FINDING THE TORQUE–SPEED CHARACTERISTIC OF AN INDUCTION MOTOR FOR VARYING SLIP The equivalent circuit of an induction motor is shown in Figure 14.8. Use PSpice to plot the torque developed against frequency for slip s = 0.1, 0.25, 0.5, and 0.75. The supply frequency is to be varied from 0.1 to 100

Question

FINDING THE TORQUE–SPEED CHARACTERISTIC OF AN INDUCTION MOTOR FOR VARYING SLIP

The equivalent circuit of an induction motor is shown in Figure 14.8. Use PSpice to plot the torque developed against frequency for slip s = 0.1, 0.25, 0.5, and 0.75. The supply frequency is to be varied from 0.1 to 100 Hz. The motor parameters are:

Stator resistance, [latex]R_{s}[/latex] = 0.42 Ω
Stator inductance, [latex]L_{s}[/latex] = 2.18 mH
Rotor resistance, [latex]R_{r}[/latex] = 0.42 Ω (referred to stator)
Rotor inductance, [latex]L_{r}[/latex] = 2.18 mH (referred to stator)
Magnetizing inductance, [latex]L_{m}[/latex] = 58.36 mH (referred to stator)

Accepted Answer

The effective resistance due to slip is

[latex]R_{slip}=R_{r}\frac{1-s}{s} [/latex]

By varying the slip, [latex]R_{slip}[/latex], the torque developed, and [latex]T_{L}[/latex], can be varied.

The PSpice schematic is shown in Figure 14.9. The slip is varied by using the PSpice parametric command .PARAM.

The listing of the circuit file is as follows:

Example 14.3 Torque–speed characteristic of induction motor

SOURCE VS 1 0 AC 170V ; input voltage of 170V
CIRCUIT .PARAM SLIP = 0.05 ; Slip

.PARAM RRES = 0.42    ; Rotor resistance
.PARAM Freq=1kHz          ; switching frequency
.PARAMRSLIP = {RRES* (1-slip)/slip}
.STEP PARAM SLIP LIST 0.1 0.25 0.5 0.75 ; Slip values
VX 1 2 DC 0V                       ; Senses the stator current
RS 2 3 {RRES}
LS 3 4 2.18MH
LM 4 0 58.36MH
LR 4 5 2.18MH
RR 5 6 {RRES}
RX 6 7 {RSLIP}
VY 7 0 DC 0V ; Senses the rotor current

ANALYSIS .AC DEC 100 0.1HZ 100HZ ; Ac analysis

.OPTIONS ABSTOL = 1.00N RETOL = 0.1 VNTOL = 0.1 ITL5 = 0
.PROBE ; Graphics post-processor

. END

The plots of the torque developed [latex]T_{d}[/latex] = V(5) × I(Vy)/2/(2 × 3.14 × Frequency) vs. frequency for various slips are shown in Figure 14.10. The torque developed increases as the slip is reduced. For a fixed slip, there is a region of constant torque.

Question 14.3: FINDING THE TORQUE–SPEED CHARACTERISTIC OF AN INDUCTION MOTO...

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