| E. H. P. = | 220 x 65 | = 19.16 |
| 746 |
and since the output is 17.1 horse power,
| efficiency = | output | = | brake horse power | = | 17.1 | = 89% |
| input | electrical horse power | 19.16 |
Speed of a Motor.—The normal speed at which any motor will run is such that the sum of the reverse electromotive force and the drop in the armature will be exactly equal to the electromotive force applied at the brushes. The drop in the armature is the difference between the applied voltage and the reverse voltage.
Mutual Relations of Motor Torque and Speed.—The character of the work to be done not only determines the condition of the motor torque and speed required, but also the suitability of a particular type of motor for a given service. There are three general classes of work performed by motors, and these require the following conditions of torque and speed:
1. Constant torque at variable speed;
Suitable for driving cranes, hoists, and elevators, etc., where the load is constant and has to be moved at varying rates of speed.
Fig. 429.—Two path method of speed regulation of series motor. A rheostat is connected in shunt to the field coils as shown. The current passing from a to b divides between the magnet coils and the rheostat coils; the higher the resistance of the rheostat the less current passes through it, and the more through the magnet coils, hence the stronger the field magnet.
2. Variable torque at constant speed;