Dynamometer type wattmeter

Dynamometer type wattmeter or electrodynamometer type wattmeter works for both DC as well as AC power measurements. This makes it suitable for using it in many places. The electrodynamometer instrument is generally used as an ammeter and voltmeter.

And one of the advantages of the electrodynamometer wattmeter has over the induction wattmeter, is as the induction wattmeter can only be used for AC measurements.

Construction of electrodynamometer type wattmeter

Parts of the dynamometer wattmeter are discussed below:

Fixed coil

Also known as a current coil and it is an air-cored coil. The role of the current coil is to carry the current of the circuit. The current coil is divided into 2 halves and the moving is placed between the 2 halves. The fixed coil is wound by heavy wire. when carrying heavy current this wire is laminated to reduce the effect of eddy current losses.

Moving coil

It is also an air-cored coil and it is placed in between the 2 halves of the fixed coil. The moving coil is mounted on the spindle and the pointer attached to the spindle moves with the moving coil over the scale.

Control

In this instrument, spring control is responsible for controlling torque.

Damping

In the dynamometer wattmeter, air friction damping is applied which dampens out the unnecessary oscillation by providing the required damping torque. The moving system carries a light aluminum vane which moves in the sector-shaped box at the bottom of the instrument.

Dynamometer type wattmeter working principle

The current coil is connected in series with the load and caries load current. The potential coil is connected parallel to the load and it carries the current that is proportional to the voltage across the load.

When the electric current passes through the fixed coil and moving coil. Both the coil produces a magnetic field and due to the magnetic field, a deflecting torque is induced which tends to rotate the pointer.

The pointer stops on the scale when the deflecting torque becomes equal to the controlling torque.

Torque equation

Errors in Dynamometer type wattmeter

  • Errors due to Pressure coil inductance
  • Errors due to mutual inductance effect
  • Errors due to Pressure coil capacitance
  • Errors due to poor connection
  • Errors due to eddy’s current
  • Errors due to stray magnetic fields
  • Errors due to temperature variation
  • Errors due to vibration of the moving system

Advantages

  • It can be operated with both AC and DC supply.
  • These instruments possess a uniform scale.
  • It can give a very accurate reading.

Disadvantages

  • Reading of this instrument can be affected by the stray magnetic field and by providing it shielding the effect of the stray magnetic field can be minimized.

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