Torque-speed Characteristics:

As seen from the axis of the winding, the MMF at any angle, θ, is

F = F_P  cos θ

Where F_P = Peak MMF, and

θ = Angle measured from winding axis.

We know that

F_P  = F_M  cos ωt

 Where            FM = Maximum value of MMF. From Eqs. (60) and (61), we have

F =( ½) F _M cos (θ - ωt ) + (½) F_M cos (θ + ωt )

where  ½  F_M cos (θ- ωt ) is the forward rotating field, F _fd , and

½  F_M   cos (θ+ ωt ) is the backward rotating field, F_bd

If the rotor rotates at the speed N_r, in the direction of F_fd, the slip of rotor with respect to the F_fd is then

S _fd   = (N_s  - N_r  )/ N_s  = S

 Whereas the rotor slip with respect to the F_bd is

S_bd  = N_s  - (- N_r )/ N_s  = (2 - S )

Therefore the rotor slips with respect to the backward and forward rotating fields are different.

The torque-speed characteristics of a single-phase induction motor is illustrated in Figure

Figure: Torque-speed Characteristics of a Single Winding Single-phase Induction Motor

When N_r = 0; S = 1, the two rotating fields have the same strength and generates equal and opposite torques resulting in net starting torque of zero value. However, If the rotor is made to run at speed N_r in the direction of the forward field, the two slips are how S and (2 - S). The result of weakening of one field and simultaneously strengthening of the other leads to a torque-speed characteristics like that of a three-phase induction motor in the speed region close to N_s.

Single-phase induction motors may be classified in the following way

1. Resistance split-phase motor

2. Capacitor split-phase motor

3. Capacitor-start motor

4. Two value capacitor motor.