Frequency inverter M-Max - Moeller

Engineering04/10 MN04020001Z-ENMotor and ApplicationMotor selectionGeneral recommendations for motor selection:• Use three-phase powered asynchronous motors with shortcircuitrotors and surface cooling, also called asynchronousmotors or standard motors for the frequency-controlled drivesystem (PDS). Other specifications such as external rotormotors, slip-ring motors, reluctance motors, synchronous orservo motors can also be run with a frequency inverter butnormally require additional planning and discussion with themotor manufacturer.• Use only motors with at least heat class F (155 °C maximumsteady state temperature).• 4-pole motors are preferred (synchronous speed: 1500 min -1 at50 Hz or 1800 min -1 at 60 Hz).• Take the operating conditions into account for S1 operation(IEC 60034-1).• When operating multiple motors in parallel on one frequencyinverter, the motor output should not be more than three powerclasses apart.• Ensure that the motor is not overdimensioned.If a motor in speed control mode is underdimensioned, themotor rating must only be one rating level lower.Connecting motors in parallelThe M-Max TM frequency inverters allow parallel operation ofseveral motors in U/f control mode:• U/f control: several motors with the same or different ratedoperational data. The sum of all motor currents must be lessthan the frequency inverter’s rated operational current.• U/f control: parallel control of several motors. The sum of themotor currents plus the motors’ inrush current must be less thanthe frequency inverter’s rated operational current.Parallel operation at different motor speeds can be implementedonly by changing the number of pole pairs and/or changing themotor’s transmission ratio.Figure 12:F1Q11M1aQ12F2F3U1 V1 W1 U1 V1 W1 U1 V1 W1M3˜M2M3˜Q13Parallel connection of several motors to one frequencyinverterh Caution!Debounced inputs may not be used in the safetycircuit diagram.If you are connecting multiple motors on one frequencyinverter, you must design the contactors for the individualmotors according to utilization category AC-3.Selecting the motor contactor is done according to therated operational current of the motor to be connected.Connecting motors in parallel reduces the load resistance at thefrequency inverter output. The total stator inductance is lower andthe leakage capacity of the lines greater. As a result, the currentdistortion is greater than in a single-motor circuit. To reduce thecurrent distortion, you should use motor reactors (see a infigure 12) in the output of the frequency inverter (see also Section“Motor chokes”, page 171).M3M3˜hhhThe current consumption of all motors connected inparallel must not exceed the frequency inverter’s ratedoutput current I 2N .Electronic motor protection can not be used whenoperating the frequency inverter with several parallelconnected motors. You must, however, protect eachmotor with thermistors and/or overload relays.The use of motor protective circuit breaker at thefrequency inverter’s output can lead to nuisance tripping.28