Dynamometer type wattmeter | what is Dynamometer type wattmeter?

In this article, we will discuss the dynamometer type wattmeter and its applications. The electro dynamometer type wattmeter is an electrical device that allows one to measure both current and voltage with high accuracy. In addition, it is used for measuring current in electronic and electrical circuits.


In the above pic, you can see an image of a dynamometer type wattmeter.you cant define by just only seeing it whether it is an electrodynamometer type or not until you are not seeing the inside connection and its features so let’s start with its construction, working and additional features.

Dynamometer type wattmeter

Short Definition -An Electrodynamometer type wattmeter is a device for measuring power (electricity) and its measurement accuracy is of the order of 1%. It is also known as a wattmeter. An Electrodynamometer type wattmeter uses two coils one is called a fixed coil and another is called a moving coil. The fixed coil is in series with the load to generate a magnetic field and the moving coil is attached with the pointer to show the reading.

Constructional Details:-

Electrodynamometer type wattmeter is made up of various elements to provide deflecting torque, controlling torque and Damping torque in order to give an accurate reading. Let’s see them one by one ………….

dynamometer type wattmeter
Dynamometer type wattmeter

The above picture is the exact inside connection diagram of the electrodynamometer type wattmeter. Here you can see the coils, pointer, series resistor, scale supply, and Load.

Let’s get into the coils first. In an electrodynamometer type wattmeter, we have two types of coil one is a fixed coil and another is a moving coil. We have two fixed coils to produce the magnetic field in which the moving coil is placed and here fixed coils are made up of thick wire because as you can see fixed coils are directly connected with load so high current is flowing through them. Two fixed coils are insulated by air-cored to prevent hysteresis loss when used for AC. And the other side we have one moving coil which is attached with the pointer and the moving coil is made up of a thin coil because it is parallelly connected with the load so apparently less current is flowing through it. the orientation of the coil is such that the moving coil is always placed in the magnetic field created by two fixed coils.

The series resistor is connected with the moving coil to control the current flowing through the moving coil. As the current through the moving coil decides the pointer movement.

The pointer is nothing but a needle that basically shows the rating on an analog meter scale. It should be lightweight so that does not contribute to measuring error.

The supply voltage is nothing but the voltage for which we want to measure the power.

Dynamometer Type Wattmeter Working principle

when the current flows through the fixed coils fixed coil develops its own magnetic field and in the same way, the moving coil developed its own, and then due to the interaction of these fields a torque is getting produced called deflecting torque which is responsible to move the pointer .developed deflecting torque directly proportional to the current flowing through these coils and their mutual inductance. expression for the torque is………                                                                 

I1=current through fixed coils / current coil

I2=current through moving coil /voltage coil /pressure coil

M= mutual inductance

Now coming to the Controlling method, out of two controlling techniques (gravity control, and spring control) here we use the spring controlling method. A spring is connected to a moving coil in order to make the pointer a steady state. At steady-state damping tarque=controlling torque.

controlling torque…………      Tc=kx

k=spring constant

x=deflection length

In this instrument Damping torque is provided by the air friction damping technique.

How does it measure power?

Here as the current coil is in series with a load, it measures current and the voltage coil measures the voltage with the help of this current, and voltage power can be calculated for a circuit.

Advantages of Dynamometer Type Wattmeter

  • It can measure the parameter for AC and DC both.
  • It has a uniform scale.
  • Accuracy is high compared to another analog wattmeter.

Disadvantages of Dynamometer Type Wattmeter

  • At low power factors, the inductance of the potential coil causes serious errors.
  • The reading of the instrument can be affected by stray fields acting on the moving coil. To prevent it, magnetic shielding is provided by enclosing the instrument in an iron case.

Errors in Dynamometer Type Wattmeter

  • Frictional error is there. As the pointer moves on the scale due to frictional loss reading can be affected.
  • Error due to potential coil inductance: The inductance of the potential coil is liable to cause an error in the reading of the wattmeter. The wattmeter gives a high reading on the lagging power factor and low reading on the leading power factor.
  • Error due to power loss in the potential coil or current coil: as both types of coils are electrical conductors so resistance loss or voltage drop must be there because of them and this may affect the deflecting torque.
  • Error due to eddy currents: AC magnetic field is produced inside the instrument and as this field passes through the metal part of the conductor eddy current is produced and after some time eddy magnetic field is introduced which may oppose the main field.

some other errors are…………………..

  1. Temperature error
  2. Stray magnetic field error
  3. Errors in the pressure coil inductance.
  4. Errors may be due to pressure coil capacitance.
  5. Errors may be due to the mutual inductance effect.
  6. Errors are caused by the vibration of the moving system.


Current circuit:-    0.25A  – I00 A without employing current transformers.
Voltage circuit:-  5 V – 740 V without using a potential transformer.

Now it’s time to clear the most frequently asked question about this instrument…

Which supply is used in Dynamometer type wattmeter?

Both AC and DC. the deflecting torque for this instrument is always unidirectional that’s why it can work on AC and DC both.

Why Dynamometer type wattmeter can be used for AC and DC?

When the negative half of the AC cycles act, the flux also changes its direction hence deflection torque always acts in one direction. So that pointer can show the deflection for DC and as well as AC.

What are the two types of wattmeter?

Two types of wattmeter are there…

  • Induction wattmeter– for AC power measurement only
  • Electrodynamometer wattmeter– for both DC and AC power measurement.

I hope you have found the answers to all your questions through this post. About Dynamometer type wattmeter  After that, if you have any questions, you can comment in our comment box.

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