A good starting point would be to check this first.

If you are sure that the capacitor requires replacement the best thing to do is contact the manufacture or perhaps download a data sheet for this particular motor. Randomly trying different capacitors would not be a good idea!

Good luck with it.

Billy Bunting

Thanks for the quick reply Lads.

Dublins a bit of a treck, and not 3 phase. Forgot to add that this Motor is 35yr old, there are no details/colours at all on the Cap, i have seen simular "size" Motors in the past with 20UF ish size Capacitors so i have tried a 20 and a 25, no go! and i'm a little reluctent to go bigger in case it goes wooofff!! Is there any way of doing this with a meter

2011

Is there any way of doing this with a meter

Yes, but if you can measure it accurately it is not blown!

2011

IMHO there is a higher chance that it is the starting switch that is faulty. They sometime stick open so the capacitor is out of circuit. They are only required to start the motor.

Billy Bunting

Is there a way of sizing the Capacitor by measuring Motor ?

2011

Billy Bunting wrote: »

Is there a way of sizing the Capacitor by measuring Motor ?

In theory, yes. In practice it would be soooo much easier just to check on the internet or with an agent.

Besides IMHO there is nothing you have described that points to the capacitor being faulty. If the capacitor was indeed a dud the motor would never be self starting.

Billy Bunting

The thing is 2011 that when this occures if you manually spin it it will start, would it if it was the starter switch ? I get this on other motors and changing the Cap usually cures the problem.

2011

the Motor occasionally sticks and just humms

This could be as a result of the load being too big, or the bearings are failing or the start switch is open circuit. Without a working capacitor in circuit the motor would never start on it's own. From what I have seen capacitors either work or they don't. I have never come across intermittent problems with them.

Billy Bunting

We double posted...

Bearings are fine, no extra load.

2011

Billy Bunting wrote: »

The thing is 2011 that when this occures if you manually spin it it will start

Then it is not self starting. The capacitor is used with a starting winding to provide a phase shift just to start the motor. Once the motor is turning it is no longer required. From your earlier description my impression was that this motor "occasionally sticks" and therefore started without assistance normally.

would it if it was the starter switch ?

Yes as the capacitor is essentially not there for startup if the switch is open.

I get this on other motors and changing the Cap usually cures the problem.

If this is your experience, do what works! It is not what I have found. But like I said if you are sure it is the capacitor then get the data sheet or talk to the agent would be my advice.

2011

Billy Bunting wrote: »

We double posted...

Bearings are fine, no extra load.

OK, so the motor is free to turn. Not that

Billy Bunting

OK 2011, chaps! i'll see about getting it out and off to someone who knows about these things, thanks again,

Bruthal

Billy Bunting wrote: »

The thing is 2011 that when this occures if you manually spin it it will start, would it if it was the starter switch ? I get this on other motors and changing the Cap usually cures the problem.

If it will start when you manually spin it then it is something gone in the start circuit, either the capacitor, the start winding or the centrifugal switch which is supposed to be closed until the motor is up to speed, or a connection is gone along this start circuit. Capacitors are usually very reliable, so the start centrifugal switch or a connection on the capacitor or start winding or switch may be the problem.

You can test the capacitor with a multi meter be disconnecting it and make sure its discharged. Then test with multi meter on lowest ohms setting or 1k setting. Then reverse the test leads on the capacitor. Each time you reverse you should see some readings then it will change up to a 1 or whatever the meter ohms reads with the leads attached to nothing (open circuit). If it shows 0 ohms or a fixed reading all the time the capacitor is probably gone. Once it shows a reading then goes up to open circuit reading each time you reverse the leads then the capacitor should be ok.

Bruthal

Billy Bunting wrote: »

Thanks for the quick reply Lads.

Dublins a bit of a treck, and not 3 phase. Forgot to add that this Motor is 35yr old, there are no details/colours at all on the Cap, i have seen simular "size" Motors in the past with 20UF ish size Capacitors so i have tried a 20 and a 25, no go! and i'm a little reluctent to go bigger in case it goes wooofff!! Is there any way of doing this with a meter

If its not starting with them capacitors then its likely another problem in the run circuit like a connection or that run(centrifugal) switch being stuck open.

