JPH0614571A - Driving circuit for induction motor - Google Patents

Abstract

PURPOSE:To eliminate trouble due to heating of phase control element by providing a phase control means for gradually increasing the conduction angle of phase control element at initial stage of starting to increase power supply to an induction motor thereby suppressing rush current at the time of starting. CONSTITUTION:A power section 6 comprising a bidirectional switching element is inserted in series into each phase of AC three-phase power supply for an induction motor M and the conduction angle of the power section 6 is controlled by means of a control circuit 4 through a gate circuit section 5 thus gradually increasing current supply to the induction motor M. In such drive circuit, a contact section 7 comprising a plurality of mechanical switching contacts is disposed in parallel with the power section 6. A section 10 for detecting current supply to the induction motor M is also provided and a timer 8 detects the time to be elapsed after the point of time of current detection. Rush current is suppressed by closing the switching contacts at a point of time when the timer produces a time up output, i.e., when the induction motor M enters into stabilized operation.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a drive circuit for an induction motor which suppresses a rush current at the time of starting.

[0002]

2. Description of the Related Art An induction motor generates a rotating magnetic field by a stator winding wound around a stator, generates an electromotive force by an inductive action in a rotor winding wound around a rotor, and this electromotive force causes the rotating magnetic field. The flowing current acts on the rotating magnetic field to generate torque. In this type of induction motor, there is a star-delta starting method as a method of suppressing the inrush current at the time of starting. As shown in FIG. 9, the control device for performing this star-delta starting method includes stator windings L _U , L _V , L of the induction motor.
The _W end of connecting the respective phases of the respective 3-phase AC power source, the stator winding L _U, L _V, L _W other end provided star connecting magnetic contactor 1, the stator winding L A delta connection contactor 2 for delta connection is provided between appropriate ends of _U , L _V , and L _W , and a timer 3 is used to control the opening and closing of each electromagnetic contactor 1, 2.

During the time limit set by the timer 3 from the time of starting, the contacts of the star connection electromagnetic contactor 1 are closed, and the stator windings L _U , L _V , and L _W are shown in FIG. 10 (a). Star connection as shown in. When the star connection is made in this way, the phase voltage applied across the stator windings L _U , L _V , and L _W becomes
If the line voltage of the three-phase AC power supply is V, then V / √3 (√3
Represents the square root of 3), and the phase current is the line current I
Flows.

When the induction motor accelerates and the starting current decreases, the time limit of the timer 3 ends at that point,
Each contact of the delta connection electromagnetic contactor 2 is closed, and the stator windings L _U , L _V , and L _W are delta connected as shown in FIG. In this case, each stator winding L _U , L _V , L
A line voltage V of a three-phase AC power source is applied to _W, and I / √3 flows as a phase current.

[0005]

However, even when the star-delta starting method is performed as described above, when switching from star connection to delta connection, a current exceeding the rating shown in FIG. 11 flows and other devices are connected. There was a problem that had an adverse effect. Therefore, a method using a phase control circuit has been proposed to improve this point. As shown in FIG. 12, the control device inserts a phase control element composed of a bidirectional switching element in series with each phase of a three-phase AC power supply to be supplied to the induction motor M to change the conduction angle of the phase control element. By controlling, the current flowing through the induction motor M is gradually increased. here,
The power unit 6 is configured by the phase control element inserted in each phase,
The control circuit 4 controls the driving of the power unit 6 via the gate circuit unit 5. As the bidirectional switching element, a triac or an SCR connected in antiparallel is used.

The control circuit 4 includes a phase detector 41 for detecting the phase of each phase voltage of the three-phase AC power supply, and a phase detector 41.
The phase control circuit 42 that controls the phase of the power unit 6 according to the output of the pulse control circuit 42 and the pulse generation unit 43 that converts the output of the phase control circuit 42 into a pulse signal. The phase detection unit 41 compares the respective phase voltages shown in FIG. 13A with 0V to output the binary signals shown in FIGS. 13B and 13C, and also based on these signals. The negative pulse shown in 13 (d) is output. That is, the phase detecting section 41 detects the zero-cross point of each phase voltage.

The phase control circuit 42 controls the phase angle corresponding to the time from the zero cross point to triggering the phase control element. Here, in the phase control circuit 42, the sawtooth wave and the reference voltage corresponding to the phase control angle are compared, and FIG.
An output corresponding to the conduction angle of the phase control element shown in (f) is generated. The pulse generator 43 generates a pulse in synchronization with the rising edge of the output shown in FIG. 13F, and the gate circuit unit 5 triggers the phase control element based on the output of the pulse generator 43. Here, two SCs are used as phase control elements.
When Rs connected in anti-parallel are used in FIG.
As shown in (h) and (i), the phase control element is triggered according to the polarity of the phase voltage.

