LEC Mechanical and Electrical Systems 2 — Questions and Answers
Question 1: What is the primary function of the dynamic brake grid resistors on a diesel-electric locomotive?
- Store electrical energy generated during braking
- Dissipate heat produced by dynamic braking current (Correct answer)
- Regulate voltage to the traction motors
- Cool the main alternator during braking
Correct answer: Dissipate heat produced by dynamic braking current
Grid resistors convert electrical energy from dynamic braking into heat, which is then dissipated into the atmosphere.
Question 2: On a modern AC traction locomotive, what device converts the alternator's AC output to DC before inverters redistribute power to the traction motors?
- Transformer
- Rectifier (Correct answer)
- Exciter
- Load regulator
Correct answer: Rectifier
A rectifier converts the main alternator's AC output to DC, which inverters then convert back to variable-frequency AC for the AC traction motors.
Question 3: A locomotive's air compressor fails to build pressure. Which system is most immediately affected?
- Engine cooling system
- Fuel injection timing
- Automatic and independent brakes (Correct answer)
- Main generator output
Correct answer: Automatic and independent brakes
The automatic and independent brake systems rely on compressed air; without adequate pressure, brakes cannot be applied or released properly.
Question 4: What does the term 'transition' refer to in the context of diesel-electric locomotive operation?
- Switching from diesel to battery power
- Changing traction motor electrical connections to optimize power at different speeds (Correct answer)
- Transferring load between two prime movers
- Activating dynamic braking from motoring mode
Correct answer: Changing traction motor electrical connections to optimize power at different speeds
Transition changes traction motor connections (series to parallel or changes in field strength) to deliver optimum tractive effort across different speed ranges.
Question 5: Which safety device prevents a locomotive engine from overspeeding?
- Low-water cutout
- Engine overspeed governor (Correct answer)
- Wheel slip relay
- Ground relay
Correct answer: Engine overspeed governor
The overspeed governor shuts down the engine if RPM exceeds a safe limit, preventing mechanical damage to the prime mover.
Question 6: What is indicated when the 'Ground Relay' trips on a diesel-electric locomotive?
- The locomotive has lost ground-level traction contact
- An electrical fault to ground has occurred in the power circuit (Correct answer)
- The locomotive brakes have grounded the train
- Engine coolant level is critically low
Correct answer: An electrical fault to ground has occurred in the power circuit
A ground relay trip indicates an unintended electrical path to ground in the high-voltage traction circuit, requiring isolation before proceeding.
Question 7: On a diesel locomotive, what is the purpose of the 'load regulator' (or load control)?
- Regulate fuel flow to each cylinder individually
- Match generator electrical load to engine horsepower to prevent stalling (Correct answer)
- Control the amount of dynamic braking applied
- Limit current to the auxiliary systems
Correct answer: Match generator electrical load to engine horsepower to prevent stalling
The load regulator adjusts generator excitation to balance electrical load with available engine horsepower, preventing the engine from stalling under heavy loads.
LEC Mechanical and Electrical Systems 2
What is the primary function of the dynamic brake grid resistors on a diesel-electric locomotive?