For all you circuit-heads out there… I answered a Quiz post about MU controls and the use of them in trains where different cars had different controllers. After re-reading my post I find I made at least one error, which I will correct here, along with supplying more info. For those of you interested in this sort of thing there’s lots more to add, including MU control of diesel engines. To start with, we’re going to look at Rapid Transit, in particular the Chicago L, but also Boston’s T. I’m sure you New York guys will weigh in here, too. Info from Cera Bulletin 115, some excellent drawings and diagrams by Seashore Trolley Museum’s Gerry O’Regan, and an industrial electical handbook set one of my office mates has in his bookcase.
Sprague Control (1897)
This is the granddaddy of MU control. The simple idea was to replace the handle on a platform controller (Type L on the first cars) with a pilot motor. The operator’s controller and the pilot motor were powered by using line power through dropping resistors. The pilot motor had windings for both rotational directions (on and off, so to speak). As long as the motor current stayed below a preset value, the pilot motor was free to advance the controller. Brakes on the shaft and other relays prevented the controller from being advanced to parallel when the operators handle was set to series. Putting the operator’s handle in the coast or off position caused the motor to “rewind”. The reverser, which was a pair of steel bars controlled by springs and electromagnets, would open if the controller was put in the “off” position, generating a loud bang under the passenger seat the reverser was under.
GE Type M (1901)
Type M used switches held open by springs and gravity and closed by electromagnets. Since blowout coils could be near all of the switches, larger motors could be handled. Type M also did
Original-type Sprague controls were used only on Chicago’s South Side Rapid Transit and on some New York cars. Interoperation of Spragues with later control systems was not completely straightforward. Most of the difference had to do with the coasting control (Sprague) which was not present on the Type M. A fix for that was to have the Type M’s treat the coast wire as if it were an instruction to shut off the controller rather than simply to remove power. As far as I can determine, this wasonly done in New York.
South Side Rapid Transit[.] had a series of AB equipped cars that were specifically set up to MU with Spragues. The line-controlled Sprague cars were plugged into a special receptacle on the AB cars. The control current from the Sprague cars operated relays that switched the battery-powered control leads on the AB cars, causing the AB controller to operate the motors. Presumably there was a similar set of relays to control the Sprague cars form the AB-equipped cars, but I haven’t seen a diagram.
Boston had Center-entrance cars with both AB and PC control. They were designed to interoperate with each other, but training them often resulted in bucking.
There is a little more history to this. Early 10 wire interurban MU control, evolved into the 37 wire DMU train lines which are accomplished through electrical contact boards attached to the couplers. Typical modern EMUs can have couplers with over 100 wires for MU control. There are also many other variations depending whether units were permanently coupled through articulation, whether they had multiple operating controls and hostling controls…
The Boston Center-Entrance cars with Westinghouse controls had type ABPC, which was specifically designed to interwork with GE PC-5 controls. WH also made an ABM variant that would work with either GE M or PC-10. The key to interoperation without bucking in either case was to use the master controller in the WH equipped car.
Chicago Rapid Transit 4000 series steel cars were mostly equipped with ABLFM control with some cars having PC-10 instead. Line Power on the Met, Battery everywhere else. A CRT mix of wood and steel cars could have AB (South Side), AL (Met), AB (North Side) or M (North Side, Lake Street or Met). Met cars were rarely mixed with North Side, South Side or Lake Street cars. The only cars not mixable with the 4000s were the original South Side Sprague cars, which would only MU with each other and South Side AB-equipped cars.