Electricity: motive power systems – Synchronous motor systems – Armature winding circuits
Patent
1984-08-17
1986-12-16
Smith, Jr., David
Electricity: motive power systems
Synchronous motor systems
Armature winding circuits
318722, H02P 540
Patent
active
046299580
DESCRIPTION:
BRIEF SUMMARY
CROSS-REFERENCE TO RELATED APPLICATION
This application is related to our copending U.S. application Ser. No. 637,220 filed July 23, 1984.
BACKGROUND OF THE INVENTION
This invention relates to a synchronous motor control system for controlling a synchronous motor in such a manner that the torque thereof is held constant. More particularly, the invention relates to a synchronous motor control system capable of reducing a follow-up delay in the current flowing through each of the windings of the synchronous motor.
Servomotors are employed widely in a variety of fields, and even AC servomotors have been developed in recent years. Synchronous motors also can be utilized as servomotors. In particular, since synchronous motors which use a permanent magnet as the rotor, are of the brushless type, they are simple in construction and do not generate noise. For these reasons, such synchronous motors are coming into ever wider use. In a synchronous motor of this kind, it is necessary that torque be controlled so as to be constant. To this end, there has been developed a technique in which control is exercised in such a manner that a current of the same phase as an electromotive force induced by the rotor is caused to flow into the windings of the armature, which serves as the stator. This technique will now be described using the drawing of FIG. 1, which shows the construction of a synchronous motor. The magnetic flux density B at a position displaced by .theta. degrees from the q axis of the magnetic field generated by a rotor 1, namely a permanent magnet, is given by the following: in FIG. 1 is expressed as follows: 1.
Accordingly, the electromotive force e.sub.1 induced in the a winding is expressed as follows: ##EQU1## (where .theta.=P.theta.m=P.multidot..omega..sub.m .multidot.t).
Likewise, the electromotive forces e.sub.2, e.sub.3 induced in the b and c windings of the stator 2, which are disposed at angles of 1/3.pi. and 2/3.pi. relative to the a winding, respectively, are expressed by the following: ##EQU2##
If we let the currents flowing in the armature windings a, b, c be i.sub.1, i.sub.2, i.sub.3, respectively, then the output torque T of such a three-phase synchronous motor will be expressed by the following: .multidot.i.sub.3) (6) ##EQU3## To render the torque T constant, it should be so arranged that T is independent of the angle .theta.. Therefore, if the following relations hold, namely: ##EQU4## where I is the current amplitude, then the torque T of Eq. (7) may be written as follows: ##EQU5## Thus, the torque T is constant, being independent of the rotational orientation of the rotor 1.
To carry out such control, it is necessary to detect the rotor angle of the synchronous motor and regulate each of the armature current values in accordance therewith.
However, if the current flowing through each armature winding is delayed by .phi..sub.o from the ideal value, then the currents i.sub.1, i.sub.2, i.sub.3 of the respective armature windings will take on the form: ##EQU6## In consequence, the output torque T will be take on the form: ##EQU7## Thus, the torque will decrease in value.
In the control circuit of a synchronous motor, the gain of a current control loop is finite and cannot be made infinitely large. A delay in response is, therefore, inevitable. Consequently, even though a rotary encoder for sensing the rotational angle of the synchronous motor may produce an output which is an accurate indication of the rotational angle, the current in the armature winding is delayed from the ideal value. The delay, which is proportional to velocity, becomes large in magnitude.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a synchronous motor control system capable of readily reducing the above-mentioned current follow-up delay.
In a motor control system provided with a sensor for sensing the rotational angle of a synchronous motor, and a control circuit for generating a sinusoidal value corresponding to the sensed rotational angle from the sensor, and for multiplying the
REFERENCES:
patent: 4358722 (1982-11-01), Iwakane et al.
patent: 4384244 (1983-05-01), Matsumoto
patent: 4456868 (1984-06-01), Yamamura et al.
Kurakake Mitsuo
Sakamoto Keiji
Fanuc Ltd
Smith Jr. David
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