protection of a.c. motors

7/25/2019 Protection of a.c. Motors

1/23


2/23

1.

MOTOR PROTECTION

IMAODUCTION

Motor

applications eover a wid.e range

of

characteristics,

starting

requirenentst

loading and anblent

tenperatule.

Proteetion

of

notorg

therefore

becones

less

standardised tharr

protection

of

say

generators.

There

are

nany

protective

devices

ranging

fron

simple

fuses

to

sophis-

ticated.

solid

state relaysr all of

which

provid"e

different

degrees

of

protectlon.

The

cost anii

the

extent

of

such

protective

devices

must

be

weighed

against the

potential

hazard.s

bearing

in mind the

si.ze

of

the

motor

and the type

of load. Any

d.evice

or

scheme,

though

reliable in

operation under

abnomal

eonditions, must

not

affect

the

continuous

operation of the

motor

under

normal

conditions.

Unnecessatry

interrup-

tions

cause

not only

inconvenience

but

serj.ous

loss

of

prod.uction

may

result.

Fractional

horsepower

and

very small

notors

are

generally

protected

only

by fuses.

Larger

notors

(up

to

about

10

nf)

are

protected

by

slnple

thernal overload

trlps

or

dashpot overload.

d.evices

p1us

undernoltage

teleases

bullt

into

the

starter

gar.

Such

deviees

opexate in

an

inverse

fashion

depend.ing

on the degree

of

excess

cunent

and

nay be

fitted

with

an instantaneous

hlgh

set

unit.

For

somewhat

larger motors

and/or

where

the function

of

the

rnotor is

of

great

inportance

requiring

a

more

discrininative

fo::m

of

protection

themal

relaye

are

used.

For notors

vith

ratings

above

1500

HP savings

in

repair

costs

by

high speed

clearance

of

aL1

faults

justifies

the

cost

of

differential

proteetion

in

addition

to the:mal,

protection.


3/23

2.

The

types of fault

or abnormality

can be

divlded

broadly

into

imposed

external

condltiona

and

i-nternal

faults. hternally

inposed conditions

include nechanical

overloadr

unbalanced supply

voltages,

undervoltage,

single

phasing

and

reversed

phase sequence

starting

and

loss

of

synchronisn

in

the

case

of synchronous

motors.

Internal

faults

include bearing failure,

winding

phase

and

earth

faults.

This

present

leeture

will

be

linited

to

a.c.

motors

and.

in

particular

ind.uction notors.

MOTOR OPERATION

ft is

inportant

to

understand the behavlor:r

of

the notor under certaln

conditions

to be

able

to apply

protection

succeasfully.

For

example,

the

nagnitud.e

and

d.uration

of

the

starting

cr:rrent

affects the

application

of

overload

protection;

the

magnitude and

maximun

allowable

duration

of

stalling

current

in relatlon

to

those

of

startlng

current

deternine

whether separate stalling

protectlon

is

required.

The

following

motor

parameters

and

operating

cond.itions sha1l

be

exqnined.

MOTOR

CURRn{TS DIIRING STARTING AND

STAI,L

CONDITTONS

lihen

the

rotor is

stationa:ry

the motor

functions

like

a

transformer with

a

distributed.

vinding

and. a

small air

gap

in

the

magnetic

circuit.

There

will be

a certain

transfonnation

ratio

K

between

the

stator and rotor.

If

VS

is

the

stator

applied voltager

the

induced.

rotor

enf

V,

=

KVg.

Rotor cunent

per

phase

=

Rotor

enf

Rotor inpedance

per phase

The

stator fie1d.

slreeps

past

the

rotor

conductors with

full

synchronous

speed and

the

rotor

current frequency

ie

the

line

frequency

f.

trrlhen

the

motor

is r:unning

the

relative

speed

of

ttre

rotor and

stator

field is

proportional

to

the

slip

s.

Ibequency

of rotor

cu:rent

=

sf.

Rotor

emf

=

sK

Vg.


4/23

,.

rfXr=

sx,

=

rotor

reactance

lltr

pe"

phase

t,tl

to

lr

crJxlent

n

of

line

frequency

then,

'r

slip

''

per

phase,

=Rr+js

.'.

Rotor

eurrent

per

phase

=

R2

*x2

when

s

is

very

snal1.

Therefore

rotor

cunent

and

hence

stator

crurent

renalns

substantially

eqr:al

to

cu*ent

at

standstlll

'nti1

the

notor

has

reached.

its

nonoal

running

speed.

rt

is

a

nomal

practice

to

assuae

a

constant

starting

current

for

the

whole

of

the

starting

period

when

the

cu*ent

and

tine

settlnge

of

overroad

proteetion

are

to

be

deterrnined.

0n

the

same

basj's

as

above

when

a

notor

starls

while

*rnning

or

is

unable

to

start

dr:e

to

excessive

load

the

cr:rrent

d.rawn

by

the

nachine

can

be

assuned

to

be

approri.mately

equal

to

the

starting

er:*ent

1f

exact

info:matlon

is

not

available.

rf

the

motor

is

unable

to

start

d.ue

to

an

open

cireuit

in

one

of the

supply

phases

the

current

dravn

is

reduced

to

0,865

tines

the

no::naL

startlng

cunent.

It

ean

be

shown

the

+ve

and.

_ve

sequence

cu*ents

resultlng

are

each

equaL

to

0.5

the

nornal

starting

current.

If

R2

=

lotor

resistance

rotor

inpeda^nee

per

phase

xzD

.'.

R2

s


5/23

4.

OPERATION

ON

UNBAIANCE SUPPLY

The

supply voltage

to

a

J-phase

induction

motor

ca:r

become

unbalanced

due

to

such

reasons as

single

phase

loadr inperfect

transpositlon

of

feeders

etc.

The

degree

of

unbalance

is

srna11 in

nornal

installation

except r*hen

one

phase

becomes open

circuited.

This

would

not

affeet

at

first

sightt

the

motor

to

any

large

extent,

but

a

smalI

voltage r:nbalance

could

produce

a

rnuch

larger

negative

phase

sequence

current

in the

winding

due to the

relative smal1 negative

phase

sequence

imped.ance

of the

machine compared

with the

positive

phase

sequence

inped.ance.

Consider

the

following

equivalent

circuits for

positive

and negative

phase

sequence

cunentsl

the

nagnetising

inpedance

being neglected

:-

R,r

Q2

i

(Y,+

x;

)

R.,r

cj

i(x,{)

With

positive

phase

sequence

voltages a

rotating field ulll be set

up

and

the rotor

will rotate in

the

direction of

rotation

of

the field

giving

a

slip

s

and. slip

frequency sf.

With negative

phase

sequence

voltages

the

field will

rotate in

the

opposite

direction

cutting

a

rotating

rotor

conductor

at

aLnost

tvice the

frequDClr

fhe

actual

frequency

of

negative

phase

sequence

voltage

and current

in

the

rotor circrrit is

(Z

-

s)f.

tr"rom

the

equivalent

cireuits :

T*i

Motor +ve

sequence

impedance

at

a

given

slip

s

=

[r.,

*

*i)'

+

(xr

*

$'1b

L=.J

=

[,*,

* n))2

+

(xr

*

*;)'J+

when

s

=

l'

at

standstllr.


6/23

Motor

-ve

=

[t"'

.

=

[to'

+

sequence

*L)'

+

f-e

\Lt'

+

2

5.

impedance

at a

(xr

*

lt]t

J

(xr

*

x;l'?J+

given

slip

s

when

s

4

protection of a.c. motors

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