Buzzy buzz buzz motor 7

[weemantam]

I know this is only a small domestic motor but I could do with a tiny spot of advice. I have a cement mixer with an ATB Morely motor fitted in it (looks like a SRBF0,75/2-C50R) 1ph 240V 0.55kW (so it's only a wee thing). Recently it stopped working properly and made only a buzzing noise. I’ve dismantled the motor enclosure and removed the drive belt. Both the motor itself and the gearbox to which it is connected move freely. Despite this the motor, when powered with no belt connected, gives a satisfying whoosh of air as it spins. As soon as I put a load on (in this case the gearbox with no cement drum on the spindle – so almost no load) the assembly gives the buzzing noise again. If, however, I give it a little spin with my finger, away it goes into normal operation.

My question is whether the motor needs servicing (if it is indeed serviceable) or if it is more likely to be the big capacitor connected to the motor which may be at fault?

I'm a mechanical engineer with enough ability to avoid being electrocuted (mostly) so any advice would be properly useful.

Steve

 

[flexoprinting]

Hi Steve

Your start capacitor may have failed, or the centrifugal switch which
takes the cap out of circuit when the motor gets up to speed may be
stuck open. You can check the capacitor and switch on a multimeter
with capacitance and ohms functions.

 

[weemantam]

Thanks Flex...

I'll wire the thing back up and see what the meter says (pretty sure my Fluke has the necessaries). I'm away on my jollies so it'll be a couple of weeks but I'll let you know once I get back.

Thanks for the help.

 

[dArsonval]

Single phase motors have been known to put the brightest, most electrical savvy people in their place.

Named for how they electrically start, (shaded pole; permanent split-capacitor run, split phase, capacitor start, and so on...)
these motors have numerous (necessary) components required for them to get moving and run.

Capacitor (s)
Internal Stationary Start Switch (s)
Shaft mounted Centrifugal Mechanism

All of the above components (including some not listed), link a chain of functioning events required for the motor to start.

Single phase motor exploded view URLs found via Google for reference.

http://ecatalog.weg.net/files/wegne...xploded-view-50036028-quick-guide-english.pdf

http://www.jameselectric.ca/wp-content/uploads/2013/04/Single_Phase_Motor_Exploded.jpg

Most single phase motor troubleshooting efforts begin with what can be checked externally on the motor before opening it up.

As already mentioned, the capacitor(s) are a good place to start checking first, followed by a quick mechanical inspection.

While there is a useful lifetime to capacitors, and they do indeed fail due to age... a bad capacitor is often attributed to
a failure in one of the other component "links" including the driven load.

In your case, we'll rule out any mechanical issue with the driven load because it appears to turn freely.

So; if you find the capacitor to be bad, you have to also inspect the stationary switch, and shaft mounted rotating
switch (Centrifugal Mechanism) to determine if either of these played a role in the capacitor testing [no good].

Stationary switch contacts wear out from age, become welded together, or can be obstructed from operating properly by debris inside the motor.
The insulated tabs on these switches are subject to wear as well.
Hence, a malfunctioning switch can keep a capacitor in the motor start mode causing it to electrically fail.
In addition, the start winding itself can electrically fail due to any of these other start mode components failing.

A single phase motor with a failed capacitor will typically exhibit some shaft turning during the energizing of it under no load.
And yes, giving the motor shaft a "spin" by hand can often get it started.

The outright "buzzing" you're referring to would tend to point toward another investigative path to take.

Mechanical Inspection.

With the motor assembled, attempt to move the output shaft by hand up-n-down or side-to-side to determine if the bearings, or bearing housings are loose.
Out of tolerance / loose bearing housings allow the rotor to be pulled to one side when the motor is energized, which prohibits it from starting.
Noting that this is a belt drive application, typically the opposite drive end motor bearing pocket will wear out first over time due to excessive belt tension.
The front bearing acts as a pivot point as the rear housing wears out. Yet! There's also the condition of the front bearing journal wearing out first...
allowing the rotor to move side-to-side during the starting process.

