Electrodynamometer instrument: Construction, Working principle, and more

Electrodynamometer instruments are very similar to PMMC instruments except that the permanent magnet is replaced by the coil. These instruments can be used for both AC and DC

Electrodynamometer instruments are used as a voltmeter, ammeter, wattmeter, power factor meter, and frequency meter with some modification. But most commonly these instruments have been used as wattmeters.

Electrodynamometer instrument

Construction of electrodynamometer type instruments:

  • Fixed coil

There are two fixed coils are used in the instruments. The fixed coil is divided into two halves which are connected in series with the moving coil. The coils are air-cored to avoid errors.

  • Moving coil

A moving coil is placed in between the two fixed coils.

  • Pointer

The pointer is connected to the moving coil. Due to deflection torque, the pointer deflects on the scale and shows the measured value

  • Control

Controlling torque or restoring torque is produced by the spring.

Working principle of electrodynamometer instruments:

Electrodynamometer instrument consists of two coils one is a moving coil and another is a fixed coil. When the current passes through the moving coil and the fixed coil, then both the coils produce a magnetic field respectively. When these magnetic fields (one produced by moving coil and another by the fixed coil) interact with each other they produce a deflecting torque. This deflecting torque is balanced by the restoring torque exerted by a spring. The torque is dependent upon the strength of the magnetic field produced by the moving coil and the fixed coil which are proportional to the current.

Errors in electrodynamometer type instruments:

  1. Friction error
  2. Temperature error
  3. Frequency error
  4. Stray magnetic fields error


  • Electrodynamometer instruments can be used for both AC and DC and voltage
  • No sign of eddy current error and hysteresis error
  • These instruments are very useful for accurate measurements of the RMS value of voltage irrespective of the waveform.
  • Dynamometer are precision grade instruments


  • The scale is not uniform for current and voltage measurements
  • Electrodynamometer instruments are more expensive than PMMC and MI instruments
  • These instruments have small torque to weight ratio
  • Power loss and power consumption of these instruments are relatively high

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