Question

A dynamometer type wattmeter with a single scale marked for the smallest power range, has two current ranges, namely, 0-5 A and 0-10 A as well as two voltage ranges, namely, 0-150 V and 0-300 V. To carry out a load test on a 230 V/115 V, 1kVA, single phase transformer, the wattmeter is used on the high voltage side. The voltage and current ranges are chosen for maximum utilization of the scale. The multiplying factor to be used in this case is

• A. 0.5
• B. 1.0
• C. 2.0
• D. 4.0

Answer 1

The correct option is D 4.0

Upper voltage of a 230/115 V, 1 kVA transformer is 230 V.

Power of transformer = 1 kVA = 1000 kVA

$\therefore I=\frac{1000}{230}=4.34A$

$\therefore$ The current comes in the (0 - 5 A) range.

$\therefore$ I, for wattmeter = 5 A

V, for wattmeter = 230 V

Full scale deflection for 230 V = 300 V (FSD)

Multiplying factor (MF)

$=\frac{V\times I\times p.f.}{FSD}$

$=\frac{230\times 5\times 1}{300}(forunityp.f)$

$=3.83\simeq 4.0$