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If the motor is operated at the frequency converter, a short circuit protection by a circuit breaker is
adequately.
In addition to 1) or 2) full motor protection with PTC (thermistor)
As standard a PTC thermistor (specified in the technical data sheet as PTC) is used for:
Motors for belt-driven fans capacity >= 11 kW
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As an option for smaller capacities available
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For all plug fan motors
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To prevent motor damages, the PTC must be connected to a PTC relay. The
PTC relay does not replace the motor protection switch or circuit breaker and is
needed in additional. The connection to a PTC relay is a prerequisite for the re-
ceipt of the warranty in case of winding damage.
The full motor protection consists of a temperature sensors and a PTC relay (on site). On frequen-
cy converters this function is integrated.
How it works: For single-speed three-phase AC motors, 3 temperature sensors are installed in se-
ries on the exhaust air side of the motor in the winding head. At 135 °C, a sharp increase of re-
sistance occurs, which switches the PTC relay off. For an example of connection diagram refer to
Figure 112.
The full motor protection switches off the motor in case of:
Overload of the motor
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Poor cooling
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Bearing damage
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Block of the rotor
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Winding problems
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Not apply voltages above 5V at the temperature sensor. This leads to its destruc-
tion!
Alternative to PTC: Motors with integrated bimetal sensor (thermal contact, Clixon) - optional
Bimetal detectors are used for thermal monitoring of motor windings and consist of two successive
rolled metals, with unequal thermal expansion coefficients. When they are heated, they expand
unevenly and can switch a contact. They have the advantage that they can be placed directly on
the switch and so no special relay (as for PTC) is needed.
Figure 112: Wiring diagram for thermistors
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