AN2281

PMAC motor control basics

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PMAC motor control basics

PMAC machines are synchronous so the average torque can be produced only when the

excitation is synchronized with the rotor frequency and instantaneous position. By

continuously detecting the rotor angular position and rotational speed, the excitation can be

properly switched among the PMAC motor phases in exact synchronism with the rotor

motion.

This concept, commonly known as self-synchronization, uses direct feedback of the rotor

angular position to ensure that the PMAC machine never loses synchronization. Generally,

Hall sensors are used to get information about the angular position of rotor, detecting the

magnetic field direction generated by the rotor. In particular, the usage of only one sensor is

supported with the system presented in this document.

Figure 3 shows the block diagram of the PMAC self-synchronization algorithm implemented

in the software library.

Figure 3.

PMAC motor control basics: the block diagram

Each of the three phases of the motor is supplied by a sinusoidal waveform whose

frequency, amplitude and phase have been respectively indicated with f, A* and

Φ.

Every time an Hall sensor signal transition occurs, the algorithm estimates the rotor

frequency f* and utilizes this value as statorical frequency (f) for the successive electrical

semi-period. Meanwhile, the phase of the sine wave is also updated and set equal to phase

angle

Φ or Φ+π depending on the Hall sensor edge transition (rising or falling). Generally, for

a large operating speed range, the proper value of

Φ is strongly dependent on the motor

speed affecting the driving efficiency. The provided library allows you to set the optimum

Φ

as a linear function of the speed (in rpm).

Since there are no direct information on current and torque, a V/F limitation has also been

implemented in order to allow you to limit the maximum flowing current for a given speed.

Phase

synchronization

Rotor Position

Phase angle

optimization

A

V/F

Limitation

A*

f

f=f*

f*

Motor

Hall

sensor

Φ

A* sin(2

πf+Φ)