Electric

sum loads on each phase. These loads may be composites of single-phase services and three-phase motors being fed by the distribution system. N is the neutral or return.

To determine the power and power factor of any phase A, B, or C, consider that phase as if it were a single-phase system. Measure the real power, kW, delivered by the phase by use of a wattmeter and measure and compute the volt-ampere product, apparent power, kVA, using a voltmeter and ammeter.

The power factor of the phase can then be determined and corrected as needed. Each phase can be treated independently in turn. The only caution to note is to make the measurements during nominal load periods, this will allow power-factor correction for the most common loading.

If heavy motors are subject to intermittent duty, additional power and power-factor information can be gathered while they are operating. Capacitors used to correct power factor for these intermittent loads should be connected to relays so that they are across the motors and on phase only when the motor is on; otherwise, overcorrection can occur.

Table III.2 How to Select Capacitor Ratings for Induction Motors/Source: 2.

Table III.2 How to Select Capacitor Ratings for Induction Motors/Source: 2.

In the special case of a four-wire wye-connected system with balanced loading, two wattmeters may be used to monitor the power consumed on the service and also allow computation of the power factor from the two wattmeter readings.

III.6.1.1 Balanced Four-Wire Wye-Connected System

Figure III.20 shows a balanced system containing two wattmeters. The sum of these two wattmeter readings are the total real power being used by the service:

Further, the angle of displacement between each line current and voltage can be computed from ?! and P2:

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