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AN2281

Library parameters

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5.2

Start-up phase parameters

The motor start-up procedure implemented in the library presented in this application note

basically consists of two different phases:

●

Rotor alignment phase

●

Sinusoidal three-phase voltage ramp-up

5.2.1

Rotor alignment phase

This phase pre-positions the rotor in order to put it in a known position. This allows the

proper alignment of the statorical flux during motor start-up, avoiding any unwanted rotor

vibrations. Moreover, putting the rotor in a known position causes the current waveform

during the motor start-up to be repetitive and deterministic, and thus easily adjustable. In

spite of all this, in some applications, it can be useful not perform the alignment phase. This

usually produces a higher and more uncontrolled current but it can lead to a faster ramp-up

of the motor.

In the library supplied with this application note, you can disable or enable the rotor

alignment by commenting the command line in or out.

#define ROTOR_ALIGNMENT

The rotor pre-positioning is achieved by applying a linearly increasing voltage (starting from

zero volts) to one of the three phases of the motor while the other two are grounded.

The two command lines:

#define ALIGNMENT_DURATION

xxx

//Alignment duration in msec

#define FINAL_DUTY

xxx //Final duty cycle in 1/255 units

//(85 <=> 33%)

set the alignment phase duration and the final duty cycle (normally it should not exceed

33%).

5.2.2

Sinusoidal three-phase voltage ramp-up

This procedure initializes the sinusoidal three-phase voltage generation and, in order to

reduce the start-up motor current, carries out a linear increase of the sine amplitude. The

command lines:

#define START_VOLT

xxx

// in 1/255 unit. 255 <=> maximum

// modulation index

#define START_SPEED

xxx

// in rpm

allow to you initialize the three-phase sinusoidal voltage amplitude and frequency. Note that

statorical frequency initialization is necessary because the self-synchronization algorithm

discussed in Section 2 can work properly only if the rotor frequency f* can be measured.

The initialized value of statorical frequency will be, therefore, applied to the motor until a

valid f* measurement has been performed.

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