The rotating magnetic field mentioned above does not refer to a rotation of
mechanical parts, but a rotation of magnetic lines of force. These magnetic lines of force
are set up by the current in the stator windings. Each phase has a separate winding for
each pair of poles. The phase windings are spaced successively around the circumference
of the stator so that the three windings (phase windings) cover two poles (which is one-half of the circumference in the case of a four-pole machine). The voltage waves of the
separate phases are not in unison but follow one another at equal time intervals. As the
phase windings successively produce magnetic lines of force in different parts of the
stator the effect of a rotating field is produced.
Synchronous motors are highly satisfactory for use in driving projectors equipped for
sound reproduction, because of the extremely uniform speed which they maintain. The
reason these motors maintain a very uniform speed is that their speed depends entirely
upon the frequency of the power supply and is not effected by voltage variations. The
frequency of the power supply is held within very close limits by sensitive governors on
the generating equipment at the power house. Furthermore, even though the frequency of
the power supply may change from 59 cycles to 61 cycles the change of frequency takes
place very slowly due to the great weight of the generating equipment used in power
houses. A change of projector motor speed changes the pitch of the reproduced sound,
but slight variations of pitch are not discernible if the change of pitch takes place slowly.
A rapid change of pitch, even if only a small amount, will cause "wows."
29. Synchronous Projector Motors.- Figure 18 shows a synchronous projector drive
motor used on RCA Photophone equipments. This motor consists of a three-phase
armature wound on the stationary part of the motor, and a rotor that has four equally
spaced shallow slots. The rotor also has a winding, which consists of copper bars through
the periphery of the rotor and bonded by copper rings on both sides. This type of winding
is called a squirrel cage winding because of its similarity to a squirrel cage. This winding
does not enter into the action of the motor once it is up to speed, and is useful only for
starting the motor. The action of this motor is as follows: When a three-phase
alternating voltage is applied to the stator (stationary winding) a rotating field is produced.
This rotating field drags the rotor along with it, so that the parts of the rotor which form
the smallest air gap between the rotor and stator are always in the positions of maximum
field strength. This can be demonstrated by placing a horseshoe magnet under a piece of
window-pane glass, and placing on the top of the glass a small oblong piece of steel about
as long as the distance between the poles of the magnet. The piece of steel will assume a
position so that the distance between its ends and the poles is the shortest. If the
magnet is moved the piece of steel will move with it, and, if the magnet is rotated about
an axis mid-way between the two poles, the piece of steel will rotate also, so as to
remain always in the same position with respect to the poles of the magnet. This
action takes place only when the motor is at synchronous speed. The motor comes
up to speed as an induction motor, the action of which will be elaborated on later
in this chapter.