GATE 2024 EE – Question 65
A single-phase full-controlled thyristor converter bridge is used for regenerative braking of a separately excited DC motor with the following specifications:
| Rated armature voltage | 210 V |
|---|---|
| Rated armature current | 10 A |
| Rated speed | 1200 rpm |
| Armature resistance | 1 Ω |
| Input to the converter bridge | 240 V at 50 Hz |
The armature of the DC motor is fed from the full-controlled bridge and the field current is kept constant.
Assume that the motor is running at 600 rpm and the armature terminals of the motor are suitably reversed for regenerative braking. If the armature current of the motor is to be maintained at the rated value, the triggering angle of the converter bridge in degrees should be ____________ (rounded off to 2 decimal places).
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Correct answer: 114.05 to 115.19
Explanation
Rated back-emf: $E_{rated}=210-10\times1=200$ V at 1200 rpm, so at 600 rpm $E=100$ V. In regenerative braking the machine acts as a generator and the converter must operate as an inverter with $V_{conv}=-(E-I_aR_a)=-(100-10)=-90$ V. The converter output is $\dfrac{2V_m}{\pi}\cos\alpha=\dfrac{2\sqrt2\times240}{\pi}\cos\alpha=216.1\cos\alpha$. So $\cos\alpha=-0.4165$ and $\alpha=114.62^\circ$.