Pivoted Coil, Direct Indicating Electro Dynamometer Type Wattmeter

PIVOTED COIL, DIRECT INDICATING ELECTRO DYNAMOMETER TYPE WATTMETER

Construction and Operation

The constructional details of pivoted coil direct indicated electro dynamometer is shown in Fig. 6.15. These instruments are similar in design and construction to a dynamometer type voltmeter or Ammeter. The two coils namely, fixed coils and moving coils are connected in different ways for the measurement of power.

1. Fixed coil and moving coil

The fixed c coils are connected in series with the load so as to carry the rated load current and is called as “current coil" of the wattmeter. The moving coil is connected across the voltage and therefore, carries a current proportional to the voltage. A high non‒inductive resistor is added

in series with the moving coil to limit the current to a small value and this coil is called as "pressure coil" or "voltage coil" of the wattmeter.

2. Moving system

The moving coil is mounted on a spindle which rests in the jewelled bearing. A pointer and the damping vane are attached to the spindle. Balancing weight is also attached to the moving system and is adjusted such that the centre of gravity lies in the centre of the spindle.

3. Control

The moving system of the dynamometer type wattmeters are usually spring controlled. This makes the instruments to a portable one.

4. Damping

Air friction damping is used. The moving system carries a light aluminium vane which moves in a sector shaped box or a piston which moves in an aluminium cylinder.

5. Shielding

The field produced by these instruments is very weak. Even earth's magnetic field considerably affects the reading. So shielding is done to protect it from stray magnetic fields. It is done by enclosing in a casing of high permeability alloy.

6. Cases

Laboratory standard instruments are usually contained in polished wooden or metal cases which are rigid. The case is supported by adjustable levelling screws. A spirit level may be provided to ensure proper levelling.

7. Scales and pointer

The pointer is made up of aluminium, the pointer is made thin and the front portion is flattened. Behind the scale, a mirror is usually provided to avoid parallax error.

Principle of operation

The pressure coil produces a flux which is proportional to the applied voltage. The flux produced by the pressure coil is very nearly in phase with the applied voltage. The current coil produces a flux which is in phase with the load current. These two fluxes link each other and a torque is produced on the moving system.

Theory of Electrodynamometer Type Wattmeter

Let,

i₁ = instantaneous current in pressure coil.

I₂ = instantaneous current in current coil.

θ = Angle by which the applied voltage lags the current, through the current coil.

T₁ = Instantaneous torque.

T_av = Average torque

Tc = Controlling torque

Vp = Voltage across the pressure coil.

Rp = Resistance of the pressure coil.

Since the deflection is directly proportional to the power being measured and the scale is uniform over a wide range.

Errors in dynamometer type wattmeters

Errors are introduced in dynamometer type wattmeters due to the following errors:

1. Error due to pressure coil's inductance and capacitance

2. Error due to mutual inductance

3. Error due to connection of wattmeter

4. Error due to eddy currents

5. Error due to stray magnetic field

6. Error due to vibration of moving system.

Advantages and Limitations

Advantages

(i) Dynamometer type wattmeters are portable instruments.

(ii) Power consumption is low.

(iii) These instruments can be operated on both ac and dc circuits.

(iv) By careful design we can have sub‒standary accuracy.

(v) Since no iron parts are used, they are free from hysteresis and eddy current losses.

Limitations

(i) Torque produced on the moving system is low.

(ii) Production cost is high.

(iii) These instruments are affected by the stray magnetic fields.