Parallelling capacitors

[T.M.Haja Sahib]

It is KVAR rather than capacitance

It is KVAR rather than capacitance

It is KVAR rather than capacitance that is important for power factor improvement for start and run circuits in a single phase motor. So I think if Phil used KVAR in places where he used capacitance in his posts here, he would be correct.

 

[mivey]

Phil knows what he is doing. He was just having one of those moments the other day.

 

[T.M.Haja Sahib]

Voltage variable capacitance

Voltage variable capacitance

The capacitance of a capacitor does not depend on the voltage in general. But take a capacitor with various voltage rating in series with an inductance just like in start circuit of a single phase motor. The circuit can behave as a variable capacitance or a variable inductance or a pure resistance depending on the voltage applied. This is what Phil meant, IMO.

 

[Phil Corso]

Ice, Mivey...

Trying to upload 23kB Doc. Geting maeeage "invalid file" Any advice you can provide?

Phil

 

[T.M.Haja Sahib]

Ice, Mivey...

Trying to upload 23kB Doc.

May not be required. I convinced them about your being right on behalf of you.

 

[kwired]

It is KVAR rather than capacitance that is important for power factor improvement for start and run circuits in a single phase motor. So I think if Phil used KVAR in places where he used capacitance in his posts here, he would be correct.

I will not argue on the fact that KVAR is what is important - typically don't hear that term except when talking power factor correction. But power factor correction is not the primary reason for using capacitors in a single phase motor, they are there to help create a 2nd phase. Single phase motors are called single phase because that is what the input power is, but they operate as two phase motors with the 2nd phase being created within the motor and/or its accessories.

 

[mivey]

Ice, Mivey...

Trying to upload 23kB Doc. Geting maeeage "invalid file" Any advice you can provide?

Phil

Try a different file type, like jpg.

 

[solarcell]

Think

Think

Safety first always and what costs more to replace.

 

[T.M.Haja Sahib]

I will not argue on the fact that KVAR is what is important - typically don't hear that term except when talking power factor correction. But power factor correction is not the primary reason for using capacitors in a single phase motor, they are there to help create a 2nd phase. Single phase motors are called single phase because that is what the input power is, but they operate as two phase motors with the 2nd phase being created within the motor and/or its accessories.

I used the term power factor improvement rather than power factor correction. Even if no capacitor is put in the start circuit, the second phase is already created due to difference of the inductance of the start circuit with the run circuit in a single phase motor. But to improve its performance, a suitable capacitor is inserted to improve the phase difference between the two phases and as a result, the power factor of the start circuit.

 

[kwired]

I used the term power factor improvement rather than power factor correction. Even if no capacitor is put in the start circuit, the second phase is already created due to difference of the inductance of the start circuit with the run circuit in a single phase motor. But to improve its performance, a suitable capacitor is inserted to improve the phase difference between the two phases and as a result, the power factor of the start circuit.

Starting only lasts for seconds at the most per start, I'm sure the capacitors are there for reasons other than power factor, like maybe to improve the amount of phase difference between main and aux winding to help produce more torque, if it improves power factor so what, it is getting switched out of the circuit as soon as rotor reaches a certain speed within very short time period. Now a run capacitor will stay in the circuit - but any power factor improvement is likely a by product of why the capacitor is there in the first place. The capacitor is needed for other reasons - otherwise it would be sold separately if for power factor only - just like three phase motors don't have included power factor correction/enhancement, that is a whole different product line and one can make more profit selling it separately - not to mention you can fine tune it to the load by installing it separately.

 

[gar]

120713-1428 EDT

To add to what kwired said.

The use of capacitors in single phase motors is not for power factor correction, but to achieve certain intended torque characteristics.

First, a quote from Alternating-Current Machinery by Bailey and Gault, Chapter 17, on Capacitor-start Induction-run Type.
" ---- In fact, by a proper choice of capacitance, the currents Ia and Ib may be made to be nearly or exactly 90 deg apart. The starting torque is therefore much greater than in the split-phase motor, since the angle a is closer to 90 deg. Also, the line current I is very much smaller since the two currents add vectorially at an angle of 90 or more. As a mater of fact, a 2-phase motor operating in this way on a single-phase line is capable of developing about 25 per cent more torque than it will on a 2-phase line and will take about 25 per cent less volt-amperes from the line. Moreover, the power factor at the instant of start is very nearly 100 per cent. ----".

Further on in the chapter is a discussion on the motor performance with a running capacitor. Here efficiency and power factor are mentioned in a test at full load with varying capacitance. Maximum efficiency occurred at a lower capacitance than maximum power factor.

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