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13
all
traces of abrasive powder
.
If
lhe
fault cuntinues, the
corresponding
wire
of
the
coil
should be
disconnected. It is
then possible
to ascertain
if the fault
is
in the winding or in the
commutator
.
If
the
winding has short circuited,
a
new
rotor
should be ordered from us. On the other hand, it is often possible
to repair the commutator by passing a
very
large
current through the short
circuit to break
it down.
The discharge of a
100
microfarad condenser charged
to
300
vo
lts
will generally be sufficient.
4. Mechanical
Fnction
Mechanical friction
of the rotor increases
both
the
wow
and
the power taken.
One
can
distinguish between
eleclric braking,
such as
is shown in the preceding
paragraph,
because mechanical
friction
is constant,
whereas the
ele~tric
braking
increases
quickly when
the motor speeds
up. Typical
valuE's of the
consumption
of the motor
on no load,
that is without
tape and with
the pinch
wheel not in
contact with
the capstan (but
not on rapid
re~wind)
are given below:
Speed
Normal no
load
current
No load current with
two
segm ents short
circuited
15
i.p.s.
26 ma
62 ma
7 1 /2
i.p.s.
20
ma
39 ma
33/4i.p.s.
17
rna
27
rna
5.
Dirty Commutator
The commutator is obviously the most critical
part
of the motor.
It is
designed
to work dry.
It
is therefore necessary to make sure that the
commutator
is
perfectly
dry.
It
can be cleaned by means of
a
piece of
rag
saturated with a solvent such as trichlorethyline or, better still, special
cleaning fluid
.of
electrical contacts. Certain of these products
leave
behind
a very
thin deposit of
lubric
ant
which seem s not to prevent good running.
6.
Misalignment
of the Axis of Commutation
This
axis
of commutation,
that
is,
the
exact position
of the brushes with
respect
to the
centre
magnet is
very critical.
A misplacement
will provoke
sparks
at
the
commutator
and
electrical
noise, and a considerable increase in current
consumed.
This increase
may be sufficient
to overheat the
servo
amplifier
sufficiently
to cause
irregular running of
the motor.
Use
the following procedure for
adj
usting
this position:
Before moving the brushes mark
their previous
position.
Start
the
motor at 15
inches
per second m
~as
uring
the current taken.
The correct alignment corresponds
to
minimum
consumption.
If you find a
noticeable
difference
(more
than 5 mm on the external
circumference of the motor), examine the fixing of the magnet.
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