PCC Streetcar - Performance

Performance

Westinghouse developed the XD-323 rotary accelerator with 99 points; it was installed in the first PRCo car, #100, and minor modifications allowed use in the last PCCs produced in North America for San Francisco in 1952. Resistance ribbons were mounted to each point around the outside edge of the accelerator. An arm rotating in the center had rollers on either end which cut out resistance alternately as it rotated approximately 180 degrees. This same accelerator was also used for dynamic braking; when the power pedal was released the accelerator sought optimum braking for the speed, which prevented a lag when the brake pedal was depressed. General Electric Developed a control system for PCC cars that mirrored the Westinghouse scheme in function, although not in simplicity or maintainability. With the GE commutator motor controller operating by air pressure, it had to be redesigned with the advent of the All-Electric PCC. Acceleration was variable between 1.5- and 4.75-mph per second depending upon the depression of the power pedal with the accelerator advanced automatically by a low-voltage pilot motor. Service braking was also variable and the maximum dynamic application decreased speed by 4.75-mph/s; pressing the brake pedal into emergency also brought the friction and magnetic brakes into play providing a maximum deceleration of 9.0-mph/s. Compared to a maximum of 14 points on old time equipment, the PCC was considerably smoother.

Most PCCs employed three pedals with a dead man's switch to the left, brake in center, and power pedal on the right. Depressing the brake about half way and then releasing the deadman pedal put the PCC in "park". Lifting the deadman alone would apply all brakes, drop sand, and balance the doors so they could be pushed open easily. Chicago used "bicycle-type levers" for power and brake but converted some cars to two pedals. (SLPS) used two pedals, both with heel interlocks. The right pedal is the brake; depressing this pedal about half way while lifting away from the heel applied "park". Once the brake is released the heel need not be engaged with the interlock (although a professional driver is to cover the brake at all times.) The left pedal applied the power and the heel interlock had to be engaged at all times since it was the deadman; only when the brake was in "park" could the deadman be disengaged.

SLPS is unique in that all 300 of their PCCs are All-Electric with the 1500s ordered in late 1939, the 1600s ordered late 1940s and the 1700s in January 1945. SLPS was the rolling laboratory for All-Electrics and what was learned here was applied to the post-WW2 All-Electric Demonstrator in the Fall of 1945.

From 1936 to 1945, PCC cars were 'Air-Electrics' with friction brakes, doors, and windshield wipers operated by air pressure. PRCo PCC 1600 of 1945 was the post WW2 All-Electric Demonstrator which eliminated the air compressor and associated piping while incorporating such features as standee windows, a sloped windshield to eliminate night time glare, redesigned back end, forced-air ventilation, and other features. Dynamic brakes were the service brake on all PCCs; when almost stopped, friction brakes completed the stop and held the car in "park". Dynamic brakes slowed the "Air" cars to 3.0-mph at which point a lock-out relay allowed automatic application of air-applied friction brakes against each of the eight wheels. On All-Electric cars the dynamics were effective to 0.75-mph where the lockout relay then allowed a spring applied friction brake to engage a drum on each of the four motor drive shafts; this completed the stop and held the car in park. Drum brakes were released by an electric solenoid operating from low-voltage battery power; a power failure would prevent the drums from releasing which would prevent power application, a fail-safe feature. Drum brakes were quite popular and greatly reduced maintenance thus some "Air" cars were retrofitted with drums. Four magnetic brakes, one between the wheels on each side of each truck, applied additional braking for emergency stopping where all brakes were generally employed.

"These performances enable the P.C.C. car to out-pace the average automobile which, in America, is of substantially higher performance than the typical British vehicle."