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Old 5th Jul 2018, 5:42 pm   #1
flyingtech55
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Default Single Phase AC Compressor Motor

My old mate Alex owns a garage where he repairs/services cars. His premises only has a single phase 240V mains supply ergo his ramp and compressor are both powered by single phase motors.

Recently his compressor started popping the breaker in the consumer unit when he switched it on. It had been working but then developed this fault. I've had a look and can confirm that the fault is solid. He had the motor repaired/rewound some years ago but it has been working fine ever since.

I've taken the cover off the box on the side of the motor which contains the capacitors and found inside two 12.5uF 250V capacitors and one 220uF 250V capacitor. The motor appears to have eight wires associated with it. Would I be correct in thinking that six of these wires are from the two coil sets and the other two are from the centrifugal switch and start winding.

Would I also be correct in thinking that the 12.5uF capacitors are for power factor correction and 220uF is for the start circuit? The fault I've found so far is that the 220uF can has chaffed through on part of the casting in the box and its insides can be seen through the hole. I disconnected this capacitor and removed it from the box, insulated the wiring and switched on the power and the breaker immediately pops as before.

I've metered the (isolated) supply and there seems to be a very low resistance between the phase and earth. If I meter between phase and neutral and turn the motor by hand the resistance (about 2 ohms) varies up and down indicating, I believe, that the motor windings are intact and are generating back EMF. I can't easily access the motor windings because I'd have to disassemble the fan assembly but will do so if necessary.

Anyone have any suggestions?

Thanks

Tim
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Old 5th Jul 2018, 5:46 pm   #2
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Default Re: Single Phase AC Compressor Motor

Do the terminals have any letters on them? Are the leads coloured?

Most British motors used standard terminal designations and wire colours.
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Old 5th Jul 2018, 5:50 pm   #3
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Default Re: Single Phase AC Compressor Motor

Any smell?
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Old 5th Jul 2018, 6:59 pm   #4
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Default Re: Single Phase AC Compressor Motor

Boater Sam suggestions could save a lot of time but if okay the next step would be to get hold of a suitable diagram.
In the first instance the two capacitors would suggest start and run caps especially since there is a centrifugal switch. It would therefore be prudent to use google or otherwise to seek a suitable connection diagram.
In this you would need to type in "single phase motor capacitor start and run diagram" and note what is available.
There will be several to choose from as loading often determines certain designs of motors; especially compressors.
The 220uF cap will be the start capacitor and the smaller the run capacitor.

The trick is to then wear goggles or ear muffs or both when re-energising.
Good luck
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Old 5th Jul 2018, 7:44 pm   #5
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Default Re: Single Phase AC Compressor Motor

Capacitor might have been knocked out by this hot weather. It happens to fluorescent lighting capacitors. If the motor stalls on startup it will take a lot of current.
Easiest way is to check the capacitors by substitution or else find a capacitance tester.

Compressors take quite a bit to start them. Some have a loading valve which is delayed to allow the motor to start under light load before making air. If yours has one of these it's worth checking its action.
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Old 5th Jul 2018, 8:36 pm   #6
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Default Re: Single Phase AC Compressor Motor

As Kevin says, do check the unloading valve. If this fails, the cylinder head is at tank pressure and the motor can't start. This is the valve that once the tank reaches the pressure preset by the pressure switch, a sharp hiss is heard as pressure in the head is released allowing a restart. The valve is very simple and can be replaced by any rubber plug etc.
Neil
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Old 5th Jul 2018, 9:10 pm   #7
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Default Re: Single Phase AC Compressor Motor

Usually the delivery pipe from the head to the tank has a non-return valve at the tank fitting.

The pressure switch, when it decides the tank is at proper pressure switches off power to the motor AND opens a little valve which is connected to the head/delivery pipe side of the non-return valve. This unloader valve lets out the pressure from the head/delivery pipe. The non return valve stops the main tank emptying itself through the unloader valve.

If this lot isn't working properly, the pressure in the head isn't released and the torque required of the motor is impossible for it unless it is pretty much up to speed. The usual outcome is a popped breaker if you have one, (or a burned-out motor if you don't).

