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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Chand ” is the best in industry. The controlling torque is provided by springs. Hence external magnetic field can interact with the operation field to cause change in the deflection, causing the error.

Electrodynamometer Type Wattmeter

From the above two cases, we can have following conclusions: Where, x is the angle. The average torque for one time period of the currents are given by. If there is phase difference between voltage and electric current, then expression for instantaneous current through current coil can be written as As current through the pressure coil is very very small compared to epectrodynamometer current through current coil hence current through the current coil can be considered as equal to total load current.

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Electrodynamometer Type Instruments ~ your electrical home

As the currents flowing in fixed and moving coils are same therefore there will not be any phase instrumentz between them. The instrument whose working depends on the reaction between the magnetic field of moving and fixed coils is known as the Electrodynamo-meter Wattmeter. Due to moment of inertia of moving system, the pointer will not follow rapidly changing alternating torque and will fail to show any reading. The above equation gives the deflecting torque in electrodynamics or electrodynamometer instruments.


It can be seen ty;e deflecting torque depends upon the multiplication of instantaneous value of current and rate of change of mutual inductance between the fixed and moving coil. Thus the deflection is decided by the product of r.

Electrodynamometer Wattmeter

With some electrodynammeter modifications, it can be used as a wattmeter for the power measurements. Deflecting Torque in Ammeter. It is done by enclosing in a casing high permeability alloy. These springs act as leads to the moving coil.

The fixed coil is usually air-cored to avoid hysteresis effects when used on AC circuits. Since, there is no iron, field strength and hence flux density is proportional to I 1. Sometimes a suspension may be used, in case a high accuracy is desired. Subscribe to our mailing list and get interesting stuff and updates to your email inbox.

Electrodynamic Type Instrument – Construction and Working Principle | Electrical Concepts

Subscribe to our mailing list and get interesting stuff and updates to your email inbox. Hence the instantaneous value of torque can be written as Average value of deflecting torque can be obtained by integrating the instantaneous torque from limit 0 to T, where T is the time period of the cycle.

Because of this change of magnetic flux, the torque on moving coil will also change in each half cycle of AC quantity to be measured. For understanding the working principle, first ask yourself what will happen if we connect a PMMC instrument for the measurement of AC quantity?

Due to the inertia of tgpe, the needle cannot follow the rapid change in direction of torque at power frequency and therefore merely vibrates around zero position. If, however, we were to reverse the direction of the flux each time the current through the movable coil reverses, a unidirectional torque would be produced for both positive and negative halves of the cycle.


Therefore we can write as. As the deflecting torque is produced due to the interaction of these two fields, the deflection is proportional to the power supplied to the load. The case is supported by adjustable levelling screws.

Really very gud looking. From the principle of conversation of energy. So the expression for the torque can be written as: If you carefully observe the figure above, you will notice that Fixed Coil as well as Moving coil is in series. The controlling torque exerted on the spring is given by. Gravity control Spring control, only spring controlled systems are used in these types of wattmeter. But the sensitivity of such instrument is low when compared from DC instruments. Thus temperature compensating resistors can be used in the precise instrument to eliminate the temperature errors.

Thus ideally the needle of PMMC instrument shall swing back and forth around zero position. It would have a torque in one direction during one half of the cycle and an equal effect in the opposite direction during the other half of the cycle. Hence moving coil is connected across the voltage and thus the current flowing through this coil is always proportional to the voltage.

Circuit Globe All about Electrical and Electronics. Instead of a permanent magnet, the electrodynamometer type instrument uses the current under measurement to produce the necessary field flux.

This fixed coil is divided into two sections so that a uniform magnetic field may be achieved at the centre.