Motor Protection

Tutorial

Presented by:
Jakov Vic ,P.Eng.
GEMultili

Patrick Robins n, CET

Altelec Engineering
~ Motor initial cost could be as low as 2% of the lifetime
operational costa
~ The driven process downtime in most cases is more
expensive than motor..
~ Motor downtime contributors are:
v Power system failures..
• Inadvertent shutdown because of human mistake or
motor protection maloperation
Motor failure
'V

'VLoad failure
~ Motor failure rate is
conservatively
estimated as 3-50/0 per
year.
~ In Mining, Pulp and
Paper industry motor
failure rate is up to
12%.
~ Motor failure cost
contributors:
• Repair or
Replacement.
• Removal and
Installation.
• Loss of Production.
y Many of the motor failure contributors (IEEE Survey) and failed
motor com.ponents (EPRI Survey) are related to motor overheating .
y Thermal stress potentially can cause the failure of aU the major
motor parts: Stator, Rotor, Bearings, Shaft and Frame..
 ermal
The motor thermal limits curves consist of three distinct segments which are based
on the three running conditions of the motor:
• The locked rotor or stan condition.
Ideally, curves have been
• Motor acceleration. provided for both a hot and
Motor running overload.
4111
cold motor. A hot motor is
defined as one that has
been running for a period of
time at fun load such that
the stator and rotor
temperatures have settled
at their rated temperature.
Conversely, a cold motor is
defined as a motor that has
been stopped for a period
of time such that the rotor
and stator temperatures
have settled at ambient
temperature. For most
molars, the motor thermal
limits are formed into one
smooth homogeneous
curve.
 ermal
The are an indication of the amount of current and
associated time for the motor to accelerate from a stop condition to a
normal running condition. In this particular example, there are two
acceleration curves: The first is the acceleration
curve at rated stator voltage
while the second is the
acceleration at a given level
of rated stator voltage, 80% i
this case; a soft starter is
commonly used to reduce the
amount of inrush voltage and
current during starting. As
can be seen on the curve
shown, since the voltage and
current are lower, it takes
longer for the motor to start
Therefore starting the motor
on a weak system can resu
in voltage depression,
providing the same effect as
a soft-start
 POWER :8000 HP TYPE :K
POLES :4 FRAME : 8713Z
VOLTAGE :13200 V ENCLOSURE :WPII
.,:;n u..,. •1 lU\

f
l TEMPERATURE RISE:80 C /RTD @ SF 1.0
INSULATION CLASS:F (POLYSEAL)
j
r ~ ~ft j

DRIVEN LOAD :FAN CLOSED VALVE

MAX. ALTITUDE :3300 Ft
LOAD WK2 REF. TO MOTOR SHAFT : 49249 Lbft2
,---------------------------------------- Calculated Performance ------------
NEMA STARTING CODE :F
RATED RPM :1780 LOCKED ROTOR CURRENT :540 %
RATED CURRENT : 297 A LOCKED ROTOR TORQUE : 77 %
RATED TORQUE :23571 lbft PULL UP TORQUE : 77 %
RATED KVA :6790 BREAKDOWN TORQUE : :2 45
STATOR CONNECTION :V COUPLING TYPE : DIRECT
MIN. STG. VOLTAGE :70% V ARRANGEMENT :Fl
TIME RATING : CONTINUOUS
ROTATION : DUAL
AMB. TEMP. (MIN/MAX) :-18/40 C MAX. BRG.VIBR. (PK-PK):O.0016
TOTAL WEIGHT (calc.) :53700 Ib BEARING TYPR : SLEEVE
ROTOR WK2 (calculated) :10422 Lbft2 BEARING LUBRICATION :OIL
Y :0.50
NOISE LEVEL (dBA) : 85.0 @ 3.3 ft ROTOR TIME
MAX CAPACITOR KVAr :1000 COLD
STATOR RESIST. @ 25C :0.1910 Ohms L-L
X/R RATIO : 33.960
OPEN CIRC. CONSTANT :1.5680 S
ACCELERATION TIME :15 Sec

