what is the Relation of Motor Starting capacitor with Motor speed?More 2 questions in Desc

Question

Pre-Knowledge: I know we use capacitors in Single phase induction motor because with one phase the the net torque is zero and Rotor won't rotate so we add another winding perpendicular(with capacitor) to that so that net torque is non-zero.

I have celling fan after some years celling fan max speed is not what it used to be so electrician change its capcitor and replace it with the same value new one and fan is back on its speed.

My question is that:

• By time what made fan to lose max speed(how is it related to capacitor)?
• What is the relation of capacitor value with the speed of motor?
• What are the capacitance limits for a certain winding on motor how much higher and lower value capcitance i can install.

Sorry for long question but i had to state things straight forward that are messing up my mind.

Answer 1

My answer to this other question provides a diagram and explanation of the why and how a capacitor in series with one winding affects the speed of a single-phase motor.

Increasing the capacitor value causes the current to increase in the winding that it is connected to. The maximum value of the capacitor is thus determined by the heating effects of increasing the current. The amount of speed increase is limited by the motor's synchronous speed, the speed of the magnetic field rotating in the motor. The synchronous speed is determined only by the number of poles created by the windings and the frequency of the applied power.

Selecting the capacitor value is part of designing the motor. There is no easy way to determine an appropriate range of values.

Answer 2

The speed is essentially related to the design of the motor (how many poles) and the frequency of the mains power. However if the phase-shift capacitor dries out and loses capacitance, then the "offset pole" created by the capacitor gets weaker and it is easy to see how that could weaken the motor so that the load will make it run slower. The tolerance of the capacitance depends on several factors. The primary one in your example would probably be the load on the motor.