JPS6231380A - Starting method for ac motor - Google Patents

Abstract

PURPOSE:To suppress as excess rush current by holding the neutral point of an autotransformer connected during the prescribed period around the connection of an impedance element. CONSTITUTION:In case of starting a motor 5, a switches 1, 2 are closed to applying stepdown tap voltage of an autotransformer 4 to the motor 5. When the motor 5 is accelerated to the vicinity of a no-load rotating speed, a switch 7 is closed to connect an impedance element 6 between a power source terminal and a stepdown tap (t). Then, a switch 3 is closed to apply the full voltage to the motor 5, the switches 1, 2, 7 are opened to disconnect the autotransformer 4 and the element 6 from a current path. Thus, an excess rush current at motor starting time can be suppressed to the sufficiently small value.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a method for starting an AC motor such as an induction motor, a synchronous motor, etc., and particularly a method for suppressing the starting current using an autotransformer. This invention relates to a method for starting an AC motor.

[Background of the invention]

One of the starting methods used when it is necessary to suppress the starting current or starting torque for AC motors such as squirrel-cage induction motors is a so-called starting method that uses a three-phase autotransformer called a starting compensator. There is a Condolfa starting method.

This starting method is configured to control the starting of the motor 5 using three-circuit, one-contact switches 1, 2, and 3 and a three-phase autotransformer 4, as shown in Figure 4. , At the time of starting, each switch 1 to 3 is operated as shown in Figure #5.

That is, when starting the electric motor 5, first switch 1 is turned on.
.
As a result, the neutral section of the autotransformer 4 is cut off, and a part of its winding is made to act as a series reactor, and 1! Finally, close switch 3 and close switch 1.
is opened and full voltage is applied to the motor 5 to complete starting.

According to this Condorfa starting method, the terminals of the motor 5 are never disconnected from the power supply from the start to the end of the start, so by appropriate selection of the tag voltage of the autotransformer 4, the start starts quite smoothly. It is possible to suppress the starting current.

However, in this starting method, depending on the inductance of the autotransformer 4, the switch 1 may be turned off.

When the switch 3 is turned on and the voltage applied to the motor 5 is switched from low voltage to full voltage, the phase of the voltage applied to the motor 5 also changes before and after switching, and this changes along with the change in the applied voltage. This has the disadvantage of generating a large transient inrush current.

Therefore, as a method for suppressing this transient rush current, there is a starting method proposed in Japanese Patent Publication No. 51-5163.

This starting method, as shown in FIG. The switches 1 to 3, including this switch 7, are connected in parallel with the line part, and the switches 1 to 3, including this switch 7, are operated in the order shown in Figure 7 at the time of starting. , electric motor 5
Immediately before the voltage applied to 1 is switched from the step-down tap voltage to the full voltage, once a part of the winding of the autotransformer 40 is made to act as a series reactor, the reactance due to this part of the winding is This is intended to reduce the above-mentioned transient inrush current by using the impedance element 6.

By the way, the equivalent circuit at each time point a, b, c, and d in FIG. 7 in the starting method shown in FIG. 6, and the relationship between voltage and current at each part are shown in FIG. 8 (a).

(b) * (c) y (d).

As can be easily understood from these figures, in the starting method shown in Fig. 6, each switch is switched to A a - h b - * c - e d at phase 7 during starting. , then the voltage E at each point in time. and electrician.

As is clear from the vector diagram in FIG. 8, the phase difference between 〉θ, and eK time a
Electrical work when switching from to b. Since a large delay appears in the phase of C, the phase change at the next point in time when switching from S to C is from θb to θ. This large phase change has the disadvantage of generating a large transient inrush current.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for starting an AC motor, which eliminates the drawbacks of the prior art described above, allows appropriate control of starting current, and sufficiently suppresses transient inrush current.

[Summary of the invention]

To achieve this objective, the present invention provides a starting method using an autotransformer in which an impedance element is connected at the time of switching from step-down tap voltage to full voltage. The main feature is that the neutral point of the autotransformer is kept connected.

[Embodiments of the invention]

Hereinafter, a method for starting an AC motor according to the present invention will be explained in detail with reference to illustrated embodiments.

FIG. 1 shows an embodiment of the present invention, and the configuration and connection state of each part is the same as the conventional example shown in FIG.

However, the opening and closing order of the switches 1 to 3.7 at the time of starting is as shown in FIG. 2, which is different from that in the conventional example shown in FIG. 7.

Next, the operation of this embodiment will be explained in detail with reference to Fig. F2.

When starting the electric motor 5, first turn on the switch].

Close 2.

