DC EMU — EMU and MEMU / Metros Notes

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The 1500V DC EMU was India's original suburban EMU technology, introduced on the Bombay VT–Kurla section in 1925. Even though most sections have since converted to 25kV AC, DC EMU fundamentals (rheostatic series-parallel control) remain a real exam topic, and understanding them makes the later AC EMU chapters easier — many of the same safety-device concepts carry over.

Technical Data

Nominal voltage: 1,500V DC, averaging 1400V in normal working (varies 1200–1600V); can also work down to 800V if a substation is temporarily out of action.
4 traction motors per motor coach. Continuous rating: 187 kW × 4 at 750V, 275A, 1100 rpm. One-hour rating: 210 kW × 4 at 750V, 310A, 1050 rpm.
Wheel diameter: 952mm. Length over headstocks: 20,726mm.
Auxiliary/control supply: a motor-generator set on the underframe, driven at 1500V DC, generates 110V DC. If this set fails, a 110V/70Ah battery covers 3 hours of normal service.

Power Circuit — Series-Parallel Control

Current from the pantograph passes through the roof fuse to the main isolating switch, then to traction-isolation switch and line switches LS1/LS2, which connect supply to starting resistors and traction motors. The 4 motors are arranged as two circuits of 2 motors each, permanently series-connected within each circuit — the notching sequence then controls how those two circuits are combined.

Shunting — Series notch 1: after selecting direction via the reverser key, moving to notch 1 closes LS1, LS2, G2, JR1 — current through the motors is small, giving a smooth start. Contactor RS then closes to establish the shunting notch.
Series notch 2: remaining resistance contactors close in sequence, governed by the Current Limit Relay (CLR).
Transition: when full series notch is reached, contactors P and G1 close and J1 opens — motors move into parallel combination with all resistance still in series.
Parallel notch: resistance contactors cut out progressively (RR1→R1, then RR2→R2 as CLR permits) until fully cut out — motors now run in parallel at full speed.

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Exam Focus — Overload Relay Settings

The overload relay has 3 settings: 395A and 460A for parallel and series operation respectively, and a higher 490A setting brought in specifically for steep gradients — a frequently tested trio of numbers.

The 6 Safety Devices

Every DC EMU power circuit carries these six protective devices

Roof Fuse — rated 900A, on the roof between pantograph and main isolating switch. Protects both traction and auxiliary circuits from earth fault/short circuit.
Overload Relay — an electro-magnetic relay that trips instantaneously above 700A, protecting traction motors from overloading.
Current Balance Relay — works on Hall's principle; detects current finding an abnormal path (e.g. earthing/insulation leakage) and disconnects OHE supply.
No-Current Relay — protects against a high current inrush if OHE supply drops and resumes before back-EMF has built up in the traction motors.
Thermal Relay — a bimetallic relay over the starting resistance, trips the line switches if the resistance compartment temperature exceeds 190°C.
Current Limit Relay (CLR) — governs the sequence of bypassing resistance as the motors accelerate, controlling current to 395A.

Retrofitment to AC-DC

DC EMU stock with bogies still in good condition has been retrofitted with AC-DC equipment for dual-voltage operation — the first such rake was commissioned on 25.03.2011. This involved designing a new 1250 KVA traction transformer, a main silicon rectifier, dual-voltage control system, HT/LT cable layout, roof modification, and undergear modification.