BRUEL & KJAER 4915 - Determination of Phase Delay or Lag

BRUEL & KJAER 4915

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An

alternative

method

of

establishing

the

true

motion

frequency

which

may

be

employed

if

difficulty

is

experienced

in

obtaining

a

reliable

motion

cycle

count,

is

to

continue

turning

the

INT. GENERATOR

knob

anticlockwise

until

a

second

stationary

single

image

is

obtained.

The

true

motion

frequency

may

then

be

calculated

using

the

relation:

f =

m

(f 1 -

f2)

where

f

1

is

the

frequency

indication

obtained

with

the

first

stationary

single

image

and f

2

is

that

obtained

with

the

second

stationary

single

image.

Example:

First

frequency

indication

f

1

= 75 Hz

Second

frequency

indication

f

2

= 50 Hz

75 X 50

True

motion

frequency

f m =

75 - 50

f

m = 1 50 Hz

or

9 k r

/min

3.2.2.

Detemination

of

Phase

Delay

For

examination

of

a

moving

object

at

any

point

in

its

motion

cycle, the

"Phase

Delay"

function

of

the

4913

may

be

employed.

In

addition

the

phase

relation

of

two

or

more

reciprocating

or

rotating

machine

parts

which

have

the

same

motion

frequency

may

be

determined.

1.

Carry

out

items

1

to

7

of

section

3.2.1

to

obtain

a

stationary

single

image

of

one

of

the

synchronously

moving

parts

to

be

examined

and

determine

its

true

motion

frequency.

2.

Using

the

DELAY

knob

move

the

observed

image

of

the

synchronously

moving

part

to

a

convenient

reference

point

in

its

motion

cycle

and

note

the

phase

indication

given

by

the

4913.

3.

Looking

at

the

second

moving

part,

adjust

the

DELAY

knob

to

move

the

observed

image

so

that

it

has

the

same

relative

position

as

the

first

and

note

the

phase

indication

on the 4913.

4.

Provided

that

the

motion

frequency

of

the

two

moving

parts

investigated

is

identical

and

not

greater

than 100 Hz, then the phase

delay

of

the

second

part

with

respect

to

the

first

may

be

determined

by

subtracting

the

first

phase

reading

from

the second. If

the

motion

frequency

is

greater

than 100 Hz, then the

phase

delay

must

be

calculated

using

the

relation:

where

Pd

is

the

true

phase

delay

in

degrees

P

1

is

the

first

phase

indication

in

degrees

,

noted

in

item

2

P

2

is

the

second

phase

indication

in

degrees,

noted

in

item

3

fi

is

the

frequency

indicated

on

the

4913 in Hz

or

r

/min

fm

is

the

true

motion

frequency

in Hz

or

r/min

13

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