The three-phase voltage for the traction motors is generated on the vehicle by two traction converters connected between the main (single-phase) transformer and the traction motors. Both frequency and amplitude of the 3-phase converter output are continuously changed per driver demand, giving continuous, smooth torque/speed adjustment. During electric braking, traction motors act as generators — the converter turns the resulting 3-phase energy into single-phase energy fed back into the line (regenerative brake).
Real Abbreviation Glossary (Converter Documentation)
| Abbreviation | Real Meaning |
|---|---|
| ALG | Drive control unit |
| ASR | Motor converter |
| BUR | Auxiliary converter |
| FLG | Vehicle control unit |
| GU | Gate unit |
| GUSET | Gate unit transmitter/receiver test unit |
| GUSP | Gate unit power supply |
| MUB | Over-voltage protection circuitry |
| NSR | Line converter |
| SLG | Converter control unit |
| SR | Converter |
| Ud | DC-link voltage |
| VS | Valve set |
| ZK | DC-link |
| ZP | Pair of arms |
4 Main Functional Groups & Real Component Designations
The traction converter is modular, consisting of 4 main functional groups: Line converter, DC-link, Over-voltage limitation circuitry (MUB), and Motor converter.
| Real Tag | Component |
|---|---|
| A11, A12 | Line converter — oil-cooled valve sets, 2 pairs of arms each |
| A111-A114, A121-A124 | Gate units (forced-air cooled) |
| K01 | Charging contactor |
| K11 | Converter contactor |
| R04 | Charging resistor |
| U11-U14 | Current transducers (line converter) |
| A31 | DC-link capacitor bank |
| A33 | Series resonant circuit capacitor bank |
| L02 | Series resonant choke (outside converter block) |
| Q21 | Earthing switch |
| R51, R52 | Earthing resistors |
| R71 | Over-voltage limitation resistor (MUB) |
| U01, U02 | Voltage transducers (DC-link voltage measurement) |
| H08 | Voltage indicator |
| U05 | Earth-fault monitoring voltage transducer |
| A21 | Motor converter — oil-cooled valve set, 2 pairs of arms |
| A22 | Oil-cooled valve set — 1 arm for motor converter + 1 arm (MU23) for MUB |
| A211-A222, A223 | Gate units for motor converter / MUB-arm |
| U21-U23 | Current transducers (motor converter) |
| A01 | Converter bus station — houses SLG + ALG |
| A04 | Primary voltage transformer module |
| A08 | Gate unit power supply (GUSP) |
Line Converter (NSR) — Real Function
The line converter consists of two pulse-controlled single-phase full-bridge circuits (A11, A12) connected to a transformer secondary winding. It is a self-commutating 4-quadrant converter: the AC terminals see square-wave voltage pulses of identical amplitude, produced by PWM of the DC-link voltage. Converter input current is in quadrature with EL (voltage across the transformer reactor); since Es equals the vector sum of EL and Ec, changing Ec's amplitude and phase (via PWM) keeps Is in phase with Es, achieving cos φ = 1 in either driving or braking mode. The full-bridge GTOs switch at a frequency much greater than line frequency; switching signals for the 4 pairs of arms are shifted 90° (a quarter switching period) relative to one another, keeping the transformer's primary AC current nearly sinusoidal and line harmonic currents minimised across the whole operating range. The line converter maintains DC-link voltage at a value dependent on power, energy-flow direction, and line voltage. A transformer with high stray inductance (30% ≤ uk ≤ 50%) is used specifically for this converter.
Each of the two converter bridges (A11, A12) has two bipolar switches — realised by two GTO-thyristor arms plus two diodes in anti-parallel. These two pairs of arms, with their snubber-circuit components, are housed in a square aluminium tank forming an oil-filled, forced-oil-cooled valve set. The 4 gate units don't belong to the valve set but sit in its immediate vicinity. Line converter input currents are measured by current transducers U11, U12, U13, U14, whose output feeds the converter controller in the drive control unit (ALG).
