Railway motors have usually four poles because this permits of a symmetrical and economical arrangement of material around the armature, and hence permits the motor to be placed in the small space available on the car truck. Two-pole motors have been used in the past, but they were not as compact as the four-pole type.
Characteristics of Railway Motors. The curve sheet, [Fig. 4], for the Westinghouse No. 69 motor represents in general the characteristics of all direct-current railway motors.
The figures for each curve are found with names corresponding to the curve to which they apply, at each side represented by vertical distance on the sheet. The amperes, represented by the horizontal distance, are marked at the bottom, and apply in common to all the curves.
The tractive effort at different current consumption is represented by a line curving upwards somewhat. This shows that the tractive effort increases, in a proportion greater than directly, as the current increases.
The torque required in starting may be many times greater than that necessary to maintain the car at full speed. The series-wound motor, therefore, furnishes this great starting torque more economically than a shunt-wound motor the torque of which is proportioned to the current. This feature of the series-wound motor makes it especially adapted to street railway work.
WESTINGHOUSE
No. 69 RAILWAY MOTOR
500 VOLTS
GEAR RATIO, 14 TO 68. WHEELS, 33″
CONTINUOUS CAPACITY, 25 AMPERES AT 300 VOLTS,
OR 23 AMPERES AT 400 VOLTS.
Fig. 4. Characteristic Curves of Railway Motor.
The efficiency curve shows the motor to have an efficiency of about 83 per cent with gears. Much other information may be obtained by a proper study of the curves. The fields are worked near the point of magnetic saturation. This economizes metal and space and is also an advantage because of the fact that when so worked the armature reactions have very little effect on the fields. The neutral points between fields are consequently shifted very little and it is therefore not necessary to shift the brushes when the motor is reversed.
