Electricity: motive power systems – Induction motor systems – Primary circuit control
Patent
1989-08-04
1991-12-03
Ip, Paul
Electricity: motive power systems
Induction motor systems
Primary circuit control
318773, 318812, 318778, H02P 748
Patent
active
050702912
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
1. Technical Field
This invention relates to a connection switching apparatus which performs delta (referred to hereinafter as .DELTA.)-star (referred to hereinafter as Y) connection switching for a three-phase induction motor which is used for a machine tool, for example, a lathe, a drill, a tapper and the like.
2. Background Art
Recently, a built-in motor is formed so that an axle of the motor is connected to a work piece or a tool not through a change gear but directly so as to rotate the work piece and the tool directly when used for a machine tool such as a lathe, a drill, a tapper and the like. The motor is driven by an inverter circuit which is formed using a PWM (pulse width modulation) technique using a variable frequency, and its rotation rate is varied by varying the frequency of the output of the inverter circuit.
FIG. 1 represents a rotational speed control circuit for a three-phase induction motor using the PWM inverter technique. FIG. 2 represents only one-phase among the pseudo-three phase alternating current which the PWM inverter shown in FIG. 1 generates, wherein (A). shows a pseudo-sine wave and (B) shows a sine wave which the pseudo-sine wave assumes.
In FIG. 1, a direct current supplied from a direct current source 10 is switched on/off by six power transistors 11 to 16 so that the waveform of each phase becomes as shown in FIG. 2 (A) having a 120.degree. phase difference in relation to the other phases, to generate a pseudo-three phase alternating current. The frequency and effective current of this pseudo-three-phase alternating current can be varied by varying the timing of the switching. Accordingly, the rotational speed of the three-phase induction motor can be controlled to any value by adjusting the timing of the switching based on a set rotation speed at a rotation speed setting means 17, and a real rotation speed calculated from a pulse generated in pulse generator 19 which rotates coaxially with a motor 18 so as to adjust the frequency and the effective power of the pseudo-three-phase alternating current.
FIG. 3 shows the relation between the rotation speed and the effective power. "A" represents the case where windings of the motor form a .DELTA. connection. Referring to FIG. 3, when the rotational speed exceeds a speed No, the effective power becomes constant and independent of the rotation speed, because as the pulse width reaches its upper limit the upper limit of the power which can be supplied is also reached. When the rotational speed becomes lower than the speed No, the current flowing through the windings decreases so that the effective power decreases proportionally to the rotational speed.
In FIG. 3, "B" represents the case where the windings of the motor form a Y connection. In this case, the rotational speed N.sub.1 where the effective power becomes constant is lower than the speed No in the .DELTA. connection. The reason is that, although the impedance of the winding in each phase in the Y connection is larger than the same in the .DELTA. connection, the current in each phase is controlled so that the effective current becomes a predetermined value by widening the pulse width. Then, for example, at the rotational speed N shown in FIG. 3 the effective power supplied is P.sub.0 in the .DELTA. connection, while it becomes P.sub.1 in the Y connection, at a value larger than P.sub.0.
As there is a relation P=kTN (k is a proportional constant) between torque T which a motor generates and an effective power P when a rotational speed is N, the relation between the rotational speed and the torque is as shown in FIG. 4. Therefore, it can be clearly realized that the torque obtained in the Y connection of "B" is larger than the same in the .DELTA. connection of "A" in the region where the rotational speed is relatively low.
Conventionally, this .DELTA.-Y connection switching has been carried out by means of a magnetic conductor requiring a considerable changeover time. The reason why so much time is required in .DELTA.-Y connection switching is mainly that pr
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Nakamura Kosei
Numai Kazuhisa
Fanuc Ltd.
Ip Paul
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