Protection Devices
The motor should be provided with protection devices to guard against non ordinary working conditions. The use of protectiondevice on the line is particularly advisable (i.e.
varistors) for those motors running at low speed (8, 12, 16 poles) to preventearly wear of windings and of contacts caused by voltage peaks during the switching on.
Operation conditions Protection typeFusesProtective circuit breakersThermal protective device on the windings Excess currents 200% InHeavy starts, reversing operation
Stalling
Starting on two phases
Voltage deviations
Frequency deviations
Insufficient motor cooling excellent protectionpartial protection
partial protection
partial protectionexcellent protection
excellent protectionno protectionno protectionno protectionpartial protectionno protection
no protection
no protection
no protectionexcellent protectionexcellent protectionpartial protectionexcellent protection
excellent protection
excellent protection
excellent protection GENERAL CHARACTERISTICS ° C. The contact closes again with a temperature reduction of at least 35 ° C. The bimetallic thermal protectors leads are connectedto a terminal board located in the main terminal box. Bimetallic Thermal Protectors (N.C.): three bimetallic sensors in series with normally closed contacts, fitted on the windingsheads. They control a switch (not provided with the motor) that interrupts the power supply when getting close to dangerous
temperature. The nominal voltage and current are 250 V and 2,5 A A.C. respectively while the temperature of intervention is 140
Thermistors (PTC): three thermistors in series (conforming to DIN standards 44081 and 44802), fitted on the windings heads.The resistance of the thermistors changes with temperature and when getting close to the nominal intervention temperature the
sharp increase of resistance guarantees a precise intervention of the safety devices. The thermistor only senses the temperatureso a cut-out device (not provided with the motor) must be added to interrupt the power supply to the motor. The maximum PTC
operating voltage is 30 V DC while the intervention temperature is 130 ° C on class F motors and 140 ° C on class H motors. ThePTC leads are connected to a terminal board located in the main terminal box. Over-voltage protection Low speed motors: when starting motors having a high number of poles (i.e. 8, 12, 16), voltage peaks can be generated damagingthe motor insulation materials and contacts. In these cases it is advisable to install safety over-voltage protection devices.
On request MGM provides over-voltage protection devices such as RC04 for motors up to 4 kW and RC10 for motors up to 10
kW. Please note that these devices should not be installed if the motor is controlled by an
industrial inverter. Brake coil: DC brake coil is supplied as standard with a rectifier fitted with a protection device against over-voltage and with afilter against the radio frequency emissions. The AC brake coil doesn’t generally need this type of protection devices. In case ofa very high start/stop frequency or in case of critical line voltage situation it is recommended the use of RC04 filter in order tolimit the electrical interference on the brake.
On request MGM is able to supply motors equipped with thermistors PTC or bimetallic N.C. thermal protectors:
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