Induction type Instrument:
Induction type instruments are used for a.c. measurements only. They can be used as ammeters, voltmeters, wattmeters i as well as energymeters or watt hour meters.In induction instrument the detlecting tonque is produced due to the interaction of the magnetic tlux of
electromagnet and the eddy currents induced due to this tlux. Fig 49.1 shows the arrangement of an induction type i ammeter. An electromagnet is excited by the a.c. current to be measured.
Half the pole shoe of this magnet is shaded as shown in Fig. 4.9.1. Due to this the fluxes , and produced by this magnet have some phase difference between them. These two tuxes react with eddy currents induced in the disc D and produce necessary detlecting torque.
In case of an induction type wattmeter the arrangement is slighty different, as shown in Fig. 4.9.2. There are two i electromagnets carrying independent windings, one of them is used as a current coil and the other as a pressure coil. A thin aluminium disc is mounted on a spindle such that it cuts the tluxes produced by both the magnets.
Hence two eddy currents are induced in the disc. The deflecting torque is produced by the interaction of the twoi eddy currents.
The controlling torque is normally provided by spiral springs and the damping is provided by a piston moving in an air Chamber. Te scale of this instrument is uniform and extends i over 300
Energy Meter: 4.9.1
Energy is defined as the product of power and time. It is measured in Watt-hour or kilowatt-hour (kWh). The meter used to measure the energy consumption is known i as the energy meter.
Therefore an energymeter must consists of a current coil and a voltage coil (siilar to a wattmeter) and an addiuonal device which will count the tume. i
Induction Type Energymeter :
The induction instrument descnbed above is very commonly used as an energymeter. Construction of the energymeter is similar to that shown in Fig. 4.9.2.
The two exciting coils, as shown, act as current coil and voltage coil and the disc acts as a time counting device. The dise is kept free to rotate continuously. Speed of the disc depends on the power supplied to the load. More the load, higher is the disc speed. i
In this instrument a gear train is provided (not shown in Fg. 4.9.2 for clarity) to count the revolutions of the dise. Number of revolutions of the disc are directly recorded in terms of the energy consumed.
Advantages and disadvantages of Induction Instruments:
The advantanes are as under1. When used as ammeter or voltmeter, its scale extends over 300"
2. The instrument is free from stray field effects.
3. The instruments is also free from frequency errors.
4. Less maintenance is required.
5. Simple and robust construction
6. It is a cheap instrument
The disadvantages are as under:
1. The instrument cannot be used for measuring d.c. quantities.
2. When used as ammeter or voltmeter, its cost is high as compared to moving iron instrument. Therefore in practice it is popularly used as energymeter only.
3. The scales of induction ammeters and voltmeters are nonuniform. They are crowded at both the ends. Therefore it is difficult to record accurate readings in the end portions of the scale.
Outstanding
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