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Electric motor question

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Robin Graham03/06/2016 22:58:01
945 forum posts
295 photos

I had a non-functioning Vax vacuum cleaner off Freegle today - the donor said it fired up intermittently on wiggling the power cord, so I thought it might be an easy fix. Not so, no amount of wiggling worked for me and I can't find anything wrong with connections or switches.

I've stripped it down to the motor housing and there are three wires coloured blue, black and brown running from the switchgear to the motor. The blue wire actually runs directly from the power cord neutral. I've measured resistances and all combinations are effectively open cicruit (greater than 2 M-ohms).

Any ideas? I don't actually need another vac, but I'm reluctant to tip it if it can be restored and passed on to someone who does. It's only a few months old, so it isn't wear and tear.

Regards, Robin.

Paul Lousick03/06/2016 23:38:11
2015 forum posts
712 photos

If it's only a few months old, it should still be under warranty. Have it replaced.

Robin Graham04/06/2016 00:27:16
945 forum posts
295 photos

The guy who gave it to me had it replaced under warranty but they didn't take the faulty machine back. I'm trying to make it work so I could give it to someone who needs it but can't afford to buy new.


John McNamara04/06/2016 02:08:23
1336 forum posts
125 photos

Sounds like the brushes are not connecting to the commutator?
2 meg is almost open circuit.

Maybe the commutator has a burnt out winding? you can test each opposite pole with a multimeter you should find the same resistance across each pair.

You can test the field winding for open circuit.

If you can wiggle the power lead it may be faulty?
Did you test the lead from end to end. through the power switch as well.


Steven Vine04/06/2016 03:34:41
340 forum posts
30 photos

Have you connected a good known cable directly to the motor and fired it up, to take the other cable out of the picture?


Colin Whittaker04/06/2016 06:25:13
142 forum posts
17 photos

I agree with John, it smells like an intermittent commutator problem that may just need a bit of jiggling or spring retensioning to start working. It could also be a burnt out field or commutator winding, in which case a repair will normally be very problematic. If you are lucky then it may be the power switch.

Even with 40+ years of electrical experience I can still scare myself with live testing of appliances like this so I don't really want to suggest anything to someone with less electrical knowledge and quite possibly a weaker heart.

Cheers, Colin

Johnboy2504/06/2016 07:58:36
260 forum posts
3 photos

Just a thought...

I bought a Henry vacuum cleaner at an auction with the same fault many years ago...

If it powers up when you wriggle the power cable - remove plug from supply and cut back say about 6" from the cable from the entry point. Remake the electrical connections inside the Vax. (It could be either the Vax end or plug end - replace moulded plug with an MK tough plug).

I've seen this manytimes in the past where the flexing of the cable breaks the conductor inside the insulation causing an intermittent fault. I experienced this on automotive sensors as well.

I have a name for this - I call it a 'rubber' joint!

Failing that diagnosis - check out the bushes. If they've warn down the retaining spring may have bottom out not providing contact pressure on the commutator.


daveb04/06/2016 08:30:30
626 forum posts
10 photos

Broken flex is one of the most common faults on portable equipment, wire breaks inside the insulation at the stress point, usually where it enters the machine. Dave

Robin Graham04/06/2016 14:06:42
945 forum posts
295 photos

Thanks chaps, it is indeed a break in the neutral line of the power cable - in this case the internal cable clamp, which takes the cable through a tight 180 degree bend and holds it with three vicious prongs, had been over tightened. Strange world where it makes more economic sense to ship out a new replacement machine than fix a broken wire. It's been saved from a premature journey to the the tip though!


Vic04/06/2016 14:12:12
3060 forum posts
8 photos

Our vac had a broken cable inside the moulded plug. A simple plug replacement fixed the problem.

NJH04/06/2016 17:14:20
2314 forum posts
139 photos

Hi Robin

" Strange world where it makes more economic sense to ship out a new replacement machine than fix a broken wire"

Yes, on the face of it it would seem so - but then I got to thinking a bit..........

A quick search shows Curry's selling various models of Vax cleaner for around the £100 mark. Assuming yours was one of these Curry's must take a profit from that as must the wholesaler and the manufacturer - so the real cost of the item must be well below £100. On the other hand the cost of shipping back to the manufacturer, his diagnosis and rectification of the fault ( simple in your case I know - but maybe not always so) and the subsequent return to you will have to be taken out of the manufacturers profit. He will need to employ staff to carry out this work and they will carry overheads for his business ( and , down the line also for you.) In the mean time you will be without a vacuum cleaner! Much better that he sets up good quality control in his manufacturing process - which should ensure low levels of defect - and just replaces faulty items which should then occur infrequently.