DC-Link (ZK) — Real Function
The DC-link connects the line converter to the motor converter, primarily compensating both periodic and non-periodic power differences between the motor-side and line-side terminals. Periodic pulsation arises because power in a symmetric 3-phase system (the traction motors) is constant, while power in a single-phase system (the line converter) pulsates at double the line frequency — so the DC-link is fed a pulsating current at double line frequency from the line converter, while the motor converter draws almost pure DC current. The A33-L02 series resonant circuit filters out this double-line-frequency current — it must be tuned to that exact frequency. Non-periodic power differences — irregular transient surges from sudden power-equilibrium disturbances (pantograph bounce, wheel spin) — cannot be fully avoided but can be minimised; power equilibrium after a disturbance cannot be instantaneously recovered, so there's a certain delay.
The DC-link capacitor (A31) caters to non-periodic power differences and absorbs harmonic currents (frequency higher than double line frequency) from both converters; it's rated so DC-link voltage stays as constant as possible under all operating conditions.
Over-Voltage Limitation Circuit (MUB) — Real Function
If capacitor A31 alone can't prevent sudden inadmissible transient over-voltages, the R71 over-voltage limitation resistor is connected across the DC-link almost immediately by firing a GTO in the A22 valve set, as soon as a certain over-voltage threshold is reached; the GTO turns OFF again once the over-voltage decays. Real causes of DC-link over-voltage: wheel spin, pantograph bounces, and a detuned/defective series resonant circuit (L02, A33). The MUB circuit also discharges the converter DC-link when the vehicle powers down. The MUB-resistor value is 2.5 mΩ, with its temperature monitored by a thermal model in the control electronics. The DC-link additionally carries: DC-voltage measurement transducers U01/U02 (for converter control), voltage indicator H08 and converter earthing switch Q21, and earth-fault monitoring (U05, R51+R52).
Motor Converter (ASR) — Real Function & 3 Control Ranges
The motor converter is a pulse-controlled 3-phase bridge circuit (valve sets A21, A22) connected to the DC-link on one side and the 3 motor stator windings (star-connected) on the other; 2 or 3 motors are connected in parallel per bogie. Each pair of arms generates an AC voltage of square pulses at constant amplitude from the DC-link voltage; both fundamental frequency and amplitude can be changed continuously and independently. The 3 output voltages (at terminals 1U2, 1V2, 1W2) are phase-shifted 120° from each other. Motor torque/speed is controlled by continuously varying both frequency and amplitude of these pulse-shaped phase voltages: in motoring mode, terminal-voltage frequency exceeds the frequency matching motor speed (positive slip → positive torque); in braking, terminal-voltage frequency is lowered below that matching speed (negative slip → braking torque).
| Real Control Range | Real Voltage/Speed Coverage | Real Mechanism |
|---|---|---|
| ISR (Indirect Self-control) | Standstill (0 V) up to ~30% of nominal voltage/frequency | PWM keeps voltage-amplitude-to-frequency ratio roughly constant; motors constantly magnetised at nominal induction, loadable with nominal torque throughout. GTO switching frequency constant over the whole range — motor voltage per half-wave has a variable number of pulses of the same amplitude but differing widths. |
| TB-DSR (Tolerance Band control) | ~30% up to ~98% of nominal voltage/frequency | Motors fully magnetised, loadable with full torque. Torque is controlled to a SET VALUE, not an average; actual-vs-set torque must stay within a preset tolerance band whose width is set by admissible GTO-switching frequency — the converter always runs at the largest possible switching frequency for the smallest possible torque pulsation. |
| Field-Weakening DSR (square-wave operation) | Between nominal speed (nominal voltage) and maximum motor speed | Motor-voltage amplitude held at maximum throughout; only frequency changes, so motor flux and pullout torque are inversely proportional to frequency. Line-line motor voltage in square-wave operation is formed by ONE voltage pulse per half-wave. |
Real Control Hierarchy & Automatic Self-Tests
Control electronics is fully microprocessor-based, all units connected via the MICAS vehicle data bus. Each vehicle has one central vehicle control unit (FLG); each converter has its own converter bus station (A01), containing both the converter control unit (SLG) and drive control unit (ALG, which also holds separate controllers for the line converter and motor converter). The converter has no hand-operated component except the earthing switch Q21.
- Real control chain: FLG calculates required tractive/braking effort from cab demand + instantaneous speed, sends demanded torque to the converter bus station via the vehicle data bus. SLG compares FLG's demanded torque with the effective load torque calculated in the ALG; the difference determines the demanded torque for the ALG. ALG determines the required GTO firing/turn-off pulses, sent to gate-units (A211-A222) via fibre optics — gate-unit outputs are at high potential; actual firing pulses are only generated in the Gate Units (GU) themselves, powered by Gate Unit Power Supply A08 (GUSP). GTOs are also monitored (feedback signals) — a GTO only fires if necessary conditions are met, and the converter shuts down immediately (or won't power up) if a GTO fails.
