Page 299 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

In the method of connection shown in Fig 3a, the pressure coil is connected on the ‘supply’ side of the current coil,
and hence, the error in power measurement is due to the fact that the voltage applied to the voltage coil is higher
than that of the load on account of the voltage drop in the current coil. As such the wattmeter measures the load
power in addition to the power lost in the current coil.

On the other hand, in the method of connection shown in Fig 3b, the current coil carries the small current taken
by the voltage coil, in addition to the load current, thereby introducing errors in power measurement. As such the
wattmeter measures the load power in addition to the power lost in the pressure coil.
If the load current is small, the voltage drops in the current coill will be small, so that the method of connection,
shown in Fig 3a, introduces a very small error and, hence, preferable.

On the other hand, if the load current is large the power lost in the pressure coil will be negligible when compared
to the load power in the method of connection shown in
Fig 3b, and, hence, a very small error is introduced resulting in the preference of this connection.

Fig 3

3-Phase Wattmeter

Objectives: At the end of this lesson you shall be able to:
• describe the various types of 3-phase wattmeters, their connections
• state how to connect different types of 3 phase watt meter.

In single-phase wattmeters there will be one set of pressure and current coils driving a single aluminium disc,
whereas in 2-element, three phase wattmeters there will be two sets of pressure and current coils driving a single
aluminium disc (Fig 1a) or driving two aluminium discs mounted on the same shaft (Fig 1b) thereby providing a
torque proportional to the 3-phase power.

Fig 1

On the other hand a 3-element, 3-phase wattmeter will have three sets of pressure and current coils kept at 120°
to each other but driving a single aluminium disc (Fig 2) or alternatively 3 sets of pressure and current coils driving
three discs one over the other but mounted on the same single spindle (Fig 3).

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CITS : Power - Electrician & Wireman - Lesson 50-53