Dynamometer Type Wattmeter Working

Hi friends, in this article, I am discussing the dynamometer type wattmeter working principle and related information, if you are interested in it then you at the right place.

These are basically moving coil instruments. But in these instruments, the operating field is produced by the fixed coil instead of a permanent magnet.

The instruments in which fixed coils produce operating field are known as dynamometer type instruments.

The dynamometer type wattmeters are very important because we use very commonly for power measurement in AC circuits.

Dynamometer Type Wattmeter Working Principle

In these wattmeters, the field produced by the current-carrying moving coil tries to come in line with the field produced by the current-carrying fixed coil, and a deflecting torque is exerted on the moving system. As a result deflection takes place in the pointer.

dynamometer type wattmeter working principle

Construction of Dynamometer Type Wattmeter

A dynamometer type wattmeter primarily consists of two coils called fixed coil and moving coil. The fixed coil is splitted into two equal parts, which are placed parallel to each other. The two fixed coils are air-cored to avoid hysteresis effects when used on AC.

The fixed coil is connected in series with the load and carries the circuit current. It is, therefore, called the current coil. The moving coil is pivoted between the two parts of the fixed coil and is mounted on a spindle.

A pointer is attached to the spindle, which gives deflection. The moving coil is connected in parallel with the load and carries the current proportional to the voltage. It is, therefore, called the potential coil.

Generally, a high resistance is connected in series with the moving coil to limit the current through it. By limiting the current, the moving coil is made lightweight, which in turn increases the sensitivity of the instrument.

The springs provide the controlling torque.  They also serve the additional purpose of leading the current into and out of the moving coil. Air friction damping is employed in such instruments.

Dynamometer Type Wattmeter Working

We use the wattmeter for power measurements. Its current coil is connected in series with the load, carries the load current, and the potential coil, connected in parallel with the load, carries the current proportional to the voltage across the load.

The fixed coil produces a field Fm, and moving coil creates a field Fr. The field Fr tries to come in line with the main field Fm, which provides a deflecting torque on the moving coil.

Thus, the pointer attached to the spindle of the moving coil deflects. This deflection is controlled by the controlling torque produced by the springs.

Advantages and Disadvantages of Dynamometer Type Wattmeter


  • It can be used both on AC and DC circuits.
  • It has a uniform scale.
  • We can obtain a high degree of accuracy through careful design.


  • At low power factors, the inductance of the potential coil causes serious errors.
  • The reading of the instrument may 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

The following are severe errors in this type of wattmeter:

1. Error due to potential coil inductance: The inductance of the potential coil is liable to cause an error in the reading of the wattmeter. Because of this error, the wattmeter gives a high reading on lagging power factor and low reading on leading power factor.

The high non-inductive resistance connected in series with the coil swamps the phasing effect of the potential coil inductance.

2. Error due to power loss in the potential coil or current coil: Another possible error in the indicated power may be due to some voltage drop in the current coil or the current taken by the potential coil.

We can overcome this defect by using an additional compensating winding. This winding is connected in series with the potential coil and so placed that it produces a field in the opposite direction to that of the current coils.

3. Error due to eddy currents: The alternating field of fixed or current coil induces eddy currents in the solid metal parts which set up their own magnetic field. This alters the magnitude and phase of the magnetic field, causing deflection.

Thus an error is introduced in the instrument reading. To reduce this error, the solid metal parts are placed far away from the current coil as possible.

4. Error due to the stray magnetic field: The dynamometer type wattmeter has a relatively weak operating field; therefore, stray fields affect the reading of this instrument considerably and cause serious errors.

Hence, this type of instrument must be shielded against stray magnetic fields try using iron cases or providing thin iron shields over the working parts.


  • Current circuit 0 – 0.25A to 0 – I00A without employing CTs.
  • Potential circuit 0 – 5 V to 0 – 750 V without using PTs.

Thanks for reading about the “dynamometer type wattmeter working principle.”

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