Schweitzer Engineering Laboratories SEL-701-1, Instruction Manual

Page: 377 / 452

Date Code 20011009
SEL-701-1 Monitor
Motor Thermal Element
The Basic Thermal Element
E.7
approximately double-frequency currents in the rotor: negative-sequence slip equals
twice the fundamental frequency.
Based on these observations, negative-sequence slip equals (2–S). Substituting
this value for S in Equation E.3 , calculate negative-sequence rotor resistance, R
r–
.
Equation E.4
Where:
R
1
= Positive-sequence rotor resistance at slip S = 1
R
0
= Positive-sequence rotor resistance at slip S = 0
To obtain factors expressing the relative heating effect of positive- and negative-
sequence current, divide Equation E.3 and Equation E.4 by R
0
. For the locked rotor
case (slip, S = 1).
Equation E.5
When the motor is running (S
≈
0), the positive-sequence heating factor, K
1
, is
found.
Equation E.6
The negative-sequence heating factor, K
2
, at S
≈
0 is found.
Equation E.7
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
1
, when the motor
is running is 1 per unit.
The heating factor of negative-sequence current, K
2
, when the motor
is running is 5 per unit.
Both K
1
and K
2
are 3 per unit when the rotor 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 SEL-701-1 Monitor 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 monitor uses the starting state. When current is less than
2.5 times rated full load current, the monitor uses the running state.
R
r-
R
1
R
0
–
( )
2 S –
( ) •
R
0
+ =
R
r+
R
0
---------
S 1 =
R
r-
R
0
-------
S 1 =
R
1
R
0
------ 3 = = =
R
r+
R
0
---------
S 0 =
R
0
R
0
------ 1 = =
R
r-
R
0
-------
S 0 =
2
R
1
R
0
------
è ø
æ ö
•
1 – 5 = =