AN863 Datasheet, PDF (13/22 Pages) STMicroelectronics – Improved sensorless control with the ST62 MCU for universal motor

IMPROVED SENSORLESS CONTROL WITH THE ST62 MCU FOR UNIVERSAL MOTOR
5 REGULATION PERFORMANCE
Figures 7, 8, 9 show the regulation performance at three speeds typical of a drill. The
horizontal axis shows the motor rms current, which is proportional to the load. The
first two figures show reasonable performance: speed is kept constant within +- 10%.
The last figure, corresponding to a very low speed of 400 rpm, shows a speed in-
crease when the load increases. This can be improved somewhat by fine tuning the
compensation table. Creating a different table for each set speed would also improve
the performance for a given set speed (for example, one table for set speeds 400-600
rpm, one for 600-900 rpm, one for 900-1300 rpm, etc... the example program pro-
vides only one table for the whole speed range). However, it should be noted that the
difficulty to find the best compromises increases considerably when we want to regu-
late the motor at slower and slower speeds. This is easy to understand conceptually,
as the quantity that we actually measure and regulate is k.Ω + r. When we move to-
wards slower speeds, the relative importance of the term k.Ω compared to the resis-
tive term r becomes smaller and smaller. Also, at very low speeds, friction becomes
predominant. This limits the usefulness of the system to moderate speed range appli-
cations.
6 CONCLUSION
This document showed how to regulate the universal motor speed without speed
sensor, with the help of a low cost micro controller. The price to pay is some accuracy
loss in the speed regulation, in the order of 10%. In most home appliance applica-
tions, such speed variation with varying load is fully acceptable. When the micro in-
vestment has been made, it is a good idea to make use of its inherent benefits, flexi-
bility, ease of customization, fast time to market. In a washing machine for example,
the micro can also be used to generate the speed ramps and washing / rinsing se-
quences, and control the unbalance; in a drill, it can be used to control or limit the
torque. In the long term, the universal motor should be replaced more and more by
brushless "electronic" motors, such as the DC permanent magnet brushless, or the
switched reluctance motor, due to their higher efficiency and dynamic performances.
However, cost reductions such as the one described in this document should allow
the universal motor to compete successfully for many years in the most cost sensitive
applications.
13/22