MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - Faulty grinder

I was going to ask Bill about this one, as my 'go to' for electrics advice. But then I thought I would open it up to all.

Photo is of my much used bench grinder. It's packed up. For a long time (years that is) on switching on it would dither and make a grumbling noise but a slight flick forwards of the wheel and it would work perfectly. That is just how it was. But then, as of last night it now runs but at low speed and with erratic bursts of energy but never enough to get going properly. There have been no sparks or smell of burning which I hope is a good sign!

So, before I strip it down to see what's inside, any clues please? Would it be something a simple as needing new brushes, or does this type of motor not have brushes. As you see from the label it says "condenser start". Might the condenser need replacing?


GuyW

The likely candidate is a failed capacitor, once you open it up you can check it if you have a meter with a capacitance range. You may actually find the cap underneath in the base. Some motor types may have a centrifugal switch inside to disconnect the cap once up to speed and they can get dirty or burn out.
David Billington

I always treat capacitors with caution as if they are charged it is possible to get an electrical shock.

As David has noted that sounds like a good place to start.
I have used external mounted ones on a few of my small motors.

R.





richard b

Is there's procedure for discharging the capacitor then,just in case?
GuyW

Didn't realise images don't upload unless there is some text with them


GuyW

I don't think you need to worry as any potential on the capacitor will discharge through the windings when the centrifugal switch is closed as the motor is not running but you can always check for any voltage across the cap connections to be on the safe side.

I've noticed that before Guy, need some text or pic won't post.
David Billington

I only have basic meters. A digital that does ohms, AC and DC volts. And an old Russian analogue needle one that does the same, plus low amps. Presumably none of these ranges is going to tell me much about the capacitor?
GuyW

IIRC you can use the resistance range to tell something about the state of the cap as it will charge from the applied voltage. IIRC on a high range it'll appear low resistance initially and then go to infinity or very high resistance as the cap charges if good. If you look online you should find more info.
David Billington

As new ones are so cheap, is it worth the trouble of working out what the problem is, buying the suspected broken part and fitting it?
Dominic Clancy

You are probably right Dominic. My only other experience was fixing a washing machine a few years ago for my daughter. She was about of spend £450 on a replacement one but I thought it worth replacing the capacitor at around £4.50 and sure enough that was all it needed.

She was, naturally, upset at not getting a new machine! Though she got think it not so bad when I calculated how many bottles of Prosecco she could buy with the difference!

So, I now need to track down a capacitor like that one.

Just realised it says 125v AC. But this has always been run on a standard UK single phase, 230v. !!!
GuyW

I only mentioned it as people who are much more into electrics have warned me of potential risks.
I think they said if you short the two capacitor wires together that would discharge it - but I would need to check with them.

Yes as Dominic said just get another one with the same ratings - when I last looked there were loads on E bay.
R.
richard b

I've helped a few people fixing this sort of problem and they usually just get a suitably size cap off ebay for about £5, so far cheaper than a new machine and a small expenditure of time. While apart maybe take the end bells off and check any centrifugal switch assembly if it has one and possible lube the bearings. Good point on cap voltage but the motor plate says 240V so maybe the cap has been replaced before.

BTW washing machines that indicate a low speed error is usually just worn brushes and many machines have them easily replaceable on dovetails. Bread and butter money for machine repairers and one that supplied me a pair of brushes admitted that he had to make the job seem longer as it's such an quick job to do once the motor is accessible.
David Billington

Thanks David. I am the first and only owner of this machine. It hasn't had a replacement capacitor in that time and its never given any trouble before. So that 125v on the label is a bit of a mystery.

When I fixed my daughter's washing machine it was juts the capacitor. The motor made abuzzing noise, but wouldn't start up. But it was an easy fix though it was just pure guesswork. I had no idea what I was doing!
GuyW

I *think* the "start" capacitor will be in series with a "start" winding so will not see 240v across it. This start circuit has a centrifugal switch which might be sticking on (should switch off when motor accelerates) which might cause erratic running but I'm not certain - need another opinion.
Bill Bretherton

Just a word on capacitors , never assume that it's not charged, 'always' treat it as charged, short it out before you handle it
It wants to get you----and it'll hurt
William Revit

you can check for resistance and a rough test connect a 9v battery for a few seconds and see what voltage is there when you remove battery should be around 9v fading to zero as it discharges
mark heyworth

Back to square 1!
I ordered one of these capacitors, but have now had a message from the seller (Hong Kong) saying that CV-19 restrictions on logistics prevents them from shipping to the UK. They have refunded me, but it leaves me the problem of finding another.

I had spent ages looking for one as I wanted to buy from UK in order to get a speedy delivery, as all of the ones listed were from China (or derivatives)taking 1- 2 months. In the end I couldn't find a UK supplier.

