motor for scribd

8/8/2019 Motor for Scribd

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y-Tarun Goel


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Contents

Motor Failures Thermal Protection

Start/stall protection

Earth Fault Protection Negative Phase sequence Protection

Other Protections


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Motor failures

Motor Induced Insulation

Bearing

Mec anica Load Induced

Overload

High Inertia Environment Induced

High ambient temperature


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Motor failure (cont..)

Source Induced Under voltage

Phase failure/ reversal

Vo tage un a ance Operation Induced


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Thermal (overload) protection

Thermal replica model of a motor is used. The temperature T at any instant t is given by:

T = Tmax(1-e-t/t

h)

= R 1-e

-

hCold characteristics

Hot characteristics

( ) =21ln IIt R

( )( )( )

=

22

22

.

1

lnB

P

IkI

ICHI

t


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Start/stall protection

A motor may fail to accelerate from rest due to : Loss of a supply phase

Mechanical problems

Low supp y vo tage Excessive load torque

Start time < safe stall time

Definite time overcurrent protection Start time >= safe stall time

Definite time overcurrent protection + Speed sensing.


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Time

Stalled motor

Running motor

thermal withstand

characteristic

Current

Motor

starting

characteristi

c

characteristic

start time less than withstand time


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Time

Running motor

thermal withstand

characteristic

Current

Motor

starting

characteristi

c

Stalled motor

thermal withstand

characteristic

start time greater than withstand time


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Earth fault protection

Current is measured with a Residual CT connection andMotor is tripped when this current exceeds the setting.

A lower limit is imposed by

System capacitive currents

Stabilizing Resistor: to prevent operation from the spillcurrent resulting from the unequal CT saturation

( )rlctst

stab RkRRI

IR ++=0


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Negative phase sequenceprotection

Negative phase sequence current is generated due to Unbalanced loading

Loss of a single phase

ng e-p ase au s I2 Z2/Z1 = Rated current/Starting Current = 1/6 (usually)

Also, negative sequence current is at twice supplyfrequency. Hence, heating due to negative sequence is

much larger than that due to positive sequence. Operating time is generally, inversely proportional tosquare of unbalance.


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Other protections

RTDs temperature detection: A rise in temperaturemay denote:

Hot spot

ver oa ng

Beginning of a fault in the affected part.

Undervoltage protection

Loss of load Protection


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Protection of a Sample motor

A pump using a HV induction motor. Details of the motor :

Rated O/P : 580 kW

a e o age : .

Rated frequency : 50 Hz

Rated p.f. : 0.91

Rated efficieny : 95.8%

Stall withstand time (hot) : 12 secStall withstand time (cold) : 18 sec


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Details(cont..)Starting Current : 560%

CT Ratio : 60:1

tart t me @ 100 vo tage : 2 secStart time @ 80% voltage : 4 sec

Heating time constant : 4 mins

Control Device : VC


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Thermal Protection

Hot operating characteristics:

AefficiencypfV

PIFLC 58

958.0*91.0*3*6.6

580

**3*===

( ) 22 1 ICHI

where, I = relay current

IB = basic current = full load current = 1pu

Ip = steady state current prior to load = 1puk = 1.05

H/C = 18/12

{ } =

22 ).(og

B

e

IkI


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Locked Rotor Protection

Starting Current = 5.6*IN Hence Current Setting = 2*IN

Start time = 4 sec and safe stall time = 12 sec

Hence Time Setting = 10 sec


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Earth Fault Protection

Current setting = 20% of motor rated Instantaneous protection with a relay inherent time of

60ms.-

maloperation due to CT spill current during starting. Calculation of Stabilizing Resistor:

RCT= 3 Ohm, Rl= 1 Ohm, Rr= 0.2 Ohm

Hence, Rstab = 140 Ohms.

( )2.0*60

2.01*23*58*6.5 ++=stabR


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Negative Phase sequence Protection

The setting current is limited by the followingconsiderations:

Unbalance in CT secondary currents due to saturation.

External unbalanced faults which may permit the motorto act as a generator and feed negative sequence currentinto the faults.

Hence a setting of 20% with a TMS = 1.0 can be used.


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RTD Protection

The motor has class F insulation Six resistance thermometers (Pt100) are provided

within the motor.

= e . temperature + a owa e temp. r se +allowance for hot spot winding

= 40C + 105C + 10C

= 155C


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motor for scribd

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