Parameter descriptions
7.10.6 Motor Protection
The protection of the motor against impermissible temperature rise requires monitoring mechanisms
for recognizing a thermal overload to prevent a possible damage to the motor.
The thermal state of a motor can be evaluated by different ways.
1.) Direct monitoring by temperature sensors inside the motor winding (Please check chapter 7.4.6
“Motor temperature”)
- PTC
- KTY
- PT100
- Thermal contact
2.) Indirect monitoring of the motor temperature
- Monitoring of the motor current based on the K characteristic of an integrated
motor circuit breaker
- Emulation of the motor heating by using a temperature-relevant mathematical model I
2
t
The choice of thermal control is mainly determined by type and operating conditions of the motor.
For safe motor protection it is generally sufficient using one of the available possibilities.
A combination of the two groups and their simultaneous operation is possible.
7.10.6.1 Motor protection by Motor Circuit Breaker
571 Operation Mode (motor circuit breaker)
M
otor circuit breakers are used for protecting a motor and its supply cable against over-
he
ating by
overload. Depending on the overload level, they disconnect the motor from power supply immediately
in the case of a short-circuit or they disconnect the motor if an overload has occurred for some time.
C
onventional motor circuit breakers are commercially available for various applications with different
trigger characteristics (L, G/U, R and K), as shown in the diagram below. As frequency inverters in
most cases are used for supplying motors which are classified as operating equipment with very high
starting currents, only the K-characteristic was realized in this function.
1/1000 seconds seconds minutes
x nominal current
245
Special functions 06/2013 Operating Instructions
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