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 modifications for a.c. measurements. Such a transfer instrument has same accuracy for a.c. and d.c. measurements. The electrodynamometer-type instruments are often used in accurate a.c. and ammeters, not only at the powerline frequency but also in the lower audio frequency range. It can be used as a wattmeter for power measurements with some little modifications.
Why PMMC instruments cannot be used for a.c. measurements?
The PMMC instrument cannot be used on a.c. currents or voltages. If a.c. supply is given to these instruments, an alternating torque will be developed. Due to the moment of inertia of the moving system, the pointer will not follow the rapidly changing alternating torque and will fail to show any reading. In order that the instrument should be able to read a.c. quantities, the magnetic field in the air gap must change along with the change in current. This principle is used in the electrodynamometer-type instrument. Instead of a permanent magnet, the electrodynamometer-type instrument uses the current under measurement to produce the necessary field flux.
Construction
Fixed coils: The necessary field required for the operation of the instrument is produced by the fixed coils. A uniform field is obtained near the centre of the coil due to the division of the coil in two sections. These coils are air-cored. Fixed coils are wound with fine wire for use as voltmeter, while ammeters and wattmeters it is wound with heavy wire. The coils are usually vamished. They are clamped in place against the coil supports. This makes the construction rigid.
Ceramic is usually used for mounting supports. If the metal parts would have used then it would weaken the field of the fixed coil.
Moving coil: The moving coil is wound either as a self-sustaining coil or else on a non-metallic former. If a metallic former is used, then it would induce eddy currents in it, The construction of a moving coil is made light as well as rigid. It is air cored.
Controlling : The controlling torque is provided by springs. These springs act as leads to the moving coil. The controlling torque is provided by springs. These springs act as leads to the moving coil.
Moving system: The moving coil is mounted on an aluminium spindle. It consists of counterweights and a pointer. Sometimes a suspension may be used, in case high accuracy is desired.
Damping: The damping torque is provided by air friction, by a pair of aluminium vanes which are attached to the spindle at the bottom. They move in sector-shaped chambers. As the operating field would be distorted by eddy current damping, it is not employed.
Shielding: The field produced by these instruments is very weak. Even the earth's magnetic field considerably affects the reading. So shielding is done to protect it from stray magnetic fields. It is done by enclosing in a casing of high permeability alloy.
Cases and scales: Laboratory standard instruments are usually contained in polished wooden or metal cases which are rigid. The case is supported by adjustable levelling screws.
A spirit level may be provided to ensure proper levelling. For using an electrodynamometer instrument as an ammeter, fixed and moving coils are connected in series and carry the same current. A suitable shunt is connected to these coils to limit the current through them up to the desired limit. The electrodynamometer instruments can be used as a voltmeter by connecting the fixed and moving coils in series with high non-inductive resistance. It is the most accurate of the voltmeter.
For using an electrodynamometer instrument as a wattmeter to measure the power, the fixed coils act as a current coil and must be connected in series with the load. The moving coil acts as a voltage coil or pressure coil and must be connected across the supply terminals. The wattmeter indicates the supply power. When current passes through the fixed and moving coils, both coils produce magnetic fields. The field produced by the fixed coil is proportional to the load current while the field produced by the moving coil is proportional to the voltage. 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.
Torque Equation
Thus the deflection is decided by the product of r.m.s. values of two currents, cosine of the phase angle (power factor) and rate of change of mutual inductance. For d.c. use, the deflection is proportional to the square of the current and the scale is nonuniform and crowded at the ends. For a.c. use the instantaneous torque is proportional to the square of the instantaneous current. The i2 is positive and as current varies, the deflecting torque also varies. But the moving system, due to inertia cannot follow rapid variations and thus finally meter shows the average torque. Thus the deflection is the function of the mean of the squared current. The scale is thus calibrated in terms of the square root of the average current squared i.e. r.m.s. value of the a.c. quantity to be measured.
If an electrodynamometer instrument is calibrated with a d.c. the current of 1 A pointer indicates I A d.c. on scale then on a.c., the pointer will deflect up to the mark but 1 A, in this case, will indicate r.m.s. value. Thus as .it is a transfer instrument, there is a direct connection between a.c. and Hence the instrument is often used as a calibration instrument.
Fig. : Electrodynamotype Wattmeter
Advantages of Electrodynamic Instruments:
2) They have a precision grade accuracy.
3) These instruments can be used on both a.c. and d.c. They are also used as a transfer instrument.
4) Electrodynamometer voltmeters are very useful where accurate r.m.s. values of voltage, irrespective of waveforms are required.
5) Free from hysteresis errors.
6) Low power consumption.
7) Light in weight.
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