What is a motor capacitor?

[cupoftea]

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,

Motor capacitor - Wikipedia

en.wikipedia.org

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?

 

[cupoftea]

sorry because this has already partly, and kindly , been answered here...

https://www.edaboard.com/threads/ripple-current-rating-of-film-capacitor.401043/

...but i am just wondering whether motor capacitors are so incredibly tough that they can just survive really high ripple current anyway?

After all, motor capacitors are known for fantastic transient overvoltage withstand.

 

[betwixt]

They are for phase shifting the waveform to the second coil of a single phase motor. Mains goes to the main coil and a live spur through the capacitor goes to the second coil. The phase shift provides the rotating magnetic field to keep it turning.

In my experience, the main failure of water pumping systems is the capacitor dropping in value so the motor struggles to generate enough initial torque. I run several pumped systems and after a few years of being constantly turned on and off, they start to pop fuses and a check of line current shows it to be much higher than it should be. A new capacitor always fixes the fault.

Brian.

 

[Easy peasy]

they are also fitted for PFC, these are called motor run caps, motor start caps are short term rated and are switched out after a few seconds, motor run are self healing and designed to handle the rigours of the 230/400 mains supply - but not rated for 4x current ...

 

[ljcox]

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,

Motor capacitor - Wikipedia

en.wikipedia.org

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?

I can't answer your first question. But the purpose of the capacitor is to introduce a phase angle to give the effect of a rotating magnetic field in order to start the motor.