This chapter describes the real power and auxiliary circuit of an older-generation, tap-changer-controlled AC EMU (as distinct from modern IGBT-based three-phase EMUs) — the exact roof-to-motor circuit path, notch-by-notch tap-changer sequencing, and every auxiliary sub-circuit.
Roof Equipment & Transformer — Real Circuit Path
The pantograph connects to the roof bushing of the 25kV cable via the air-blast circuit breaker (ABB), whose live side carries a surge diverter to earth. A roof-mounted earthing switch has two earthing blades that, in the closed position, contact both sides of the ABB isolator arm — earthing both the pantograph and the transformer primary winding simultaneously.
The 25kV cable from the roof bushing passes through the emergency driving cab and enters the transformer through another bushing, connecting to the primary winding. The transformer's return (secondary-side earth path) is brought out by an insulated cable connected to earth brushes on all four axles' traction motors — these earth brushes are insulated from the motor frames, keeping the transformer return independent of the earthing of equipment cases and underframe.
Primary overload protection uses a relay operated through a current transformer: the primary overload relay (OLP) combines with the earth-fault relay (EFR) as a double-element relay sharing a common set of interlocks and reset coil. Tripping either element opens the ABB; both trip elements carry flags to show which relay caused the trip — the flag stays after an electrical reset and must be reset manually.
| Real Parameter | Value |
|---|---|
| Transformer secondary windings | Two separate windings, each 350V |
| First-half tapped section | Divided into 5 sections of 70V each |
| Surge-protection capacitors across secondaries to earth | 2 capacitors, 0.05 μF each |
| Winding-grouping switch contacts | W1 (LV) / W2 (HV) — off-load operation only |
| Tap-changer contactors | T1 to T9 |
| Reactor used for alternate-notch transitions | TL |
In the first half of the notching sequence, only the tapped portion of the secondary is used; for the remaining notches the two sections connect in series. Tap-changer contactors T1-T6 connect to the transformer tappings and select the required voltage; T7 and T8 (with reactor TL) give alternate notches with and without the reactor in circuit — a series resistor is also in circuit with TL, cut out immediately when T9 closes, to ease duty on the opening contactor by reducing circulating current. Selected output voltage feeds through overload-relay trip coils OL5 and OL6 to the main rectifier unit.
Dropping Reactor DL — Real 3 Purposes
Reactor DL sits in series with the output for all notches where only the tapped half of the secondary is in circuit. Its three real purposes: (1) steepen the notching curves on low taps, reducing notching-current swing; (2) increase the transformer's short-circuit reactance on low taps; (3) provide two extra notches, 11 and 12.
Two tertiary windings on the main transformer feed the AC auxiliary machines and DC low-tension circuits respectively.
Tap-Changer Notch Sequence — Real 23-Notch Structure
The tap changer runs through 23 real notches, tied to 4 master-controller positions. Rather than listing every notch's exact contactor combination, the structural landmarks below are what matter most for understanding (and for exam purposes):
| Real Landmark Notch | Real Significance |
|---|---|
| Notch 1 | T1, T7, T9 closed — one 70V section (M3-M4) in circuit with reactor TL in series |
| Notch 2 — SHUNT notch | T1, T8 closed — section M3-M4 direct. Moving the master controller handle to position I lets the equipment proceed automatically up to this notch. |
| Notch 10 | T5, T8 closed — full voltage of the tapped half (M3-M8) applied to rectifiers, DL still in series |
| Notch 11 | T6, T8, T9 closed — untapped half of winding (M1-M2) now in circuit with TL in series, but DL no longer in circuit |
| Notch 12 — HALF POWER connection | T6, T7 closed — untapped winding section to rectifier direct. Moving the master controller handle to position 2 (half power) lets notching proceed automatically up to this notch. |
| Notch 13 | Moving the handle beyond position 2 throws the winding-grouping switch from LV to HV (W1 opens, W2 closes); transition completes when T1 closes and T6 opens, then T9 closes — connecting the untapped winding plus one tapped section (M1-M4) |
| Notch 22 — FULL (PUT.I) POWER connection | T5, T8 closed — connecting the full secondary winding (M1-M8) direct. Selected by moving the master controller handle to position 3 (full power). |
| Notch 23 — WEAK FIELD | Moving the handle to position 4 (weak field) closes the four field-weakening contactors F1-F4, giving the highest running speed. |
- Odd → even notch (e.g. notch 1 → notch 2, with T1/T7/T9 closed): transition starts when T8 closes, short-circuiting reactor TL. T7 and T9 then open off-load in preparation for the next transition, setting up the notch-2 circuit (T1, T8 closed).
- Even → odd notch (e.g. notch 2 → notch 3): transition starts when T2 closes, setting up (in addition to load current through T1 and T8) a circulating current through T2, reactor TL, and the transition resistor (M39-M11). T1 then opens, and the transition completes when T9 closes — this short-circuits the transition resistor, giving the notch-3 circuit (T2, T8, T9 closed).
Transformer Overload Protection — OL5/OL6 Real Logic
Two overload relay coils OL5 and OL6 form a double-element overload relay: OL5 protects against a direct overload from a fault in the rectifier or motor circuits; OL6 protects against four tap-change contactors being closed together simultaneously (e.g. T1, T2, T7 and T8) — electrically impossible under normal interlocking, and detectable only if a contactor fails mechanically. Tripping either OL5 or OL6 opens the ABB.
