Nagra III - Mechanical Friction and Alignment

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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

perfectl

y

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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