Interesting responses so far. Thank you. Kinda what I expected. Also what’s a tap changer? Hadn’t considered locos so that’s an interesting point.
A tap-changer is the control method used in older AC locos (Classes 81-87 for example) and some older AC EMUs (Class 303, 309 for instance).
On these locos the power handle controls the motor voltage, provided from a number of "taps" on the secondary coil of the main transformer, which is increased or decreased in fixed steps to provide more or less power in the motors. (This is different to DC locos/EMUs which used resistance control with camshafts.)
On 81-85s there are 38 voltage steps plus two weak fields notches (40 power settings)
On standard 86s there are 38 voltage steps and no weak field, the motors were designed to provide sufficient current at higher speeds without impairing performance.
On 87s and the 86s with Class 87-style motors/transformers there are 38 voltage steps and one weak field step. The motors on these are in a permanent weakened field of ~17% iirc, the weak field notch adding a further ~19% field weakening.
The power handle has the following positions:
OFF - tap changer runs down if not already done so, no voltage
Run-down - tap changer automatically reduces taps to Zero
Notch-down - runs tap changer down one notch, sprung position returning to "Hold"
Hold - maintains current tap-changer setting
Notch-up - increases voltage tap by one notch, sprung back to "Hold"
Run-up - Tap changer automatically increases voltage taps to full.
Ordinarily, getting under way, the driver adds one or two notches by using the "Notch-up" position. Then when underway adds notch by notch, paying attention to the ammeter (green is continuous rating, yellow is the one-hour rating and used while accelerating, red is the overload and should be avoided but it is permissible to briefly allow the ammeter to enter the red so long as no further notches are added until the meter drops well back into the yellow sector).
Run-up is used to quickly add power at speed, such as after passing a neutral section or returning to linespeed after a speed restriction.
Notch up/down are used to fine control the power as necessary to maintain running speed.
Run-down is used to quickly reduce power, such as in preparation to shutting off completely.
It is permissible to go straight to "OFF" but doing so from a high voltage step can result in severe jolts. Also, the tap changer will automatically run down in this position and power cannot be reapplied and the tap changer run-up, until it has completed running down.
Depending on the equipment fitted, it could take over 35 seconds to fully run up or down the tap changer, the older locos taking longer than 86/87s generally.
On EMUs with this system it is massively simplified, with 4 power notches on the handle:
1 - Shunt, first voltage tap only
2 - Half-voltage or half power, enough voltage taps to provide half the rated voltage are notched in
3 - Full-voltage, all voltage taps are added
4 - Weak field, as above but the weak field contact is opened when the motor current has fallen to a prescribed level after all the voltage taps have been added.
On 303s there are 11 voltage taps and one with weak field added giving 12 notches:
Notch 1 gives 1 notch
Notch 2 gives 7 notches
Notch 3 gives 11 notches (ie all voltage taps)
Notch 4 gives 12 notches (all 11 voltage taps, plus weak field)
Note that, unlike tap changer locos, on the EMUs you cannot reduce the power by going back a step. You have to shut off completely and re-open the handle to the required position. You can however hold a certain voltage tap by allowing the tap changers to notch-up to a certain tap and then moving the handle back to "Shunt"; in poor adhesion hand-notching can be achieved in the same way as on locos.
Standard practice in good conditions was to simply move the handle on EMUs straight to position "4" where road conditions and permitted speed allowed it. The automatic notching equipment would run up the taps at a defined rate with each successive tap opened when motor current fell to a prescribed level.
