Master Controller & Tap-Changing — EMU and MEMU / Metros Notes

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The master controller is the driver's single point of control for an AC EMU/MEMU — it closes the motor contactors, operates the tap changer, controls the notching sequence, and (via the reverser) sets the direction of current flow through the traction motor fields.

Dead Man's Handle

A safety-critical part of the master controller — if the driver takes his hand off the handle, it lifts and disconnects supply to the tap changer and traction motors, and simultaneously exhausts BP (brake pipe) pressure through a pilot valve, applying the brakes automatically.

22-Notch Tap-Changing Sequence

The controller has 22 notches beyond OFF, grouped into three power ranges: Shunt (notches 1–2), Half power (notches 3–12), and Full power (notches 13–22). Each notch selects a specific winding (W1 or W2), a tap-changing contactor, and a pair of transfer switches — the table below gives the notch sequence.

Notch	Range	Winding	Tap-Changing Contactor	Transfer Switches
1	Shunt	W1	T1	T7 & T9
2	Shunt	W1	T1	T8
3	Half	W1	T2	T8 & T9
4	Half	W1	T2	T7
5	Half	W1	T3	T7 & T9
6	Half	W1	T3	T8
7	Half	W1	T4	T8 & T9
8	Half	W1	T4	T7
9	Half	W1	T5	T7 & T9
10	Half	W1	T5	T8
11	Half	W1	T6	T8 & T9
12	Half	W1	T6	T7
13	Full	W2	T1	T7 & T9
14	Full	W2	T1	T8
15	Full	W2	T2	T8 & T9
16	Full	W2	T2	T7
17	Full	W2	T3	T7 & T9
18	Full	W2	T3	T8
19	Full	W2	T4	T8 & T9
20	Full	W2	T4	T7
21	Full	W2	T5	T7 & T9
22	Full	W2	T5	T8

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Exam Focus — W1/W2 Changeover

W1 (winding section 1) stays closed up to half power; on the transition to full power, W2 closes and W1 opens. This changeover is done by the Winding Change-Over (WCO) switch — a frequently tested distinction from the tap-changing contactors T1–T9, which select voltage taps within whichever winding is active.

Overall Voltage Steps

Because the secondary winding has 5 tapped sections plus 1 untapped section, the 22 regular notches of the tap changer (via various connections of transformer tappings and voltage-dropping reactors) give a total of 10 equal voltage steps for actual traction power delivery — the 22 notches and 10 voltage steps are two different counts describing the same system, and exam questions sometimes conflate them.