Page 173 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Single phase wattmeter connection
WA power measuring meter called wattmeter.Wattmeter mainly consist of two coils ,they are current coil and
pressure coilIn wattmeter current coil connected in series with the supply and pressure coil connected in parallel
to the supply line. The wattmeter takes the power factor of the circuit in to account and always indicates true power
Three phase system
In a three phase system three electrically insulated windings are wound on the armature and a phase difference
of 120 degree electrical is kept between them.

The emf generated has same max value, frequency and wave length. The induced emf has a phase difference
of 120 deg electrical.

Instantaneous values of phase voltages are,
E =EmSinwt
R
E = EmSin(wt-120)
Y
E = EmSin(wt-240)
B
Sum at any instant ER+EY+EB = 0
A three phase system can either supply three single phase circuits or a single three phase circuit. The current in
different phases depends upon the nature of load and the phase angle between voltage and current.
Balanced load
If the load is such that the same current flows through each of the three phases and the angle of phase difference
between each current and voltage is also same, this load is said to be a balanced load.
Unbalanced load
If the magnitude or phase angle of the currents is different, the load is unbalanced load.
Phase sequence
The sequence of attaining the maximum value of the induced emf is called phase sequence, RYB, ABC, 123.
Definitions of fundamental terms

• Line voltage (V ).
L

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