Siemens SIMPRO-100 Instruction Manual page 210

E
Motor Thermal Element
When the motor is running (S ≈ 0), the
positive-sequence heating factor K
The negative-sequence heating factor, K
S ≈ 0 is found.
To summarize, based on the assumption that the
locked-rotor rotor resistance is three times the
running rotor resistance:
• The heating factor of positive-sequence
current K , when the motor is running is
1
1 per unit
• The heating factor of negative-sequence
current, K , when the motor is running is
2
5 per unit
• Both K and K are 3 per unit when the rotor
1 2
is locked
The differences in the positive- and
negative-sequence heating factors immediately
suggest that the thermal element should have
two states representing the starting and running
states of the motor. The SIMPRO-100 Relay
thermal element automatically selects which state
to use based on the measured positive-sequence
current. When the positive-sequence current is
greater than 2.5 times the motor rated full load
current setting, the relay uses the starting state.
When current is less than 2.5 times rated full load
current, the relay uses the running state.
208
E.4
, is found.
1
Equation E.6
, at
2
Equation E.7
Figure E.4 Motor Starting Thermal Element
Figure E.4 shows the thermal element used when
the motor is starting. Locked rotor heating occurs
over just a few seconds, so the model assumes
that no heat is lost to the surroundings and the
resistor is removed from the thermal circuit. The
thermal trip value is defined by the motor rated
locked rotor current I
hot motor locked rotor time To. The thermal
capacitance is selected to match the heat source
heating factor 3. By setting the capacitance equal
to 3, when the motor positive-sequence current
I , equals locked rotor current I
1
estimate U, reaches the trip value in exactly
locked rotor time To.
When a successful motor start occurs and
positive-sequence current drops below 2.5 times
full load current, the relay switches from the
starting thermal element to the running thermal
element. The present heat estimate U, is
transferred directly to the running element,
representing the heat build-up that occurred
during motor starting.
Motor Starting
Protection
, squared, times the rated
L
, the heat
L
SIMPRO-100
PRIM-2400C