~ Motor Data Sheet Parameters

E. Temperature Rise, Insulation Class
F. locked Rotor Time; Cold/Hot
G. Number of Starts; Cold/Hot
 Efficiency:
An indication of how much electrical energy is converted
to output shaft mechanical energy expressed as a
percentage. losses
Core loss
Stator loss
Rotor loss
Electric
al Mechanical ctio
Energy Energy and
in Windage
Stray loss

Electrical Energy in =Mechanical Energy out + losses (mostly

heat)
•
~ Typically motor manufacturer provides the values of the locked rotor
thermal limits for 2 motor conditions:
• COLD: motor @ ambient temperature
• HOT: motor @ rated temperature for specific class and service
factor.
~ NEMA standard
temperature rises
for motors up to
1500HP and
Service Factors 1
and 1.15
respectively.

~ HeR defines the proportional increase of Thermal Capacity Used

(TCU) of the motor running fully loaded at settled temperature in
comparison to the motor resting at ambient temperature"
Information required to set Thermal
Model:
• Motor FLA
• Locked Rotor Current
• Locked Rotor Time Hot
• Locked Rotor Time Cold
• Safe Stall Time old
• Service Factor
• Motor .Damage Curve
Motor Management Relays have three
basic categories of protection elements:

- TRIPS

-ALARMS

• BLOCKS
Thermal modeli g is ,
..

Some of the other rotection n i ns t e .,. . . . . . .-. ,.,. ,.

digital motor relay ase an grou are
limit the damage caused by inadeq ate r i
set thermal i

U ose t Ig r
pposed n I
revent a t e r
trough system co ns,
keepin t e rocess i
•
>- Motor Start Inhibit
>- Standard, Custom and Voltage Dependant Overload
Curves
>- Thermal Model Biasing by Current Unbalance
>- Thermal Model Biasing by RTD Inputs
>- Separate Thermal Time Constants for Running and
Stopped Motor Conditions
>- Independent Current Unbalance Detector
>- Acceleration Limit Timer
>- Mechanical Jam Detector
>- Start and Restart Supervision
 Insulation Ii time
For F class decreases by half
insulation stator
~-:-A-C~LA~SS~(l
=-+--+--1-1
05="C~)H-..J-.U--J.-\.--I--l--\-.H----\-1-1--!--l temperatu re of
motor operating
H-+-4-+-H-+--+--lJ---l-PH--+-H 165QC causes motor
-S-CLASS (130'Cj
H-+-----J++--+-\l----i---<-\-1-J--.'l+-+---J....-l lifetime to decrease
temperature
-F-CLASS (155 'C)
to 50%
excee ermal
limit by 10
~ Rotor
In most cases, rotor therma/limit is defined by the allowed motor stall
time.. These motors are classified as rotor limited"

~ Stator limited motors are uncommon... voltage rating is 10 times

greater than HP rating: For example: 500Hp, 6900V
If the motor has been designed conservatively, the portion of the acceleration
curve under the motor thermal limits curve is less than a third to a half in terms
of trip time, and the motor has been applied conservatively (during
acceleration or running the acceleration and thermal limits curve do not cross)
then thermal model settings can be set easily.

If the acceleration curves

and the thermal overload
curves are very close,
accuracy in the settings
becomes very important
in order to ensure reliable
motor protection without
nuisance tripping.
~ Thermal Capacity Used (TCU) is a criterion selected in
thermal model to evaluate thermal condition of the moto
~ TCUisdefinedaspercentageof motor thermal limit
utilized during motor operation.
~ Thermal Limit of the model is. dictated by overload curve
constructed in the motor·protection device in the
reference to therm.aldamage curves normally supplied by
motor manufacturer.
~ IEEE Std620-1996 provides the gUidelines for the
presentation of motor thermal lImit curves..
~ Motor protecti.on device is equipped with set of standard
curves and capable to construct customized curves for
any motor application..
We will use the following model to aid in a better understanding of motor
thermal modeling concepts. The motor's thermal capacity, that is to say, the
amount of heat energy the motor can hold, win be represented by the glass
vessel. The lava like fluid filling the vessel will represent thermal energy or
heat energy that has been absorbed by the motofl
 The sources of thermal energy that win fiU the vessel or heating the motor
are:
o Ambient temperature

