Ans. The speed of a series motor may be nearly doubled, that is, if the lowest permissible speed of the motor be 250 revolutions per minute it can be readily increased to 500 revolutions per minute by changing the field coil connections from series to parallel. It is on this account, as much as on their powerful starting torque, that series motors have been until recently almost exclusively employed for electric traction purposes.
Series Parallel Controller.—When two motors are used in electric railway work, their armatures are connected in series with each other and an extra resistance which prevents the passage of an excessive current through the armature before the motor starts. As the speed of the car increases, the extra resistance is gradually cut out of circuit and the field winding connections changed from series to parallel by means of a series parallel controller, which finally connects each motor directly across the supply mains, or between the trolley line and the track or ground return.
Efficiency of a Motor.—The commercial efficiency of a motor is the ratio of the output to the input. As a rule, the power developed by a motor increases as the reverse voltage generated by it decreases, until this voltage equals one-half of the voltage applied at the brushes. After this point is reached, the power developed by the motor decreases with the decrease of the reverse voltage. Therefore, a motor performs the largest amount of work when its reverse voltage is equal to one-half the impressed voltage.
Fig. 431.—Double-throw, double-pole switch for reversing direction of rotation of a motor. The direction of rotation can be reversed by changing the direction of current in either the armature or the field coils. It is preferable, however, to reverse the direction of rotation by changing the direction of current through the armature. The switch is wired as shown, means of reversal being provided by running the wires as indicated by the dotted lines.
The efficiency of a motor as just stated is the ratio of the output to the input; this is equivalent to saying that the efficiency of a motor is equal to the brake horse power divided by the electrical horse power.
The electrical horse power is easily obtained by multiplying the readings taken from volt meter and ammeter, which gives the watts, and dividing the product by 746, the number of watts per horse power. That is:
| Electrical horse power = | volts × amperes | = | watts |
| 746 | 746 |
Fig. 432.—Wiring diagram, showing electrical connections between the armature, field, and interpoles of an interpole motor. As the name implies, an interpole motor has in addition to the main poles, a series of interpoles which are placed between the main poles, and whose function is to assist in the reversal of the current under the brushes. They provide a separate commutating field of a correct value at all loads and speeds, and their windings are for this purpose connected in series with the armature. The proper functioning of the interpoles is independent of the direction of rotation of the armature, also of the load carried over the whole speed range. In an ordinary motor without interpoles, commutation is assisted by a magnetic fringe emanating from the main poles, but as the value of this fringe is altered by the load of the motor and by rheostatic field weakening, if higher speeds be desired from such a machine, commutation becomes imperfect and sparking results, making a readjustment of the brushes necessary.