Page 298 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

• This instrument has better accuracy.
• When used as wattmeter, the scale is uniform.
Fig 1

Disadvantages
• It is more expensive than PMMC and moving iron instruments.
• When used as voltmeter or ammeter the scale will not be uniform.
• It has a low torque/weight ratio-as such has low sensitivity.
• Sensitive for over loads and mechanical impact. Hence careful handling is necessary.

• It consumes more power than PMMC meters.
Induction type single phase wattmeter: This type of wattmeters could be used only in AC circuits whereas a
dynamometer type wattmeter could be used in both AC and DC circuits.
Induction type wattmeters are useful only when the supply voltage and frequency are almost constant.

Construction: Induction wattmeters having two different types of magnetic cores (Figs 2a and 2b).

Fig 2

Both the types have one pressure coil magnet and one current coil magnet. The pressure coil carries a current
proportional to the voltage whereas the current coil carries the load current.
A thin aluminium disc is mounted on a spindle in between the space of the magnets and its movement is controlled
by springs. The spindle carries a weightless pointer at one end.

Working: The alternating magnetic fluxes produced by the pressure and current coils cut the aluminium disc and
produce eddy currents in the disc. Due to the interaction between the fluxes and the eddy currents a deflecting
torque is produced in the disc and the disc tries to move. Control springs attached to the two ends of the spindle
control the deflection and the pointer shows the power in watts on a graduated scale.
Shaded rings provided in the pressure coil (shunt) magnet could be adjusted in order to cause the resultant flux
in the magnet to lag in phase by exactly 90o behind the applied voltage.
Method of connecting wattmeter in single phase circuits - pressure coil connection to reduce erroneous
measurement.
There are two ways of connecting the pressure coil of the wattmeter (Fig 3).
Both the methods shown in Figs 3a & b need correction in power measurement due to the reasons stated below.

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