- Line converter (NSR) is also controlled by ALG to maintain a constant DC-link voltage — reduced if needed power is below rated value, so line-converter loading must match motor-converter loading (active power flow on both sides of the traction converter is the same value and direction, whether driving or braking).
- The converter is regularly auto-tested (at first power-up, and whenever re-powered after e.g. a protective shutdown) with 2 real test phases. OFF-LINE test (before turn-ON, converter de-energised): tests current transducers U11-U23 via simulated signals (test windings), comparators for protection thresholds, and gate-unit/control-electronics power supplies. ON-LINE test (during DC-link charging and operation): tests K01 charging-contactor closure, DC-link voltage reached within a set time, K11 main-contactor closure, U01/U02 plausibility, and MUB readiness — and continuously monitors main-breaker/K01/K11 position, oil-cooling pressure/temperature, DC-link voltage, transducer plausibility, and gate-unit supply voltages during operation. ON-LINE test only starts once OFF-LINE test succeeds.
Converter Turn-On/Off — Real Sequence
Powering-up: after the OFF-LINE test, K01 charging contactor closes so the DC-link precharges through charging resistor R04 and the diodes in valve set A11; immediately afterward K11 main contactor closes. GTO control pulses for the line converter release next, and approximately 200 ms later the pulses for the motor converter (ASR) release. Control-pulse release requires: closed vehicle main circuit breaker, converter powered-up (OFF-LINE and ON-LINE test OK), gate-unit supply (GUSP) OK, reversing switch set to forward/reverse, and DC-link charged.
Powering-down happens if: the vehicle is put out of operation, converter protection stages 4 or 5 have reacted, or line voltage stays too low for a considerable time. During power-down: demanded torque ramps to zero, both converters' firing pulses inhibit, main circuit breaker opens, K11 opens, and the MUB is turned ON to discharge the DC-link.
Converter Protection — Real 5-Stage Concept
| Stage | Real Trigger | Real Effect |
|---|---|---|
| 1 | Converter reaches its control limits (GTO minimum switching-time/interlock monitoring within a pair of arms) | Normal operation maintained; diagnostic system indicates a fault only |
| 2 — power/current limitation | Overhead line under-voltage, OR motor converter operated with fundamental frequency below 1.1 Hz | Reduction of motor torque; no fault indicated |
| 3 — instantaneous voltage limitation (MUB) | DC-link over-voltage threshold reached | Normal operation transiently affected but maintained; no fault indicated |
| 4 — full reduction of load | Transducer plausibility-test failure (thermal drift/defective transducer), MUB-resistor overheat, cooling-medium temperature too high/inlet pressure too low, or converter control circuits reach an abnormal state (e.g. a GTO can't be fired) | Torque ramped to zero (controlled), all GTOs of both converters turned off; converter re-tested via OFF-LINE test and restarted if it passes — else fully powered down (DC-link discharged) |
| 5 — immediate converter shut-down | Max. DC-link voltage exceeded despite MUB (stage 3) activation; MUB responds repeatedly; max. current exceeded at a power terminal; gate units signal an inadmissible switching state/defective GTO; DC-link voltage drops inexplicably below minimum; or gate-unit/control-electronics power supply fails | Converter shut down without delay — MUBs fired via continuous pulse, all GTOs of both converters turned off, vehicle main circuit breaker opened; undergoes OFF-LINE test after shutdown, stays shut down if unsuccessful |
Behaviour Under Line Under-Voltage — Real Power-Derating Curve
If line voltage drops below the minimum allowable value during operation, the motor converter — then immediately after, the line converter (or vice versa, depending on motoring/braking) — turns off. In this blocked state DC-link voltage gradually drops; if line voltage returns inside tolerance before DC-link voltage drops too much, firing pulses release again. If line voltage stays below minimum for more than approximately 10 seconds, the converter powers down; once line voltage recovers, the normal power-up sequence restarts.
