Single phase Electrodynamometer type Wattmeter

The instrument used to measure active power ‘P’ drawn by a load or circuit is called ‘watt-meter’. Three types of watt-meter are in use. They are:

1.       Dynamo-meter type,

2.       Induction type, and

3.       Electrostatic type.

The most commonly used watt-meter and available in labs are the dynamo-meter type. Although digital watt-meter are also in use and are mainly found in industries.

Lets’ have a look into the Electro-dynamo-meter type Watt-meter…

An electro-dynamo-meter type watt-meter has two coils; a fixed coil and a moving coil. The fixed coil is also called the current coil (CC) since it carries the load current or a fraction of it. The current coil, which is connected in series, is made up of thick wires of few turns and is divided into two identical parts (as shown in the figure). The current coil is divided into two to have a uniform magnetic field. The terminals of these fixed or current coils are marked ‘M’ and ‘L’.

The second coil is movable and is called the pressure coil (P.C.). It is located inside the current coil and is made up of large number of turns of very fine wire. A very high resistance is also sometimes added in series with the pressure coil (also called voltage coil) which makes the resistance of pressure coil in kilo-Ohm range; usually 5, 10 or 20 kilo-Ohm. The pressure coil is connected in parallel to the load and carries a definite very low value of current .The terminals of pressure coil are marked ‘COM’ and ‘V’.

Fig 1 and 2: Two different views of Dynamo-meter-type watt-meter.

Working of Electro-dynamo-meter type Watt-meter:

The pressure coil or the moving coil, which is suspended on a spindle, moves in between the two halves of the fixed coil. The movement is due to the interaction of the magnetic fields of the two coils; fixed and the moving. The controlling torque is provided by two fine springs which also serves as leads to pass the current into the pressure coil. A pointer is attached to the moving coil which directly indicates the value of active power recorded by the watt-meter. 

The deflection of the watt-meter is given by:

T = K . V . I. cos(phi)

 where ‘K’ is a constant,

V and I are the r.m.s. value of supply voltage and load current, and

‘phi’ is the phase difference between V and I.  

Multiplying Factor of Electro-dynamo-meter type Watt-meter:

Watt-meters usually have selection facility i.e. one can select the range of voltage as well as current of the watt-meter. Suppose we have a 2.5/5 A watt-meter and by properly connecting the links on the watt-meter we can select either 2.5 A or 5 A capacity range.

Similarly, we can select the voltage range also. Suppose we have a watt-meter with voltage range 75 V, 150 V, and 300 V. One can select any one voltage according to the voltage applied to the circuit. Let’s make you more clear. 

The voltage applied in the short circuit test of a single phase small transformer is very low, usually 10 – 20 V, so in this case we have to select the 75 V range. On the other hand, in the open circuit test of the same transformer normal rated voltage of 230 V is applied, hence we have to select the 300 V range.

Depending on the selection of voltage and current, we have to consider the ‘multiplying factor’ for further calculation. In simple, a ‘multiplying factor’ is a factor which is to be multiplied into the watt-meter reading to obtain the correct value of active power in the circuit.    

An example for ‘multiplying factor’ is given below:

Current selected	Voltage selected
	75 V	150 V	300 V
2.5 A	1	2	4
5 A	2	4	8

The figures in ‘bold’ are the multiplying factors. For  example, when we select (connect to) 150 V and 2.5 A, the ‘multiplying factor’ is 2 and for a selection of (connection to) 150 V and 2.5 A, the ‘multiplying factor’ is 4. Multiplying factor for the same values of current and voltage may vary according to the construction of the watt-meter.