AN1362 STMicroelectronics, AN1362 Datasheet - Page 6

• Manufacturer Part Number: AN1362
• Description: SWITCHED RELUCTANCE MOTOR CONTROL BY ST FIVE
• Manufacturer: STMicroelectronics

AN1362
Figure 8. CURRENT MODE MODULATION AND INSERTION OF OVER-CURRENT PROTECTION
Some hardware protections are implemented on board to prevent drivers and switches from damage
caused by over-current on Bus. The previous picture shows how to protect them. The current sensing
resistor, which is printed on-board, enables to detect the Bus current by using one channel of the micro A/
D, the same one used for the current mode modulation. In the picture, the gate signal “off” is a direct result
of one over-current in a no-showed phase, but it shuts down all the drivers L6386.
DRIVING STRATEGIES
Many strategies are checked to drive the motor phases, by manipulating the encoder signal with some
logic operators.
SRM control is often described in terms of “low-speed” and “high-speed” range. Low-speed operations are
based on the control of the current.
As the motor speed increases, it is very difficult to control the current because of a combination of the back
EMF effects and a reduced amount of time for the commutation interval.
The motor speed can often be increased by setting the turn-on and turn-off angles, so that the phase
commutation begins early by producing current in the winding, while the inductance is at the decreasing
phase. Besides, the extra time obtained allows the reduction of the current in the winding before the rotor
reaches the negative torque region. The control of the firing angles can be carried out in different ways in
relation to the type of position feedback available and the specifics of the application.
The analysis of the logic combination of the encoder signals is a correct way to drive the SR Motor. In this
document, the logical AND between Encoder E1 and Encoder E2, or E2 & E3 or E1 & E3 is used to
produce a set of three signals, which allows an appropriate firing sequence. At the same time, the logical
function AND between E1 and the complementary E3, is another sequence representing an appropriate
pre-fired control signal to drive the SR Motor at high speed. The choice of the correct logic combination
function derives from the comparison between the Encoder Signal chart and the measured Inductance
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