Page 297 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

• This postitive triggered train is fed to the inverter which converts it into a negative triggered train.
• Output of the inverter is fed to a counter which counts the number of triggers in the duration which is proportional
to the input signal i.e. voltage under measurement
This counter can be calibrated to indicate voltage in volts converts an analog signal into a train of pulses, the
number is proportional to the input signal. So a digital voltmeter can be made by using any one of the A/D
conversion methods (Fig 3)
Now-a- days digital voltmeters are also replaced by digital multi meters due to its multitasking feature.

Figs 1 - 3

Wattmeters

Objectives: At the end of this lesson you shall be able to:
• state the advantages of measuring power directly
• explain the construction and working of the induction type single phase wattmeter.

Advantages of measuring power supply
Power in a single phase AC circuit can be calculated by using an ammeter, a voltmeter and a power factor meter
with the help of the formula
Power in a single phase circuit = EI Cos ø watts.
To get an on the spot true power reading, a wattmeter is used. The power dissipated in the circuit can be read
directly from the scale of the meter. The wattmeter takes the power factor of the circuit into account and always
indicates the true power.
Types of wattmeters

There are three types of wattmeters in use as stated below.
• Dynamometer wattmeter
• Induction wattmeter
• Electrostatic wattmeter
Among the three, the electrostatic type is very rarely used. Information given here is for the other two types only.

Dynamometer type, single phase wattmeter: This type is commonly used as a wattmeter.
Dynanometer used as a Wattmeter: The dynamometer is commonly used as a wattmeter to measure power
in both AC and DC circuits and will have uniform scale.

When this instrument is used as a wattmeter, the fixed coils are treated as current coil, and the moving coil is
made as pressure coil with necessary multiplier resistance (Fig 1).
Advantages
• This instrument can be used both in AC and DC.

• As this is an air cored instrument, the hysteresis and eddy current losses are eliminated.

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