Multiphase Motor

A motor driven by multiphase currents. It is arranged in general terms for distribution of the multi phase currents in coils symmetrically arranged around the circle of the field. These coils are wound on cores of soft iron. A rotating field is thus produced, and a permanent magnet or a polarized armature pivoted in such a field will rotate with the field, its poles following the poles of the rotatory field.

The cut, Fig. 245, illustrates the principles of action of a four phase current motor, connected to a four phase current dynamo or generator. The generator is shown on the left hand of the cut and the motor on the right hand. In the generator the armature N S is supposed to be turned by power in the direction shown by the arrow. Each one of the pair of coils is wound in the reverse sense of the one opposite to it, and the two are connected in series with each other, and with a corresponding pair in the motor. The connection can be readily traced by the letters A A', a a' for one set of coils and B B' b b' for the other set.

For each rotation of the armature two currents, each in opposite direction, are produced in A A', and the same is the case for B B'. These currents which have an absolutely constant relation of phase, and which it will be seen alternate four times for each rotation of the armature, regulate the polarity of the field of the motor. The resultant of their action is to keep the poles of the field magnet of the motor constantly traveling around its circle. Hence the armature N S of the motor, seen on the right hand of the cut, tends to travel around also its north and south poles, following the south and north poles of the rotatory field respectively.

Fig. 245. FOUR-PHASE CURRENT GENERATOR AND MOTOR.

It is not essential that the armature should be a magnet or polarized. Any mass of soft iron will by induction be polarized and will be rotated, although not necessarily synchronously, with the rotatory field. Any mass of copper, such as a disc or cylinder, will have Foucault currents induced in it and will also rotate. The only components of such currents which are useful in driving the motor are those which are at right angles to the lines of force and to the direction of motion. A very good type of armature based on these considerations is a core of soft iron wound with insulated copper wire in one or more closed coils; and so wound as to develop the currents of proper direction.

Such an armature is used in the Tesla alternating current motor. An efficiency of 85 per cent. has been attained with some of the Tesla motors.