It says something dire about the degradation of UK's economic independance when a simple commonplace device like this isnt available in the UK
GuyW

Have you tried RS Components?

https://uk.rs-online.com/web/p/aluminium-capacitors/2508851525/
Dave O'Neill 2

Is that the same as CPC these days?
In which case. yes, but they didn't have one!
GuyW

RS is not the same as CPC, they teamed up with Farnell. I looked at RS but you'll notice the extended range details and it carries an added charge for items under £100. On ebay I only saw 1 likely option in NI.
David Billington

David, Quite right, I was consing them with Farnel.

Visiting their site again I recognised that I had looked there before and only found ones on their "extended range" option which are imported, with an extra price penalty plus a minimum order number of 5 units. Gives the impresion that they really don't want to do business!

I did before find 2 or 3 which initially I thought were UK as on eBay they list a UK address. But drill down a bi and they all shipped from China!

At present, I don't even know for sure if the part is faulty!
GuyW

https://www.ebay.co.uk/itm/ELECTRIC-MOTOR-START-CAPACITOR-80UF-to-100UF-220-275v-NEW/192645994170 is the one in NI. I think I've dealt with him before.
David Billington

Wouldn't your local washing machine/appliance repair joint have something
William Revit

David, mine is rated 125v. That looks good, but is a 220 / 275v one.

Willie, I did look at washing machine ones on the internet, but where the quoted figures rather than just appliance reference numbers, they were much lower capacitance.
Local repair shop - what's that!
GuyW

As Lou Reed warned we've reaped what we sown.

I used to know RS at Corby and they'd have skips full of perfectly good items with very minor faults or just lack of packaging, returns and such like to be "recycled". We are such a wasteful society, all of us me included.

We also lowered our standards to get very cheap, shiney things from one side of the world to perhaps in future look to the other side of the world for lower standards of much more important basic or very important things in life.

Didn't help with finding this part, but helped me in part - a poet and didn't know it.

Nigel Atkins

The working voltage (WV) of a capacitor is the MAX voltage it should see across it and it generally accepted to be chosen with a 50% headroom.

A higher voltage rating than 125V is OK, the important rating is its capacitance of 100 mfd.
Chris at Octarine Services

How about this supplier

http://www.capacitor-sales.co.uk/start_capacitors.html
Alan Cotterill

Searching ebay for 'motor start capacitor' in the UK turns up quite a few but trying to find 100uf doesn't, the vendors all seem to sell them covering a range such as 88uF - 106uF which would likely be fine as capacitors typically have a large capacity tolerance.
David Billington

Thanks Chris (and David). That widens the scope considerably. I had been trying to match all of the parameters at the same time. Plus find one with the same terminals.
GuyW

The 'site that Alan has linked to looks like it should have something suitable.
Dave O'Neill 2

Guy, I'd still check to see if the motor is one with a centrifugal switch that may be stickng on. All the start capacitor does is to initially start the motor which you could do manually to see if it runs. You could disconnect the capacitor and still start the motor manually, in fact I'd try that because, IF I'm right and it has a centrifugal switch which is stuck on, then removing the cap would take the start coil out of circuit and verify if the motor will otherwise run normally. And at zero cost.

This type of motor is called a squirrel cage induction motor which is brushless, it works using electromagnetic induction (not all ac motors are the same so I'm guessing which yours is but it's worth a shot imv).
Bill Bretherton

Bill, check my first post on this thread. I have been doing an 'assisted' flick start for several years so maybe the capacitor has been faulty all along. Now that doesn't work any more. Even with encouragement it dithers and won't pick up speed.
Does that give any clues?

When you say to try it without the capacitor, should the 2 disconnected capacitor wires be shorted together for this test?
GuyW

Guy, I did read the first post. It may be (and this is purely conjecture) that the centrifugal switch was stuck open for years and has recently stuck closed (the motor wouldn't like that).

I mean to disconnect the cap altogether and see if the "flick start" still works. If it does you have the answer. If it doesn't then it may be the cap is faulty. Electrolytc capacitors (the motor start cap is a version of this) tend to fall in value over the years. I have a 1974 hi-fi amplifier in which I've replaced all the electrolytics (it works fine now).

Give it a go, it's free. If I'm wrong then nothing is lost surely? (Just my pride!).
Bill Bretherton

If you do remove the cap, you could check it for short circuit with a meter i.e. zero ohms although if it was "breaking down" it might only do it with mains voltage applied (DON'T DO THAT!)
Bill Bretherton

Guy,

How do the bearings feel are they at all stiff? I get problems in my central heating blower from time to time as the lube in the bearings can weep out or go stiff over time and it causes the motor to not run up to the proper speed and so no CH. The bearings in the motor are only shielded so a small application of oil quickly seeps into the bearings and they're good to go again without the need for an expensive replacement fan.

Have you pulled the end bells on the grinder yet to check for a centrifugal switch and the bearings. Motor types with low starting torque are more prone to suffer from the bearing drag causing poor starting, I have one that isn't happy in to winter but fine in the summer when the lube is warmer.
David Billington

Apologies everyone else for thread drift.

