Motor malfunction

I am assuming this is an electronics question!

I have a Harrison 11" retro-fitted, I presume, with a 240v motor. There are just go and stop buttons. The motor has a small box on top plus what looks like a capacitor.

Recently I have noticed intermittent slight hesitations (a bit like a car "missing" on one cylinder), accompanied by a sort of quiet knocking noise.

My immediate assumption was that the capacitor was at fault, because the machine then began failing to start properly - it hesitated, then got up to speed after a few seconds.

However, I have no info whatever about the motor or the capacitor, with no legible markings on either. My capacitor tester says 38mf, but I do not know if this is significant.

Is my diagnosis likely to be correct?

How can I establish the rating of a) the capacitor and b) the motor?

If the worst comes to the worst and the motor is defective, what do I need to know to specify a replacement?

Any guidance would be much appreciated.

Edited By Martyn Stevens on 17/07/2021 12:54:34

Welcome to the forum Martyn.

Be good if you can manage some photos of the guilty motor, as described here.

Sounds very much like a single-phase motor, which exist in a few different configurations. Tubal Cain's Model Engineer's Handbook has a useful few pages on the subject.

38mF is old-fashioned speak for 38 microfarads, and any start or run capacitor ±30% of that value should do, like this example from Mr Amazon. (Some motors have two capacitors)

The symptoms may be of a faulty centrifugal switch. These are usually under a cover at the rear end of the motor; worth giving it a clean and confirming it's in good nick.

In the worst case, motors are standard sizes,so not too difficult to replace. If necessary to go to that much trouble worth considering replacing the motor with a 3-phase type and VFD: just a though at this stage.

As I'm not a motor expert, I hope one of the forum gurus will be along shortly.

Dave

Martyn

If it is a motor issue I'd suspect the centrifugal start winding switch assembly. Although effective these tend to be a bit on the flimsy engineered side, often with dose of optimistic design concept. Elderly ones are prone to bits working loose or shifting out of position.

Theoretically easy to examine.

Just disconnect the motor wires from the terminal block having marked them so you know where to re-connect and remove the closed end of the motor after unscrewing the shaft end and bearing cover. The screws that hold this cover on frequently do double duty and also hold the switch in position. In practice a certain amount of bumping with soft hammer and suitable verbal encouragement is needed to shift the end on a motor that has been together for many years. Need to be rather careful to ensure that the switch doesn't get caught up on anything. It should be a simple pull off job but sometimes bits get wgere they shouldn't.

Generally its not too difficult clean up and repair a centrifugal switch given a bit of low cunning. The only one that defeated me was basically built on curved arms made from flat spring steel. One of which was fractured.

If you do decide that the nuclear option of changing the motor makes more sense the best, albeit most expensive, way is to use a three phase motor and VFD. Plenty of choice. Although many folk have done just fine with low cost direct from China imports I've never thought the price differential relative to a name brand from UK suppliers such as Inverter Drive Supermarket is worth the lack of local support and wresting with a Chinglish manual. Capacitor quality is always the worry at the low end. Inverters are hard on their capacitors and the proper things aren't cheap. Less expensive economy range ones won't last as long but, given the difference between industrial and home shop use, not as long may well be long enough.

Although a speed display and infinite variability by turning a dial is sexy I'm unconvinced as to the advantages on a gear head lathe with a decent choice of speeds. I like the Eaton DE1 series "intelligent contactor replacement" devices for this sort of thing. Just a grey box that can be hidden anywhere out of the way with no display, knobs or buttons. Being designed to operated by simply switching on and off at the power socket in the most basic single speed application they can be super easy to install. Although thats how the one on my car lift is connected for a lathe I'd go a bit up market with stop start control buttons and switch selection of a useful range of speeds to fin in between the gears. Three or five speeds I think. Central native and either one or two up and down. Twiddling a knob gets tedious, switches you are more likely to use.

Clive

Edited By Clive Foster on 17/07/2021 13:54:51

Some more detail would be helpful, Maker Etc. The 38mf could be right, though 40 is more likely given +/- 10%, on old motors the spec may be on an inner end cover. Voltage will be 450V. As others have said it could be the centrifugal switch. This device is normally internal and fragile. There will be 2 windings, start and run, note all connetions and take pictures then carefully dismantle. Check the Cent switch for burnt contacts or any damage and repair as needed. What starting switch do you have DOL, just a plug, simple switch ? could this be faulty ? Only large frame industrial motors seem to have an external Centrifugal switch, I've only seen 1.

What test gear do you have, or can get your hands on ?

Good luck Noel.

Martyn

When you have the motor running at full speed press the stop button or other switch you use for stopping the motor and as the motor slows down but before it stops you should hear the centrifugal switch contacts close, normally with a distinct click.
If you don't hear anything then do as suggested by others, if you do hear a click then just obtain and fit a new start capacitor.

Emgee

It might not be an electronics question. I've had something very similar. It took me far too long to find a small piece of debris jammed in one of the slots of the windings (in my case jammed in the rotor) which was fouling the gap. The gap isn't much so it only took a miniscule bit of crud to jam up the works.

HTH Simon

Once it running, make up some form of cover so that swarf annot find its way into the Centifugal switch.

(That nearly ruined my single phase motor. Fortunately, the local motor rewinders, lacking a replacement switch, were able to clean up the original to get it working again. )

Howard

Thank you all for your comments. Here is a progress report.

I removed the cover to the box on top of the motor. The wiring seemed a bit awkwardly done and the plastic base for the connectors was partly melted. One wire, between a microswitch and a red pushbutton on the outside of the box, was almost rotted thru so I replaced that. I disconnected then reconnected the capacitor and put everything back together. The hesitant start-up has gone, but it don't sound quite right and there is still that "missing" and the intermittent thumping noises.

I was interested in the comments about automatic centrifugal switching of the capacitor, which is a feature of Debrie projectors and there is indeed a very characteristic "click" as the motor runs down.I have fixed one or two of these so may be able to figure out what to do. I am puzzled, however, by some aspects of this. Most older sound projectors, where a fairly constant speed is necessary, use an induction motor with a start/run capacitor, which is always in circuit. My experience of the Debrie and a few other machines that use a start cap and a centrifugal switch is that if the centrifugal switch fails, it very quickly leads to destruction of the cap circuit - afterwards, the motor is still capable of running but will not start on its own. If this is the problem, why is the motor still working?

I will do some pix of the motor and probably put them on my website. One of my problems here is that access to the motor is very restricted and my ability to work in confined spaces is very limited nowadays. Also, like I suspect most hobbyists, I have far to many things/projects to do than I am likely to complete in my lifetime and I really don't need this. Nor did I (or my wife, I suppose) need for her to fall over and break her upper arm, naturally on the dominant side, leaving her severely incapable for a couple of months. I am temporarily a carer and housekeeper. Life? Don't talk to me about life!

We are assuming that the on/off switch isn't the culprit. The contacts inside the switch could be arcing ??

Update

.http://cinerdistan.com/2-uncategorised/617-engineering

This link should enable access to a special page I have added to my website with a couple of pix to show the motor and the very restricted access. I'm certainly not going to try anything until it's a lot cooler, but I will consider taking the back off and looking at the centrifugal switch, if such there be. I shall also have a look at the main switch to check if there is arcing. If I find any info about the motor, I will add to this thread, but at the moment I can see no legible info anywhere on the motor or the capacitor, so I'm just going to wait for a bit until a) it's cooler and b) I feel strong.- at the moment I can still get by.

Thanks again for all the comments and suggestions

Martyn

Edited By Ronald Morrison on 21/07/2021 13:10:23