Electrical Engineering ⇒ Topic : Dynamometer Type
Dynamo meter type of watt meters are classified according to the method in which the moving coil is mounted. They are: (a) pivoted coil type, and (b) suspended coil type or torsion type.
In this type of dynamo meter watt meter, the moving coil is supported on a small shaft on which it can rotate. In general, a dynamo meter type of watt meter consists of two fixed identical coils called the current coils, which are connected in series such that the magnetic field established by the two will aid each other, as shown in Fig.(a).At the center and in between the two fixed coils is mounted a moving coils by pivoting it at both the ends. The moving coil is also called the pressure coil. A high non-inductive resistance is connected in series with the pressure coil. In a watt meter, M and L, the two ends of the current coil are called current terminals, and ± and V, the two ends of the pressure coil are known as the potential terminals (see Fig.(a). In this type of instruments, the damping is generally provided by air (i.e. air damping).
FIGURE (a) Pivoted-coil type dynamo-meter watt-meter
These instruments have similarity with moving coil permanent magnet type instruments. In these instruments, an air-cored coil (generally two fixed air-cored coils are used) instead of the magnetic field that carry the current to be measured (or a definite fraction of it or proportional to the voltage to be measured) is used as shown in Figure (a).
These instruments consist of fine wire moving coil placed in the magnetic field that is produced by another fixed coil due to carrying current. When these instruments are used for AC,hysteresis, and eddy current losses, the errors occur. To avoid errors, the coils are usually air-cored.
Figure (a) depicts that the fixed coil is divided into two halves and placed close together.To provide a fair, uniform magnetic field within the range of the movement of the moving coil, the two halves of the fixed coil placed parallel to each other are used.These instruments can be used either as an ammeter or as a voltmeter. These instruments in general are used as wattmeter since these are costlier than other types of ammeters and voltmeters.
To measure small currents by this type of instrument (as an ammeter), the moving and fixed coils are connected in series as shown in Figure (b). It shows that the current through the fixed and moving coils is equal.For measuring higher currents, the fixed coils are wound with thick wire. The moving coil will be very heavy if it is wound with thick wire.
Figure (b) Connection of fixed and moving coils of dynamometer type
instrument for measuring small currents.
In this case strong magnetic field is required to move it. The moving coil is connected to the terminal of a shunt to overcome this difficulty as shown in Figure (c).
Figure (c) For measuring higher current
When the instrument is used as a voltmeter, the coils are wound with high resistance of fine wire. The fixed and moving coils in this case are connected in series with a high non-inductive resistance R as shown in Figure (d).
Figure (d) Connection of dynamometer type instruments to measure voltage.
An expression for TD may also be derived in terms of mutual inductance M between fixed and moving coils.
The energy in the magnetic field of the coil is:
where I1 is the current in the fixed coil and /2 is the current in the moving coil.
The deflecting torque is given by
because M is a function of θ only.
In ammeters and voltmeters, each of the coils carries current in proportion to the quantities to be measured. The deflecting torque is:
..... .................. (3)
Therefore, the instrument has square law response. It is expressed in terms of R.M.S. value of current or voltage.
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