Errors in Electrodynamometer Type Wattmeter

Error in Electrodynamometer type Wattmeter are caused mainly due to inductance and capacitance of pressure coil, mutual inductance effect between current coil and pressure coil & error due to improper connection of wattmeter in the circuit. In this post we will mainly focus on Error in Electrodynamometer Type Wattmeter reading due to improper connection.

We know that current coil of wattmeter is connected in series with the load and pressure coil is connected across the load. This philosophy permits us to have two different connections of wattmeter in a circuit for power measurement. In one connection, the pressure coil is connected in the load side while in another connection pressure coil is connected toward source side. So the question arises that which method gives the accurate measurement?

In fact none of the two method gives the accurate measurement but there is always a way which gives almost accurate value i.e. less error. We will examine both the methods and will check for their suitability.

Connection-1: Pressure Coil connected toward load side.

In this type of connection, current coil of Electrodynamometer type wattmeter do not only carry load current but it is also carrying pressure coil current. Thus the total current flowing through the current coil = (I +Ip)

Voltage across the pressure coil = Voltage across the load

Therefore, the power measured by wattmeter will be sum of power consumed by load and pressure coil.

= Power consumed by load + (V/Rp)2

Where V = voltage across pressure coil

Since the resistance of pressure coil is very high therefore the current flowing through the pressure coil will be very low. Hence current flowing through the current coil will be approximately equal to the load current. Thus the power measured will be approximately equal to the power consumed by load. Alternatively, we can say that the power loss in pressure coil will be less compared to power consumed by load. Hence measurement will be almost accurate.

Thus this method of connection is suitable for measurement of power in a circuit carrying high current.

Connection-2: Pressure Coil connected toward source side.

In this connection,

Current through current coil = Load Current

Voltage across pressure coil = Voltage drop across current coil + voltage drop across load

Thus this method of power measurement gives the sum of power consumed in current coil and load. If the current flowing though the circuit is low, the voltage drop across current coil will be low and hence voltage drop across the current coil may be neglected when compared with voltage drop across load. Therefore,

Voltage across pressure coil = voltage drop across load

Hence, this method of connection of wattmeter is suitable for circuit carrying low current.

It is a good practice to connect wattmeter as shown in connection-1. This is due to the fact that power consumed in pressure coil is constant (V/Rp)2 which can be subtracted from the wattmeter reading to get accurate measurement. Constant power loss in Pressure Coil may found from the wattmeter reading by disconnecting load and taking the wattmeter reading.

Compensating Winding in Electrodynamometer type Wattmeter

Let us now consider a case where we need to measure power of a circuit carrying high value of current but the power factor of load is poor. As per our discussion, we will use connection-1 for better accuracy.

Since the power factor of load is poor, the power consumed by load will be low. Thus the constant power loss in pressure coil of wattmeter will introduce appreciable amount of error. Thus neither connection-1 nor connection-2 will give accurate reading for this case.

To account and handle such cases, an electrodynamometer type wattmeter is provided with compensating winding. This compensating winding is connected in series with the current coil. Since current coil carries Ip, this current produces a magnetic field. The compensating coil is connected in such a manner that magnetic field due Ip in current coil and compensating winding cancel each other. In this way the error in power measurement due to flow of Ip is eliminated.

Precaution:

Current Coil should never be connected across the source or across the load. It should always be connected in series with load. Similarly pressure coil should always be connected across the load.