The E. M. F. of a Dynamo
المؤلف:
GEORGE A. HOADLEY
المصدر:
ESSENTIALS OF PHYSICS
الجزء والصفحة:
p-414
2025-12-14
21
depends upon three things: speed, number of conductors, and number of lines of force cut. The relation in a two-pole dynamo is expressed by the formula 
in which n is the number of revolutions per second, C is the number of conductors on the armature, N is the number of lines of force in the magnetic circuit, and 10s is a constant necessary to reduce the product of nCN to volts.
The above formula is also applicable to the back electromotive force of .a motor. By this is meant the E. M. F. that is generated in the armature of the motor because of the armature wires cutting the lines of force in its own field.
The existence of this back E. M. F. is shown by coupling a voltmeter to the terminals of a motor and sending current from a dynamo through the motor. The first reading of the voltmeter, before the motor starts, will be small and will be due to the potential drop caused by the resistance of the armature. As soon as the motor starts, the reading of the voltmeter begins to increase, and when the motor is running at its rated speed, the back E. M. F. is nearly as great as the E. M. F. of the dynamo. The voltage sending current through the motor is the difference between the E. M. F. of the dynamo and the back E. M. F. of the motor. This means that as the speed of a motor increases the current decreases.
An electric motor should be run from a current source having a constant electromotive force. From formula 60 we find the value of n to be 
which may be written, 
Since E is constant and since C is also constant, as it is the number of conductors on the armature, it follows that if we wish to change the speed, it must be done by changing N, the number of lines of magnetic force in the field of the motor. Increasing the number of lines of force decreases the speed, and decreasing the number of lines of force increases the speed. This means ww that a motor runs faster with a weak magnetic field.
This speed control is obtained in a shunt motor by putting a rheostat in series with the shunt field winding as shown in Fig. 1. In this way a wide obtained.
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