Method and apparatus for controlling AC motors

An AC motor control apparatus including:

• a speed sensor for sensing the actual speed of the AC motor;
• angular frequency detection means for providing an angular frequency signal in accordance with the actual speed of the AC motor;
• torque command generating means for generating a torque command on the basis of a speed deviation between a speed command and an actual speed from said speed sensor;
• memory means for storing a torque-slip frequency characteristic, a slip frequency-amplitude characteristic and a slip frequency-phase characteristic which take secondary leakage reactance into account, for providing a slip frequency in accordance with said torque command and said torque-slip frequency characteristic, for providing a primary current amplitude in accordance with said slip frequency and said slip frequency-amplitude characteristic, and for providing a phase signal in accordance with said slip frequency and said slip frequency-phase charcteristic;
• an arithmetic circuit for computing a three-phase current command from said slip frequency, said primary current amplitude, said phase signal, and from said angular frequency signal;
• and
• a drive circuit for driving the AC motor by means of the three-phase current command from said arithmetic circuit.

A method for controlling an AC motor in accordance with a slip frequency, a phase difference, a primary current amplitude, a commanded speed and an actual speed, said method comprising the steps of:

• (a) computing a torque command in accordance with the difference between said commanded speed and said actual speed;
• (b) computing the slip frequency in accordance with the torque command, and the following relation ##EQU11## wherein T corresponds to said torque command, and ##EQU12## where r.sub.2 is the secondary resistance, 1.sub.m is the excitation inductance, 1.sub.2 is the secondary leakage inductance, w.sub.s is the slip frequency and I.sub.o is the excitation current;
• (c) computing the primary current amplitude using said slip frequency and the following equation ##EQU13## wherein k.sub.3 1.sub.2.sup.2, k.sub.4 =1/r.sub.2.sup.2 .multidot.21.sub.2 (1.sub.2 +1.sub.m)+r.sub.2.sup.2 1.sub.m.sup.2),
• and ##EQU14## (d) computing a phase difference using said slip frequency and the following relation ##EQU15## (e) determining an angular frequency in accordance with the actual speed of AC motor;
• (f) generating a three-phase primary current command using the slip frequency, the phase difference, the primary current amplitude and the angular frequency;
• and
• (g) driving the AC motor in accordance with said three-phase primary current command.