o Motor losses due to current unbalances and I squared T

• Motor heating due to a start

•
Motor cooling will be represented by:
• The vapour evaporating from the surface of the liquid when the motor is
running or stopped will represent the motors ability to dissipate heat.
• The fan is representative of the additional cooling effect of the motor's
cooling system which is commonly a fan mounted on the motor shaft
Upon a start, the inrush current is above the motor's full load current causing
the thermal capacity used within the motor to rise rapidly. Intelligent Motor
Management Relays "learn" the amount of thermal capacity required to start
the motor and if the start inhibit function is enabled, use this information in
addition to the thermal capacity used data to ensure that there is enough
thermal capacity within the motor for a successful start before a start is
attempted.
 Assume that a motor requires 40°10 of it's
thermal capacity to start.

If the motor had been running in an overload

condition prior to stopping, the thermal capacity
would be some value; say 80%.

Thermal Capacity must decay by 20% (from

80% to 600/0 Used) in order to start the
,KJ'- ... ---- ........ --
motor.
• etermine verload Pickup
• at/Cold Safe Stall Ratio
• Unbalanced Bias
• Cooling Times and Sta Inhibit
• RTD Biasing
 TIME-CURRENT AND THERMAL LIMIT' CURVES
taaa

eoo '\

tn
400
aoo \\
\
...
- Thermal limit curve
~.
"

when motor is cold

1100

"C "'He
\~
oc: '.SOO ;
(.) IIiCOj

(1) ..
tn ·' <.

c: "'"
a
\,
"
"~
>.
~, '.
--. Thermal limit curve
-- aO
(1)
'\ ~
.~,"- when motor is hot
E 'toa

i= I
III
<II
,
I '
,.
\
"-
\
Acceleration curve @
'. •
'v
.,.....
J / •
\ \

\J - ---- 900/0 rated voltage

.. ~
Acceleration curve
@ 100% voltage
----
.4

•• .-- ..
QlII
.• a Oll.

o 150 300 450 600 750 900 'CURRENT

. A = thermal limit - cold B = thermal limit - hot
Cl = time-curre.nt 100 % V C2 = time-current 90 % V

Phase current in multiplies of FlC

)p- Thermal limit plot includes
10000 -MOTOR START (1) - i ' l l I ' D OVERUlIAD CURVE (2)

hot and cold running -COLD ROTOR LIMIT (3) - H O T ROTOR LIMIT (4)

• • • COLD STATOR LIMIT (5) • • - HOT STATOR liMIT (6)

overload limit curves (5 & 6) '" !

and hot and cold locked 1000

)---<
5
1

rotor limit curves (3 & 4 ) as

a standard..
100
!
v;
r-
•
-j
l
m I
I

)P- In special cases plot can be .....

~
u ~r-
w
furnished with acceleration !e
w
:lE
I
I

!
4
curves for the range of -~~lr1
i=
'-
10 1

operational voltages"
>- MPD (motor protection
tEE I
~
~I

~
I
I
device) Overload curve (2) 1 I

should be selected to fit in

i
between cold and hot motor I I
0.1
thermal limit curves.. 0.00 1.1:11:1 2.00 3.00 4.00 5.00 6.00 1.00
CURRENT (FlA)
 T = 87.4xTDM
. 1 2 -1

Where:
TOM is relay setpoint "TO
Multiplier"

I .. current in multiples of FlA

10

Multiples of Full Load Amps

 TYPICAL. CUSTOM CURVE
6500 HP, 13800 VOLT INDUCED DRAFT FAN MOTOR

If the motor
starting current
beg i ns to i nfringe
on the thermal
damage curves or
if the motor is
called upon to
drive a high inertia
load such that the
acceleration time
exceeds the safe
staB time, custom
or voltage
dependent
overload curve
may be required.

