Question

A moving coil ammeter a thermal ammeter and a rectifier are connected in series with a resistor across a 110V ac supply

The circuit has a resistance of 50ohm to current in one direction and due to rectifier on infinite resistance to the current in infinite direction calculate

The reading on the ammeter

The form and peak factor of the current wave.

Due to rectifier only half wave of ac is conducted and the moving coil reads the average over a whole cycle even though current passes for half a cycle.

Peak value applied voltage

Vmx = √2 x 110 = 155.54V

Resistor current = V/ R = 155.54 / 50 = 3.11 amp

Moving coil ammeter reads current = ½ x [ 2/π x 3.11] = 0.99amp

Thermal ammeter would give a reading dependant on the average power

It is for full cycle in this case

= ½ [ rms value]

= ½ [ Vm / R]

= ½ x 100 / 50 = 1.1Amps

Form factor = rms value / average value = 110/√2 / 2/π x 110 = 110 / √2 x π/ 2x110 = 1.11.

Peak factor = maximum value / rms value = 110/ 110/√2 = √2 = 1.414