ROTOR EMF

The revolving magnetic field set up in the stator by polyphase currents is common to both stator and rotor winding. This field induces emfs in both windings. The stator-induced emf per phase is given by the relation

 

E₁ = 4.44 × kw₁ × T₁ × f × ɸ_m        (12.1)

Where kw₁ = winding factor, that is, product or coil span factor k_c and distribution factor k_d.

T₁ = No. of turns/phase of stator winding

ƒ= stator or supply frequency and

ɸ_m= maximum value of flux.

The rotor-induced emf/phase, E₂ = 4.44 × kw₂ × T₂× ƒ_r × ɸ_m        (12.2)

Where ƒ_r is the rotor current frequency, and under stationary condition, that is, at the start ƒ_r= ƒ.

Therefore, rotor-induced emf/phase at standstill or start, E_2s= 4.44 × kw₂ × T₂ × f × ɸ_m

Dividing equation (12.2) by (12.1), we get,

 (i.e., transformation ratio)

From equation (12.2), induced emf in the rotor under running condition,

 

E₂ = 4.44 × kw₂ × T₂ × (S × f ) × ɸ_m = S × E_2s

The induced emf in the rotor circuit is maximum at the start and varies according to the value of slip under running condition. Since the value of normal slip under loaded condition is nearly 5 per cent, the rotor-induced emf is, therefore, nearly 5 per cent of the maximum value.