Motors with enclosures in protection type “d” or “de” are designed to work with feeding by means of a frequency convertor (inverter).
Euromotori designed and certified the motors Ex dand Ex deusing a conver-tor with a switching frequency above 3kHz (type PWM).
The motors supplied by converters and intended to be installed in rooms with atmosphere potentially explosive must be:
• explicitly required for this type of feeding
• appropriately selected according to load and speed range
• provided with appropriate passive thermal protection (thermistors, PTC, PT100)
When using a Ex dor Ex demotors with a frequency converter, the follow-ing points must be taken into account in addition to the general selection criteria:
1) A motor fed by converter is subjected to a non-sinusoidal feeding (voltage, current) leading to increased additional losses, noise, vibration and insulation solicitations.
The increase of additional losses gen-erates an increase of motor heating and consequently the rise of bearings temperature. Of these effects must be taken into account (through an appropriate declassification compared to the mains supply) in particular for potentially explosive atmospheres as there are limits on the maximum surface temperature.
The increase of the magnetic noise de-pends mainly on the converter switching frequency and on the harmonic content.Self-cooled versions, with frequencies higher than the network, including the noise due to ventilation is increased about 3-5 dB (A) for frequency of 60 Hz and 8-9 dB (A) for 75 Hz compared the values given in the tables.
The insulation system of a motor is subjected to higher dielectric stress when using a frequency converter.According to the characteristics of the feeding system, it must be utilized filters dU/dt in order to ensure voltage peak of less than 1000V between terminals (motors with 500<Un<600V).
A particular attention must be paid for motors with long feeding cable between the frequency converter and the motor or in the case of rated voltage of 690 V: the motor insulation must be strength-ened due to voltage peaks.
2) In a frequency converter drive the actual operating speed of the motor may deviate considerably from its rated speed. For higher speeds it must be ensured that the highest permissible motor rotation speed, or the critical speed of the entire equipment, is not exceeded. In addition, bearing lubrifica-tion will require special attention.
3) A motor with rated voltage 400V, 50Hz with a star connection, can be used in constant torque up to the frequency delta connection of 87Hz.In this case, the power of converter must be greater than 1.73 times the rated power of the motor and the converter configured to provide 230V at the frequency of 50Hz.
4) It must use cables and connections in accordance with requirements under the EMC (electromagnetic compat-ibility). The feeding cables must be symmetrical and insulated. The grounding of the motor and of the operated system must be properly done in order to avoid possible tensions and currents on motor bearings.
The additional instructions provided by the manufacturer’s converter should be followed.
The performance of 2 and 4 poles motors using a frequency converter (constant torque and quadratic torque) are given in specific tables together with the loadanility curves. For motors of 6 and 8 pole take the advice of EUROMOTORI.