Electrodynamics or Dynamometer Instrument :
The basic action of this instrument depends upon thei electromagnetic force exerted between fixed and moving coils carrying current. Fig. 4.8.1 shows fixed coil FF and a moving coil M carried by a spindle S.
The controlling torque is provided by two spiral springs mounted on the spindle which act as the leads of coil M. The deflecting torque is due to the interaction of the magnetic fields produced by fixed and moving coils.
Damping is provided by a piston moving in an air chamber (not shown in Fig. 4.8.1 for clarity.)
When the instrument is used as an ammeter, the two coils FF and M are connected in parallel. When used as a voltmeter, they are connected in series.
In each case the deflecting torque is proportional toi square of the current. The scale of the instrument isi therefore nonuniform. The damping torque is generally provided by air friction principle.
Dynamometer type wattmeter:
The instrument descnbed above (Fig. 4.8.1) is very commonly used as a wattmeter where the fixed coil is treated as a current coil and the other as a pressure coil (also called voltage coil).
In that case, the deflecting torque is proportional to the product of the current in the current coil and voltage across pressure coil. So Ta is proportional to power.
Damping is often provided by the air piston on vane moving in the air chamber. This is called as the air piston damping. The scale of this instrument is uniform.
It is used for power measurement in ac an de circuits. Advantages and disadvantages:
The instrument has the following advantages: It can be used for a.c. as well as d.c. measurements. It is easy in construction.
The instrument is free from hysteresis errors. Uniform scale.
Light weight. i Consumes less power.
It is possible to change the current and voltage ranges by adding series or parallel resistors respectively.
The disadvantages are as under: Hish cost. i
Lower sensitivity when instrument is used as ammeter and voltmeter as compared to wattmeter. Therefore this instrument is seldom used as ammeter and voltmeter but very commonly used as a wattmeter.
It gets affiected by the exteral stray magnetic fields. So shielding is essential.
These instruments are expensive.i
Errors can occur due to friction, changes in temperature etc. Large errors at low power factors.
Schematic Diagram of a Wattmeter:
The wattmeter is an instrument which can measure the power in single or three phase ac circuits. It gives its reading directly in watts.
The wattmeter has two different coils namely voltage coil and current coil. The voltage coil is also called as pressure coil. It is the moving coil.
Current coil
The coil shown between the terminals M (mains) and L (load) in Fig. 4.8.2 is the current coil. The current coil is the fixed coil.i
It is to be connected in series with the load.
The resistance of this coil is small due to its large cross sectional area and small number of turms.
No comments:
Post a Comment