GATE EE 2024 Set 1 — Question 65
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Electrical Machines → AC-DC Converters → DC Motor Braking
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Question
A single-phase full-controlled thyristor converter bridge is used for regenerative braking of a separately excited DC motor with the following specifications:
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).
| Parameter | Value |
|---|---|
| Rated armature voltage | 210 V |
| Rated armature current | 10 A |
| Rated speed | 1200 rpm |
| Armature resistance | 1 |
| Input to the converter bridge | 240 V at 50 Hz |
Correct answer
113 to 116
Solution
1.Calculate the back EMF at rated conditions:
At rated voltage , rated current , and armature resistance :2.Calculate the back EMF at 600 rpm:
Since the field current is constant, the back EMF is directly proportional to the speed ():3.Determine the required converter output voltage for regenerative braking:
In regenerative braking, the armature terminals are reversed. The motor acts as a generator feeding power back to the AC source through the converter. The average output voltage of the converter must satisfy:Maintaining the rated current :
4.Calculate the triggering angle :
For a single-phase full-controlled bridge, the average output voltage is given by:Given the input RMS voltage , the peak voltage is :
The triggering angle should be approximately .
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