Question

Explain the construction and working of dynamometer type wattmeter. Mention its merits and demerits.

Answer 1

 It very essential to know the principle of working of electrodynamometer type wattmeter. Dynamometer type wattmeter works on very simple principle and this principle can be stated as" when any current carrying conductor is placed inside a magnetic field, it experiences a mechanical force and due this mechanical force deflection of conductor takes place".

Construction and Working Principle of Electrodynamometer Type Wattmeter:

 Now let us look at constructional details of electrodynamometer. It consists of following parts there are two types of coils present in the electrodynamometer. They are moving coil moves the pointer with the help of spring control instrument.

 A limited amount of current flows through the moving coil so as to avoid heating. So in order to limit the current we have connect the high value resistor in series with the moving coil. The moving is air cored and is mounted on a pivoted spindle and can moves freely.

 In electrodynamometer type wattmeter, moving coil works as pressure coil. Hence moving coil is connected across the voltage and thus the current flowing through this coil is always proportional to the voltage.

 Fixed Coil The fixed coil is divided into two equal parts and these are connected in series with the load, therefore the load current will flow through these coils. Now the reason is very obvious of using two fixed coils instead of one, so that it can be constructed to carry considerable amount of electric current.

 These coils are called the current coils of electrodynamometer type wattmeter. Earlier these fixed coils are designed to carry the current of about 100 amperes but now the modern wattmeter are designed to carry current of about 20 amperes in order to save power.

Control System Out of two controlling systems i.e.

i. Gravity control

ii. Spring control

 Damping System Air friction damping is used, as eddy current damping will distort the weak operating magnetic field and thus it may leads to error. Scale there is uniform scale is used in these types of instrument as moving coil moves linearly over a range of 40 degrees to 50 degrees on either sides.

 Now let us derive the expressions for the controlling torque and deflecting torques. In order to derive these expressions let us consider the circuit diagram given below:

Advantages of Electrodynamometer Type Wattmeter

Following are the advantages of electrodynamometer type wattmeter and they are written as

follows:

1. Scale is uniform upto certain limit.

2. They can be used for both to measure ac as well dc quantities as scale

is calibrated for both.

Disadvantages of Electrodynamometer Type Wattmeter

1. Errors in the pressure coil inductance.

2. Errors may be due to pressure coil capacitance.

3. Errors may be due to mutual inductance effects.

4. Errors may be due connections.(i.e. pressure coil is connected after current coil)

5. Error due to Eddy currents.