A 3-phase, wound-rotor induction motor has a rating of 150 hp (~110 kW), 1760 r/min, 2.3 kV, 60 Hz. Under locked-rotor conditions, the open-circuit rotor voltage between the slip-rings is 500 V. The rotor is driven by a variable-speed dc motor.

Question

Calculate

a. The turns ratio of the stator to rotor windings
b. The rotor voltage and frequency when the rotor is driven at 720 r/min in the same direction as the revolving field
c. The rotor voltage and frequency when the rotor is driven at 720 r/min opposite to the revolving field

Accepted Answer

a. The turns ratio is

[latex]\begin{aligned}a &=E_{1} / E_{ oc }=2300 / 500 \&=4.6\end{aligned}[/latex]

b. The slip at 720 r/min is

[latex]\begin{aligned}s &=\left(n_{ s }-n ight) / n_{ s }=(1800-720) / 1800 \&=0.6\end{aligned}[/latex]

The rotor voltage at 720 r/min is

[latex]\begin{aligned}E_{2} &=s E_{ oc }=0.6 imes 500 \&=300 V\end{aligned}[/latex]

The rotor frequency is

[latex]\begin{aligned}f_{2} &=s f=0.6 imes 60 \&=36 Hz\end{aligned}[/latex]

c. The motor speed is considered to be negative (-) when it turns opposite to the revolving field. The slip at - 720 r/min is

[latex]\begin{aligned}s &=\left(n_{ s }-n ight) / n_{ s } \&=(1800-(-720)) / 1800 \&=(1800+720) / 1800 \&=1.4\end{aligned}[/latex]

The rotor voltage and frequency at - 720 r/min are

[latex]\begin{aligned}E_{2} &=s E_{ oc }=1.4 imes 500 \&=700 V \f_{2} &=s f=1.4 imes 60 \&=84 Hz\end{aligned}[/latex]

Question 14.7: A 3-phase, wound-rotor induction motor has a rating of 150 h...

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