Sony PS-X7 - Page 38

PS-X6/X7 |
Fig. 4-3.
ICi-3
When an S pole corresponds to the Hall element
H2, current flows through L3 and L4, thereby
generating the same electro-magnetic force to con-
tinue spinning the rotor.
Hence, N and §S poles are repeatedly positioned
oppositely to the Hall elements H! and H2 every
22°30’ of a rotation thereby generating the voltages
in these elements, resulting in the production of a
current in the motor coils, and subsequent rotation
of the motor.
magnetic flux (B) 0
Ss
hall element (V) o
coil (1) 0
Joa (electrical phase angle)
90°
Fig. 4-5.
Rotational Force
The rotational force (torque F) varies directly
with the strength of the magnetic field (B) cutting
across the coils, and the amount of current (J) flowing
through the coils.
That is, F=B-1
But since the magnetic field produced by the
magnets of the rotor varies in accordance with a
sine wave relation with the angle of rotation, and the
pairs of motor coils are arranged 90° out of phase
each other, the combined torque will be constant.
Constant Speed Rotation (See Figs. 4-6 and 4-7)
In order to. maintain constant turntable speed,
this model features a magnetic-disc detection system
in the speed-detector stage, and a crystal oscillator in
the phase control stage of a crystal-locked servo
system.
Changes in rotational speed of the turntable are
detected by the speed-detection head fixed on the
frame. (S and N magnets are mounted around the
outer rim of the turntable platter with a very high
degree of precision.) The reference standard fre-
quencies detected by this head are 284 Hz at 33 rpm,
and 384Hz at 45 rpm. Any changes in frequency
detected by this head are converted into voltage
signals, and fed to the servo amplifier.