Re: WHAT DOES A CAPACITOR DO IN A CAPACITOR START MOTOR TO MAKE IT START

An electric motor is basically composed of windings around a magnet.  
Motors are either multi-phase or single-phase.  Multi-phase motors generate 
starting torque along the various windings by applying out of phase 
voltages to each winding in a pattern that generates a torque force in the 
desired direction.  Single-phase motors must generate the same starting 
torque however they have only one phase to work from.  This means they have 
to have a method to generate a shifted version of the single phase voltage 
to send to one of their windings.

There are three common methods of creating single-phase electric motors:  
capacitor start, split-phase, and shaded pole.  Each other these motors has 
some method to provide starting torque to the motor by shifting the voltage 
given to one of the windings on the motor by some angle.  This phase shift 
corresponds to one winding of the motor having a voltage before another 
coil.  The difference in time between when one coil has a voltage and 
when a second coil has a voltage causes the torque force and begins the 
movement of the motor.

To start to solve why capacative start motors work we can generalize Ohm's 
Law, V = IR, and say that V = IZ where Z is a generalized impedance.  The 
impedance is composed of an the inductance, capacitance, and resistance.  
Inductance will cause the current to lead the voltage, capacitance will 
cause the current to lag the voltage, and resistance has no effect on the 
timing between the current and voltage.  

In all motors the windings of a motor are highly inductive so the voltage 
always arrives after the current.  The capacitor changes the relative 
impedance of the circuit on one winding causing the shift in the 
relationship between the voltage and current on one winding and the 
voltage on current on another winding.  This difference in the time the 
electrical energy is dispersed in a winding allows the motor to rotate.  

In summary, the capacitor provides a delay in the energy given to 
one of the windings.  This delay causes the forces of the motor to 
be unbalanced and the motor then starts.  

Economically, capacitor start motors are often more costly due to the 
inclusion of the capacitor however they have the most starting torque This 
means that you probably have one in your refridgerator, washer, dryer, or 
other application where you may need a lot of starting force but you won't 
find them in your electric fan.