Rectifier & Traction Motor Circuits
The silicon-rectifier assembly consists of six bridge circuits in parallel, each bridge protected on its AC side by fuses, with a surge-absorbing capacitor and series resistor across the AC input to the rectifier bridges. Rectifier output passes through a smoothing reactor to the motor circuits. The four traction motors are arranged in parallel, connected via four motor contactors M1-M4. Each motor has its own overload relay tripping its motor contactor on overload; direction of rotation reverses by reversing the motor field-winding connections.
| Protection Feature | Real Value/Function |
|---|---|
| Current-limit relay (motors 1 & 3), normal setting | 625 A — controls automatic acceleration |
| Current-limit relay, reduced setting | 550 A — reduced immediately before the notch-10→11 transition, and again before weak field |
| Current-limit relay setting with any motor cut-out (via MCOS) | 450 A |
| Earth Fault Relay (EFR) | Trips ABB on an earth fault anywhere in the motor circuit; carries a flag indicating a trip occurred |
| No-Volt Relay (NVR) | Opens traction-circuit contactors on failure of line voltage |
Motor cut-out switches (MCOS) let the unit operate with any one motor cut out, or with a whole bogie pair of motors cut out.
Auxiliary Circuits — 266V AC and 141V AC
Two separate real auxiliary circuits exist: nominally 266V AC and 110V DC (fed from a 141V AC circuit). The 266V AC circuit is fed from tertiary winding aux.2 and feeds the static battery charger plus induction-motor-driven auxiliary machines: Oil Pump (OP), two Radiator Fans (KF1, KF2), and Main Rectifier Fan (RF). These are capacitor-start-and-run single-phase induction motors, each protected by its own MCB fuse on the live side. A no-volt relay (NVR) — a DC relay fed by a bridge-connected rectifier — sits directly across the 266V supply, detecting transformer-live failure; on line-voltage failure, NVR drops out and disconnects the traction circuit via contacts NVR1/NVR2, protected by its own MCB. Aux.2 winding is protected against wiring faults by two main fuses, AF1 and AF2, inside the transformer connection chamber.
The 141V AC circuit is fed from a second tertiary winding, aux.1, and protected by fuses AF3/AF4 inside the transformer connection chamber. A rectifier-fed relay LTR proves fuses AF3/AF4 are intact and that aux.1's voltage reaches the auxiliary supply rectifier (ASR); LTR's circuit is protected by an MCB. A surge-suppression capacitor with series resistor sits across the ASR input.
Compressor, Fan & Battery Circuits — Real Train-Line Wires
- Main-compressor motor: protected by fuse AF10 + a permanent series resistor; its contactor CC1 is controlled by a trip-and-set relay CR, via the compressor governor. Compressor synchronising across the train uses train-line wire 13. Pressing the COMPRESSOR SET button in any driving cab energises train line 12, setting all compressor relays in the train, and contact CR/1 closes each unit's compressor-contactor coil (CCI) circuit, closing the compressor contactor. Pressing COMPRESSOR TRIP energises train line 42, tripping compressor relays and contactors. A sealed switch GS3 short-circuits the main-compressor governor if it sticks open; a blocking rectifier between wires 1445-1446 stops two units' batteries from paralleling when working in multiple.
- Ventilating fans: fed from two unit lines 29 and 30, energised via a double-pole fan contactor under Guard control (latched type — momentary pulses from FANS ON/FANS OFF buttons in the driving-trailer cab operate it). Protected by fuses VF1/VF2 or MCB.
- Battery circuit: battery positive/negative through fuses BF1/BF2 to a 200A double-pole rotary battery-isolating switch with FOUR positions — OFF, NORMAL, OFF, CHARGE. In either OFF position the battery is fully isolated. In CHARGE, the battery connects to a charging socket in the emergency cab for external-supply charging. In NORMAL, the battery connects to wires DIG and GD, supplying the auxiliary compressor, emergency lights, and control circuits. The static battery charger feeds the battery once the 266V AC auxiliary circuit is energised.
- Auxiliary compressor motor: protected by an MCB fuse, controlled by auxiliary-compressor contactor CC2 (fed from control positive via an MCB). A sealed switch GS4 on the auxiliary-compressor governor can be closed on governor failure. Interlock ABR/1 isolates the auxiliary-compressor circuit whenever the ABB is open.
Emergency Lights, Control Supply & Main Lights
- Two emergency lighting circuits are fed direct from the battery. Emergency head/tail lights are fed through fuse EHTF and a contact on the emergency head-and-tail-lights relay — if supply to the normal head/tail lights fails, this relay opens and lights the emergency lamps automatically. Emergency coach lights are fed through fuse ELF and one pole of a double-pole lighting contactor under Guard control, plus contacts of an emergency-lighting relay — if the main lighting circuits' supply fails, the relay opens and emergency lights come on. Every coach has its own relay controlling its own emergency lighting.
- Control-circuit supply to the whole unit is fed through control fuse CF to unit wire 14; the auxiliary compressor-governor circuit and the LT voltmeter are also fed through this same fuse.
- Main lighting circuits are fed from the regulated 110V supply (wires D8 and GD), protected by fuse MLSF, controlled by the second pole of the same double-pole lighting contactor used for emergency lights (latched, Guard-controlled, momentary LIGHTS ON/LIGHTS OFF pulses from the driving-trailer cab). The circuit splits into two halves, each supplying half the train's lights, each protected by its own fuse — MLF1 and MLF2.