ST7MC1 datasheet(139/308 Pages) STMICROELECTRONICS

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MOTOR CONTROLLER (Cont’d)

10.6.3 Application Example: PM BLDC motor

drive

This example shows a six-step command se-

quence for a 3-phase permanent magnet DC

brushless motor (PM BLDC motor). Figure 74

shows the phase steps and voltage, while Table

24 shows the relevant phase configurations.

To run this kind of motor efficiently, an autoswitch-

ing mode has to be used, i.e. the position of the ro-

tor must self-generate the powered winding com-

mutation. The BEMF zero crossing (Z event) on

the non-excited winding is used by the MTC as a

rotor position sensor. The delay between this

event and the commutation is computed by the

MTC and the hardware commutation event Cn is

automatically generated after this delay.

After the commutation occurs, the MTC waits until

the winding is completely demagnetized by the

free-wheeling diode: during this phase the winding

is tied to 0V or to the HV high voltage rail and no

BEMF can be read. At the end of this phase a new

BEMF zero-crossing detection is enabled.

The end of demagnetization event (D), is also de-

tected by the MTC or simulated with a timer com-

pare feature when no detection is possible.

The MTC manages these three events always in

the same order: Z generates C after a delay com-

puted in realtime, then waits for D in order to ena-

ble the peripheral to detect another Z event.

The BEMF zero-crossing event (Z), can also be

detected by the MTC or simulated with a timer

compare feature when no detection is possible.

The speed regulation is managed by the micro-

controller, by means of an adjustable reference

current level in case of current control, or by direct

PWM duty-cycle adjustment in case of voltage

control.

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