If you look at 2011`s motor diagram (post #7) you will notice that if you remove the capacitor, and get your multi meter on the 1k ohms setting, you should have a reading of 0 ohms between one of the connectors for the capacitor and either the live or neutral supply terminals of motor (top feed terminal where it says line -to- left side of capacitor in his diagram), and also a test between the other capacitor terminal and the remaining supply terminal will give an ohms reading through the start winding.

On one of the above tests the reading should be 0 ohms. On the other you are measuring through the start switch and start winding and this should have an ohms reading depending on the start winding impedence.

So find the capacitor terminal and supply terminal (usually Live) that is giving a 0 ohms reading, and the reading on the other capacitor terminal to other supply terminal should be an ohms reading, if its the same reading as if the leads are attached to nothing then the problem is in the start circuit in the motor, either the start winding switch stuck open or winding open circuit, or a connection open.

Make sure the motor is isolated from the supply for them tests.

The start winding switch is on the shaft inside the motor housing and the motor runing up to speed usually opens it as the capacitor-start winding are not needed once its running.

Billy Bunting

Thank's Robbie, I'll give this a go and see how i get on.

Bruthal

Billy Bunting wrote: »

Thank's Robbie, I'll give this a go and see how i get on.

Dont forget to make certain the motor is isolated. Just being stopped at the stop button is not good enough for that sort of testing.

Bruthal

The fact it seems intermittent would suggest the starting switch is sticking open or there is a loose connection at an educated guess anyway.

2011

robbie7730 wrote: »

The fact it seems intermittent would suggest the starting switch is sticking open

Yeah, that is the conclusion that I came to in post 7.

Capacitors are usually very reliable

+1
Exactly what I was saying.

make sure its discharged

Good point Robbie

Bruthal

2011 wrote: »

Originally Posted by robbie7730
The fact it seems intermittent would suggest the starting switch is sticking open

Yeah, that is the conclusion that I came to in post 7.

Capacitors are usually very reliable

+1
Exactly what I was saying.

Sounds the likely cause alright, ye just never know for certain with electrical all the same.

brightspark

I always thought capacitors either worked or didn't until recently.

An oil fired burner with a 4uF capacitor would sometimes (often a few times a day, or else once every couple of days) just sit and hum, until the lockout circuit activated.

However as several others had similar problems I decided to try just replacing the capacitor (a whole lot easier than dismantling the pump!)

It's been working fine since.

Using a Fluke multimeter I measured the old capacitor, it only read 1.8uF! despite originally being 4uF.

BUT that dosen't mean that the OP's problem is the capacitor, just that I now wouldn't rule it out as quick as I would have previously.

(perhaps OP you could measure the existing capacitor and use one just slightly larger, assuming everything else checks out ok?)

Bruthal

brightspark wrote: »

BUT that dosen't mean that the OP's problem is the capacitor, just that I now wouldn't rule it out as quick as I would have previously.

(perhaps OP you could measure the existing capacitor and use one just slightly larger, assuming everything else checks out ok?)

It could be the capacitor, but its a simple enough test to check the motor start winding through the centrifugal switch. And the run winding also.

Billy Bunting

brightspark wrote: »

I always thought capacitors either worked or didn't until recently.

An oil fired burner with a 4uF capacitor would sometimes (often a few times a day, or else once every couple of days) just sit and hum, until the lockout circuit activated.

However as several others had similar problems I decided to try just replacing the capacitor (a whole lot easier than dismantling the pump!)

It's been working fine since.

Using a Fluke multimeter I measured the old capacitor, it only read 1.8uF! despite originally being 4uF.

BUT that dosen't mean that the OP's problem is the capacitor, just that I now wouldn't rule it out as quick as I would have previously.

(perhaps OP you could measure the existing capacitor and use one just slightly larger, assuming everything else checks out ok?)

Snap!! have experienced this dozens of times on burners and never had a call back.
Will have a play with the multi meter though and refresh the grey matter.

DublinDilbert

If there is a centrifugal switch (most large Ac single phase motors will have one) definitely check it as Robbie says a above. The reason the caps last so long on these is cause they are only used for a few seconds on start up.

Some smaller Ac motors will leave the capacitor connected permanently in the circuit. This is not very efficient, but that typically doesn't matter on smaller motors...

brightspark

Yes definately check the switch, only when you've checked the rest of the circuit replace the cap