What is an Electrodynamometer Wattmeter & Its Working

An electrical instrument used to measure electric power in watts of any circuit is called Wattmeter. It consists of two coils like the current coil and voltage coil. The current coil, which is connected in series & voltage coil is connected in parallel. Wattmeters are mainly used in electrical circuit measurement, debugging, transmission, distribution of electrical power, power rating, consumption of electrical appliances, utility frequency measurement, home appliances and many more. These are classified into three types. They are Electrodynamometer wattmeter, Induction type wattmeter, Electrostatic type wattmeter. Let us discuss an overview of the Electrodynamometer wattmeter.

What is Electrodynamometer Wattmeter?

Definition: Electrodynamometer wattmeter is an instrument whose working is related to the reaction between magnetic fields of the fixed coil and moving coil which is connected across the voltage (current is directly proportional to voltage). Electrodynamometer wattmeters are similar to the Electrodynamometer ammeters and voltmeters. These are mainly used to measure power.

Working Principle

The Electrodynamometer wattmeter working principle is very simple and easy. It is based on the theory of a current-carrying conductor experiences a magnetic force when it is placed in a magnetic field. Hence there will be a deflection of pointer that took place due to the mechanical force. It contains two coils such as fixed coil (current coil) and moving coil ( pressure coil or voltage coil).

The fixed coil is used to carry the current and connected in series with the load in any circuit. The moving coil carries the current directly proportional to the voltage and connected across the voltage. The value of current limited to a minimum value due to large non-inductive resistance connected in series. The circuit diagram is shown below.

Electrodynamometer Wattmeter Working Principle
Electrodynamometer Wattmeter Working Principle

Construction of Electrodynamometer Wattmeter

The construction of the Electrodynamometer wattmeter includes fixed coil, moving coil, control, damping, scales, and pointer. The construction of the Electrodynamometer wattmeter is shown below.

Construction of Electrodynamometer Wattmeter
Construction of Electrodynamometer Wattmeter

Fixed Coil

It is connected in series with the load, which is considered as the current coil. To make construction easy and simple, it is divided into two parts. Those are two elements connected parallel to each other. It produces a uniform electric field, which is very essential for working. The current coil is designed in such a way that it carries approximately 20 Amperes.


Moving Coil

Considered as pressure coil in this instrument, that is connected parallel with the supply voltage. So, that current flows directly proportional to supply voltage. A pointer is mounted on the moving coil with the help of spring to control the movement. The temperature increases when current flows through the coil. So, in order to control the flow of the current resistor is connected in series with the moving coil.


It provides controlling torque onto the instruments. Gravity control and spring control are the two types in this control system. Among these two Electrodynamometer wattmeters uses a spring control system as it helps in the pointer movement.


The effect which reduces the pointer movement is called damping. In this, damping torque is produced because of the air friction. Other types of damping are not used as they destroy the useful magnetic flux.

Scales and Pointers

It uses a linear scale as the moving coil moves linearly. The apparatus uses knife-edge pointers in order to remove parallax error caused due to oversights.

Working of Electrodynamometer Wattmeter

The Electrodynamometer wattmeter has two coils i.e., fixed and moving coils. The fixed coil is connected in series with the circuit in order to measure power consumption. The supply voltage is applied to the moving coil. Current across the moving coil is controlled with the help of a resistor, which is connected in series with it. Moving coil on which pointer is fixed is placed in between fixed coils. Two magnetic fields are generated due to the current and voltage in the fixed coil and moving coil. The pointer deflects as the two magnetic fields interact. The deflection is proportional to the power that is flowing through it.

Theory of Electrodynamometer Wattmeter

The circuit diagram of the Electrodynamometer wattmeter is shown below.

Circuit Diagram
Circuit Diagram

The instantaneous torque acting on the pointer is given by,

T1 = i1ip dM/dθ

Where ‘ip’ is the current flowing through pressure coil

The equation for the voltage in a circuit across the pressure coil is,

V =√2Isin(ωt-Φ)

The current will be in phase with voltage if a purely resistive pressure coil is used. The value of the current is,

Ip = v/Rp = √2(VI/ Rp)sin ωt = √2IpSin ωt

The current flowing through the current coil when it is lagged by the voltage in phase angle is,

‘ip’ = √2Isin(ωt-∅)

The current value is very small in the pressure coil. Hence it is considered as the total load current. The torque acting on the coil is,

Ti = √2Isin(ωt- Φ) dM/dθ

0 to T limit is integrated to get average defection torque and it is given by,

Ti = √2(VI/Rp) cosΦdM/dθ

The controlling torque on the spring is,

Tc = Kθ

Errors in Electrodynamometer Wattmeter

Pressure Coil Inductance: The pressure coil has some inductance because of which current is lagged by voltage. Hence power factor becomes lagging and leads to a high reading.

Pressure Coil Capacitance: Pressure coil also has capacitances that increase the power factor. This leads to reading errors.

Errors Caused by Mutual Inductance Effect: In between pressure and current coil, the mutual inductance produces an error.

Eddy Current Error: It creates own magnetic field in the coil, which effects the main current flowing through the coil.

Stray  Magnetic Field Error: The main magnetic field is disturbed due to this. This affects the reading of the instrument.

Temperature Error: Change in the pressure coil resistance is caused due to variations in temperature. Due to this the temperature variation, the controlling torque produced by the spring movement is also affected.


1). What is a dynamometer type wattmeter?

The instrument in which the operating field is generated by fixed coils is known as dynamometer type wattmeter.

2). How is a wattmeter connected?

The current coil is connected in series with the load to carry circuit current and the potential coil is connected across the load in order to carry current proportional to the voltage.

3). What does a wattmeter indicate?

Wattmeter measures the electric power in watts of any circuit.

4). What happens if an electrodynamic type of wattmeter current coil is connected across the supply?

When electrodynamic type wattmeter is used to measure AC power fixed coil splits into two parts, which are air-cored. This avoids hysteresis loss.

5). What 2 factors determine the turning force of a Wattmeter?

It depends on the strength of the magnetic field of both stationary and moving coils.

Thus, this is all about the definition, construction, working principle, working, theory and errors of Electrodynamometer Wattmeter. Here is a question for you, What are the applications of Electrodynamometer Wattmeter?”