IGBT-Based Traction Converter Upgrade — Real Modern Architecture
A newer IGBT-based traction converter is designed to replace the existing GTO traction converters of WAP5, WAP7, and WAG9/9H — including their traction control electronics — with modern IGBT drives, while interfacing/communicating with the existing MICAS-S2 vehicle control electronics. System description: 25kV/50Hz catenary feeds the main transformer primary via pantograph, main circuit breaker, primary CT and roof cable bushing. Two sets of secondary windings (4 windings total) connect to 2 identical IGBT main converters via line and charging contactors, supplying traction power. Secondary AC is rectified/regulated by a PWM converter (IGBT bridges), then the regulated DC feeds a VVVF IGBT 3-phase PWM inverter — each inverter feeds 3 individual traction motors (i.e. axle/independent control).
- Real electric braking: regenerative, with generated power fed back to catenary via the VVVF inverters and PWM converters. Electrical braking is supplemented by EP brakes via brake blending, with electrical braking given priority; EP braking supplements only when needed to meet total brake demand.
- The Traction Control Unit (TCU) is based on a Digital Signal Processor (DSP), a supervisory microprocessor, and IO cards — handling IGBT main-traction-converter/inverter control and braking control. Real switching frequencies: 250 Hz for the Four-Quadrant Controller's IGBTs; approx. 500 Hz for the inverter (reduced at higher motor speed due to the inverter's pulse pattern).
- Real traction performance is guaranteed for line voltage between 19 and 27.5 kVAC, continuous at full power throughout — so 1-hour rating and continuous rating are the same (though existing traction motors/components may still limit continuous power by design). Below 19kV line voltage, line current is limited to the value at 19kV, down to 16kV; converter power is limited accordingly (auxiliary load excluded from this current-limitation figure).
- The power circuit uses IGBT water-cooled power modules, each built with 2 identical phase legs (usable in parallel without reducing per-module power rating), each with its own DC-link capacitor, plus 2 current transducers and 1 voltage transducer per module.
- Real architecture: DCU (Drive Control Unit) controls the traction converter; VIU is the interface between 'standard' traction control and locomotive control (via train-line wires/Vehicle Bus), coupled to the DCUs via a dedicated point-to-point high-speed traction network. The locomotive's fibre-optic MVB network converts to electrical/copper via the MFI (MVB Fibre Interface) PCB. The TCU is built from 1 VIU (Vehicle Interface Unit) + 3 DCUs: DCU1 controls a Four-Quadrant Controller (FQC) + an inverter; DCU2 controls an inverter + a Soft Crow Bar (limits DC-link transient over-voltage during slip, to avoid tripping the converter); DCU3 controls a FQC + an inverter. The 3 DCU boards are arranged so a failure of any one loses at most one motor. DCU/VIU real processor basis: DCU uses a DSP + FPGA; VIU uses the same FPGA type but NO DSP. Both have an Ethernet maintenance connection for laptop-based software/parameter upload and fault-log/diagnostic download. Each traction converter uses 2 power supplies in parallel, diode-paralleled for built-in redundancy, monitored by control electronics (error message if one is defective).
- Real modern inverter-control functions: speed-sensor or sensor-less vector control (each motor individually controlled, with automatic sensor-less fallback if a speed sensor is defective); modern slip/slide control using 'creepage control' (also called 'inching control') to guarantee optimum use of available track adhesion, individually per traction motor; maximum-torque limiting; wheel-diameter correction (equal effort per axle regardless of actual wheel diameter); line-break control (inverters enter light braking mode on OHE-voltage loss, keeping DC-link capacitors energised); jerk-rate limitation; effort curves; fault registration with time-stamped malfunction records; and fault handling.
WAP7 Hotel Load System — Real Detail
WAP7 is essentially a passenger version of the freight WAG9, with modified gear ratio and software for higher-speed operation. It reuses WAP5's transformer, which has a 945 kVA hotel-load winding. This winding feeds 2 × 500 kVA static converters, converting single-phase 750V supply into 750V 3-phase supply, transmitted to the feeder of the existing EOG (End-On-Generation) train via an IV coupler. The hotel-load circuit's real protective/monitoring chain includes surge arresters, earth-fault detection, current sensors, an input contactor for the hotel load, 2 hotel-load converters each with sine-filter chokes/capacitors and output contactors, delivering 3-phase 3-wire 750V/50Hz output.