Single Phase Motor Problems

[Happy Harry]

I have a 240v single phase motor which seems to have seriously lost power. It is a budget motor fitted to a cement mixer so it is probably 4 pole about 375watt.

I have dismantled it and can see no obvious signs of damage or overheating so I made a few measurements with my multimeter and put them in a little sketch. The first problem is that I cannot find a way of downloading the sketch (jpeg file) onto this page so I can only describe what I have done and ask if you can convert my description into a picture for yourself so that you can understand what I am talking about.

The picture is an illustration of the arrangement in the terminal box and is as follows:

Draw 4 terminals at the corners of a square and since there is no lettering against these terminals label them A and B on the top left and top right and C and D on the bottom left and bottom right.

Terminals A and C are connected to the windings at the left hand side of the motor and terminals B and D are connected at the right hand side of the motor to the windings.

There is a 12uF capacitor between terminals A and C.

There is a link between terminals B and D.

The mains supply is connected to terminals A and D.

The capacitor was replaced some years ago and the mixer has been seldom used since. I cannot remember what prompted me to use a 12uF capacitor so I hope that I chose an appropriate value for this size of motor. Anyway, I'm certain that the motor was running at full power after changing the capacitor.

Now for the measurements:

With the capacitor disconnected but the link still connected.

A - B 9.8 ohms

A - C 31.6 ohms

B - C 21.9 ohms

With both capacitor and link disconnected:

A - D 9.8 ohms

B - C 22.0 ohms, also

A - B 66 Megohms

A - C 70 Megohms

C - D 68 Megohms

B - D 70 Megohms

All measurements were made with a digital multimeter and insulation resistance of windings to case was outside the top 200 Megohm range. Sorry , no megger available.

Has anyone got any ideas what is at fault to cause this loss of power and is it repairable? Apart from the expense for a seldom used piece of equipment it would be nice if instead of purchasing a new motor I could keep the old one and avoid having to fabricate a different mounting and belt adjustment arrangement.

 

[ProDave]

I can't see anything wrong with the resistance readings.  I would start by replacing the capacitor again.

 

[Happy Harry]

Thanks Dave,

The capacitor has a plastic case and measures up as 11.85 uF on my multimeter. Do you think it would be worth trying a different value?

 

[Technician]

Looking at this as a ladder diagram you would

have the capacitor and the start winding in one

rung in series and the run winding in the other rung.

Disconnecting each winding in turn, testing for

continuity should give open circuit on the start

winding and there should be a read on the run

winding, because the capacitor would be an open

circuit to your test current.

It LOOKS as if the start winding is between A & C

and the run winding is between B & D.

If as Pro Dave suggests the capacitor has failed,

there will not be the required phase shift of current

in the start winding and this may explain the loss

of power.

 

[ProDave]

What was the original one's value?  What's the voltaeg rating of the capacitor?

Are you sure the motor shaft spins freely, and the mixers drum and gearbox are free?

 

[Doc Hudson]

Thanks Dave,

The capacitor has a plastic case and measures up as 11.85 uF on my multimeter. Do you think it would be worth trying a different value?

Welcome to the forum Harry, on your previous post I think you accidentally clicked the strike through button,

which leaves a line through text,  so I have removed it for you.



Doc H.

 

[Happy Harry]

That's not adding up for me Technician, A - C measures 86 Megohm (70 Meg yeterday) and with capacitor connected it's down to 1 meg. If you reckon that the start winding is between A and C then it will be in parallel with the capacitor. Also if the run winding is between B and D then this would be shorted out since there is a link between these terminals.

 

[Technician]

I can understand what you are saying, but that it how

it LOOKED and yes, from your instructions I sketched

out the terminals, and yes, from that it looked as if the

capacitor WAS in parallel with the start winding and that

B to D was shorted out by the link.

I re-drew the circuit as a ladder diagram but the whole

purpose of the capacitor is to provide a phase shift for

the current in the start winding so that the rotor has the

"sense" of a rotating magnetic field.

 

[OnOff]

There'll be grease nipple on the drum bearing................if it's been laid up for a while have you tried that?  :lol:  Is the belt slipping? they're notorious for eating plastic pullies!

 

[Happy Harry]

Don't know what the original capacitor size was ProDave. It was a long time ago that I replaced it and my long term memory is now almost as bad as my short term memory. It is 440 volt rating, I think it may have been one of Maplin's motor capacitors. The bearings are ball races and are free and smooth but could do with re-packing with grease. The mixer drum turns freely and is driven by a pinion which in turn is belt driven via pulleys to the motor. It all works freely without undue resistance.

Thanks for the help Doc H, I'll work out how to do things properly one day.

 

[Happy Harry]

I can understand what you are saying, but that it how

it LOOKED and yes, from your instructions I sketched

out the terminals, and yes, from that it looked as if the

capacitor WAS in parallel with the start winding and that

B to D was shorted out by the link.

I re-drew the circuit as a ladder diagram but the whole

purpose of the capacitor is to provide a phase shift for

the current in the start winding so that the rotor has the

"sense" of a rotating magnetic field.

Tried drawing this as a ladder diagram Technician but I still can't get my head round it. Is not a phase shift also produced by a capacitor in parallel?

 

[NozSpark]

Does it run if you "start it off" (with the capacitor removed)???

 

[OnOff]

What's the make and model of the mixer?

 

[Happy Harry]

Still don't think I am managing to upload  my sketch yet Canoeboy. All I'm seeing is a square box with a cross in it.

Could be a pricey job getting hold of a selection of capacitors and trying each one.

I will reassemble and refit the motor NozSpark and see what happens if I try it without the capacitor. Presumably I will have to give the drum a little push by hand to get it started.

I seem to remember buying the mixer from an advert in Exchange and Mart Onoff. I have just looked on Machine Marts website and they do a mixer that is the same size and looks identical http://www.machinemart.co.uk/shop/product/details/ccm125c-concrete-mixer

They have a product manual that you can download and that gives a part number for the motor but unfortunately no information about the capacitor. Perhaps I can give them a ring in the morning and see if I can find out anything.

Having tried to post this Canoeboy it won't let me upload my sketch. Perhaps I'll work out how to do it one day.

 

[Happy Harry]

Hi Fella's,

Thanks Canoeboy I'll try and upload my original sketch now.



I had a word with Machine Mart this morning and they want £120 for a new motor, which is pretty well double what you would pay elsewhere for one of similar specification. They don't supply capacitors seperately as they are now integral with the motor and if the capacitof fails then you have to buy a new motor complete.

Thought I would try running with the capacitor removed as you suggested NozSpark. The replacement capacitor had spade terminals so I had protected these with self amalgamating tape and rather than fiddle about in the terminal box I decided to remove the tape from the spades. What I found was one spade terminal with a melted plastic insulating hood. It looks as though I had inserted the blade in the gap between the hood and the back side of the female spade and the molten plastic had seeped between the two. Well that was problem number one and once disconnected properly the motor would run quite happily in either direction depending on which way you gave it a shove to start.

Having got in the mood for cleaning muck off terminals I decided to start on the extension lead that I use only for this mixer as it has an in line plug and socket at the motor end and a 13amp plug at the other end. The neutral wire in the 13amp plug was loose as the screw was not tightened and the fuse was rather black and so loose in its clips that it would almost drop out if you turned it upside down.

Well that was three problems alltogether, all of which probably contributed to the lack of power. With those issues rectified the drum now turns with sufficient power that I cannot slow it down by grabbing hold of it. I ran it under no load for about 1/2 hour and it did not get unduly hot but the proof of the pudding will be when I load it up with some cement and ballast.

Anyway, thank you all for your input and quick replies.