As a result, the step-down tap voltage of the autotransformer 4 is applied to the motor 5, and the motor 5 starts rotating. The equivalent circuit and the relationship between voltage and current at this time are as shown in Figure 3 (a), and the ratio of the voltage (total voltage) of power supply terminals U, V, W to neutral point N and the voltage of tap t is If n (n<1), the starting current flowing through the motor 5 will be n times the current when starting at full voltage, and the current on the power supply side will also be n1 times the current due to the principle of a transformer, resulting in a ! It is also possible to suppress the drive current and power supply current. In addition, Satoshi3shi (al, Zt
t is the tap t of the autotransformer 4 and the power terminals (U, V.

W), Zr2 is the impedance between tap t and neutral waste N, and ZM
represent the impedance of the electric motor 5, respectively.

If the motor 5 starts rotating due to the step-down tap voltage and accelerates to near the no-load rotation speed, you may be wondering! As shown in FIG. 2, the switch 7 is further closed.

This connects the impedance element 6 between the power supply terminal and the step-down tap t, and as a result, what is the equivalent circuit? , 3
[1(bl). Here, ZR represents the impedance of the impedance element 6. At this time, the composite impedance Zo of the impedance ZT+ of the autotransformer 4 and the impedance ZR of the impedance element 6 is If the constants of each part are selected so that they are smaller than the composite impedance Z1 of the impedance! and the impedance ZM of the motor 5, the relationship between voltage and current at this time can be determined as shown in Figure 3 (vector diagram of bl). As a result, the voltage EM° applied to the motor 5 is smoothly applied to i!, and 'Flj to the voltage Eo is
Since the phase of the current I o becomes θb, which does not differ much from 03 in the v3 diagram (al), the inrush current of the motor 5 and power supply when the switch 7 is turned on can be suppressed to a sufficiently small value.

After this, switch 3 is turned on to apply full voltage to the motor 5, and switches 1, 2.7 are opened to connect the autotransformer 4.
When the impedance element 6 is disconnected from the current path, the starting is completed. The relationship between voltage and current when switch 3 is turned on is as shown in Figure 3(c), where the voltage and current are in phase θ. is not much different from θ5, so the transient inrush current can be kept small.

Therefore, according to this embodiment, the transient inrush current at the time of starting the motor can be suppressed to a sufficiently low level, and proper starting can be performed.

In the above embodiments, no particular explanation was given as to how to operate the switches 1, 2, 3, and 7, but this is done using conventional starting methods. Using a sequential control mechanism using a timer, switch 2 and 3.7 from the electromagnetic contactor to 1.
The 11th order may be operated according to a predetermined sequence.

〔Effect of the invention〕

As described above, according to the present invention, the shortcomings of the prior art are eliminated with a simple configuration that changes the sequence of rotation switching at the time of starting, and the transient inrush current that appears at the time of starting is sufficiently suppressed. It is possible to easily provide a method for starting an AC motor that allows smooth starting without adversely affecting the 1E bullet side.

[Brief explanation of the drawing]

Fig. 1 is a wiring diagram showing an embodiment of the method for starting an AC motor according to the present invention, Fig. 2 is a sequence explanatory diagram showing the operation of the modified embodiment of Fig. 1, and Fig. #L3 is the same as that of the embodiment. Explanatory diagram consisting of an equivalent circuit and vector diagram to explain the operation, 2
) Fig. 4 is a wiring diagram showing an example of the well-known Condolfer starting method, Fig. 4 is a sequence discussion diagram showing the operation of the starting method of Fig. F4, and Fig. 6 is a conventional example of the starting method that is an improvement on the Condolfer method. 7 is a sequence diagram explaining the operation of the conventional example shown in Figure 6, and Figure 8 is an explanatory diagram consisting of an equivalent circuit and vector diagram for transferring the operation of the conventional example between stations. It is. 1. 2, 3, 7... 3 circuit 1 contact switch,
4...Three-phase single transformer, 5...AC motor,
6... Impedance element. Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 □Darkness Figure 6 Figure 7-tlt! 7Ia b c d

Claims (1)

[Claims] 1. In a method for starting an AC motor using a three-phase autotransformer, the starting current is suppressed by starting with a step-down tap voltage and switching to full voltage after a predetermined time, When switching from step-down tap voltage to full voltage,
First, with the neutral points of the above autotransformers connected to each other,
A predetermined impedance element is connected between the step-down tap terminal of each phase and the full voltage terminal, and then switching to full voltage and disconnecting the autotransformer and the impedance element from the power supply terminal are performed. A method for starting an AC motor, characterized by comprising: 2. The method for starting an AC motor according to claim 1, wherein the impedance element is a resistor.