Synchronous motor: Maximum possible speed with/without field weakening function (base speed)

Question:
As from what motor speed starts field weakening operation for synchronous motors?
Is it possible to reach motor speeds higher than the rated speed specified on the motor nameplate when operating synchronous motors (SM) on i700, 9400, 8400, 9300 or ECS controllers?
 
Answer:
At their output terminals (U, V, and W), controllers can approximately output a voltage with variable frequency amounting to the mains voltage applied (L1, L2, L3) less a voltage drop of at least 20 V.
Permanently-excited synchronous motors (PSM) always require a motor voltage proportional to their speed.
For synchronous motors, field weakening operation starts from the base speed. The base speed corresponds to the speed from which on the output voltage at the inverter terminals can no longer be increased proportionally to the speed.
The base speed is not equal to the rated motor speed. In the event of a higher mains voltage or a higher DC voltage in the DC bus, the base speed will be correspondingly higher.
The base speed (nbase(motor)) can be roughly determined according to the following rule of three formula: 
 
                nbase(motor)  =   nrated(motor) * ( Umains - 20V )  / Urated(motor)
 
As a function of the rated motor voltage in relation to the mains voltage, higher speeds can also be achieved without active field weakening function in the controller.

Example:
MDSKSRS036-13 synchronous servo motor with the following rated data from the motor nameplate:
  - Rated speed  nrated(motor)     = 4000 rpm 
  - Rated voltage Urated(motor)  = 245 V
With a mains voltage of 400 V, the field weakening range starts at the following base speed (nbase(motor)):
                 nbase(motor) =  4000 rpm * ( 400V - 20V )  / 245V    = 6204 rpm
 
Field weakening function:
To operate SM at speeds above the base speed in the so-called field weakening range, the controller must be equipped with a special field weakening function for synchronous motors.
The following Lenze controller series are equipped with a field weakening function for synchronous motors:
  • i700 servo inverter:   Function is activated by default
  • 9400 servo drives:    Function is available, but must be explicitly activated
  • 8400 TopLine:           Function is available, but must be explicitly activated
  • ECS servo system:    
  • 9300 servo controller:       Not implemented

Hint:
When selecting the maximum speed make sure that the mechanical maximum speed of the motor is not exceeded. Also ensure that the motor is always operated in the permissible speed / torque characteristic range. Please use the speed / torque characteristic provided in the Lenze motor catalogues for this. Please consider the power assignment for the servo controller used.

For information on 'base speed, rated speed, and base frequency', see article 20037715.


URL for linking this AKB article: https://www.lenze.com/en-de/go/akb/200503622/1/
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AKB-ID: 200503622
Last update: 2024/01/11
Category: FAQ, Tips and Tricks

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