Power Factor

Power factor is normally the phase difference between the voltage and current. Power factor is due to the inductive nature of the circuit, however, in resistor the voltage and current are in phase with each other. So, the power factor of the resistive load is unity and reactive power is zero. All the power is active power in resistor. Power Factor is the cosine of active power and he reactive power and maximum value of cosine is one. Therefore the power factor of a load can never be more than one.

• Power factor is said to be lagging when the current lags the voltage by some angle, due to the inductive nature of the load. 
• Power factor said to be leading if the current leads the voltage by some angle, due to the capacitive nature of the load.

Active Power:

Reactive Power:

Reactive power is the component of the total power which is travel along the active power but not used by the load. It  just travel back and froth in the load. It is used to establish the magnetic field in the motor or other alternating current equipment.

Apparent Power:

Synchronous Motor Vs Induction Motor

Speed Vs Load:

In Induction motor and DC motor, the speed decreases when we increases the load. The decrease in the speed reduces the counter EMF, the additional current is drawn from the motor. If we add more load on the motor, the motor will draw more current which increase the iron losses and more heat will be dissipated, that can cause failure of insulation, faults can occur.

In Synchronous motor, the speed doesn't decrease with the increase in the load, it runs at synchronous speed at all loads. It meets the increased load by the relative shift btw the rotor and stator poles i,e by the adjustment of the torque angle. If the motor load increases , the torque angle increase which causes a greater phase shift of back EMF w.r.t supply voltage. This increases the voltage in stator winding and consequently, armature current will be drawn .

Disconnection of one phase:

If one phase of the three phase motor is disconnected due to some faults, more current will be drawn from the remaining two phase which can cause many failure. It also increases the iron losses. If it continue to draw more current , there can be motor failure or motor burnout.

Power Factor:

Synchronous motor can operate from lagging to leading power factor while the induction operates at the lagging power factor. A synchronous motor can generate and absorb reactive power by adjusting the excitation of the motor. Overexcited synchronous motor under no load condition can be used for power factor corrective device and known as synchronous condenser.

Supply :

Synchronous motor are doubly fed motors: Rotor require dc excitation and Stator require 3 phase ac supply. Induction motor only require 3 phase AC supply.

Self-starting: