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C-2.1

Determine the required capacitor size to improve power factor in accordance

with the following expression:

where,

= Required capacitor size in kVARs

= Active power in circuit

?1

= Phase angle before applying power factor correction

?2

= Desired phase angle after power factor correction

C-2.2

Figure C-2 shows the phasor relationship for power factor correction. The

addition of kVARs by a shunt capacitor reduces the supplied kVAR.

kW

θ2

θ1

kVAR2

kVA

2

kVAR1

kV

A

1

EXAMPLE: A three-phase, 460-volt, 50 horsepower (37,300 watts) motor has a

power factor of 0.65. What capacitor rating is needed to improve the power

factor to 0.95?

First, calculate the power required by the motor at full -load conditions. NEC

Table 430.150 (2002 Edition) specifies a typical full -load current of 65 amperes.

The load power is then calculated by:

3 *V * *I * *pf*

1.73 460 65 0.65

=

= 33.7 *kW*

1000

1000

For a power factor of 0.65, cos *?*1 = 0.65, or *?*1 = 49.46, and tan *?*1 = 1.17. The

desired power factor is 0.95, or *?*2 = 18.19, and tan *?*2 = 0.33. The required

capacitor size is given by:

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