Guy,

Your post referring to central heating has touched a very big nerve as I have an intermittent central problem that seems to be similar to the one you described. When an "on" timer period occurs the fan starts, everything sounds normal but there is no final click and the boiler does not ignite. Sometimes it will ignite a couple of minutes after the fan starts, sometimes it takes much longer. Once it has fired, subsequent refiring is completely normal. On other occasions it will fire properly first time and run properly for that whole "on period". Does that sound like the fan problem you referred to? British Gas wall mounted boiler, I think a rebadged Potterton.
Cheers,
Ray
Ray Rowsell

Hi Ray
Have you got the right thread here? I have installed and fixed CH boilers but don't recall posting any messages here about them. I know that most modern boilers go through a self- check process before igniting and maybe that is the problem with yours? Perhaps a failing sensor.

Does it have an accessible error code display? Sometimes these are only available if you have the gas safe registered installers code.
GuyW

Well, its fixed! Thanks for all the advice. I am not 100% sure what actually sorted it out.

Capacitor was showing no continuity between terminals. I know thats not a full test,but it rather indicated that it was maybe ok.

Cleaned out 40 years worth of dust and metal powder generated from the grinding wheel. Some had accumulated around the soft iron armature which was dragging on the coil armature that it runs inside. Cleaned and regreased the bearings.

Stripped down and reassembled the centrifugal switch. Although the bits all moved it had a lot if sticky dirt in there. Also the contacts had a build up of debris and corrosion so I cleaned them with a needle file, washed it all down with contact cleaner and reassembled.
I also adjusted it to give a more positive 'on' as it looked like the button contacts weren't actually meeting fully.

Back together and it now works fine. It no longer even needs the encouraging flick of the wheel. So not sure what part of that list actually sorted it but probably the switch mechanism.
GuyW

Great news Guy. I'm glad it's fixed at zero cost! Yes, a good cap would have zero continuity on a resistance test.
Bill Bretherton

Guy,

Good to hear it's all going again at no cost. Normally testing a cap with resistance range will show a low resistance initially as the applied voltage charges the cap and then rise to show no resistance when fully charged.

On a CH front my engineer gave me a fan from a boiler he had recently had to replace as it wasn't running properly, I'll likely use it for a small forge. Turns out one of the bearings that supports the end of the shaft had stuck to the shaft, maybe dust, when it should have moved slightly when the motor was started and that allowed the self aligning end cup to leave its socket slightly and the rotor to touch the field winding and the slight drag slowed the motor causing the CH to fail. About 15 minutes to dismantle, clean, lube and reassemble and the fan runs fine. Image shows the rotor bearing and spring, I don't know what boiler it was.


David Billington

David and I both mean very high, virtually infinite resistance (when fully charged). But if the motor is starting properly now, then the capacitor must be doing its job well enough (which is to create a "phase shifted" voltage in the start winding. I'll leave it at that.
Bill Bretherton

I thought it is the current that is phase shifted?
Chris at Octarine Services

I was measuring the resistance of the Capacitor when disconnected, not charged, thinking that in that state if it had low resistance then something was wrong. I realise this was only a partial test but I didn't want to start messing with it when charged up!

Apart from cleaning the other thing I did was to reposition the centrifugal switch on the shaft. It is held by a grub screw and I moved it to increase the pressure on the contacts when stationary. I dont think it was switching the cap on properly.
GuyW

Guy,

Apologies for causing confusion. It was David B's post that referred to central heating. He has subsequently added to the post and what he described is certainly similar to my problem. Thanks Guy; thanks David.

Ray
Ray Rowsell

Chris, yes, strictly speaking the current "leads" the supply voltage by 90 degrees so it has phase shifted but that current causes a volt drop in the start coil which, itself, is phase shifted with respect to the supply AIUI.

In the single phase induction motor, there has to be a way of starting the motor which is achieved by this phase shifted voltage/ current which results in the magnetic flux peaking at a different time in the start coil, to cause the rotor to rotate. In effect, a 2 phase system is created. In a 3 phase motor, things are simpler as there already are 3 phase shifted voltages. Well that's my understanding of it anyway, AC circuits are complicated. We haven't mentioned reactance and impedance yet.

But I'm no motor expert, I'm more interested in electronics. I looked in the textbook that I used for HNC many moons ago and have forgotten most of the maths! Interesting trying to read it all again though.
Bill Bretherton

My understanding is that the phase shifted current in the starter winding leads the current in the main winding which causes the magnetic field to rotate - this in turn causes the rotor to revolve until it is going fast enough for the starter circuit to be switched out by the centrifugal switch.

Typically starter capacitors are high value to maximise torque and if fitted the run capacitor is low value and more to do with motor efficiency.
Chris at Octarine Services

This thread was discussed between 12/06/2020 and 17/06/2020

MG Midget and Sprite Technical index

This thread is from the archives. Join the live MG Midget and Sprite Technical BBS now