LEC Drivetrain & Transmission 2 — Questions and Answers
Question 1: What is the primary function of a locomotive's main generator (alternator) in a diesel-electric drivetrain?
- Store electrical energy in batteries
- Convert diesel engine mechanical output to electrical power for traction motors (Correct answer)
- Directly drive the axles through a gear reduction system
- Provide auxiliary power for HVAC and lighting only
Correct answer: Convert diesel engine mechanical output to electrical power for traction motors
The main generator converts the diesel engine's mechanical energy into electrical power that feeds the traction motors on each axle.
Question 2: In a diesel-electric locomotive, what component controls the amount of electrical power delivered to the traction motors?
- The fuel injector pump
- The air compressor governor
- The load regulator (throttle control system) (Correct answer)
- The dynamic brake grid resistors
Correct answer: The load regulator (throttle control system)
The load regulator adjusts field excitation and generator output to control power delivered to traction motors in response to throttle position.
Question 3: What happens to wheel slip in a diesel-electric locomotive when tractive effort exceeds the rail-wheel adhesion limit?
- The locomotive automatically switches to diesel-hydraulic mode
- The traction motor speeds up unloaded, causing wheel spin (Correct answer)
- The main generator shuts down to protect the engine
- The dynamic brakes automatically engage to prevent slipping
Correct answer: The traction motor speeds up unloaded, causing wheel spin
When adhesion is exceeded, the drive wheels lose traction and spin freely, causing the traction motor to accelerate rapidly without load.
Question 4: On a six-axle diesel-electric locomotive, how are traction motors typically connected electrically during starting operations?
- All six motors in parallel for maximum speed
- Motors grouped in series-parallel combinations for high starting torque (Correct answer)
- All six motors in series for maximum voltage
- Motors switched off alternately to reduce heat buildup
Correct answer: Motors grouped in series-parallel combinations for high starting torque
Series-parallel groupings allow high torque at low speed by limiting current through individual motors while maintaining adequate voltage.
Question 5: What is the purpose of the transition in a diesel-electric locomotive's power circuit?
- To switch from diesel to electric utility power at yards
- To change traction motor connections from series to parallel as speed increases (Correct answer)
- To transfer power between the A and B units of a locomotive consist
- To engage the dynamic brakes when descending grades
Correct answer: To change traction motor connections from series to parallel as speed increases
Transition reconnects traction motors from series to parallel to allow higher speeds while maintaining efficient power utilization.
Question 6: A locomotive engineer notices the wheel slip indicator activating repeatedly on wet rail. The BEST corrective action is to:
- Increase throttle to power through the slip condition
- Apply sand and reduce throttle to restore adhesion (Correct answer)
- Engage the independent brake and hold speed constant
- Switch to the alternate traction motor configuration
Correct answer: Apply sand and reduce throttle to restore adhesion
Sanding the rail increases friction at the wheel-rail interface, and reducing throttle lowers tractive effort to within the available adhesion limit.
Question 7: What is 'creep control' or 'adhesion control' in modern locomotive drivetrains?
- A system that limits maximum speed on curves
- An electronic system that optimizes wheel slip to maximize tractive effort (Correct answer)
- A mechanical brake that prevents rollback on grades
- A governor that maintains constant engine RPM under varying loads
Correct answer: An electronic system that optimizes wheel slip to maximize tractive effort
Adhesion control systems monitor and maintain a small, optimal amount of wheel creep that actually maximizes available tractive force at the rail-wheel interface.
LEC Drivetrain & Transmission 2
What is the primary function of a locomotive's main generator (alternator) in a diesel-electric drivetrain?