The electrodynamometer type instrument is a transfer instrument. A transfer instrument is one which is calibrated with a d.c. source and used without any. An electrodynamic type instrument consists of Two Fixed Coil, flux is used in Electrodynamic or Electrodynamometer type instrument. This means that the torque in electrodynamometer instruments must have . Electrodynamometer type voltmeters are the most accurate type of.

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Electrodynamometer Instruments are widely used as rlectrodynamometer ammeter, voltmeter and wattmeter. This instrument can measure AC as well as DC quantities.

This means that the torque in electrodynamometer instruments must have a finite value and unidirectional for AC and DC quantity unlike PMMC instruments. In this post we will be discussing the torque equation in such instruments.

Electrodynamometer type Instruments – Construction and Operation

It electroddynamometer recommended to read before going to understand the torque equation. If you are already aware of constructional feature then you may proceed. Let us assume that the current in fixed coil be I 1 and that in moving coil be I 2 as shown in figure below.

The electrical energy input to the instrument. Therefore energy input to the instrument. Some of the above input energy to electrodynamometer instruments are stored in the form of instrumenta energy in the coil while rest is converted into mechanical energy of moving coil.


Thus we can write. Thus to find the mechanical energy, we need to find the change in stored energy in the magnetic field of the coil. Let us assume an infinitesimally small time dt for the sake of calculation of change in stored energy.

Change in stored energy.

From equation 12 and 3. The above equation gives the deflecting torque in electrodynamics or electrodynamometer instruments. It can be seen that deflecting torque depends upon the multiplication of instantaneous value of current and rate of change of mutual inductance between the fixed and moving electrodynamomdter. Now we will consider two cases. Let I 1 and I 2 be the current in fixed and moving coil respectively.

But this deflecting torque is controlled by the spring. Spring provides the controlling torque. At equilibrium the controlling torque and deflecting torques are equal, hence. Therefore we can write as. Thus the instantaneous deflecting torque is given as. The average torque for one time period of the currents are given by.

From the above two cases, we can have following conclusions: Figure below shows an electrodynamometer ammeter. It can be seen that, the fixed coil and moving coil are connected in series and hence carries the same current.

As the current through the moving coil shall not exceed electrodynamometr, therefore moving coil is shunted by suitable resistance to increase the range of such ammeter.


Electrodynamometer Type Wattmeter

As the currents flowing in fixed and moving coils are same therefore there will not be any phase angle between them. Deflecting Torque in Ammeter. Figure below shows the connection arrangement for electrodynamometer voltmeter.

Therefore, Deflecting Electrodynamommeter in Ammeter. Electrodynamometer type voltmeters are the most accurate type of AC voltmeters.

But the sensitivity of such instrument is low when compared from DC instruments. Your email address will not be published. Notify me when new comments are added. This site uses Akismet to reduce spam. Learn how your comment data is processed.

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In the figure above, voltmeter is connected between points A and B to measure the voltage drop across it. Note that a high value of resistance is connected in series with the coils.

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