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Operation of motors connected in parallel with dynamic load (where the load condition changes quickly) and in the open–loop controlled range require an additional dimensioning: 1.2 (Σ rated motor currents)  rated current of the drive module – The current limit of the drive module must be increased to 150% of the rated current when commissioning the system. The motors should not be subject to torques that exceed their rated torque. For special high–speed induction motors, e.g. for woodworking, a series reactor must always be located between the drive module and the motor group: Rated reactor current: rms current of the motor group2) When the above information and instructions are taken into consideration, the individual motors are able to correct even for dynamic load and speed steps. ”Stable” operation without stalling – also for individual motors – is achieved when following the dimensioning guidelines specified above. The speeds of the individual motors depend on the load. The currently set speeds can drift apart by several percent due to the closed–loop group slip control.

Operating several induction motors in parallel

 Several motors can also be operated in parallel on a power module, for each axis. When selecting the motor and drive module, several engineering guidelines must be observed. When expanded to the maximum, a drive configuration for parallel operation can comprise up to eight motors. Motors connected to a drive module in parallel must have the same V/f characteristics. Further, we recommend that the motors have the same number of poles. If more than two motors are connected to a drive module, then these should essentially have the same power ratings. For a 2–motor configuration, the difference between the power ratings of the motors should not exceed a ratio of 1:10. The following engineering guidelines must be carefully observed: Selecting the size of the drive module – Steady–state operation of the motors connected in parallel – namely in the closed–loop controlled range (> nmin1)) and preferably in the rated speed range: Σ rated motor currents  rated current of the drive module

Load surges and overload conditions in the field–weakening range can result in oscillation and should be avoided. The drive module cannot detect if an individual motor is overloaded. Individual thermal monitoring functions must be provided to ensure that each individual motor has overload protection. We recommend that the motor is monitored using a PTC thermistor evaluation circuit.

Overload protection

Individual thermal monitoring functions must be provided for overload protection of the individual induction motors. We recommend that the motor is monitored using a PTC thermistor temperature sensor (embedded in the motor) and a 3RN1 thermistor motor protection evaluation unit. If motor feeder cables have to be protected where the rated drive converter current is significantly greater than the rated motor current then this must be implemented outside the drive converter. Notice Motors may only be changed over using the power contactors in the motor circuit when terminal 663 – pulse enable/start inhibit – is inhibited (de–energized). This means that the power contactor may only be switched when the motor circuit is in a no–current condition. For additional information also refer to circuit examples =9 in Section 9.7