Question

A d.c. ampere-hour meter is rated at 5-A, 250-V. The declared constant is 5 A-s/rev. Express this constant in rev/kWh. Also calculate the full-load speed of the meter

Answer 1

Meter constant = 5 A-s/rev 

Now, 1 kWh = 10³ Wh = 10³ × 3600 volt-second 

= 36 × 10⁵ volt × amp × second

Hence, on a 250-V circuit, this corresponds to 36 × 10⁵/250 = 14,400 A-s 

Since for every 5 A-s, there is one revolution, the number of revolution is one kWh is 

= 14,400/5 = 2,880 revolutions 

∴ Meter constant = 2,800 rev/kWh 

Since full-load meter current is 5 A and its constant is 5 A-s/rev, it is obvious that it makes one revolution every second. 

∴ full load speed = 60 r.p.m