This chapter covers three specific, real maintenance procedures for self-generating (SG) TL & AC coach auxiliary components: V-belt tensioning (the mechanical link between axle and alternator), compressor motor ratings, and Over Voltage Protection (OVP) testing.
V-Belt Tensioning
The alternator is driven off the coach axle via V-belts under spring tension. Per RDSO/PE/SMI/AC/0042-2009, the correct tensioning device and spring must be confirmed before commissioning a new belt (production, POH/IOH, or any spring/tensioning device replacement).
| Alternator Capacity | Drawing No. | Spring Turns | Spring Wire Dia. |
|---|---|---|---|
| 25 kW | ROSO/PE/SK/AC/0068-2004 (Rev.0) | 11 | 16 |
| 4.5 kW | SKEL 3940 | 14 | 14 |
- The tensioning device's indication plate is cut to 250mm regardless of alternator rating — it only signals that the spring needs replacing if proper tension isn't achieved with a 4 kg weight test.
- Correct tension check: hang a 4 kg weight from the centre of the belt span — the belt's top surface should not go below the bottom surface of the remaining belts.
- A 30cm steel scale should pass easily between the V-belts once properly tightened; if tension still isn't achieved at 250mm of spring length, the tension rod must be replaced.
- Barrel bush gap in the bogie bracket: 35mm for TL coaches, 50mm for AC coaches — a real, specific, exam-testable distinction.
Compressor Motor (Under-Slung AC Coaches)
Compressor motors on under-slung AC coaches come in three ratings, and the choice depends on coach type.
- 6.35 kW (8.5 HP) and 7.5 kW (10 HP) motors — used in standard AC coaches.
- 9.35 kW (12.5 HP) motors — specifically used in AC Chair Cars, which have a higher passenger density and cooling load.
- Direction of rotation: clockwise for the 6.35 kW motor, counter-clockwise for the 7.5 kW and 9.35 kW motors, viewed from the non-driving end — a real, specific distinction worth memorizing since it's easy to reverse-connect otherwise.
Over Voltage Protection (OVP) Unit
The alternator's induced voltage depends on rotor speed and field excitation current, which is regulated by a magnetic amplifier and electronic control circuitry inside the RRU/ERRU. If a fault in that excitation control circuit causes over-generation, the static OVP unit is the safety device that intervenes — stopping over-generation to protect coach electrical appliances and maintain fail-safe operation.
OVP units come from multiple OEMs (PIPL, HMTD, ICECPL, SIL, Stesalit, Best & Crompton, Kapson, BHELEML), each with a slightly different in-situ testing procedure for both 4.5kW and 25kW ratings — each vendor's specific test steps are documented separately, since a generic procedure doesn't apply uniformly across makes.