Fig. 395.—Circuit with generator and motor. Whenever current flows through some portion of a circuit in which there is an electromotive force, the current will there either receive or give up energy according to whether the electromotive force acts with the current or against it. In the figure, the generator and motor are rotating clockwise, and hence each generates an electromotive force tending upwards from the lower brush to the higher. In each case the upper brush is the positive one. In the dynamo, where energy is being supplied to the circuit, the electromotive force is in the same direction as the current, while in the motor where work is being done and energy is leaving the circuit, the electromotive force is in a direction which opposes the current.

Ques. In the operation of a motor what is the nature of the reverse electromotive force?

Ans. It is proportional to the velocity of rotation, the strength of the magnets, and to the number and arrangement of the wires on the armature, that is, the reverse voltage depends on the rate at which the lines of force are cut.

Figs. 396 and 397.—Water and electric circuits. Diagrams showing comparison between water motor and electric motor.

In the diagrams:
The pumpcorrespondstothedynamo.
The high level pipe"""positive conductor.
The low level pipe"""negative conductor.
The valve"""switch.
The water motor"""electric motor.
The water pressure (called head)"""electric pressure (called voltage).
The flow in gallons per minute"""amperes.
The size of pipe"""size of conductor.
The foot pounds"""watts.

The greater the difference between the height of the two pipes the higher the pressure, and the greater the difference between the pressures of the two conductors the higher the voltage. The larger the diameter of the pipes the less resistance is offered to the flow of water, and the larger the diameter of the conductors the less resistance is offered to the flow of electricity. The more water required by the water wheel, the more power is required to drive the pump. The more electricity required by the motor the more power is required to drive the generator.

Fig. 398.—Fairbanks-Morse standard TR type motor. This type is built in the smaller sizes and the design is such that the motor can be installed upon the floor, wall or ceiling, the bearing yokes being attached to the frame by four equally spaced bolts so that they can be turned to provide for proper operation of the oiling devices in either position. A substantial base is provided with a thrust screw for adjusting the belt tension. This base has clamping bolts which permit adjusting the position of the motor while suspended. There is a cast ring type frame having steel side pieces which press firmly together, the steel laminations making up the pole pieces. The field coils armature, and armature coils are illustrated in detail in figs. 399 to 401. The commutator bars are of drawn copper, insulated with mica. The lugs which extend outward from the bars to receive the lead wires from the armature windings are formed in one piece with the bars, and are of the full width of the bars with the insulator extending outward between them, so that when assembled a solid flange is formed to receive the armature connections. Self-oiling bearings are provided and the location of the bearing sleeves in the housing is adjustable so that the armature may be centered in the magnetic field. The brush rigging is carried on a skeleton rocker supported in a groove, turned in the edge of the frame. The brush holders are of the box type with independently adjustable tension spring for each. Standard shunt windings are for 115, 230 and 550 volts. The compound wound motors operate at approximately the same full load speeds as the shunt wound, but the no load speeds will be about 20 per cent. higher than the full load speeds. They have, however, the ability to exert a more powerful starting effort than shunt motors without drawing such a heavy current from the line, and are, therefore, especially adapted for driving apparatus that has to be frequently started and stopped under load and where close speed regulation is not required.

Ques. Describe an experiment which shows the existence of a reverse electromotive force in a motor.