~ 19 ~
NOVATEK-ELECTRO
UBZ-304
During start-up the protection is enabled when all phase currents are more than the set-
point Is (“Start I Coef”
parameter) during the period of time more than the ST time delay (“Start delay I” parameter
).
Figure 3.3
–
Delayed start and rotor blocking
3.4.6.2
Rotor blocking
After motor start performing (reducing the starting current lower than 1.2 of rated one) UBZ switches to control of
possible blocking of the rotor. The protection system operates when all the phase currents are more than set-points
during a period of time greater than LT time delay (“Block I delay” parameter
).
3.4.7 Thermal overload protection
Thermal overload protection is made on the basis of the equation solution of motor thermal balance under the
following assumptions:
–
before the first start the motor was cold;
–
during the motor operation, heat generates that is proportional to the square of the current;
–
after motor turning off it is cooled down exponentially.
For protection, you should enter the response time in case of double overload T2 (the parameter of
“T
hermal
delay”).
Current-time characteristic with different values of T2 is given in Fig.3.4.
For the standard recommended T2 value (60 s at 2 time overload) Table 3.7 shows the following current-time
characteristic.
Table 3.7
I/Inom
1.1
1.2
1.4
1.7
2
2.7
3
Т
sec
365
247
148
88.6
60
36.4
24.6
I/Inom
4
5
6
7
8
10
15
Т
sec
13.5
8.5
5.9
4.3
3.3
2.1
0.9
For rotating machines, cooling is more efficient during operation than during the stop of the motor, so enter the
parameter
“T
h
ermal C stop”
- the constant increase rate of cooling when the motor is stopped.
After the load relay disabling owing to thermal overload with ARS permitted, the relay will be enabled again
after the time more than the maximum of the two values:
–
time of thermal hysteresis (motor should cool down to 33% of the accumulated heat);
–
time of ARS.
Choosing different ARS time periods considering thermal hysteresis, one can reduce the number of starts per
time unit because in the intermittent mode of operation UBZ remembers the amount of heat released during the
motor start.
Delayed start time
Rotor blocking
Case of rotor blocking
