A 230 V, 10 A, single-phase energymeter makes 90 revolutions

A 230 V, 10 A, single-phase energymeter makes 90 revolutions in 3 minutes at half load rated voltage and unity pf. If the meter constant is 1800 revolutions /kWh, then its error at half load will be

Energy consumed as indicate by energy meter

=	90	= 0.05 kWh
	1800

Actual energy consumed

= 200 ×		10		× 1 ×		3		× 10^-3 = 0.0575 kWh
		2				60

∴ Error = 0.0500 – 0.0575 = – 0.0075 kWh

Hence meter is slow by =	0.0075	× 100 = 15%
	0.0500