3hp motor problems

[Sausage]

Bought a spares or repairs 50l compressor for £25, it had melted / burnt capacitor and knackered motor bearings and the motor had about 1 pint of water in it.

Motor was stripped, cleaned and rebuilt with new bearings, windings seemed good (continuity) and a cheap and likely nasty 45uf capacitor was procured.

Capacitor was wired up same as the original burnt one and the motor ran. I sorted out the rest of the compressor and let it build pressure to about half way then drained it off and double checked a few things, then fired it up again and it cut out with a blown fuse in the plug (13a) after about 10 seconds.

Replaced fuse and motor hums like the cap is dead. Normally the rcd would trip so not sure why i got a blown fuse. Motor and compressor turn freely by hand.

Is this just a cheap nasty cap and my sub 5 minutes run time was to be expected or do i have something else to test for / look at?

Pics:

Motor leads are brown and red on the cap and blue to the overload switch.
Mains lead live goes to the red motor lead on the cap.

 

[ronan]

Is the cap the right spec, i see its 450vac while the motor is 230v.

 

[rtbcomp]

The rated voltage can be the same or higher than the supply voltage.

When the motor was humming, had you switched the compressor off using the knob on the pressure switch? If not and there is pressure in the pipe between the pump and the tank the motor won't start

 

[Sausage]

On and off via red stop / start button on the pressure switch.

 

[Sausage]

I think the cap was rubbish when it arrived as i couldnt get any sensible reading on it with my multimeter (fluke 117), but as it was my 1st time trying to measure one with a meter i decided to just bung it on and hope for the best. it went to OL in about 1 second or else just showed OL immediately.

So do i buy a better cap for about £10 instead of the fiver i paid for this one? I could always complain to the seller on ebay i guess but they will probably just pass the buck to my motor.

Mod:language edit

 

[arther dailey]

you could try, remove drive belts put cord around pulley, pull cord to spin motor,turn on whilst its spinning, if it runs its either the start cap,start windings/switch.hope this helps.Arther

 

[windingroads]

If you are not getting legible reading for the cap I would exchange it for another.
If this is a start cap then fine, but if you require a run cap, then a start cap will burn out in
no time at all, so determine what kind of cap is required, and exchange the one you have
that gives those readings.

Second, when these compressors stop, you should hear a gush of air, this is a feature
to ensure no air is left above the cylinder when the compressor stops, as it is extremely
difficult and harmful for the compressor to try and start with this air still in the cylinder.

Never switch the compressor of at the wall socket as doing so leaves trapped air above the cylinder
and will of course blow a fuse trip cap or motor again on start-up.
There should be a decompression button on the compressor that you can push or pull to release the
pressure from above the cylinder, this is done automatically if you let the air build up and trip the
power and the release valve, if this mechanism is not working, there is every possibility
that the compressor will either bust its self a pipe or the tank.
Needless to say if any part of this mechanism hits someone, they will be no more.
I screwed a two inch bung from the end of a compressor which its clock said was empty,
it took off and went threw a bucket of spanners and kept going, if it had hit my leg it would
have broke it.
Run your motor without the compressor, if all goes well, ensure the compressor has the right amount of oil and run it, keep an eye on the gauge, and ensure it stops or you stop it at a safe pressure, if it wants to keep going above its recommended pressure then the safety cut-out is not working.
You get a blown fuse because of pure heat, trying to turn something that does not want to turn, fuses protect the equipment, trips protect people, that's why so many deaths occurred before the trips came in.
Thats some motor by the way, a cosine of .97, usually .8 or some worse figure.
Check if there is a mechanism, usually built in at the back of the motor that disconnects the capacitor via centrifugal forces as the motor spins up, and ensure it is working, if one is installed.

Regards, john

 

[Sausage]

Ok, belt off, rope pull then hit start button and it ran at whatever speed it started at (slow) and didnt gain speed, it was struggling and then the thermal cutout switch popped out and stopped it after maybe 5 seconds. only tried this once.

Edit: thanks for your words of wisdom chaps. There is a gush of air when it is switched off on the start / stop button yes. I replaced the cap like for like hopeful that it was the original and correct one that had melted. I have an extreme aversion to risk and messing about with mains electric, capacitors, and high pressure means i am extra cautious, but your warnings are welcome.

 

[WorkshopChris]

Was that with the capacitor connected?

 

[Sausage]

Was that with the capacitor connected?

Yes.

 

[windingroads]

If you don't trust the cap, try another and hope this will help, I would also ensure the cap you take
as original is just that, and not someone replacing a running cap with a starting one or one the wrong
size.
And if there is a mechanism that is not kicking the cap out at a certain speed, then this could also
be your problem.
Different metres work in different ways, you may have to press a button after connecting a (discharged) capacitor to your meter.
My meter starts off with low ohm reading and quickly goes to OL for a good cap.
That is a run capacitor you are using,
it is used to phase shift, other wise the motor would cog as in bind instead of rotate freely, and it would also be capable of going in either direction should it start up.
Read the tips in red at the bottom
How to measure capacitance
A multimeter determines capacitance by charging a capacitor with a known current, measuring the resulting voltage, then calculating the capacitance.

Warning: A good capacitor stores an electrical charge and may remain energized after power is removed. Before touching it or taking a measurement, a) turn all power OFF, b) use your multimeter to confirm that power is OFF and c) carefully discharge the capacitor by connecting a resistor across the leads (as noted in the next paragraph). Be sure to wear appropriate personal protective equipment.

