Capacitors on motors: why big C for starting, small for running?

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Discussion Overview

The discussion revolves around the use of different capacitance values in single-phase capacitor motors, specifically focusing on why larger capacitors are utilized for starting and smaller ones for running. Participants explore the relationship between electrical phase shift, rotor speed, and the physical configuration of the motor's windings.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant notes that large capacitance is used for starting due to the need for significant phase shift, while smaller capacitance is sufficient for running.
  • Another participant mentions that starting windings can produce much higher torque, which necessitates larger capacitors.
  • A participant questions whether more phase shift is required for starting than for running, or if the capacitance needed varies with rotor RPM.
  • Discussion includes a reference to a specific motor type (cap start/cap run split phase motor) and questions about the phase shift between start and run coils, suggesting a possible 90° displacement.
  • Participants discuss the implications of impedance at different capacitance values and how it relates to the performance of start versus run capacitors.

Areas of Agreement / Disagreement

Participants express differing views on the specifics of phase shift and the configuration of windings in motors, indicating that multiple competing perspectives exist without a clear consensus.

Contextual Notes

There are unresolved questions regarding the exact phase shift required for starting versus running and how rotor speed influences capacitance needs. The discussion also highlights potential variations in motor designs that may affect these parameters.

Jack of some trades
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Long story short, on single-phase capacitor motors, large values of capacitance are used for starting and much smaller values are used for running. Yet it would seem that the amount of electrical phase shift needed is dictated and fixed by the physical relationship between the main and auxiliary windings around the stator core. (E.g. on a small four-pole fan motor, the auxiliary winding is 45 degrees off from the main one.) Does the rotation of the rotor change the value of capacitance needed for a given degree of electrical phase shift, or is something else going on here?
 
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Usually the start windings are MUCH heavier.. it's not uncommon for capacitor start motors to have 400% or more starting torque, hence the bigger capacitors.
 
Thanks. However, that's not quite the direction I was going. To rephrase the question: Does it require more phase shift to start a motor than to keep it running (on a capacitor-run motor), or does the amount of capacitance required for a given amount of phase shift vary with the RPM of the rotor? If this isn't clear, please let me know. There are motors that use a large capacitor for starting, then once they're up to speed, a centrifugal switch cuts out the big capacitor but leaves the same auxiliary winding connected through a smaller capacitor. Back in the 1960s, when my parents owned a laundromat, we had a Frigidaire washer (remember the up-and-down agitator?) that used such a motor.
 
What you've described is a cap start/cap run split phase motor, and a 'Cadillac' among single phase motors. Are you sure about the 45° phase shift? Wouldn't the start versus run coils be displaced by 90°?

Several things factor into capacitor rating differences. A major reason is the start winding performs two different duties. It must conduct considerable current to generate high torque during start-up, but not nearly so much during normal run, or the winding would burn up.

XL=1/(2*pi*f*C).

At 60 Hz. line frequency, a 20 uF cap (typical size for a run cap) has an impedance of about 133Ω, while it is close to 33Ω for an 80 uF 'start' capacitor.
 
On a two-pole motor the start coils would be at 90°, but on a four-pole, opposite poles of the run coils appear every 90°. If I understand correctly, the start coils have to be in between the run coils, or at least that's how it appears on small fan motors.
 

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