Hawkins Electrical Guide v. 02 (of 10) / Questions, Answers, & Illustrations, A progressive course of study for engineers, electricians, students and those desiring to acquire a working knowledge of electricity and its applications

by N. Hawkins

Fig. 412.—Principle of the electric motor as illustrated by experiment showing effect of a magnetic field on a wire carrying an electric current. Let a vertical wire ab be rigidly attached to a horizontal wire gh, and let the latter be supported by a ring or other metallic support as shown, so that ab is free to oscillate about gh as an axis. Let the lower end of ab dip into a trough of mercury. When a magnet is held in the position shown and a current from a cell is sent through the wire as indicated, the wire will move in the direction shown by the arrow f, that is, at right angles to the direction of the lines of magnetic force. Let the direction of the current in the wire be reversed, then the direction of the force acting on the wire will be found to be reversed also. The conclusion is that a wire carrying a current in a magnetic field tends to move in a direction at right angles both to the direction of the field and to the direction of the current. The relation between the direction of the magnetic lines, the direction of the current, and the direction of the force, is often remembered by means of the following rule, known as the motor rule, and which differs from the dynamo rule only in that it is applied to the fingers of the left hand instead of to those of the right. Let the forefinger of the left hand point in the direction of the magnetic lines of force and the middle finger in the direction of the current sent through the wire, then will the thumb, at right angles to the other two fingers, point in the direction in which the wire is urged.

Negative lead is the amount of backward advance of the brushes against the direction of the rotation of the armature, measured in degrees from the neutral plane.

If the brushes be given positive lead, that is, placed in advance of the neutral plane in the direction of rotation, the cross magnetizing force is converted into one that tends to increase that of the field magnet, while if they be given negative lead, it tends to demagnetize the field magnet.

Since with positive lead the armature polarity strengthens that of the field magnet, it is possible, disregarding sparking, to operate a motor without any other means being taken to magnetize the field magnets, because the armature will induce a pole in the field magnet and then attract itself towards this induced pole.

Ques. What effect has the cross magnetizing force on the field?

Ans. It tends to shift the field around in a direction opposite to that of the rotation.

Fig. 413.—Current commutation in a motor. Considering the coil W which is ascending, current is flowing through it from the top brush, while it is itself the seat of an electromotive force that tends to stop or reverse its current. The condition for sparkless commutation requires that during the interval the coil is short circuited by the brush, the coil should be passing through a field that is not only sufficiently strong but one that tends to reverse the direction of its current. The coil is already in such a field, hence, commutation must take place before it passes put of this field. To accomplish this the brushes must be shifted backward, that is, given negative lead, to overcome sparking. In other words, the commutating plane must be shifted back of the neutral plane in a motor instead of being placed in advance as in a dynamo.

Ques. What are the conditions of minimum sparking?

Ans. The same conditions must obtain as in a dynamo, that is, the current in the coil undergoing commutation must be brought to rest and started again in the opposite direction. This involves that while the coil is short circuited by the brush, it should be passing through a field that tends to reverse the direction of the current. Since the coil is already in such a field, the act of commutation must take place before it passes out of this field. Accordingly, a negative lead must be given the brushes.

Fig. 414.—Railway motor. This type of motor, since it must operate under cars, has taken on the peculiar form under which it is most familiar. As illustrated, the case is of such shape that compactness and water proofing are secured, and the means of attachment to the car axle and support from the axle and truck frame are provided.