To calibrate wattmeter using D.C. potentiometer kit.
A potentiometer is an instrument which is used for measurement of potential difference across a known resistance or between two terminals of a circuit or network of known characteristic. it is also used for comparing the EMF of two cells. A potentiometer is extensively used in measurement where the precision is required, is higher then that can be obtained by ordinary deflecting instrument. power can also be measured using a potentiometer. Some important for characteristic of potentiometer for which it is used for wattmeter calibration are: –
- Potentiometer measured the unknown voltage by comparing it with a known voltage source rather than by the actual depletion of the pointer. This is capable of high accuracy.
- As a potentiometer measures using null or balance condition, hence no power is required for the measurement.
- Determination of voltage using potentiometer is quite independent of the source resistance.
In this calibration process, the current coil of two-watt meter is supplied from low voltage supply and potential coil from the normal supply through potential divider. The voltage v across the potential coil of the wattmeter under calibration is measured directly by the potentiometer. The current through the coil is measured by measuring the voltage drop across a standard resistor connected in series with the current coil divided by the values of the standard resistor.
The true power is then VI where v is the voltage across the potential coil and I is the current through the current coil or wattmeter. The wattmeter reading may be compared with this value. This type of arrangement is known as “phantom loading”.
- Apply voltage to the pressure or wattmeter & simultaneously apply to the current to the current coil or wattmeter. The current can be varied with the help of the rheostat provided. The wattmeter reads the power.
- The volt ratio box converts the applied voltage in the ratio of 1V:1mV. Observed the step-down voltage in the digital devise connect to the potentiometer kit.
- Similarly, the step-down ratio of current ratio box is 1A:10mA. Observed this reading on the current display panel of the kit.
- Check the null point by the potentiometer scale which has the specification of 1cm=1mV/Div.
- Finally calculate the power from the readings on potentiometer kit (v × I=p) & then calculates the error.
- Tabulate the reading.