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What is a motor capacitor?

Flyback

Well-Known Member
Hi,
What is a "motor capacitor". The ones i speak of are metalised polypropylene and rated for mains useage.
Why/How are they different from other types of film capacitors?
Is it literally a capacitor which is supposed to just sit across line and neutral of the mains?....upstream of a motor? What kind of motor does it sit upstream of?

Wiki tells of it,

But does not allure to its comparison of ESL and ESR when compared to other types of film capacitors.

..and why it has to be for motors?....eg why not for SMPS?..Eg why not at the output of a Boost PFC SMPS?
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Either "Motor start" or "Motor run" operating conditions.

They are basically intended for use to phase shift one of the AC power connections to a single phase motor, so one of the windings provides a directional field, in comparison to the directly powered winding.

Run caps are intended for continuous duty, Start caps for short term / intermittent use.

Some connection examples here; the switch, if used, is built on to the motor shaft.

They are pretty much just heavy duty caps; thicker dielectric to stand spikes and surges, probably heavier plates and connections to stand the current surges as power is switched and reduce heating, and often "self healing", where by design a point breakdown and short through the dielectric will burn itself away rather than cascading though other layers and fully shorting the cap.

The exact ratings are in the data sheets.

They can be used for other applications, and often are.

A capacitor for direct connection across AC power is another category, a "Class X" cap.
 

Flyback

Well-Known Member
..Thanks, so its a pretty rubbish cap with high ESL and ESR?....shouldnt be anywhere near an SMPS?
 

rjenkinsgb

Well-Known Member
Most Helpful Member
so its a pretty rubbish cap with high ESL and ESR?
No??
They have to be very low loss / low dissipation as they are intended to be handling high currents continuously.
No idea in inductance, but that's not relevant with the intended use.

There are different classes, like "snubber" grade caps, for very fast pulse handling.
 

MaxHeadRoom78

Well-Known Member
Most Helpful Member
The motor start (only) capacitors are solid electrolyte bi-polar electrolytic, and as such should only be in circuit for a very few seconds, or failure will result.
The start & run can be left in circuit all the time and should be:- Motor Start Capacitor Rated.
Usually oil filled.
 

Flyback

Well-Known Member
They have to be very low loss / low dissipation as they are intended to be handling high currents continuously.
Thanks, but would you agree, the "MAB MKP 12/400" Motor capacitor is rated for 1.5A of current at 60Hz, which is 400VAC / Xc

...Where Xc = Xc at 12uF and 400VAC.


1.5A doesnt sound like a high current rating for such a big cap?..thats why i suggest these are pretty weak capacitors?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Thanks, but would you agree, the "MAB MKP 12/400" Motor capacitor is rated for 1.5A of current at 60Hz, which is 400VAC / Xc

...Where Xc = Xc at 12uF and 400VAC.


1.5A doesnt sound like a high current rating for such a big cap?..thats why i suggest these are pretty weak capacitors?
Why are you pissing about with motor capacitors? - if you're using a motor that requires one, then use the correct one. Otherwise don't consider them for anything else! - they are specialised items for a specilised job.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
No.
1.5A is the "best possible case" figure, not allowing for inductance, switching spikes - considering it's directly wired to a large inductor - or noise and spikes from other motors on the same supply.

There is no data sheet for that example, and the price is ludicrous.

This is one with data available:
(And at a realistic price for such a part).

That's rated at 20V/uS, which is equivalent to approximately 8KHz at 400V, sine signal.
It's impedance at that is around 1.66 Ohms, so a current of ~240A ???

As I said previously, if you want something for serious high frequency handling, look at snubber rated caps; they make these look weedy.
 

Flyback

Well-Known Member
It's impedance at that is around 1.66 Ohms, so a current of ~240A ???
Thanks, though i thought a 12uF 470VAC cap had a ripple current rating of VAC/Xc

Where Xc = Xc at 12uF and 50Hz?

..ie, about 1.8A?
 

MaxHeadRoom78

Well-Known Member
Most Helpful Member
For motor run versions.
I use specifically rated Motor Rated Capacitors made by CDE and are oil filled paper.
Tip:- Do NOT use motor capacitors made in China.
 

gary350

Well-Known Member
For motor run versions.
I use specifically rated Motor Rated Capacitors made by CDE and are oil filled paper.
Tip:- Do NOT use motor capacitors made in China.

I buy motor capacitors at the local industrial supply store. I have never seen a motor capacitor that said, made in china. We have a big industrial park south of town with about 200 factories. We have several specialized business just for factories. I have been retired 20 years maybe someone could now be selling china motor caps that I don't know about.
 

MaxHeadRoom78

Well-Known Member
Most Helpful Member
If you look at many posts on the subject of those that have motor problems on M/C's imported from China, it usually comes down to cap failure.
I knew of at least one importer of Chinese machines, the first thing he did was to replace the motors before selling the M/C. !!
 

gary350

Well-Known Member
If you look at many posts on the subject of those that have motor problems on M/C's imported from China, it usually comes down to cap failure.
I knew of at least one importer of Chinese machines, the first thing he did was to replace the motors before selling the M/C. !!

Most people don't know bad caps are the reason cap start motors won't run. Replace bad cap with same value cap or a tiny bit larger cap motors run good. Most often caps are good for 10 to 20 years.
 

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