Page 286 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

The generator develops supply voltage to the electrodes and ohmmeter.
2 Current reverser
Dc produced by the generator is changed to ac through a current reverser.
3 Sychronous rotary rectifier

The ohmmeter requires dc supply for working, for this synchronous rotary rectifier is used.
4 Ohmmeter
Ohmmeter consists of two coils namely current coil and potential coil. The two coils are kept at 90o to each
other and mounted on the same spindle. A pointer is attached to this spindle. The current coil carries a current
proportional to the current in the test circuit. The pressure coil carries a current proportional to the potential across
the resistance under test. The deflection of ohmmeter reading is proportional to the current in the two coils.
Hot wire instrument
The working parts of the instruments as shown in below figure. It is based on the heating effect of current. It
consists of a platinum – iridium (it can withstand oxidation at high temperatures) wire AB stretched screw at A.
When current is passed through AB, it expands according to I2R formula. Due to heating AB expands of sag. This
sag in AB produces a slack in phosphor- bronze tension wire CD attached to the center of AB. This slack in CD is
taken up by silk fiber which after passing round the pulley W is attached to a spring S. As the silk thread is pulled
by S, pulley rotates, there by deflecting the pointer. It would be noted that even a small sag in AB is magnified
very much and is conveyed to the pointer. Expansion of AB is magnified by CD which is further magnified by silk
thread.

It will be seen that the deflection of pointer is proportional to I2.Hencedeflection is proportional to I2.If spring
control is used then Tc α Ѳ .
Hence ѲαI2.
So these instruments have a square law type scale They read the rms value of current and are independent of its
wave form and frequency.
Damping
A thin light aluminum disc P is attached to the pulley such that its edge moves between the poles of a permanent
magnet M .Eddy current produced in this disc give the necessary damping

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CITS : Power - Electrician & Wireman - Lesson 50-53 CITS : Power - Electrician & Wireman - Lesson 50-53