Electrical Engineering XYZ MCQs
The DC shunt generator, when commissioned after a major repair of the field system, failed to build up electric voltage. It was because:
- Field winding was wrongly connected
- Field resistance was higher than critical resistance
- There was a break in the armature circuit
- Field system lost residual magnetism
Correct answer: 4. Field system lost residual magnetism
Explanation: For a DC shunt generator to build up electric voltage, it requires the presence of residual magnetism in the field system. Residual magnetism refers to the remaining magnetic flux in the field poles when the generator is not in operation.
If the generator undergoes a major repair of the field system, there is a possibility that the residual magnetism might be lost. Residual magnetism is crucial for the initial generation of voltage during the startup of the generator. Without this residual magnetism, the generator may fail to build up voltage even if all other components are in proper working condition.
The other options can be ruled out with the following explanations:
- Field winding was wrongly connected: While a wrongly connected field winding could cause issues, it is more likely to result in incorrect or unstable voltage regulation rather than a complete failure to build up voltage.
- Field resistance was higher than critical resistance: The critical resistance is a value below which the generator may fail to build up voltage. If the field resistance is higher than the critical resistance, it could affect the generator’s performance, but it is not the primary reason for the failure to build up voltage in this scenario.
- There was a break in the armature circuit: A break in the armature circuit would lead to an open circuit, preventing the flow of current and, consequently, the generation of voltage. However, this is not the specific issue described in the scenario provided.
Therefore, the loss of residual magnetism is the most likely reason for the DC shunt generator’s failure to build up electric voltage after a major repair of the field system.