A small direct-current motor, such as can be purchased for about a dollar, will operate in various ways as an alternating-current motor. These methods of operation are not recommended for regular use, but they serve as excellent experiments with alternating currents.

A series-wound motor with a three-part commutator is suitable. Owing to the variety of such motors on the market, only general directions can be given here, leaving the details to the judgment of the experimenter. If the motor is to be connected to an alternating-current circuit of about 110 volts, it is necessary to have some means of limiting the current passing through the windings. The diagrams show a lamp rheostat used for this purpose. The rated voltage of the lamps should not be less than the voltage of the circuit, for the resistance of the motor may be so low that the lamps will receive almost the full voltage of the supply, and they would then be burned out if made for a lower voltage. A rheostat enabling any number not exceeding ten of [6-candle-power carbon filament lamps to be connected in parallel is large enough. In determining whether the wiring and fuses through which the current is supplied have sufficient carrying capacity, remember that each lamp takes about 1/2 ampere when supplied with its rated voltage. The current per lamp will be less than this when the motor is in series with the lamp rheostat.

Make the connections to the motor with all of the lamps turned off, and start by turning on the lamps until the motor receives sufficient current. Do not turn on so many lamps that the motor attains an excessive speed or temperature.

Of course, a suitable step-down transformer or a reactance coil may be used instead of a rheostat.

When the motor is in proper condition to operate as a direct-current series motor, it may be operated as an alternating-current series motor. With the exception of inserting the rheostat, the motor is connected to the alternating-current circuit in the same way that it is connected to a battery when run as a direct-current motor. (See Fig. 248.) To reverse the direction of rotation, transpose the wires connected to the brush holders, as would be done to reverse it when operating with direct current.

A repulsion motor consists of a stationary field magnet, through the winding of which alternating current is passed. The armature is similar to a direct-current armature. Instead of the two brushes, or, in larger machines, the two sets of brushes (corresponding to the positive and negative sets of brushes in a direct-current machine) being insulated from one another, they are connected together. To run the motor as a repulsion motor, it will be necessary to shift the brushes until the proper position for operation is found. If the brushes supplied with the machine can be readily shifted, then they may be connected together by attaching a wire to the two brush holders. If they cannot be readily shifted, remove them and bend a piece of copper wire into the shape shown in the diagram (Fig. 249) so that it can embrace the commutator and touch it at diametrically opposite points. This wire acts as two brashes connected to one another. and for experimental purposes may be held in place by hand. After the brushes have been arranged, pass current through the field winding, as shown in Fig. 249, and vary the position of the brushes until the motor runs.

Fig. 248

Fig. 249

Fig. 250

Fig. 251

Figs. 248 to 251 - Various methods of connecting the motor.

The inverted repulsion motor differs from the repulsion motor in that the alternating current is supplied to the armature, and the field winding is short-circuited. To obtain this motor (Fig. 250) connect together the two ends of the field winding and supply current to the armature. As was the case with the repulsion motor, it is here necessary to shift the brushes until the proper position for operation is found. If the brushes supplied with the motor can be readily shifted, supply current to the armature through them. Otherwise, the current may be supplied to the armature by removing the regular brushes and pressing the wires carrying the current against the commutator at two diametrically opposite points, shifting them until the proper position for operation is found. The repulsion and inverted repulsion motors are reversed by shifting the brushes.

In the single-phase induction motor current is supplied to the stationary winding, and the revolving part consists of a winding having short-circuited coils, or else a squirrel-cage winding.

To obtain the induction motor, wrap a few turns of wire around the commutator, so that each coil of the armature is short-circuited. Run without brushes, supply current to the field winding only, according to Fig. 251. Unlike the other motors here described, the single-phase induction motor is not self-starting unless special devices are provided to make it so. When these devices are absent, as in the case here, the motor will run equally well in either direction when once started. Start by giving the shaft a twist with the fingers or by wrapping a piece of string around the shaft and rapidly pulling it off.

For the theory of these motors, and also the modifications in construction used to secure better operation, text books on alternating currents should be consulted.