Page 307 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

LAG adjustments (Low power factor adjustments): The pressure coil is connected across the rated supply
voltage and the rated full load current is passed through the current coil at 0.5 P.F. lagging. The lag device is
adjusted till the meter runs at the correct speed.
Rated supply voltage: By adjusting the rated supply voltage, with the rated full load current and unity power
factor, the speed of the meter is checked and the full load unity power factor and low power factor adjustments
are repeated until the desired accuracy limits are reached for both the conditions.
Light load adjustment: The rated supply voltage is applied across the pressure coil and a very low current
(about 5% of full load current) is passed through the meter at unity power factor. Light load adjustment is done
so that the meter runs at the correct speed.

Full load unity power factor: Light load adjustments are again done until the speed is correct for both loads
i.e. full load as well as light loads.
Creep adjustment: As a final check on the light load adjustment, the pressure coil is excited by 110 percent of
the rated voltage with zero load current. If the light load adjustment is correct, the meter should not creep under
these conditions.

Multimeters

Objectives: At the end of this lesson you shall be able to:
• explain the construction of multimeter
• explain the working priniciple of analog multimeter
• explain the method of measuring direct / alternating voltages and current with a multimeter
• explain the method of measuring resistance by a multimeter
• explain the precautions to be observed while measuring voltage, current and resistance in the circuit.

A single instrument used for measuring current voltage and resistance is known as a multimeter. It is a portable,
multi range instrument.
It has a full scale deflection accuracy of ±1.5 %. The lowest sensitivity of multimeters for AC voltage range is
5 K ohms/volts and for the DC voltage range it is 20 K ohms/volts. The lowest range of DC is more sensitive than
the other ranges.
Fig 1 show typical multimeters.

Fig 1

Construction of a multimeter
A multimeter uses a single meter movement with a scale calibrated in volts, ohms and milliamperes. The
necessary multiplier resistors and shunt resistors are all contained within the case. Front panel selector switches
are provided to select a particular meter function and a particular range for that function.

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