APT-P Diesel-Alternator

The Diesel-Alternator has been included to provide a stand-by supply of 415 volts and 250 volts AC at 50 Hz if a Motor-Alternator Set should fail, or if the train has to be towed.

The output of the Diesel-Alternator is 250 KVA with the same tolerances on voltage and frequency as the Motor-Alternator Set, due to its output being lower than the M-A certain non-essential loads are shed when the D-A Set is in use. Inter-locking is provided to ensure that both alternators cannot be connected to the supply lines at the same time.

There is a small control panel at the driver's left hand side for control of the M-A and D-A sets. It consists of an on and off switch with indicator for each M-A set Local/Remote and for each D-A set Local/Remote.

M-A and D-A Set Controls
M-A and D-A Control Panel in APT-P Cab.

D-A Set
APT-P Diesel-Alternator Set, located in the equipment bay of the Driving Trailer.

Diesel Unit

The Diesel engine is a 4 Stroke, In-line, 6 Cylinder, Turbocharged CUMMINS NTA 855-G. A hydraulic governor is used to enable the speed (1500 r.p.m.) of the engine (and consequently the frequency of the alternator supply) to be maintained within close limits. It is started by compressed air from the main train air line.

Cummins Diesel Engine


  1. Corrosion Resistor
  2. Air Line Lubricator
  3. Air Line Filter
  4. Air Line Inlet
  5. Lubricating Oil Filter
  6. Lubricating Oil Drain Plug
  7. Lubricating Oil Filler
  8. Fuel Inlet Connection
  9. Air Starter
  10. Pressure Reduction Valve for Ether Start
  11. Water Heater
  12. Ether Injector for Cold Starting
  13. Ether Start Solenoid Valve
  14. Turbocharger
  15. Ether Start Thermostat
  16. Exhaust Outlet
  17. Air Start Solenoid Valve
  18. Water Pump
  19. Lubrication Oil Pressure Switch
  20. Fuel Pipe
  21. Intercooler
  22. Fuel Filters
  23. Overspeed Switch Reset Button
  24. Coolant Level Switch
  25. Governor
  26. Fuel Pump
  27. Water Filter Isolation Valves
  28. Fuel Solenoid Valve
  29. Water Make Up Hose for Engine Filling
  30. Vent Hose


The Alternator is of the brushless type 5800 made by NEWTONS of Derby. It provides 3 phase, 415 volt Alternating Current at 50 Hertz when rotated at 1500 r.p.m. with an output of 250 KVA.

Load Shedding

As the Diesel-Alternator is of a lower rating than the M-A Set the full train load cannot be taken, therefore certain loads are prevented from being connected to the A.C. supply system.

The loads that are shed are -

  1. Large tilt pumps, 11 KW each.
  2. Heating, the 12 KW and 8 KW stages would not be operative.
  3. Air Conditioning, 50% of the load is shed, the fresh air fan is kept running to ensure a fresh air supply.


Video D-A Sets Starting 48103 and 48106.

Video D-A Set Starting 48103.

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Last Revised: 24.05.2021 14:49
by R G Latham
© 1998