|Ian Richards 3||17/03/2014 22:13:27|
|5 forum posts|
Is anyone an expert on single phase induction motors (capacitor start and capacitor run)? Mine is fitted to a Chester lathe / mill, which is about 10 years old. The motor is 4 pole (1400RPM), 0.75 HP, frame size 80.
The motor has suddenly gone from running smoothly and almost silently to sounding very rough and making quite a lot of noise. I think it's still achieving normal speeds (unloaded), but I haven't dared run it for long enough to investigate the performance. So far I've checked:
- The centrifugal switch. Fairly confident this is OK. It's closed circuit at rest and goes open circuit when you spin the motor with a drill.
- The start capacitor, which was replaced about 3 years ago. When this failed, it struggled to start up.
- The run capacitor. I've replaced this since the problem started, but it hasn't had any effect.
- The bearings. There is no significant noise when you spin the motor with a drill, or when it's coasting down. It spins freely.
Running current (unloaded) is about 1.7A, which feels quite high. Is there anything else that I can investigate?
Thanks very much
422 forum posts
Silly question but its not a loose pulley by any chance.
|julian atkins||17/03/2014 22:56:09|
1238 forum posts
i had the same problem with my Fobco Star pillar drill. after asking around a very old friend suggested one of the wires to the magnets/looms (soz im crap at electrics and cant remember which) might have worked loose. sure enough i adjusted all and tighten them up and it's performed as expected ever since.
|Rick Parry||17/03/2014 23:06:24|
|5 forum posts|
Sorry to say but it sounds like the start capacitor ot me. Try spinning it up with the belt removed to some speed before you switch on with piece of string around the pully will do it. It should then start and run if everything else is working.
|Les Jones 1||17/03/2014 23:21:02|
|2225 forum posts|
|626 forum posts|
I bought a lathe some time ago, very cheap, very filthy. When I cleaned all the grease and dirt off, It looked like new, no wear at all, appeared to be unused. when I switched it on, the whole machine vibrated horribly, which probably explained its unused condition. It turned out that the centrifugal switch was shorting to earth.
|Clive Hartland||18/03/2014 07:18:18|
2667 forum posts
My saw bench motor has a centrifugal switch and it started vibrating and cutting out and getting hot! As it is a double ended spindle I could not buy a new one so had to have it rewound and the switch points remade. Very expensive but now OK. If you strip it down replace the bearings anyway as they are not expensive. maybe you have a bit of swarf sucked into the gap between the armature and the field coils?
|john fletcher 1||18/03/2014 08:46:57|
|678 forum posts|
Hello Ian and others. Before spending any money try Julians idea, but be very careful.Wrap a piece of string around the motor pulley give it a good pull and then switch on the power.This way you are eliminating the start circuit,as the weak parts are the start capacitor and the centrifugal switch. I have replaced start capacitors with ones with a higher working voltage 400 volts, the original ones were 240 volt and I haven't had any come back.I'm not familiar with your lathe or motor but the next thing is to get an electrician with his Megger to carry out an insulation test both between winding and to earth.Not often do you get a short between windings or open circuit. I have had several Chinese motors to pieces and they are well designed and well constructed, inspite of what the advert says in Model Engineer,but having aluminum cooling fins around their outer can be a bit noisy, ringing sound. One good thing is you only have to take the drive end off to gain access to the switch and the windings, very easy to dismantle and re assemble.I'm not in business fixing motors, but help out my friends as and when they need it.Keep us posted.Ted
|roy entwistle||18/03/2014 09:05:23|
|1346 forum posts|
Ian I have had the internal fan come loose if there are any cooling slots in the end cap you might be able to check that
|Colin Price 1||18/03/2014 11:29:51|
|2 forum posts|
The excessive noise level, combined with the large off-load current suggests that the motor is continuing to run on the starter windings. Although you may be able to hear the centrifugal switch "operating", the stater winding is not being switched off.
The vibration is caused by the strong magnet field brought about by the starter winding, ie strong compared to the field caused by the field windings, so the running becomes very rough indeed.
|1046 forum posts|
Had exa fly the same on my lathe a few months ago, turned out to be a broken wire on the centrifugal switch. Quick resolder fixed but then a few days ago the start cap blew quite spectacularly, suspect it was stressed not being switched out of circuit. Replaced today with a high quality component ( also checked the switch to be on the safe side) motor running g sweet and quiet again
|Ian S C||18/03/2014 12:45:52|
7468 forum posts
If the start windings don't disengage, won't the motor run as an 8 pole motor, at a bit over 720rpm, there was an article in ME a few years back about someone who found that he could get extra speeds on his drill press, for a number of years he used the start windings to run his drill at low speed until one day when the magic blue smoke leaked out, and the motor was no more. Ian S C
|Les Jones 1||18/03/2014 15:09:10|
|2225 forum posts|
It is about 5" dia and the length (Excluding the shaft.) is about 9"
From the test on my motor it does not look like your motor is drawing excessive current.
I do not agree with the suggestion that it could be the start capacitor causing the problem as the motor starts OK and when it is up to speed it is no longer connected.
|Ian Richards 3||19/03/2014 22:27:00|
|5 forum posts|
Problem solved, and peace has returned!
First of all though, thanks very much for all the advice. I logged on last night wondering if anyone had replied at all, and was amazed by the number of detailed replies!
Anyway, the prize (if there was one) would go to wheeltapper. The pulley was loose. I can't believe that I didn't spot it sooner, or that it's such a simple explanation. I also can't understand why a loose grub screw on a keyed pulley would make such a racket.
I realised the cause when I tried to load up the motor on starting (to try and test the starting capacitor and centrifugal switch). Not only did the motor have plenty of torque, but my gloved hand on the pulley also stopped the racket!
422 forum posts
WOW I got something right for a change!!!
my flabber is ghasted.
glad you have it sorted now.
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