Electricity: motive power systems – Limitation of motor load – current – torque or force
Patent
1994-06-06
1995-11-14
Martin, David S.
Electricity: motive power systems
Limitation of motor load, current, torque or force
318472, 318700, 318798, H02K 1732
Patent
active
054670012
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a control method for an alternating current motor, and more particularly, to a control method for an alternating current motor with a limiter capable of clamping a torque command at a predetermined limit value when the torque command exceeds the limit value, thereby providing thus a limited torque command, as an amplitude of a current command, to a current loop of each phase of the alternating current motor.
BACKGROUND ART
In alternating current type, motors such as induction motors and synchronous motors, current flowing through each exciting winding is alternating current, whose frequency is proportional to a rotational speed of the motor. Thus, with increasing rotational speed of the motor, frequency of the command current to be supplied to the exciting winding and frequency of the actual current of the exciting winding increase. Frequency increase of the actual current brings amplitude reduction of the actual current with respect to the command, as well as phase delay and power factor reduction.
FIG. 1 is a block diagram showing a conventional speed loop processing used for the speed control of a three-phase AC motor.
In FIG. 1, a reference numeral represents a speed loop compensatory circuit, in which a speed feedback signal of supplied from a speed detector, attached to the motor to detect the actual speed of the motor, is subtracted from a speed command Vc to obtain a speed deviation. The product of the integrated value of the speed deviation and integral constant K1, and the product of the speed deviation and the proportional constant K2 are added, and the result is outputted as torque command Tcmd1. That is, the PI (proportional-and-integral) control is applied to the speed of the motor to obtain the torque command Tcmd1. Next, in order not only to protect the components, such as transistors for controlling the motor but also to protect the motor itself, the torque command Tcmd1 is inputted into a current limiter 2, in which plus-and-minus upper limit values of the torque command Tcmd1 are clamped at predetermined limit values so as to generate a torque command (an amplitude of the current command in the current loop processing) Tcmd2 to be supplied to each phase current loop circuit which controls the current flowing through each phase winding. In a case where the speed loop control is not carried out, the torque command will be directly inputted to the current limiter 2. Then, the torque command (amplitude of the current command) Tcmd2, output by the current limiter 2, is corrected by a rotor position .theta. of the motor detected by the detector and a phase advance amount ph to be determined based on the rotational speed of the motor, and then multiplied by sine values having a 2 .pi./3 phase difference therebetween (3R, 3S, 3T), thereby obtaining torque commands (current commands) Tcmd(R), Tcmd(S) and Tcmd(T) to be supplied to the windings R, S and T, respectively.
In each phase current loop, the IP (integral-and-proportional) control or the PI (proportional-and-integral) control is carried out to control the current flowing through each phase winding.
FIG. 2 is a block diagram showing a conventional control system whose current loop comprises the IP control, corresponding to the R phase of the motor. Control systems for the other phases are substantially the same as that disclosed in FIG. 2. In the drawing, a reference numeral 4 represents an integral element with an integral gain k1, and a reference numeral 5 represents a proportional element with a proportional gain K2. Furthermore, a reference numeral 6 represents a transfer function term of the R-phase winding of the motor, with a winding inductance L and a winding resistance R. A reference character E represents a reverse electromotive force generated by the motor.
A current deviation, obtained by subtracting the actual current Ir from the torque command Tcmd(R), is integrated in the element 4, the integrated value is multiplied by the integral gain K1 and, from this product, the produ
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patent: 4375047 (1983-02-01), Nelson et al.
patent: 4443750 (1984-04-01), Sheng
patent: 4611159 (1986-08-01), Kurakake et al.
patent: 4843297 (1989-06-01), Landino
Fanuc Ltd.
Martin David S.
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