The capacitors draw a leading kVAr to compensate for the lagging kVAr drawn by the load. Thus the total kVAr required for the load is reduced.
Calculation of capacitors required
The total value of the capacitors to be connected can be calculated from
KVARneed for correction = kW X (tan φ uncorrected - tan φ target )
Where φ refers to the phase angle of the target and the uncorrected power factor
Excess Power Factor Correction and Self Excitation
The total kVAR to be connected should not exceed the kVAr required to bring the power factor to unity when the motor is running on no-load.
This is to prevent a condition called self excitation which can cause high voltages and excessive torque. Self excitation is a condition that occurs in induction motors which have capacitors connected to them. Consider a motor coupled to a load running with capacitors connected to it. If the supply to this motor is cut off.
The motor will continue to run for some time due to the inertia of the load coupled to it. During this time, the energy stored in the capacitor begins to excite the windings. The inductance in the windings and the capacitors together form a resonant circuit. The oscillations produced in this resonant circuit can produce high voltages which can cause damage.
If the supply is again switched on during this period, the motor can experience sudden movements with high torques.