If you release the pressure in the main tank, then the motor ought to be able to spin the pump relatively freely and any breaker popping indicates a motor problem.

If a start capacitor is open circuit, or the centrifugal switch is open circuit, the motor will just sit stalled and buzz loudly for a moment before the breaker goes. Compressor motors have a hard life and are run pretty much at their limits. It burned out once and it's been doing the same job for some time, so shorted turns in an overheated winding wouldn't be that unusual.

David
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Old 5th Jul 2018, 10:33 pm   #8
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Default Re: Single Phase AC Compressor Motor

Have had two motors trip RCDs in the past, one compressor and a washing machine. In both cases the problem was cured by cleaning away carbon deposits in the region of the brushes and rotor. Presumably the carbon build up caused earth leakage.

Alan
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Old 5th Jul 2018, 10:38 pm   #9
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Default Re: Single Phase AC Compressor Motor

It's a garage compressor, so it's likely to be a 2 or 2 horsepower induction motor, and so, brushless.

David
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Old 6th Jul 2018, 8:48 am   #10
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Default Re: Single Phase AC Compressor Motor

Quote:
Originally Posted by Station X View Post
Do the terminals have any letters on them? Are the leads coloured?

Most British motors used standard terminal designations and wire colours.
Please excuse a quick hijack of this thread () but can I ask if you know what those colours and designations are? I've got a Compton motor minus caps and would be interested to know which wire Goes where!

Cheers,
Steve.
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Old 6th Jul 2018, 2:13 pm   #11
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Default Re: Single Phase AC Compressor Motor

Compressor motors are usually capacitor start and run. initially both capacitors are in parallel with each other and in series with the start winding which is much higher resistance than the run winding which might be around 6 ohms or less. The capacitor with the highest value, up 120 micro farads has the centrifugal switch in series with it, when the motor is up to speed the switch opens leaving just the run capacitor in series with the start winding. As preciously mentioned check the unloader valve first before spending a lot of money. Completely empty the air receiver, then see if the motor will power up the compressor ,if it does then its the unloader valve, Machine Mart might just have one to fit. Ted
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Old 6th Jul 2018, 3:44 pm   #12
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Default Re: Single Phase AC Compressor Motor

Do you know the HP of the motor?

Single phase compressor motors are usually toast by the time you know if the capacitor has failed, as the damage to the windings is done by then.

If you're still generating back EMF, but popping breakers, it sounds as though the windings are shorted - unless as suggested pressure is stopping the motor from being able to start, which case you could try running the motor with the belt removed.

Here's a place which has reasonably decent motors

https://www.engineersmate.com/produc...Motors/C106088

But if your friend had his rewound some time ago, I'm assuming it's quite a large single phase motor.

I've got a 100 litre compressor which as a 3HP motor and a replacement motor for that was around £100, so not worth rewinding.

With replacment motors, the shaft lengths can be different, I've had to make up for that by using taper lock bush pulleys, where the bush can lock anywhere along the shaft length.
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Old 6th Jul 2018, 7:32 pm   #13
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Default Re: Single Phase AC Compressor Motor

The OP did say:-

"I've metered the (isolated) supply and there seems to be a very low resistance between the phase and earth."

So perhaps some more checks on the motor side of the isolation e.g. neutral to earth?
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Old 6th Jul 2018, 8:01 pm   #14
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Default Re: Single Phase AC Compressor Motor

Quote:
Originally Posted by fetteler View Post
Please excuse a quick hijack of this thread () but can I ask if you know what those colours and designations are? I've got a Compton motor minus caps and would be interested to know which wire Goes where!
I don't know if all motors used the same colours, but I have a 1/4HP Crompton Parkinson single phase capacitor start motor on my lathe which I restored about 10 years ago. Of course the start has now failed since, so it will have to come off for fault finding at some point (it can still be manually started, but this is not recommended for obvious reasons).

Here is a picture of the paper wiring diagram from inside the terminal box cover showing the colours used in mine, the capacitor (TCC branded) on this 1/4HP motor is 40uF.

David
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Old 6th Jul 2018, 11:35 pm   #15
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Default Re: Single Phase AC Compressor Motor

Thanks David, I'll have a look at mine and see if it's similar.

Cheers,
Steve.
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Old 7th Jul 2018, 7:42 am   #16
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Default Re: Single Phase AC Compressor Motor

One thing that's definite is that there shouldn't be a low resistance from any motor winding to earth.

Time for another motor or time for another rewind.

It might be worth considering the next frame size up if motors keep dying on this machine.

David
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Old 9th Jul 2018, 9:29 am   #17
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Default Re: Single Phase AC Compressor Motor

Quote:
Originally Posted by fetteler View Post
Quote:
Originally Posted by Station X View Post
Do the terminals have any letters on them? Are the leads coloured?

Most British motors used standard terminal designations and wire colours.
Please excuse a quick hijack of this thread () but can I ask if you know what those colours and designations are? I've got a Compton motor minus caps and would be interested to know which wire Goes where!

Cheers,
Steve.

Run winding is generally between A1 and A2 or U1 and U2, coloured red and black.

Start winding is generally between Z1 and Z2, coloured yellow and blue.
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Old 13th Jul 2018, 3:39 pm   #18
flyingtech55
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Default Re: Single Phase AC Compressor Motor

Firstly thanks for all the replies so far.

Bit of an update.....

I had another look based on your (collective) replies and with research from the Internet. Whoever rewound the motor didn't preserve the colours. All the wiring is sleeved in black. I haven't disturbed any wiring, and as the motor did work then I presume that the wiring is correct. The rating plate is very dirty and I can't make out the motor rating but I can make out that it's rated at 1440 rpm.

The compressor is 40 years old and over the years various ancillary parts have failed and have been disconnected. The pressure switch is gone and the unloading valve is also gone. The wiring goes directly into the motor via the isolator. When Alex needs the compressor he simply turns on the motor using the isolator lever and it sometimes won't start and hums. If this happens Alex manually pulls the dump valve until the motor starts. The compressor then runs until the blow-off valve opens when he switches the motor off and he then uses the air stored in the tank or if it's a high consumption tool such as an air cutoff tool the compressor stays on while the tool is in use. I think that these years of abuse have taken their tole on the motor and the insulation has broken down.

The compressor is installed in a dark corner of the workshop at the end of the ramp and is absolutely filthy. When I'm working on the compressor I'm under Alex's feet because of the position of the compressor and the 'business end' of the ramp.

The plan is, he's going to remove the motor from the compressor and I'm going to take it away to my workshop where I can clean it off using my compressor and air dusting gun and remove the covers to get at the wiring. I can have a good look at the wiring and centrifugal switch in better lighting and megger the windings. I shall report back when I have done all this.

Thank you all for your input so far

Tim
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Old 13th Jul 2018, 5:04 pm   #19
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Default Re: Single Phase AC Compressor Motor

Ah, that clears things up a lot. Starting the motor with load on the pump and sometimes stalling it will kill motors.

Unless some sort of unloader is arranged, it will keep on eating motors, and as newer motors tend to be a bit lighter on torque than older ones, it will be worse.

The fault is with the unloader, the motor failure is a symptom.

Fit a non-return valve where the feed pipe enters the tank, fit a modern pressure switch sensing the pressure in the tank, with a small pipe to unload the feed pipe. It'll cost significantly less than a motor and it will make the thing reliable once the motor has been rewound again, or a new one fitted.

David
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Old 13th Jul 2018, 6:04 pm   #20
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Default Re: Single Phase AC Compressor Motor

Absolutely agree with all of the above. Alex is a very good car mechanic and is one of my oldest friends. He's one of the very few people I trust to work on my cars when I'm unable to do the job myself, but like quite a few older mechanics he's not so good on electrics and hasn't understood the implications of applying power to a stalled electric motor.

Tim
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Last edited by flyingtech55; 13th Jul 2018 at 6:27 pm. Reason: Corrected untidy sentence construction.
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