Power factor correction of induction motors

Connection of a capacitor bank and protection settings

Individual motor compensation is recommended where the motor power (kVA) is large with respect to the declared power of the installation

General precautions Because of the small kW consumption, the power factor of a motor is very low at no-load or on light load. The reactive current of the motor remains practically constant at all loads, so that a number of unloaded motors constitute a consumption of reactive power which is generally detrimental to an installation, for reasons explained in preceding sections.
Two good general rules therefore are that unloaded motors should be switched off, and motors should not be oversized (since they will then be lightly loaded).

Connection
The bank of capacitors should be connected directly to the terminals of the motor.

Special motors
It is recommended that special motors (stepping, plugging, inching, reversing motors, etc.) should not be compensated.

Effect on protection settings
After applying compensation to a motor, the current to the motor-capacitor combination will be lower than before, assuming the same motor-driven load conditions. This is because a significant part of the reactive component of the motor current is being supplied from the capacitor, as shown in Figure L23.
Where the overcurrent protection devices of the motor are located upstream of the motor capacitor connection (and this will always be the case for terminal-connected capacitors), the overcurrent relay settings must be reduced in the ratio:
cos ϕ before compensation / cos ϕ after compensation
For motors compensated in accordance with the kvar values indicated in Figure L24 (maximum values recommended for avoidance of self-excitation of standard induction motors, as discussed in sub-clause 7.2), the above-mentioned ratio will have a value similar to that indicated for the corresponding motor speed in Figure L25.

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Fig. L23:Before compensation, the transformer supplies all the reactive power; after compensation, the capacitor supplies a large part of the reactive power

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3-phase motors 230/400 V
Nominal power		kvar to be installed
		Speed of rotation (rpm)
kW	hp	3000	1500	1000	750
22	30	6	8	9	10
30	40	7.5	10	11	12.5
37	50	9	11	12.5	16
45	60	11	13	14	17
55	75	13	17	18	21
75	100	17	22	25	28
90	125	20	25	27	30
110	150	24	29	33	37
132	180	31	36	38	43
160	218	35	41	44	52
200	274	43	47	53	61
250	340	52	57	63	71
280	380	57	63	70	79
355	482	67	76	86	98
400	544	78	82	97	106
450	610	87	93	107	117