Hitachi Uni Torque Motor Service Manual
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Page 1
@HITACHI
SERVICE MANUAL No. 127
Contents
. Outline of development ........................... l
. Features ............. l
. Comparisons with other types of motors .............. 2
. Operating principle ............................... 2
. Uni-torque ......... 4
.Servocontrolcircuit....,.......i...... ........... 5
. Specifications ............................ . . . . . 8
* Wow/flutter * Load characteristics
* S/N ratio * Start-up characteristics
* Time characteristics * Temperature characteristics
. Circuit diagram .................................. 10
1. Outline of development
The signal-to-noise ratio, wow/flutter and other
important characteristics of turntable which employ
direct drive motor directly rely on the performance
of the motor. This is because the platter and the
motor are coupled directly.
Conventionally, multi-pole DC servomotors or cup type
AC servomotors have been available for direct drive
turntables, but each type has its merits and demerits.
Hitachi set out to develop a high-performance direct
drive motor which combines the merits of both systems,
and the result of its endeavors was the flat, commutator-
less DC servomotor. (A total of 15 patents have been
applied for).
The construction of the new motor is completely
different from that of conventional motors, and in
theory, the torque is constant, there are few vibrations
and it allows the platter to rotate smoothly. Hitachi
called it the Uni-torque motor. Its construction is
shown in Fig. l.
2. Features
(1)Excellent S/N and wow/flutter
Thanks to the new core-less, slot-less flat star-shaped
coil construction, there is no slot phenomenon (whereby
ut axis (record guide)
Cover
E Outp
Rotor mag net (8«pole)
Speed detection
base board
Drive coils (2)
switching use Bat!
Fig. 1 Construction of main uni-torque motor unit
the platter does not rotate smoothly), and the output
torque is constant so that there is very little vibration
and the platter rotates smoothly. This makes for a
superb signal-to-noise ratio and wow/flutter.
(2)High starting torque
Unlike the cup type AC motor, the new motor pro-
vides a high starting torque for its small size without slip.
(3)Sirnple construction, high reliability
The motor is composed of two core-less flat, starrshaped
drive coils and a base board for speed detection, and
also of a brush~less switching mechanism which contains
two Hall elements for an overall simple construction.
'High reliability is yielded by the small number of parts
used.
September 1977
Page 2
(4)Very little rise in temperature
At starbup, the motor starts operating with its maxi.
mum torque provided by square waves, but when its
speed has become constant, vibrations are prevented by
sinusoidal waves. Furthermore, when locked together
with the platter, the protection circuit is actuated to
automatically reduce the drive current and safeguard
against rises in temperature.
3. Comparisons with other types
of motors
The table below compares the uni-torque motor with
conventional multi-pole DC servomotors and cup type
AC servomotors.
Performance required DC 10015 Uni-torque
, . AC motor
fol due d-UVB motor Brush Brush-less motor
Torque fluctuations Poor Poor Good Very good
Vibration Good Good Poor Very good
Temperature rise Good Good Poor Very good
Simple construction Fair Fair Fair Very good
Long service life Poor Good Good Good
Efficiency 3% Good Poor Good
Ease of control Good Good Fair Good
Comparison of performance of motors
4. Operating principle
Fig. 2 (a) shows a graphical representation of Flemings
Left Hand Rule. In a magnetic field, a conductor will
be subjected to the force of wire motion at right angles
to the direction of the current, and this diagram shows
the basic operation of a rotating machine. According to
this rule, the coil in Fig. 2 (b) will be subjected to the
upward force. If the coil is fixed, a repelling force will
be generated in the magnets and the direction of this
force is shown by the dotted arrows, i.e. downward.
If the length of the coil across the magnetic field is
1, then the relationship between this length and the
force F, to which the coil is subjected, the magnetic
flux density B and current I can be expressed by the
following equation. F = B11
Let us now consider two magnets which are bonded
together with their polarities reversed, as shown in Fig.
3, with a V-shaped drive coil placed beneath them.
The magnetic flux under the magnets go in opposite
directions and so even when the direction of the current
is the same, the forces on edge b under the south pole
and on edge a under the north pole will be exerted in
opposite directions. If these forces are Fa and Fb,
and the component forces in the x axis direction are
fax and fbx, then the size of these component forces
will be:
fax = fa sind
fbx = fb sine
9 Force
0 & Magnetic flux
t
\
1/92
9
/\_/
Fig. 2 (a) Flemings Left Hand Rule
Current
Fig. 2 (b)
If, as shown in Fig. 4 (a), the magnets are moved to the
left in the direction of the x axis, then the force to
which the drive coil is subjected will be canceled out,
since the forces on edges a and b under the north
pole are applied in opposite directions, and force fx
which is generated decreases.
If the magnets are moved further to the left, as in
Fig. 4 (b), forces falx and fb2x are balanced, and the
force generated is zero. Moving the magnets even
further to the left means that the value of leX will
increase more than that of falx, and that the direction
of the force applied to the drive coil will be reversed
and tend to the left. Fig. 5 shows the relationship
between the distance moved by the magnets from the
position in Fig. 3 and the forces which are applied to
the drive coil.
Let us now proceed to extend the example by aligning
four pairs of circular magnets, which were used in