Capacitors in motors

[dlt27]

Hi All,
Had a farmer I’ve done some install work for call yesterday saying overload was tripping on his vac pump and could I get some new capacitors ( thought it would be simple enough as changing like for like). When I got there to have a look he said the guy who looks after parlour ( controls etc) had changed the capacitors for 3 new ones, but said they were not the same size and I should get some the same as the originals.
I know the basics of how motor start, motor run capacitors work, but no more than that.
The ones that were taken out ( see pictures) were 2 x 35 micro farad capacitors ( presume for motor run) and 1 x 408 micro farad capacitor with a 15 kilo ohm resistor across it ( motor start).
What I was hoping for was that somebody better educated than myself could tell me;
1 How does bad capacitor cause overload to trip out?
2 Why is there a resistor across the start capacitor?
3 Because I can’t get exact match as long as new capacitors are correct size and voltage can I use any make?
4 Is the start capacitor oil filled as he said terminal box had oil in it?
5 What affect does size of capacitor have on the motor ie changing a 30 micro farad to a 35 micro farad for example?
6 Is there anyway to determine what size capacitors should be used for a motor if you can’t find the info on existing ones?
7 How do you know a bad capacitor, as when I checked the 2 x 35 micro farad capacitors they were reading 29 micro farad so presume ok, but 408 micro farad was reading 0 micro farad.
Sorry so many questions, I haven’t had a chance yet to see the motor as he was milking at the time and just gave me the capacitors.
Thanks again in advance for any information..

 

[Lucien Nunes]

1. If the capacitor is shorted, open or badly out of tolerance, the motor won't start, in which case the run winding will be in overload, and / or the current through the start winding or leading winding will be excessive and the total will be in overcurrent.

2. To discharge it. Otherwise, if the starting switch opens at a moment in the mains cycle when the capacitor is charged, that charge could remain for a long time if it is in good condition.

3. Correct capacitance, AC voltage and for the right duty. E.g. an electrolytic start capacitor can't be used as a run capacitor even if its capacitance and voltage are correct, because it is not continuously rated. Any make, yes.

4. Not oil, although they do contain a liquid because they are usually electrolytic. Failed capacitors sometimes spit out the electrolyte. Paper capacitors used to contain oil.

5. Wrong capacitance can result in reduced torque / decreased power factor / decreased efficiency. Seriously wrong capacitance can cause overheating / overload / failure to start.

6. Yes, but it tends to be approximate and requires significant knowledge of the motor to get right. Enquiry of the manufacturers is probably a better route.

7. Really bad capacitors are usually open-circuit, low in insulation or short-circuit, so it's equally important to test with an ohmmeter as a capacitance meter. Sometimes it is necessary to apply working voltage before they break down, so an insulation test is often needed too. Knowledge of capacitor construction and pathologies is needed to interpret the readings in some cases, and to avoid damage when subjecting them to stress tests.

If I get time later I'll explain why the two caps in your pictures are as different as chalk and cheese and why it's the start cap that has failed.

 

[dlt27]

Hi Lucien, really appreciate your time to explain.
One final question, how come I haven’t seen a resistor connected to one before? Is it because it’s a higher capacitance or 408 micro farad compared to 35 micro farad. Do I need to do the same to the new one I buy.
Many thanks again.

 

[Lucien Nunes]

Some capacitors have the bleeder resistor contained within the can, in which case its presence will usually be shown in the data. A run capacitor on a PSC or CSCR motor theoretically doesn't need one, because the capacitor is always shunted by the windings and discharges into them as soon as the supply is disconnected. Otherwise, yes, the larger the cap the more important it is to have a bleeder. A resistance test will tend to reveal its value once the capacitor has charged to the meter's test voltage and the reading settles.

 

[dlt27]

Do you know where I can get the replacement capacitors to match the existing ones? Thanks again
[automerge]1580672751[/automerge]
Hi Lucien, when you get time could you explain why they are so different and why the start capacitor has failed. Many Thanks

 

[Lucien Nunes]

There are many different technologies used in making capacitors for all kinds of duties, each with different pros and cons. The type usually used for run capacitors consists of long thin strips of plastic film, such as polypropylene, forming the dielectric sandwiched between two conductive foils forming the plates. The strips are rolled up and sealed into the can with one plate connected to each terminal. Different plastics and metallised papers are used in the construction for different performance benefits, but what most film/foil caps have in common is the ability to withstand a significant continuous AC voltage and current without excessive heating, over a long lifespan.

The much higher capacitance required for the start capacitor of a reasonable size motor would require an inconveniently large and expensive film/foil capacitor. An alternative technology, the aluminium electrolytic, packs much higher capacitance into the same size can at moderate cost, but with some significant disadvantages. In an electrolytic, the dielectric is a very thin coating of aluminium oxide on the surface of the foil (only on one foil for a DC capacitor, both for an AC-rated 'non-polarised' one). These layers are permanently in contact with a liquid electrolyte held in a porous separator, that both provides conductivity and electrolytically maintains / repairs the fragile dielectric coating. Many characteristics of electrolytics are inferior to other types, with high leakage, wide tolerance, short life, poor loss tangent etc. They are prone to heating with high currents so in motor start duty tend to have short maximum operation times. Anything that disrupts the dielectric coating can cause a rapid meltdown, whether it is too long a period of operation, overvoltage, too long a period standing disused on the shelf, etc.

Film/foil capacitors do fail, both open and short-circuit, and different types tend to do different things. Your run cap is P0 rated so has no particular protective mechanism and could do either. But electrolytics fail much more often because of their inherent frailties, and when they fail they can be a lot more spectacular, if the electrolyte boils up and the safety pressure vent releases before the thing goes completely S/C.

 

[David Prosser]

you can get both those capacitors direct from WEG or as it for a vacuum pump on a dairy farm I would try dairyspares.co.uk

 

[dlt27]

Many thanks again for sparing your time to explain.. greatly appreciated..