Driving an ACIM (AC induction machine)

The induction motor is outside the primary focus of our customers. Nevertheless, our driver is well suited for this type of motor. There are two supported methods:

• U/f mode. This is the simple, widely recognized control method, commonly referred to as "variable frequency drive" for induction motors. The frequency is coupled to the output voltage. This method is easy to configure, offers a stable operation, but only the voltage mode is supported. Current control / torque control is provided indirectly, through imult command / iref parameter.

• FOC mode. Employs the full FOC with axes separation, working with the slip angle and enabling more precise control. With this type, the induction motor achieves most of the drive modes we normally introduce for a PMSM.

How to configure

tip

Automatic identification is not limited for this type of motor. We recommend consulting our technical support team for assistance.

For U/F mode

• set /driver/psi to 0 (no flux linkage in the squirrel cage).

• set /driver/prest to $0$ (asynchronous mode - decouple the field angle from the rotor: execute reinit 0 to make run-time reinitialization).

• set /driver/vcf to $12$ (disable current control in direct axis, choosing VFD mode). You may experiment with values of 4 and 8 for directional preferences.

• set /driver/dac/dxc to $\sqrt{2}/2$ (roughly $0.707$) set direct axis current limit to match the quadrature axis current limit.

• set /driver/dac/mtk to $0$. Alternatively, use /driver/dac/mtk to achieve a degree of torque control: the d- current reference is mapped to the q- current measurement (through filter defined by mtk).

• set /driver/rest/omega to the desired ratio $r$ between the voltage and the frequency. It can be calculated with this equation:

  $r = \frac{2\pi \cdot f}{U_{stator,RMS} \cdot \sqrt{2/3}}$

  where $f$ is the rated electrical frequency and $U_{stator,RMS}$ is the rated phase-to-phase RMS voltage. The motor manufacturer should specify those ratings.

tip

A fine tuning by hand might be advisory for /driver/rest/omega, as different vendors rate motors differently. The tuning goal is to find the minimum viable current magnitude of unloaded motor (in voltage drive command). However, increasing omega too much will severe the dynamics and efficiency. Usually, a 20-30% upper margin for the calculated omega is advisory when the vendor provides ratings under load (e.g. value of 32 will yield into 40, etc).

For FOC mode

• set /driver/psi to 0 (no flux linkage in the squirrel cage).
• set /driver/prest to the value representing your motor sensor (usually, a quadrature encoder: execute reinit 10 to make run-time reinitialization).
  • For a quadrature encoder, set the correct ppr.
  • Provide sensor mapping in /rest/ folder manually, or execute identrun to map the encoder to the motor automatically. Note that successful spin-up of an induction machine might be tricky to tune.
  • For an encoder, rpole can be auto-identified with the identrun procedure as long as the encoder's resolution is more than 360 ppr. Other sensors work as usual.
• set /driver/vcf to $2$ to prefer the flux axis voltage feed and prevent saturation. This will silence the motor at higher speeds.
• set /driver/dac/dxc to $\sqrt{2}/2$ (roughly $0.707$) set direct axis current limit to match the quadrature axis current limit. The d- current reference is mapped to the q- current measurement (through mtk filter).
• tune the /driver/rest/omega to the desired slip frequency dependence to the stator current:
  1. Start with value of zero and feed a drive command in voltage mode (e.g. run 0.1) with moderate iref current setting, relative to the motor size. Motor should not spin. If it does, the sensor is not properly identified.
  2. Observe the D-Q currents and gradually increase the value /driver/rest/omega until the motor starts to spin (we suggest 0.01 increments in most set-ups).
  3. Increase /driver/rest/omega until the current will stop decreasing proportionally to your inrements (optionally, contact siliXcon to help with this step).
  4. Evaluate the drive, and optionally fine-tune the omega to find the sweetspot, where your motor has the best performance in your operating point.

tip

To emulate a PMSM (so all siliXcon drive modes can be used with highest dynamics), you may set set the /driver/dac/mtk to zero: /driver/dac/dxc then defines the constant flux current excitation.

Limitations

• Please use only VECTOR algorithm. BLDC has no use for this type of motor.
• For the ACIM in FOC mode, sensorless mode is not supported at the moment. You need to provide a shaft sensor.