Page 829 - Basic Electrical Engineering

terms of voltage. PMMC-type instruments are suitable for measurement of dc
voltage.
Now suppose we connect a PMMC-type instrument for measurement of ac

voltage or current. Can you imagine what would happen? If the frequency of
supply is low, the instrument will show positive reading for one half cycle

and negative reading for the next half cycle. For a centre r zero instrument
where the pointer is at the middle, on low-frequency supply the pointer will

be seen oscillating at the same frequency of the supply. For normal power
frequency, i.e., 50Hz supply, the oscillations will be very quick. However,

due to inertia, the moving system will not be able to respond to the quick
positive and negative pulses but will somewhat vibrate in its initial position
of rest indicating that the average value of the quantity to be measured is

zero. PMMC instruments are therefore not suitable for measurement of
alternating quantities.

Temperature compensation: We have seen that torque developed is
proportional to magnetic field strength of the permanent magnet used and the

current flowing through the coil. The deflecting torque is balanced by control
torque produced by spring tension. The temperature variation affects both

magnetic field strength and spring tension. The resistance of the coil used
also increases with temperature. The reduced spring tension will cause the
pointer to read higher than the actual value. The increase in coil resistance

(i.e., reduced current) and the reduced magnetic field strength (reduced flux)
will cause the pointer to read lower than the actual value. Since these effects

are not balancing each other’s effect of reduced or enhanced meter reading,
some temperature compensating arrangement has to be made to increase the
accuracy in meter readings.

If a PMMC instrument is not compensated against temperature effects, it
has been observed that for each degree of temperature rise the meter tends to

read 0.2 per cent lower than the actual value.
We have known that manganin has negligible temperature coefficient of

resistance. If we connect a high-value manganin resistor in series with the
moving coil, the total change in resistance of the combination of resistance of

manganin resistor and the copper coil will be negligible and the temperature