Page 189 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Effect of load pf on wattmeter reading

Power factor from two wattmeter reading
tan¢=√3 (W1-W2)
(W1+W2)
¢=tan~√3 (W1-W2)

Phase-sequence indicator (Meter)

Objectives: At the end of this lesson you shall be able to
• describe the method of finding the phase sequence of a 3-phase supply using a phase-sequence indicator
• explain the methods of finding phase sequence using lamps.

Phase sequence
A three-phase alternator contains three sets of coils positioned 120° apart and its output is a three-phase voltage
as shown in Fig 1. A three-phase voltage consists of three voltage waves, 120 electrical degrees apart.
At a time 0, phase U is passing through zero volts with positively increasing voltage. (Fig 1) V follows with its zero
crossing 1/3 of the period later and the same applies to W with respect to V. The order in which the three-phases
attain their maximum or minimum values is called the phase sequence. In the illustration given here the phase
sequence is U,V,W.
Importance of correct phase sequence: Correct phase sequence is important in the construction and connection
of various three-phase systems. For example, correct phase sequence is important when the outputs of three-
phase alternators must be paralleled into a common voltage system. The phase `U’ of one alternator must be
connected to phase `U’ of another alternator. The phase `V’ to phase `V’ and phase `W’ to phase `W’ must be
similarly connected to each other.
In the case of an induction motor, reversal of the sequence results in the reversal of the direction of motor rotation
which will drive the machinery the wrong way.
Phase-sequence indicator(meter): A phase-sequence indicator (meter) provides a means of ensuring the
correct phase-sequence of a three-phase system. The phase-sequence indicator consists of 3 terminals `UVW’
to which three-phases of the supply are connected. When the supply is fed to the indicator a disc in the indicator
moves either in the clockwise direction or in the anticlockwise direction. The direction of the disc movement is
marked with an arrowhead on the indicator. Below the arrowhead the correct sequence is marked. (Fig 2)

Fig 1 Fig 2

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CITS : Power - Electrician & Wireman - Lesson 26-29