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.
Motor, Prime.
A machine used for producing mechanical motion against resistance. It
may operate by converting heat or any other form of kinetic or potential
energy into mechanical energy of the moving type. A steam-engine and a
water-wheel are examples of prime motors.
Motor, Reciprocating.
The early type of motor depending upon reciprocating motion, such as the
motion of a coil in a solenoid. These were based upon the lines of a
steam engine, and have been abandoned except for special purposes where
reciprocating motion is especially required, as in the case of rock
drills.