In rare cases, an open rotor bar will not allow the motor to start, but that scenario has additional failure attributes that are likely
not the cause of your motor not working.

So having written somewhat of a mini essay for a Sunday morning... the answer to your question of,
"... whether the motor needs servicing (if it is indeed serviceable) or if it is more likely to be the big capacitor
connected to the motor which may be at fault?", is Yes.

What you may likely find though is that the cost of troubleshooting such a motor is not worth the expended
expertise required in determining the exact cause of it not working, and you're perhaps better off considering outright replacement
of the item if it's even available.

Single phase motors can be a "kicker". Especially today... due to their "Throw away, and buy new factor".

Sorry about the long-winded answer.

Always enjoying the forum,

John

 

[weemantam]

@ dArsonval,

Awesome but yes, along answer. Nevertheless I will do a step-by step approach to this. I did wonder if the motor unit was serviceable, for example it has brushes and the such like, but the more I read on this the more it's pointing to the capacitor. I have to say the motor is in really good condition, it's not stoored-up (as we say in Scotland) the contacts were a little chalky but I've since sprayed them. I don't think it's a switching thing unless the switch is inherently within the motor casement (I've completely removed the motor from the mixer).

I'll go through the various points raised and see what the end result is. Might even be worth buying a replacement capacity (for all the cost I expect them to be) just to dismiss it off the bat. I have no problem buying a replacement motor barring the obvious problem in making sure it is like for like.

Anyway, I'll check all this back when I get back from my hollybags.

Steve

P.s.

"... whether the motor needs servicing (if it is indeed serviceable) or if it is more likely to be the big capacitor
connected to the motor which may be at fault?", is Yes. It's a but like, "Would you like tea or coffee" and having the answer, yes.... but I'm guessing you wanted coffee

 

[ScottyUK]

Be careful spraying anything into motors: contact cleaner is usually slightly conductive, which isn't a great thing to put on insulators, and anything oily attracts dust like flies to the proverbial.

Did you say that this motor has brushes? Unusual for a capacitor start motor to say the least. A photo or two - including the nameplate - might be helpful. I agree with the general consensus that the capacitor and the switch are the likely suspects.

There is a lot of useful advice on small motors in a book from the Workshop Practice series:

http://www.amazon.com/Electric-Motors-Workshop-Practice-Jim/dp/1854862464.

Much of it is written for the home workshop user and for the price it isn't a bad little book. It has a fair bit of practical information on re-using scrap motors and on fault-finding.

 

[mikekilroy]

dArsonval great answer, well worth one of them there purple stars!

http://www.KilroyWasHere<dot>com

 

[waross]

My experience with electrical faults in single phase motors showed the following order of likelihood:
1- Start switch failure. It could be dirty, or it may have lost a contact button. This is located inside the end bell on the non drive end.
2- The centrifugal mechanism (which operates the start switch) may have failed. It may be worn, broken or stuck. The springs break from time to time.
3- Corroded connections in the starting circuit. Connections to the start switch or connections to the capacitor. Occasionally the connections at the terminal board will fail.
4- Capacitor failure.
5- Start winding failure.
BUT, in this case, if the motor will start consistently with no load, check the capacitor first. It may be failing and limiting the current through the start winding.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Usually if spinning it by hand makes it work, that points to the centrifugal switch. If the capacitor were bad it may exhibit the same initial failure to start, but spinning it by had generally makes no difference.

"Will work for (the memory of) salami"

 

[weemantam]

Thanks to everyone for getting back to me and giving me such helpful advice (most of which was completely over my head). I took a punt on it being the capacitor since it was a little deformed in shape which suggested it has probably been overheating. Page £12.50 on ebay and fitted it last night. Works better than ever now.

Thanks again to everyone

 

[ScottyUK]

Thanks for the feedback - and pleased you got it working.