Ans. The apparatus required consists of a small motor, battery, and ammeter. They should be connected in one circuit and the deflection of the ammeter observed when the armature is held stationary, and when it rotates with various loads.
In an experiment of this kind made on a motor with separately excited magnets, the following figures were obtained:
Revolutions per minute 0 50 100 160 180 195 Amperes 20 16.2 12.2 7.8 6.1 5.1 Apparently, if the motor had been helped on to run at 261½ revolutions per minute, the current would have been reduced to zero. In the last result obtained, the current of 5.1 amperes was absorbed in driving the armature against its own friction at the speed of 195 revolutions per minute.
| Revolutions per minute | 0 | 50 | 100 | 160 | 180 | 195 |
| Amperes | 20 | 16.2 | 12.2 | 7.8 | 6.1 | 5.1 |
Fig. 399.—Fairbanks-Morse field coil and pole piece. The field coils are wound upon iron forms, each layer treated with insulating compound. Afterward they are removed from the forms and baked hard and dry and finally wrapped with insulating materials; all but the three smaller sizes are wrapped with a protecting cord. The series and shunt coils of the compound winding here shown are wound separately, the smaller one being the series coil and the larger the shunt coil.
Ques. Explain the action of the current supplied to a motor for its operation.
Ans. The motor current passing through the field magnets polarizes them and establishes a magnetic field, and entering the armature, polarizes its core in such a way that the positive pole of the core is away from the negative pole of the magnetic field, and the negative pole is away from the positive pole of the magnetic field. The magnetic repulsions and attractions thus created cause the armature to rotate in a position of magnetic equilibrium or so as to bring its positive and negative poles opposite the negative and positive poles respectively of the magnetic field. It is evident that unless suitable means were provided to reverse the polarity of the armature core at the instant it reached the position of the magnetic equilibrium, the armature would not rotate any further. The construction is such that the polarity of the armature core, or the direction of the current in the armature coils is reversed at the proper instant automatically by the commutator, thus giving continuous rotation.
Fig. 400.—Fairbanks-Morse armature for 7½ H. P., 1300 R. P. M., TR type motor. The armature core is built up of thin sheet steel laminations with notches in the circumference, which, when the discs are placed together, form grooves or slots to receive the armature coils. The armature cores for the larger machines are mounted on a cast iron spider, which also carries the commutator, making the two parts entirely self-contained, and with this construction, it is possible to remove the armature shaft, without disturbing the core, commutator or windings. Cores of all sizes are provided with ventilating spaces, running from the surface to the central opening of the core, so that air is drawn through the core and blown out over the windings by the revolution of the armature.
Direction of Rotation of Motors.—In the case of either a motor, or a dynamo used as a motor, the direction in which the armature will rotate is easily found by the left hand rule, as illustrated in [fig. 411], when the polarity of the field magnets and the direction of currents through the armature are known.