OTIS S-71, Technical Information

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SPEED CONTROL
The accelerator pedal operates a pushrod connected to a series of switches inside the
enclosed box on which the solenoids are mounted. As the accelerator is pressed down,
power is applied to the motor through two solenoids, and through resistors (they look like
coils of heavy wire) mounted under the motor. As the accele rator pedal is pressed further
other solenoids reduce the amount of resistance in the circuit. Five solenoids and two
resistors (one with a tap) offer four speeds. This approach has been used in electric street
railways for over a century [3].
Solenoids LT and LB together in series are used to apply or remove power from the
motor. The S-71 uses these two solenoids (LT and LB) with their contacts in series,
apparently to guarantee that power can be removed from the motor in the event the
contacts on one of the solenoids should weld themselves closed. The other three
solenoids (RT, RM, and RB) are used to short out various parts of the resistance for speed
control.
The accelerator, as it is pressed, offers OFF and positions sequentially labeled (by me) A
(slowest), B, C, and D (fastest). With any one of the motor wires disconnected, and key
ON, you can hear the solenoids turn ON as the accelerator is depressed through its range.
The pushrod between the accelerator pedal and the switches can be adjusted in length so
that, at rest, NO switches are engaged, and so just before the accelerator bottoms on the
floorboard ALL switches are engaged.
The five solenoids are basically automotive starter solenoids with all terminals isolated
from the metal case. The solenoid coils are rated at 12 VDC and show a measured DC
resistance of 12 to 14 ohms. In my S-71 they are each operated on 18 VDC.
NOTE: THE S-71 MOTOR USES UP TO 55 AMPERES. FOR
SATISFACTORY OPERATION IT IS VERY IMPORTANT THAT ALL
CONNECTIONS TO THE MOTOR, BATTERIES, RESISTORS, AND
REVERSING SWITCH BE CLEAN AND TIGHT.
Solenoid ON Accelerator Position
LT, Left Top A, B ,C ,D
LB, Left Bottom A, B, C ,D
RT, Right Top B, C, D
RM, Right Middle C, D
RB, Right Bottom D