Wattmeter Connection tutorial

How to connect the wattmeter to the circuit?

In our previous posts we discussed about the wattmeter types, common errors during measurement construction and working principle. In this post we will discuss about Wattmeter Connections. It is highly recommended to read the following posts before proceeding further.

How does the wattmeter work?
Types of wattmeter
Wattmeter readings errors

The dynamo-meter type wattmeter consists of two coils namely: fixed coil (Current coil) and moving coil(Pressure Coil). The current coil is connected in series with the load hence it carries the circuit current. The potential coil is connected across the load so it carries current proportional to the voltage.

Current Coil(CC):

  • The current coil in wattmeter is connected along the circuit to carry load current.
  • The two terminals of the current coils are named as M and L.
  • M stands for Main side and L stands for Load side.
  • While connecting the current coil in the circuit, this sequence must be followed strictly.

Pressure Coil (PC):

  • The Pressure coil is a high resistance coil ( internal series resistance added).
  • It is connected across the load circuit.
  • It carries a current proportional to the voltage across its terminals.
  • The terminals of the pressure coil are marked as ± and V. In some meters it is marked as COM and V.
  • The first one is the common terminal and the second one is the specified voltage terminal with actual voltage marking as 115V, 230V or 440V.
  • The common terminal of the pressure coil may be connected after or before the current coil.
  • Based on this connection, there are two methods of wattmeter connections.
  • Each method is suitable for one particular case. These two methods of wattmeter connection is explained below;

First method:

Wattmeter_Connection

  1. The pressure coil comes before the current coil moving from supply side.
  2. In this method of connection the pressure coil reads the voltage drop in load as well as small drop in current coil.
  3. So the power measured will include the power lost in current coil resistance. This is an error.
  4. So this method of connection is suitable for the circuit having small load currents, in which case the voltage drop in current coil will be negligibly small.

Second Method:

  1. In this method the pressure coil is connected across load circuit after the current coil.
  2. Now the pressure coil reads the correct voltage, but the current coil will carry a small extra current that is drawn by the pressure coil. This is an error.
  3. This method is suitable for the circuits having large load current.
  4. Compared to a large load current, the current drawn by the pressure coil is negligible.
  5. When the second method is adopted, a compensating coil is used in series with the pressure coil.
  6. The compensating coil is just another current coil connected in reverse sequence. ie, the magnetic effects of current coil and compensating coil are made to oppose each other. This will nullify the error due to the pressure coil current.

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