By observing the motor data sheet, these concepts become clear. In fact, it is possible to see the rela-

tion between shaft position, hall sensors response and voltage profile to be supplied.

This way to supply the stator phases is very complex because it is necessary to produce a sine wave

with a proper period and delay with respect to the sensors information. As we will show later on, a sim-

pler method is possible.

INVERTER DRIVER TOPOLOGY

The stator of a brushless DC motor is generally supplied by an inverter which converts a DC voltage to a

3-phase AC voltage whose frequency is related to the speed of the rotor. Speed control is achieved by a

Pulse Width Modulation (PWM) of the phases voltage accomplished by periodically switching the phase

voltage to zero. The widely used driver to perform this switching is the six-step inverter where each pha-

se is driven by means of a couple of transistors. Fig.3 shows the basic operating principle of this drive.

The name "six-steps" arises from the finite time-steps in which the whole period can be shared. During

each time step, current direction does not change whereas current amplitude can increase or decrease

in the coil.

To better explain this operating principle, let us consider the action of one leg (phase) of the inverter,

such as for example T1 and T4.

Transistor T1 is turned on at

θ= - 90°and turned off at θ=90° while T4 is turned on at θ=90°. When T1 is

Figure 3. Inverter driver operating principle

turned off, the current it was carrying is immediately diverted to the diode in parallel with T4. This diode

re-circulates the instantaneous current of the winding until it decreases to zero.

Once the phase current reverses direction is carried by T4. In term of voltage it is easy to draw the pha-

ses voltages by conceiving the transistors as switches. Due to the triangular connection of the phases,

each phase voltage depends by the status of two legs of the bridge. Figure 4 shows one real phase star

voltage and one phase current. Six voltage steps are evident in the look like sine wave.

The same happens in the other legs of the bridge in different times. This topology drives the windings for

the whole period, avoiding the phase to be "floating" (Six-Steps Continuous Mode Inverter).

FUZZY CONTROLLER

Ph 3

U

T2

T3

T5

T6

T1

T2

T3

0

π/2π3π/2θ=ωt

T1

T4

Ph 1

V

Ph 2

W

®

Brushless Motor Fuzzy Control by using ST52x301

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