Suitable for driving line shafting in machine shops, which must run at constant speed regardless of variations of torque due to variations in the number of machines in operation at a time, or the character of work being performed.
3. Variable torque at variable speed.
Suitable for electric railway work. For example: when a car is started, the torque is at its maximum value and the speed zero, but as the car gains headway, the torque decreases and the speed increases.
Speed Regulation of Motors.—The speed of motors connected to constant voltage circuits is usually regulated by the two following methods:
Fig. 430.—Variable field method of speed regulation of series motor. The field winding is divided into a number of sections with leads connecting with switch contact points as illustrated. The speed then is regulated by cutting in or out of the circuit sections of the field winding thus varying the strength of the field.
1. By inserting resistances in the armature circuit of a shunt wound motor;
2. By varying the strength of the field of a series motor.
The first method is sufficiently explained under [fig. 418] and the second method is illustrated in [fig. 430]. The controller switch S is so arranged that a greater or lesser number of field coils can be inserted in the field circuit. When the switch arm is on point 1, the motor current will flow through all the field windings, and the strength of the field will be at its maximum. When the switch arm is moved so as to successively occupy positions 2, 3, and 4, thus cutting out of circuit a greater and greater number of field coils the strength of the field will be gradually decreased until practically all of the motor current is led or wired through the armature. Under these conditions, when the field of a motor is at its maximum strength, the motor torque will be at a maximum for any given strength of current, and the reverse electromotive force will also be at a maximum for any given speed, therefore, when the field strength is increased the speed will decrease and vice versa.
Ques. What results are obtained by this method of regulation?