-
MUL.TIPLe OF FUL.L L.OAD CURRENT SETPOINT
 _ _- H - - - - TYPICAL CUSTOM CURVE
6500 Hp, 13800 VOLT INDUCED DRAFT FAN MOTOR

A custom curve win anow

the user to tailor the relay's
thermal damage curve to
the motor such that a
successful start can occ r
(/)
o without compromising
z
o protection while at the
~ ~
(/)

Z
same time utilizing the
a.
CE
motor to its fun potential
I-
o
I-
during the running
W
~ condition. The custom
1=
overload curve feature
allows the user to program
their own curve by
entering trip times for pre-
determined current levels.

MULTIPLE OF FULL LOAD CUF~REr\lT SETPOINT

~ The main issue is that the duration of the high inertia
load starts is longer than the allowed motor safe stan
time. That is why the standard thermal algorithm
method can not be applied.
~ For these starts, thermal model must account for the
current change during acceleration and also use the
acceleration thermal limits for TCU calculations~
~ Motor thermal limit is growing along with motor
rotation speed during acceleration~
~ Starting current is proportional to system voltage
during motor acceleration, thus voltage could be a
good indication of the current level corresponding to
the locked rotor conditionslB
~ Voltage dependant dynamic thermal limit curve is
employed to enhance the thermal model algorithmlB
•

The two graphs illustrate the resultant

overload protection curves for 80% and
100% line voltage, respectively. For
voltages in between, the motor relay will
shift the acceleration thermal limit curve
linearly and constantly based on
measured line voltage during a motor
start.
v Select OIL Curve
ie
• Hot/Cold Safe Stall Ratio
• Unbalanced Bias
• Cooling Times and Start Inhibit
• RTD Biasing
Determini load "
I
• The protection engineer will typically set the overload pickup to
100% of the motors capability.. For motors with a 1"15 service
factor, a maximum pickup of 125% of the fun load current can be
selected while the maximum pickup for 1.. 0 service factor motors
is 1150/0 of full load current.. Having said this, it is commo
practice to set the pick up to no more than the rated motor full
load current plus no more than 10% of the service factor unless
there is another independent measure of motor temperature
such as stator RTOms"
• If the motor's winding temperature is also being directly
monitored by an RTD biasing function to the thermal model, the
overload pickup can be safely increased to the maximum
allowable value for that motor..
(I Note that the motor feeder cables are normally sized at 1.. 25
times the motor's full load current rating which would lim the
motor overload pickup setting to a maximum of 125%"
v Select OIL Curve
• Determine Overload Pickup
""lIlT£II"'lIl ~'II"""lIlU

• Unbalanced Bias
• Cooling Times and Start Inhibit
• RTD Biasing
 Motor Data Sheet
Method
___ • M' • ~ MM

iI Gt': MOTORS D A. T A SHE E T

CUSTOM BODO (R)
SQUIRREL CAGE MOTOR Hot/Cold Safe Stan
*---------------------------------------*----------------------------------_.
CUSTOMER
Time Ratio
GE MODEL :2Sl1R546 DESIGN :lCC7~9W284A
SO ,2870018 RI ;132-0-9522j01
QTY :4 SERIAL II :287000042/3/4/5

POWER :8000 HI' TYPE :1<

POLES FRAME
VOLTAGE
:4
: 13200 V ENCLOSURE
:8713Z
:W?Il HeR
FREQUENCY :60 Hz PHASES ::1 SERVICE FACTOR : L 00
INSULATION CLASS:F (POLYSEAL)
TEMPERATURE RISE:80 C /Rm @ SF 1.0
* ~ M_~ _

DRIVEN LOAD ,FAN CLOSED VALVE

I MAX. 1\l.Tl'fUDE

*. ---. - _.. --
,.,00
LOAD INK2 REF. TO MOTOR SUM'T : 19249 Lbft2
_.. _. _.. -
l"t

_.. - _. -.. -. Calculated Performance .