Here, in the phase control circuit 42, the phase angle is gradually narrowed (the conduction angle of the phase control element is gradually widened) in a predetermined time after starting, and the starting current is supplied to the induction motor M in a suppressed manner. The power to be applied is increased to prevent the transient current as shown in FIG. 11 from flowing. At the time of stop, the electric power supplied to the induction motor M is gradually reduced and stopped in the reverse operation to the above case.

However, in the starting control method using the phase control circuit as described above, since the phase control element is always kept in conduction, it is necessary to dissipate the heat generated by the phase control element. However, problems such as an increase in size and an increase in cost occur. The present invention has been made in view of the above points, and an object of the present invention is to provide a drive circuit for an induction motor that suppresses the inrush current at the start and does not cause the problem of heat generation.

[0010]

According to the present invention, in order to achieve the above object, a phase control element inserted in series between an AC power source and an induction motor and a phase control element of the phase control circuit are connected in parallel. A mechanical switching contact connected to, and phase control means for gradually increasing the conduction angle of the phase control element in the initial stage of starting to increase the power supplied to the induction motor,
An opening / closing control means for closing the opening / closing contacts during stable operation of the induction motor is provided.

When the induction motor is stopped, the opening / closing control means may stop the induction motor by opening a mechanical opening / closing contact at an arbitrary timing. Further, when the opening / closing contacts are opened, the phase control means sets the conduction angle of the phase control element to the maximum state and then gradually narrows the conduction angle to stop the induction motor. Can be made.

Further, in order to reduce the possibility of generating an inrush current, a current phase detecting means for detecting that the cut-off period of the load current has fallen below a predetermined width is provided, and the output of this current phase detecting means is provided at the time of starting. The opening / closing control means may close the opening / closing contact.

[0013]

According to the present invention, as described above, the conduction angle of the phase control element is gradually increased in the initial stage of starting to increase the electric power supplied to the induction motor, thereby suppressing the inrush current at the time of starting and reducing the induction motor. By closing the switching contacts during stable operation, the phase control element is prevented from always conducting, and problems due to heat generation of the phase control element are prevented.

[0014]

(Embodiment 1) FIGS. 1 to 4 show an embodiment of the present invention. The drive circuit of the present embodiment detects the current flowing in the induction motor M and the contact portion 7 which is connected to the power portion 6 in parallel and includes a plurality of mechanical switching contacts in the drive circuit of the phase control method shown in FIG. A current detection unit 10 and a timer 8 for measuring a predetermined time from the time when the current is detected by the current detection unit 10 are added, and the open / close contact of the contact unit 7 is closed by the time-out output of the timer 8. .

The power section 6 and the gate circuit section 5 can be constructed as shown in FIG. As the phase control element 61, for example, a unidirectional thyristor such as an SCR connected in antiparallel or a bidirectional thyristor such as a triac is used, and a transient voltage generated at the time of ignition at both ends of the phase control element 61. A power section 6 is configured by connecting a snubber circuit 62 that absorbs. The gate circuit unit 5 is configured by using a pulse transformer, a photocoupler, or the like to insulate the control circuit 4 from each phase voltage.

At the initial stage of starting, the open / close contact of the contact portion 7 is in an open state, and the control circuit 4 operates in accordance with the starting signal as shown in FIG.
By gradually narrowing the phase angle α as shown by, the electric power supplied to the induction motor M is gradually increased, and the induction motor M is started without generating an inrush current. Here, when the phase control element of the power unit 6 is triggered and electric power is supplied to the induction motor M, the current flowing in the induction motor M at this time is detected by the current detection unit 10,
The timer 8 starts the time counting operation.

When the phase angle α reaches a predetermined value, the phase angle α becomes constant. At this time, the timer 8 times up and the open / close contact of the contact part 7 is closed, so that both ends of each phase control element of the power part 6 are short-circuited and the phase control element becomes non-conductive. Therefore, thereafter, electric power is supplied to the induction motor M through the contact portion 7. Therefore, the heat generated by the phase control element does not pose a problem.

When the induction motor M is stopped, the control circuit section 42 which has received the stop signal resets the timer 8 and minimizes the phase angle so that the gate circuit section 5 is activated.
The power unit 6 is controlled via the. That is, the open / close contact of the contact portion 7 is opened and the maximum electric power is supplied to the induction motor M. Then, after that, the control circuit unit 42 gradually widens the phase angle to reduce the electric power supplied to the induction motor M, thereby slowing down the induction motor M and stopping it.

(Embodiment 2) FIG. 5 shows another embodiment of the present invention. Although the timer 8 is separately used in the above-described embodiment, the timer 8 is provided in the control circuit 4 in this embodiment. That is, since this type of control circuit 4 is configured using a microcomputer, the built-in timer of the microcomputer is used. In this way, the start control and the reset control of the timer 8 can be performed without the need for the current detection circuit 10.

