Motor: base speed, base frequency, rated motor speed, field weakening range

Question:
What is a base speed (base frequency), how is it determined and how is it related to the rated motor speed?

Answer:
The base speed marks the transition between the so-called base setting range (armature setting range) and field weakening range.
The base speed is not equal to the rated motor speed.

Base setting range
In the base setting range, the controller can vary the output voltage to apply the required motor current for the excitation field and the torque generation.

Field weakening range
As from the base speed the output voltage of the controller (motor voltage) has reached its maximum and cannot be increased with increasing speed to drive the required field current. The results in a 'field weakening' and the typical reduction of the achievable torque with increasing speed.

Base speed
The base speed is determined by the rated motor data and the maximum possible output voltage of the inverter.

Maximum possible output voltage of the inverter
The maximum possible output voltage of the inverter depends on the following variables:

• Amount of DC bus voltage (mains voltage)
• Voltage reduction by inverter performance
• Voltage drop at the mains choke

Empirical values for the influence of mains choke and inverter:

• 3...7 %   (inverter only)
• 6...10 % (inverter and mains choke)

The base speed can be roughly calculated using the rule of proportion from the rated motor speed, the rated motor voltage and the max. possible output voltage. In the exact formula, the rated slip would be added to the calculation.

Example (motor: MDFKA71-120):

• Rated motor speed (C0087): 3410 rpm
• Rated motor voltage (C0090): 390 V
• Mains voltage: 400 V
• Max. output voltage = mains voltage - 8 % = 368 V
• Base speed = 3410 rpm * 368 V / 390 V = 3218 rpm

In this example the base speed (beginning of field weakening range) is smaller than the rated motor speed (C0087).

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