Electrodynamometer

Electrodynamometer

1. Electro Dynamometer type Wattmeter

2. Power Measurement

3. Dynamometer type Wattmeter Principle: When a current carrying moving coil is placed in a magnetic field produced by the current carrying fixed coil, a mechanical force is exerted on the coil sides of the moving coil and deflection takes place.

4. Same as PMMC except permanent magnet is replaced by fixed coils. Same current both in fixed coil and moving coil. Torque exerted on moving coil α meter current squared. deflection α average torque α average of the current squared. Meter scale easily calibrated to read rms value.

5. Torque eqn

6. D.C. operation : For D.C current of I 1  and I 2 ,

7. For A.C operation

8. Advantages •Can be used on both DC & AC circuits . •Uniform scale . •High degree of accuracy can be obtained . Disadvantages • At low power factors, the inductances of the potential coil causes serious errors . • The reading of the instrument may be affected by stray fields acting on the moving coil.

9. Induction Type Energy Meter

10. Induction Type Energy Meter Principle When AC current flows through two  suitably located coils ( current coil & potential coil ) , they produce rotating magnetic field which is cut by the metallic disc suspended near to the coils, thus an e.m.f . is induced in the disc which circulates eddy currents in it. By the interaction of rotating magnetic field & eddy currents, electromagnetic torque is developed which causes the disc to rotate. Used on AC system only.

11. Construction 1 . Driving System i ) Series Magnet ii ) Shunt Magnet 2. Moving System 3 . Breaking System 4 . Recording Mechanism

12. Induction Type Energy Meter

13. Working Principle: When the energy meter is connected in the circuit, the current coil carries the load current and pressure coil carries the current proportional to the supply voltage. The magnetic field produced by the series magnet is in phase with the line current and the magnetic field produced by the shunt magnet is in  quadrature with the applied voltage. Thus , a phase difference exists between the fluxes produced by the two coils . This sets up a rotating field which interacts with the disc and produces a driving torque, thus disc starts rotating . The number of revolutions made by the disc depends upon the energy  passing through the meter. The spindle is geared to the recording mechanism so that electrical energy consumed in the circuit is directly registered in kWh .

14. Driving System Series Magnet: It is placed below the aluminum disc and produces the magnetic field ( Φ sc ), which is proportional to & in phase with line current. Shunt Magnet: It is placed above the aluminum disc . It produces a field ( Φ sh ) proportional to the applied voltage. This field is in phase with the current flowing through the pressure coil, but is in quadrature with the applied voltage.

15. Moving System It consists of a light aluminum disc mounted on a vertical spindle.   The disc is positioned in the air gap between series & shunt magnet.   The spindle is supported by a cup shaped jeweled bearing at the bottom end & has a spring journal bearing at the top end. Since there is no control spring, the disc makes continuous rotation under the action of deflecting torque.

16. Braking System: A permanent magnet positioned near the edge of the aluminum disc, forms the braking system.   When the disc moves in the field of the braking magnet, flux is cut & eddy currents are induced in the disc.   The direction of induced currents is such that it opposes the rotation, thus braking torque is produced.   Since the induced currents are proportional to the speed of disc ( N ), therefore braking torque ( Tb ) is proportional to the disc speed. i.e. Tb α N

17. Recording Mechanism : The function of recording or registering  mechanism is to record continuously a number on the dial which is proportional to the revolutions made by the moving system.   The number of revolutions of the disc is a measure of the electrical energy passing through the meter.