RATED RPM
RATED CURRENT
RATED TORQUE
:1780
: 2~7
:23571
A
1bft
NEMA STARTING CODE :F
LOCKED ROTOR CURRENT :540
LOCKED ROTOR TORQUE : 77
PULL UP TORQUE : 7?
%
% HeR =30s/35s .86
RATED KIIA :6790 BREAKDOWN TORQUE :245
STATOR CONNECTION :Y COUPLING TYPE : DIRECT
MIN. 81'0. VOLTAGE :70% V ARRANGEMENT ,FI
TIME RATINC : CONTINUOUS
ROTATION ,DUAL
AMB. TEMP. (MW/MAXl ,-18/40 C MAX. ERG.VIBR. (PK·PK):O.0016 in
TOTAL WEIGHT (calc:. l ,5:nno 1b BF~RTNr, TVPR ,SLEEVE
ROTOR WK2 (calculated) :10422 Lbft2 BEARING LUBRICATION :OIL
END in
NOISE LEVEL (dEA) 135.0 @ 3.3 it ROTOR TIME
MAX CAPACITOR KIIAr 1000 COLD ,35
STATOR RESIST. @ 25C 0.1910 Ohms L-L HOT :30
XjR RATIO 33.960 OF STARTS (NEMA Me
OPEN CIRC. CONS~\NT 1.56S0 S
ACCELERATION TIME 15 Sec OR HOT ,1
OUTLINE NUMBER M87C100048
INSTRUCTION BOOK GEEP-l111
11

v Select OIL Curve

• Determine Overload Pickup
• Hot/Cold Safe Stall Ratio
. . . . . . . ,.,. . . Bi

• Cooling Times and Start Inhibit

• RTD Biasing
 A

/~~
/~~ // ~
C // ~ B B /
y ~~"" ~c
~ Positive Sequence ~ Negative Sequence

Y A positive sequence set of vectors consists of three equal vectors that

are displaced by 120 degrees and have a rotational phase sequence of
ABC.
~ A negative sequence set of vectors consist of three equal vectors that
are displaced by 120 degrees, but have a rotational phase sequence of
ACB.
In the real world, the power system is never perfectly balanced
therefore negative sequence currents wiU always exist.
 A

Negattve sequence

Negative sequence currents (or

unbalanced phase currents) will
cause additional rotor heating that
will not be accounted for by
electromechanical relays and may
not be accounted for in some
electronic protective relays.
The rotating negative sequence
vectors increase the frequency of
voltage induced in the rotor
The cumulative "skin effect"
increases the apparent impedance of
the rotor r r
~ If the heating caused by unbalance is taken into account by the
thermal mod~l, the motor and process can continue to run

.~ A 3 phase motor can run with one phase completely absent or

be started with a single phase connection, by adding force

~ In these circumstances extra heat is produced, but it does not

necessarily mean the motor must be tripped immediately

~ Remember the idea is to run and not trip unnecessarily,

maintain the process as long as possible

~ Main causes of current unbalance

);> Blown fuses
);> Loose connections
);> Stator turn-to-turn faults
);> System voltage distortion and unbalance
);> Faults
 Y Equival igor is employed to bias
thermal model in response to current unbalances

1 EQ =~I~ x (1 + X x (I21 1 1 )2)

~ 1m ... real motor current; K .. unbalance bias factor; 11 & 12 ... negative
and positive sequence components of motor current
~ K factor reflects the degree of extra heating caused by the negative
sequence component of the motor current.
~ IEEE guidelines for typical and conservative estimates of

Ix =175L/~cl
t rou.
t! ILRC ... Motor locked Rotor Current @ 1000/0 oltage (in pu)
v Select OIL Curve
• Determine Overload Pickup
• Hot/Cold Safe Stall Ratio
• Unbalanced Bias
. . . . ., U I"

(I RTD Biasing
Thermal Model 1

100 100

"0
75 75
iIII

\
I

\
CI>
II>
::> Cool Time Constant::: 15 min ::> Cool Time Constant::: 15 min
>-
:i:: TCused_start::: 85% :;. rCused_start= 85%
(,,) '0
Hot/Cold Ratio::: 80% ell HollCold Ratio::: 80%
~
'"
Co
50 leq/Overioad Pickup::: 80%
Co
Cll 50 leqlOverload Pickup::: 100%

\ "- \ --
0 0

..
<a
E
I II
..
1ii
E
CIl
.c:
t= 25
I-
25 -- ~

o
o o a o o(0 o o o
(0 OJ (\J l!) co

Time in Minutes Time In Minutes

 Thermal Model COlDurlO IVIIOI()r ::;)to~)pe~d IVlocel Cooling aViatOr TriIPPE~d

100
100

~T-11-l-
-g 76
Cool Time Constant= 30 min
-
U)
"C 75 Cool Time Constant= 30 min
-+---1
::::l TCused_start= 85% 3:
~
::::l TCused_start= 100%
Hot/Cold Ratio: 80% ~
U HollCold Ralio= 80%
«l Motor Stopped after running Rated load '0
fir 50 III Motor Stopped after Overload Trip
TCused...end= 0% - Co
Co) 50 - -rCused_end= 0%
iij c'J
\
iij
E...
lU E
~
r= 25 .c:

'-'--
I- 25

o
0 0 0 0 0 0 0 0
to 0> 0l LO
~
(')
r- r- 0 0 0 0 0 0 0
<') to 0> ;;r ~ ~
Time in Minutes
Time in Minutes

When the motor is stopped, it's thermal capacity used value win decay according to
the same formula shown previously. If the thermal capacity used were at 100%
before stopping the motor, the thermal capacity used will take 5 time constants or 2.5
hours to decay. Note that after only three time constants, the motor would be within
50/0 of its final value of zero from it's initial value. If the same motor were stopped
with 850/0 of its thermal capacity used, it would decay according to the same formula
taking 5 time constants to decay to zero completely and would be within 50/0 of zero
from it's initial value after 3 time constants.
v Select OIL Curve
• Determine Overload Pickup
• Hot/Cold Safe Stall Ratio
• Unbalanced Bias
• Cooling Times and Start Inhibit
I"
 RTD Bias Curve Example RTC input is a good indicator of
THERlMl.
CAPACI1Y the thermal capacity used,
USED
dependent on stator temperature
1DDZ ------------------------------------
RTD's are very slow and so are
not acceptable for primary
instantaneous protection, nor do
they measure rotor temp and so
do not help for transitory
conditions, like starting
251:
RTCs are very good at correcting
(~~"c) ISS
the current based thermal model
• .,...----rr---,--
D'lIl' -t------..:eF--r--"!- over time
I
o"C BO"C : 12UC
•
1lD'C
•
1.55"C The relay win use the calculated
(c~itit"~) (u'l¥ :mE) thermal capacity unless the RTD
FACTORY PRESET CURVE:
thermal capacity is higher (rtd
Min.= 40" c. Center = 1"1 cr C & Mox.= "155
8
C) cannot overrule current if current
( Center Thermal Capacity = 15%. modeled TCU is higher)
 ***1
AU algorithms of the based

Improper curve selection II lead false 'lI'''U·~''' increased ,.....,.,........... mes,

increases operational costs, as the early sl

Selecting a curve that is too (more conservative) II cause

a given amount as ou.e;\111'" greater percentage the

This II cause a

Si the $ .
a
greater

example,
II extend i

case, a mOllor oo!eraltinlO in

ini
Additional Common Induction and Synchronous
Motor Protective elements:

.. Short circuit
• Ground Fault
.. Differential Trip
• Current Unbalance
• Single Phasing
• Undervoltage & vervo.ltage Protection
• Mechanical Jam Detection
• Undercurrent
• Underpower
• Acceleration Timer
'r The short circuit element provides protection for excessively hig
overcurrent faults.
'r Phase to phase and phase to ground faults are common types of short
circuits.
>- The Short Circuit trip element is coordinated with external up stream
fuses such that the element win operate first.
•
y When a motor starts, the starting current (which is typically 6 times the
Full Load Current (FLC) rating of the motor) has asymmetrical
components. These asymmetrical currents may cause one phase to see
as much as 1.7 times the normal RMS starting current. As a result the
pickup of the short circu element must be set higher than the maximum
asymmetrical starting currents seen by the phase CTs to avoid nuisance
tripping. The rule of thumb is to set the the short circuit protection pick
up to a value which is at least 1.7 times the maximum expected
symmetrical starting current of the motor. This allows the motor start
without nuisance tripping.
~ **It is important to note that the device that the relay is to control
under such conditions must have an interru tin ca acit ual to
or greater then the maximum available fault current.
11

Resistive Grounded System and a Inductive Grounded System

~ A ground fault is a fault that creates a path for current to
flow from one of the phases directly to the neutral through
the earth bypassing the load.
~ This current is sometimes referred to as zero sequence
current.
~ Damage to a phase conductors insulation and internal
shorts due to moisture within the motor are common causes
of ground faults.
~ A strategy that is typically used to limit the level of the
ground fault current is to connect an impedance between
the supplies neutral and ground. This impedance can be in
the form of a resistor or grounding transformer sized to
ensure that the maximum ground fault current is limit to no
more then 10 amps to reduce the chances of metal damage
to the motor.
 t;... (:1

***Note that
[< ---+-+--+-+---------1 same limitations the
element for interrupting
is can operate phase
Core Balance Method phase short circu faults

~ The Differential Trip element function can only be used if both sides of each stator
phase are brought out of the motor for external connection such that the phase
current going into and out of each phase can be measured.
~ The differential element subtracts the current coming out of each phase from the
current going into each phase and compares the result or difference with the
differential Pickup Level. If this difference is equal to or greater then the pickup
level for a period of time greater a user specified delay, a trip win occur.
~ Separate pickup levels and delay times are provided for the motor starting and
running conditions.
~ Some motor protective relays support both 3 and 6 CT configurations. In this
example both sides of each of the motors stator phases are being past through a
single CT. This is called core balance method and is the most desirable owing
it's sensitivity and noise immunity.
~ The level may be set more sensitive if the Differential CTs are connected in core
balance configuration (3 CTs).
 +--i.-,~L Summation Method
! +-----.._ _.
....!

WITH PHASE en

WITHOUT PHASE CTs

~ If 6 CTs are used in a summing configuration, during motor starting, the values
from the two CTs on each phase may not be equal as the CTs are not perfectly
identical.
~ Asymmetrical currents may cause the CTs on each phase to have different
outputs. To prevent nuisance tripping in this configuration, the starting differential
level may have to be set less sensitive, or the starting differential time delay may
have to be extended to ride through the problem period during start. The funning
differential delay can then be fine tuned to an application such that it responds
very fast and is sensitive to low differential current levels.
 Percent Differential Method

P"OSlT1VE WAns )a

A-_-""'"
CIRCUIT Sm:AKt'R

B 52 H-i-ffl----*i--+-.nnrn.--iI-H---A";-----4

PICKUP ~..~~~L=~-=~
restra inil1g
-.
>- This method allows different CT ratios for system/line and neutral
>- This method has a dual slope characteristic. The main purpose of the percent-slope
characteristic is to prevent a maloperation caused by unbalances between CTs during external
faults. CT unbalances arise as a result CT accuracy errors or CT saturation.
>- This method has a built-in CT Saturation Detector. External faults near generators typically result
in very large time constants of DC components in the fault currents. Also, when energizing a
step-up transformer, the inrush current being limited only by the machine impedance may be
significant and may last for a very long time. When saturation is detected the element will make
an additional check on the angle between the neutral and output current. If this angle indicates
an internal fault, then tripping is permitted.
Current Unbalance Detection Alarm Current Unbalance
Y System voltage unbalance
1% voltage imbalance translates
into a 6°k current unbalance
Y Stator turn-to-turn faults
Where, K - adjustment factor
Y If voltage unbalance is typically 20/0, then
1_1 -positive sequence current
set alarm to 15% (> 2 x 6%) with delay
1_2 -negative sequence current
Motor Sin Ie Phasin
Y Blown Fuses K AdDustment Factor
y Bad Connections then K =1
y 20-40 k Trip Level is recommended
0

ifI
lAve -
< I FLA th en K -- I Ave

Single Phasing is declared when:

~ 2 seconds after 40% current unbalance has been detectedm
~ Average current is above 250/0 of FLA and the current in one of the
phases is less then 2% of FLA.
If an induction motor operating at fun load is
subjected to an under voltage condition, the
following effects will occur (Moisey, 1997):

• Full load speed will decrease

• Efficiency will decrease

• Power factor will increase

• Full load current will increase

• Temperature will increase

When the motor is running in an overvoltage condition,
the following affects will occur (Moisey, 1997):

• Slip will decrease because slip is inversely

proportional to the square of the voltage
• Efficiency will increase slightly and power factor win
decrease because the current being drawn by the
motor will decrease
• Temperature rise will decrease because the current
has decreased (based on the formula 12t)
• Most motors are designed close to the saturation
pointuuunincreasing the V/HZ ratio could cause
saturation of air gap flux causing heating
>- Possible causes of motor mechanical jam:
• Worn motor bearings
• Load mechanical breakage
• Driven load process failure
~ Element is used to disconnect the motor on
abnormal overload conditions before motor stalls"
~ The Mechanical Jam element is designed to
operate for running load jams.
~ In terms of relay operation, element prevents
motor from reaching 100% of thermal capacity
while Mechanical Jam is detected. It helps to
avoid mechanical breakage of the driven load and
reduce sta inhibit waiting time"
 This is useful
for indicating
the of loss of
suction in a
pump
application, or
a broken belt i
a conveyor
application .

" The Undercurrent element is active only when the motor is running. is
blocked upon the initiation of a motor start for a defined time
" A trip or alarm win occurs once the magnitude la, Ib, or Ic faUs below the
pickup level for the time specified by the UNDERCURRENT ALARM
DELAY.
a.III
. . ·.

)- The motor relay's Thermal

Model is designed to protect
the motor under both starting
and overload conditionsE
)- The Acceleration Timer may
enhance the motor protection
scheme but is mainly load
protectionE
~ For example, a given motor would always complete a start within 2
seconds. If the safe stan time is 8 seconds and a failure occurred such
that the motor was held in a stan condition, the motor would normal
remain at stall for a total of 8 seconds before the thermal model would
generate a trip. The accelerator timer could be configured to generate a
trip if the motorremained at stall for more then 3 seconds thereby
reducing the stress on both the motor and driven equipment.
~ Note that some soft starts limit the motors starting current to less then the
motors rated full load current. Therefore if the relay does not see the
motor's current rise to a value greater than the motors rated full load
current within 1 second after a start, the acceleration timer will be ignored.
 r Company

TIME-CURRENT AND THERMAL LIMIT' CURVES

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rnUilor RB\J!~a\ir~ oloer'ate~s in
" Time between starts
" RTD alarm and trip settings (stator, bearing, etc.)
" Two speed motor protection
" Load averaging filter for cyclic load applications
" Reduced voltage starting supervision
" Variable frequency fUter allowing accurate sensing and calculation
the analog values in VFD applications
" Analog input differential calculation for dual drives applications
" Speed counter trip and alarm
" Universal digital counter trip and alarm
" Pulsing KWh and Kvarh output
" Trip coil supervision
" Emergency Restart Input
" Undervoltage auto restart (additional element per special order).
" Experimental broken rotor bar detection system
 rotectio :
v Standard Protection Cu ll-u.,J" Start-up
I!l Custom Protection rves
I!l RTD Biasing

IllUnbalance Biasi
1$ 3 Phase Voltage

1$ Safe Stall

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• Motor Learned LJn lin

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Q&A