To safely discharge a capacitor: After power is removed, connect a 20,000 Ω, 5-watt resistor across the capacitor terminals for five seconds. Use your multimeter to confirm the capacitor is fully discharged.

1. Use your digital multimeter (DMM) to ensure all power to the circuit is OFF. If the capacitor is used in an ac circuit, set the multimeter to measure ac voltage. If is used in a dc circuit, set the DMM to measure dc voltage.
2. Visually inspect the capacitor. If leaks, cracks, bulges or other signs of deterioration are evident, replace the capacitor.
3. Turn the dial to the Capacitance Measurement mode (
   
   ). The symbol often shares a spot on the dial with another function. In addition to the dial adjustment, a function button usually needs to be pressed to activate a measurement. Consult your multimeter’s user manual for instructions.
4. For a correct measurement, the capacitor will need to be removed from the circuit. Discharge the capacitor as described in the warning above.
   
   Note: Some multimeters offer a Relative (REL) mode. When measuring low capacitance values, the Relative mode can be used to remove the capacitance of the test leads. To place a multimeter in Relative mode for capacitance, leave the test leads open and press the REL button. This removes the residual capacitance value of the test leads.
   
   
5. Connect the test leads to the capacitor terminals. Keep test leads connected for a few seconds to allow the multimeter to automatically select the proper range.
6. Read the measurement displayed. If the capacitance value is within the measurement range, the multimeter will display the capacitor’s value. It will display OL if a) the capacitance value is higher than the measurement range or b) the capacitor is faulty.

Capacitance measurement overview
Troubleshooting single-phase motors is one of the most practical uses of a digital multimeter’s Capacitance Function.

A capacitor-start, single-phase motor that fails to start is a symptom of a faulty capacitor. Such motors will continue to run once operating, making troubleshooting tricky. Failure of the hard-start capacitor for HVAC compressors is a good example of this problem. The compressor motor may start, but soon overheat resulting in a breaker trip.

Single-phase motors with such problems and noisy single-phase motors with capacitors require a multimeter to verify properly functioning capacitors. Almost all motor capacitors will have the microfarad value marked on the capacitor.

Three-phase power factor correction capacitors are typically fuse protected. Should one or more of these capacitors fail, system inefficiencies will result, utility bills will most likely increase and inadvertent equipment trips of may occur. Should a capacitor fuse blow, the suspected faulty capacitor microfarad value must be measured and verified it falls within the range marked on the capacitor.

Some additional factors involving capacitance are worth knowing:

• Capacitors have a limited life and are often the cause of a malfunction.
• Faulty capacitors may have a short circuit, an open circuit or may physically deteriorate to the point of failure.
• When a capacitor short circuits, a fuse may blow or other components may be damaged.
• When a capacitor opens or deteriorates, the circuit or circuit components may not operate.
• Deterioration can also change the capacitance value of a capacitor, which can cause problems

Regards, john

 

[carbon]

As the capacitor doesn't show anything like the correct value when tested then I would think that the capacitor is useless. Test the capacitor out of circuit but short the connections on the capacitor just to make sure it's discharged, as they can store quite a charge

 

[arther dailey]

Going back to your first question, you stated motor was half full of water, yes you cleaned it,did you dry it out in an oven afterwards? and check earth readings.
as you had it apart did it have a o/c switch, basically a centrifugal switch to do as it says,when speed gets up it throws out to make the start windings open circuit, if it jams shut start windings will burn out on continuous run.Arther

 

[Sausage]

I'm confused, there is no way that i can see that the motor can stop using the capacitor as it doesnt have a centrifugal switch or a switch of any sort in it, hence it cant be a start capacitor. But it must be a start capacitor as the motor wont start or run without it.

The motor plate says 45uf so it's the right size. I will try and measure the cap again and any other ones of differing sizes laying about (as i dont have another like this one) and see if i can get any readings.

Edit: re the water, several days passed between me cleaning it out and running it, but no i didnt put it in the oven, although i considered doing so. I will strip the motor again tomorrow and take some pics.

 

[carbon]

As it's only got one cap listed in the plate I wouldn't expect it to have a centrifugal switch to disconnect the capacitor. Most of the single phase motors that I come across for work only have one cap that is constantly in circuit. The value when testing does need to be within the + - 5% range that's on the cap

 

[arther dailey]

windings should be dried out properly,ok,so if it has no switch its a cap run mtr, that means the capacitor is constantly in circuit as is the start windings, so as long as motor hasnt blown to earth due to dampness you need a new capacitor. Arther

 

[windingroads]

The oven would be a good idea, there is only one outcome from running a damp motor, and its not good, but you are correct, the capacitor has no way of leaving the circuit unless
it is defective, which it absolutely could be, capacitors are not easy to test, you can only test for shorts
or extreme leeks, but you can not test without specialized testers for capacitors whether or not it will break
down under load, they can and do, if they short, that's easy, no motor will turn, if they go the other way
they will start to what ever extent they can, they may hold this value, or they may and usually do get worse
as they heat. Give me the capacitor out of the known working machine any day, made quite a few return
visits back with ones that drove me nuts, along with bridge rectifiers just went wallop.

Regards, john

 

[arther dailey]

Google all name plate details and you might find correct spec for cap.

 

[arther dailey]

I should say the oven is more than an idea! by trade I was an armature winder,lol.stoved thousands...

 

[rtbcomp]

The plate says uF R 45 which translates to 45 uF run capacitor