Nagra III - Rotor and Commutator Faults

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12

2.

Open

Circuit

of

Half

the

Rotor

Below

there

is

a

schematic

of

the

motor.

It

can

be

seen

that

the

current

passes

through

two

paths

in

parallel.

Interruption

of

one

of

the

paths

does

not

prevent

the

motor

from

functioning

but

doubles

its

resistance.

This

is

indicated

by

a

less

vigorous

rewind

and

partly

by

an

increase

of

the

limiting

voltage

required

for

operation

at

15

i.

p.

s.

One

localize

this

fault

by

measuring

of

the

rotor.

The

normal

value

is

21

ohm

s

but

a

rotor

half

open

circuited

will

read

42

ohms.

3.

Short

circuit

between

segments

A

short

circuit

between

two

segments

of

the

commutator

has

very

serious

repercussions;

wow

at

double

the

speed

of

rotation

of

the

capstan

and

increase

in

power

co

nsumption.

Such

a

short

circuit

can

happen

in

the

windings

or

inside

the

commutator

,

but

such

cases

are

less

likely

in

recorders

which

are

in

ser

v

ice.

On

the

other

hand,

it

is

much

more

likely

that

a

metallic

deposit

has

appeared

on

the

insulating

segments

of

the

commutator.

To

localize

this

kind

of

fault,

it

is

necessary

to

measure

the

resistance

between

adjacent

segments.

It

is

usuall

y

between

6

and

8

ohms

through

o

ut

the

series.

It

increases

regularly

from

the

first

to

the

last

winding

progressi

v

ely

from

1

to

1,5

ohms

in

all.

Any

abnormally

low

resistance

found

here

indicates

a

fault.

First

of

all

the

commutator

should

be

cleaned

with

a

very

fine

emery

cloth,

then

washed

carefully

to

eliminate

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