(Embodiment 3) FIGS. 6 to 8 show still another embodiment of the present invention. In the present embodiment, the current phase detection unit 11 that detects the current phase of the current flowing through the induction motor M is provided, and the time point at which the opening / closing contact of the contact unit 7 is closed is determined according to the output of the current phase detection unit 11. It was done.

The voltage waveforms of the respective phases when the phase control element of the power section 6 performs the phase control are shown in FIGS.
It shows in (e). Here, the voltage waveform of each phase is the induction motor M
The waveform is as shown in the figure due to the inductance component of. Here, the current-carrying period shown by the shaded areas in FIGS. 7A, 7C, and 7E coincides with the conduction period of the phase control element, and is shown in FIGS. 7B, 7D, and 7F. The current cutoff period changes depending on the impedance of the induction motor M that is a load. Then, the time when the current cutoff period becomes equal to or less than the predetermined width or completely becomes 0 is the time when the induction motor M enters the substantially stable operation.

Therefore, in this embodiment, the current phase detector 11 detects the current interruption period, and the control circuit 4 controls the output of the current interruption period to close the open / close contact of the contact 7. In the case of the present embodiment, the timer 8 is used to measure the period during which the phase angle is gradually decreased, and is not used to control the switching contacts of the contact portion 7. By doing so, the switching contacts can be closed according to the operating state of the induction motor M, so that the possibility of further inrush current flowing can be reduced, and the induction motor M can be reduced.
It is possible to deal with the case where the types are different.

[0023]

As described above, the present invention provides a phase control element inserted in series between an AC power source and an induction motor, and a mechanical switching device connected in parallel to the phase control element of this phase control circuit. A contact point; a phase control means for gradually increasing the conduction angle of the phase control element to increase the electric power supplied to the induction motor in the initial stage of starting; and an opening / closing control means for closing the opening / closing contact during stable operation of the induction motor. By gradually increasing the conduction angle of the phase control element in the initial stage of starting to increase the power supplied to the induction motor,
It is possible to suppress the inrush current at the time of starting, and by closing the switching contacts during stable operation of the induction motor,
It is possible to prevent the phase control element from always conducting and prevent a problem due to heat generation of the phase control element.

When the induction motor is stopped, if the opening / closing control means opens the mechanical opening / closing contact at an arbitrary timing,
The induction motor can be stopped. Furthermore, when the phase control means makes the conduction angle of the phase control element to the maximum state when the switching contacts are opened, and then gradually narrows the conduction angle to stop the induction motor, the induction motor is slowed down and stopped at the time of stop. Can be made.

Furthermore, a current phase detecting means for detecting that the cut-off period of the load current has fallen below a predetermined width is provided, and the opening / closing control means closes the opening / closing contact at the time of starting with the output of the current phase detecting means. As a result, the switching contacts can be closed when the induction motor enters a substantially stable operation, and the occurrence of inrush current can be further reduced.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an embodiment of the present invention.

FIG. 2 is an explanatory diagram of a phase control operation of the above.

FIG. 3 is an explanatory diagram of an operation at the time of starting the same as above.

FIG. 4 is a circuit diagram showing a specific configuration of a power section.

FIG. 5 is a circuit diagram of another embodiment.

FIG. 6 is a circuit diagram of still another embodiment.

FIG. 7 is a voltage waveform diagram of each phase.

FIG. 8 is an explanatory diagram of an operation at the time of starting the same as above.

FIG. 9 is a circuit diagram of a starting circuit for performing star delta starting.

10A and 10B are explanatory diagrams of star and delta connection.

FIG. 11 is an explanatory diagram of the above problem.

FIG. 12 is a circuit diagram of a drive circuit that starts by phase control.

FIG. 13 is an operation explanatory diagram of the above.

[Explanation of symbols]

 4 Control Circuit 6 Power Section 7 Contact Section 8 Timer 11 Current Phase Detection Section M Induction Motor

Claims (4)

[Claims] 1. A phase control element inserted in series between an AC power supply and an induction motor, a mechanical opening / closing contact connected in parallel to the phase control element of the phase control circuit, and phase control at the initial stage of starting. An induction motor comprising: phase control means for gradually increasing the conduction angle of the element to increase electric power supplied to the induction motor; and opening / closing control means for closing the opening / closing contacts during stable operation of the induction motor. Drive circuit. 2. A drive circuit for an induction motor according to claim 1, wherein the opening / closing control means stops the induction motor by opening a mechanical switching contact at an arbitrary timing. 3. The phase control means sets the conduction angle of the phase control element to the maximum state when the switching contact is opened, and then gradually narrows the conduction angle to stop the induction motor. 2. The drive circuit for the induction motor according to 2. 4. A current phase detecting means for detecting that the load current interruption period has fallen below a predetermined width is provided, and the opening / closing control means closes the opening / closing contacts at the time of starting by the output of the current phase detecting means. The drive circuit for an induction motor according to claim 1, wherein: