-ETHOD OF OPERATION
3*
6
# ' #
4HERMAL OVERLOAD PROTECTION
4HE THERMAL OVERLOAD PROTECTION PREVENTS THE PRO
TECTED OBJECT EG IN CASE OF CABLES OR MOTORS FROM
DAMAGE CAUSED BY THERMAL OVERLOADING
4HE UNIT COMPUTES THE TEMPERATURE RISE ACCORDING
TO A THERMAL SINGLE BODY MODEL AS PER THE FOLLOWING
THERMAL DIFFERENTIAL EQUATION
D
DT
Ç
Ç
)
WITH
ACTUAL TEMPERATURE RISE REFERRED TO THE FI
NAL TEMPERATURE RISE FOR THE MAXIMUM PER
MISSIBLE CURRENT OF THE PROTECTED OBJECT
Kw)
.
Ç
THERMAL TIME CONSTANT FOR HEATING UP OF
THE PROTECTED OBJECT
)
ACTUAL CURRENT OF THE PROTECTED OBJECT
RMS VALUE REFERRED TO THE MAXIMUM
PERMISSIBLE CURRENT OF THE PROTECTED OB
JECT )
MAX
K w )
.
7HEN THE TEMPERATURE RISE REACHES THE FIRST SET
THRESHOLD A WARNING ALARM IS GIVEN IN ORDER TO REN
DER POSSIBLE AN EARLY LOAD REDUCTION )F THE SECOND
TEMPERATURE THRESHOLD IS REACHED THE PROTECTED OB
JECT CAN BE DISCONNECTED FROM THE NETWORK
4HE TEMPERATURE RISES ARE CALCULATED SEPARATELY FOR
EACH INDIVIDUAL PHASE ! CHOICE CAN BE MADE
WHETHER THE MAXIMUM CALCULATED TEMPERATURE RISE
OF THE THREE PHASES OR THE AVERAGE TEMPERATURE RISE
SHOULD BE DECISIVE ! TRUE RMS VALUE MEASUREMENT
IS PERFORMED IN ORDER TO INCLUDE FOR THE EFFECT OF HAR
MONIC CONTENT
4HE MAXIMUM PERMISSIBLE CONTINUOUS THERMAL OVER
LOAD CURRENT )
MAX
IS DESCRIBED AS A MULTIPLE OF THE
RATED CURRENT )
.
)
MAX
K
)
.
)N ADDITION TO THE K VALUE THE TIME CONSTANT
Ç
AS
WELL AS THE WARNING TEMPERATURE
WARN
MUST BE EN
TERED INTO THE PROTECTION UNIT
!PART FROM THE TEMPERATURE DEPENDENT WARNING
STAGE THE OVERLOAD PROTECTION ALSO INCLUDES A CUR
RENT DEPENDENT WARNING STAGE 4HIS LATTER ALARM
STAGE CAN GIVE AN EARLY ANNUNCIATION OF AN IMPEND
ING OVERLOAD CURRENT EVEN WHEN THE TEMPERATURE RISE
HAS NOT YET REACHED THE WARNING OR TRIP TEMPERATURE
RISE VALUES
4HE TIME CONSTANT DURING STAND STILL OF A MACHINE
MAY BE MUCH LONGER THAN DURING OPERATION IN CASE
OF SELF VENTILATED MACHINES BECAUSE THE ROTOR DOES
NOT VENTILATE 4HAT IS WHY THE PROTECTION USES FOR THE
STANDING MACHINE THE COOLING DOWN TIME CONSTANT
WHICH CAN BE SET AS A FACTOR OF THE HEATING UP TIME
CONSTANT FACTOR K
Ç
)N THIS ASPECT THE MOTOR IS AS
SUMED TO STAND STILL WHEN THE CURRENT CONSUMPTION
IS LESS THAN THE SETTABLE THRESHOLD ) "2+ #,/3%$
)N CASE OF FORCED VENTILATED MACHINES TRANSFORM
ERS OR CABLES IS K
Ç
4HE THERMAL OVERLOAD PROTECTION IS SUPPLEMENTED BY
THE START UP TIME MONITOR SEE 3ECTION AND THE
RESTART LOCKOUT FUNCTION SEE 3ECTION FOR USE ON
MOTORS $URING START UP OF THE MOTOR THE OVERLOAD
PROTECTION DOES NOT OPERATE IE CHANGE THE CALCU
LATED TEMPERATURE RISE 3TART UP CRITERION IS THE SET
TABLE CURRENT THRESHOLD ) 3424 $%4
&IGURE SHOWS THE LOGIC DIAGRAM OF THE THERMAL
OVERLOAD PROTECTION
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