When, after frequent and diligent operation in your hands, this Vax reaches the end of its life it will, no doubt, make an honourable exit to the local tip where it will be sent to be recycled into other "stuff" for our homes!


Edited By NJH on 04/06/2016 17:16:10

Mark P.04/06/2016 18:09:15
625 forum posts
8 photos
Maybe it's time to go back to having engineers in a workshop at the back of the shop!
Mark P.
Muzzer04/06/2016 18:56:26
2904 forum posts
448 photos

I doubt you'd want engineers at the back of the shop unless they'd been very naughty. Repair men perhaps...

Robin Graham04/06/2016 21:39:51
945 forum posts
295 photos
Posted by Muzzer on 04/06/2016 18:56:26:

I doubt you'd want engineers at the back of the shop unless they'd been very naughty. Repair men perhaps...

But they'd call themselves engineers I expect. The admin in University Departments used to be run by the Senior Administration Assistant, it's the Director of Business Administration now. But I'm feeling a grumpy old man rant coming on, I shall resist it!

Norman, I'm sure you're right in your explanation of the economics. Apart from the motor and ten self tapping screws, the machine is entirely made of plastic mouldings, so I imagine the production cost is dominated by the cost of the motor. No idea what that would be, but I can believe that if I were to charge Vax a commercial rate even for this straightforward repair it might be cheaper for them to supply a replacement. Especially after I'd paid consultancy fees to you guys wink.

To get back to technical matters, I think this may be a two speed motor - there are two (mechanically) latching switches operated by push-rods from the top of the machine. One is clearly the on-off switch (I don't have the manual), t'other connects mains live to the third wire into the motor. Would this be a second set of windings? Doesn't matter now I've found the fault, but I'm interested - always been a bit hazy on electric motors.

Thanks again for advice, Robin.

Tim Stevens05/06/2016 18:11:56
1587 forum posts

One problem for the supplier running a repair service is his guarantee. How and where does he get certificated repair-persons, and then carry the cost of their inevitable failures including the houses that get burnt down?

An automatic replace-with-new policy also avoids having to employ staff who know what a screwdriver is for.

Silly old world ...

Cheers, Tim

Robbo05/06/2016 19:23:37
1504 forum posts
142 photos

The repair persons would be called "Technicians" and they would cost far too much to keep at the back of the shop in case of repairs under guarantee.

In the days when repairmen were on premises, they also did repairs that customers paid for.

Isn't it great going off-thread?


Edited By Robbo on 05/06/2016 19:25:42

Dod06/06/2016 21:39:42
114 forum posts
7 photos

Don't get me started angry 2, I begun and spent 26 years as a Domestic Appliance Engineer angel, technicians are overcharging lowdown, guessing part pushing replacers, certainly not engineers.

Wiggling the cable 99% of the time means, as suggested cutting a 6 inch (or metric equivalent) off one end and if that dont work off the other end. That way you can have a 2 foot cable and if it still wont work have a look at the brushes.

Vacuum cleaner flexes break regularly as people try to do the extra inch before plugging into another socket

Robin Graham07/06/2016 23:15:33
945 forum posts
295 photos

Well, it's back together albeit with a cable shortened by 153mm wink. Quite a nice vac for the price I paid (zero), but I wouldn't want to part with 200 quid (asking price for this model) for it.

When I put it back together I realised that it wasn't designed to be taken apart and reassembled - several of the self-tappers holding plastic to plastic couldn't be convincingly retightened. I'll find a way of getting it solid again I expect, but if anyone's been there before and can advise, that would be great.


StephenS08/06/2016 00:56:57
38 forum posts

A good trick that I was taught many years ago works especially well for self-tappers into plastic, where the problem is that they will all too readily cut another new thread when reinserting them and after a couple of times there is no more material left for them to grip on to.

Place the screw into the hole, and with the screwdriver carefully and lightly turn the screw backwards, or anti-clockwise. You will feel the screw drop into the previously cut thread, when you can tighten it in the normal direction once more. This means that you are screwing into the previously cut thread, not making another new one.

Try it, it really does make a difference.

Cheers, Stephen S.

Robbo08/06/2016 09:28:46
1504 forum posts
142 photos

Stephen S post is a good idea, have also used it for years.

If you've already cut a second thread which doesn't grip well, fill the hole with some epoxy filler and insert the screw. The filler won't stick to the plastic but will fill the gaps

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