User Guide

AN1292 Tuning Guide
 2010 Microchip Technology Inc. DS70640A-page 9
STEP 5 – Optionally, Tune the High-Speed Field Weakening Parameters
a) Set Up Initial Parameters
• Nominal and Maximum Speed
Start with a value for nominal speed RPM (i.e., a couple of hundred RPM less
than the motor rated speed). In this example, the motor is rated for 3000 RPM;
therefore, we set NOMINAL_SPEED_RPM to 2800. Consult the motor
specification for the maximum field weakening speed, and enter this value into
MAXIMUM_SPEED_RPM.
userparms.h
Be aware of the fact that for these values above (over) Nominal speed, the field
weakening strategy is enabled, and therefore, lowering the nominal speed used
for smoothing this transition implies additional energy is spent on airgap flux
decrease, which overall, leads to lower efficiency.
CAUTION
Usually, the motor manufacturer indicates the maximum speed achievable by the motor
without it being damaged (which could be higher than the brake point speed at rated
current). If not, it is possible to run it at higher speeds but only for small periods (inter-
mittent) assuming the risks of demagnetization or mechanical damage of the motor or
of the devices attached to it.
In Field Weakening mode, if the controller becomes lost due to a miscalculation of the
angle at high speed above the nominal value, the possibility of damaging the inverter is
imminent. The reason is that the Back Electromotive Force (BEMF) will have a greater
value than the one that would be obtained for the nominal speed, thereby exceeding the
DC bus voltage value, which the inverter's power semiconductors and DC link capaci-
tors would have to support. Since the tuning proposed implies iterative coefficient cor-
rections until the optimum functioning is achieved, the protection of the inverter with
corresponding circuitry should be modified to handle higher voltages in case of stalling
at high speeds.