25. Speed Control of a DC Motor.- While a DC motor is rotating, it generates a voltage in the same way that a voltage is generated in a DC generator. The generated voltage is in the opposite direction as the applied (line) voltage, but not quite equal to it. The voltage which causes a flow of current through a motor is the voltage difference between the line voltage and the generated voltage of the motor. If the line voltage is 110 volts and the generated voltage of the motor is 105 volts, the difference of voltage (110-105, or 5 volts) is used in causing a current to flow in the motor. The resistance of a motor is rather low, and, as a result, a fairly large value of current will flow through the motor armature for a slight difference of voltage between the line voltage and generated voltage of the motor. The generated voltage of a motor, like the voltage of a generator, depends upon the speed of the motor and the number of lines of force in the field poles. The speed of the motor automatically regulates itself so that the generated voltage will allow just enough current to flow through it to produce the necessary torque (turning power) to keep the motor running. If a braking force is applied to the motor, it will slow down a little so as to allow more current to flow to increase the torque. If the number of lines of force in the field of the motor is increased, the motor does not need to run at as high a speed to produce the necessary generated voltage, and the motor slows down. If the number of lines of force in the motor field is decreased, the motor must speed up to produce the necessary generated voltage. Therefore, if a rheostat such as described for regulating the voltage of a DC generator is used in the field circuit of a, motor, the motor can be made to "speed up," or "slow down," by changing the amount of resistance in the field circuit. See Figure 13. When the resistance of a motor field circuit is decreased, the motor "slows down " and if the resistance is increased, the motor will "speed up." Therefore, the use of a variable resistance in the field circuit is a very practical method of controlling the speed of a DC motor.

26. Rating of Motors and Converting Equipment.- Motors are rated in horsepower (hp.) or kilowatts (kw.). One horse-power is equal to approximately three-quarters of a kilowatt.

A kilowatt is a thousand watts, and a watt is a unit of electric power. The number of watts of power in a DC electric circuit, or in an AC circuit of pure resistance, is equal to the product of the voltage across the circuit and the amperes through the circuit.

DC generators are usually rated in kilowatts.

AC generators and converters are usually rated in kilovolt-amperes (kv-a). The kv-a of a circuit is the product of the volts and amperes divided by 1000. For example: An AC machine that will deliver 20 amperes at 440 volts has a rating of 20 multiplied by 440 and divided by 1000, or 8.8 kv-a. In DC circuits or in AC circuits of pure resistance the kv-a. and kw. are the same, but in some AC circuits this condition does not hold, and the product of the voltage and current is not equal to the watts of power delivered to, or taken from the circuit. In all such cases, the product of volts and amperes is always greater than the watts of power. The reason for this is very complicated and will not be taken up here.

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