JPS5838080B2 - Induction motor control device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a control device for an induction motor, and more particularly to a control device for an induction motor using a variable voltage variable frequency inverter.

FIG. 1 shows a general slip frequency control device for an induction motor.

In this figure, a J DC power supply 1 is connected to a variable voltage variable frequency inverter 4 via a smoothing circuit consisting of a reactor 2 and a capacitor 3, and the AC output of this variable voltage variable frequency inverter 4 drives an induction motor 5. ing.

A pulse generator 6 is connected to the induction motor 5, whereby the equivalent speed frequency fm of the motor 50 is detected.
It is added to adder 7.

On the other hand, the slip frequency fs is added to the adder 7, and the gate control circuit 8 is driven by the sum or difference of the two.

1. Output measurement of variable voltage variable frequency 4? A current transformer 9 is inserted, thereby detecting the input current IM.

The adder 10 outputs the deviation between this IM and the current reference value 0 to the gate control circuit 8.

Therefore, the gate control circuit 8 has a slip frequency fs of fs
The variable voltage variable frequency inverter 4 is controlled so that -=f±fm becomes constant and the input current IM becomes equal to the current reference IQ.

In this way, the current standard is set to obtain the desired torque,
When performing control such that the slip frequency is constant, the output torque TM is as follows if the primary winding resistance R of the induction motor 5 is ignored, and from equation (1), the output torque TM is independent of the input frequency f. As shown in Figure 2, the slip frequency f
It is known that it is a function of s.

By the way, in the general slip frequency control device shown in FIG. 1, the motor torque at startup is the desired torque value O and rises rapidly. The shock results in a relatively dog-like disadvantage.

In view of the above-mentioned points, the present invention aims to provide an AC induction motor control device that can gradually increase the torque of the induction motor at a desired torque value 1 during startup to realize a soft start.

Embodiments of the present invention will be described below with reference to the drawings. In FIG. 3, a self-oscillation circuit 2 is added to the configuration of FIG.
A zero is inserted between pulse generator 6 and adder 7.

The configuration of other parts is the same as that in FIG.

A specific configuration of this self-oscillation circuit 20 is shown in FIG.

In this figure, the equivalent motor frequency fm from the pulse generator 6 is applied to the up-count input of the up-down counter 21.

On the other hand, the output of an oscillator 23 having an oscillation frequency fosc is applied to the down-count input via a frequency divider 22.

Then, the carry signal CRY of the up-down counter 21 is applied to the J input of the JK flip-flop 24.

Here, when fosc≧fm, the up-down counter 2
Since the 1 carry signal CRY is O, flip-flop 24 is not set.

Therefore, the Q output is 0 and the Q output is 1.

As a result, the oscillation frequency fosc of the oscillator 23 is added to the adder 7 via the AND gate 25 and the OR gate 26 as the output frequency fm' of the self-oscillation circuit 20.

On the other hand, when fosc<fm, the up-down counter 2
The output of the carry signal CRY of 1 becomes 1, the JK flip-flop 24 is set, and as a result, the Q output becomes 1 and the Q output becomes 0.

Therefore, the detected speed frequency fm is outputted as the output frequency of the 11 self-oscillation circuit 20 via the AND gate 27 and the OR gate 26.

Equivalent motor frequency fm and oscillation frequency fos explained above
The relationship between c and the output frequency fm' of the self-oscillation circuit 20 is the fifth
as shown in the figure.

That is, immediately after the induction motor 5 is started, the equivalent motor frequency fm is smaller than the oscillation frequency fosc, so in the initial stage when the motor is started, fosc is the output frequency f m
' becomes.

After starting, as time passes and the rotational speed of the induction motor 5 increases, the equivalent motor frequency fm becomes smaller than fosc, and the equivalent motor frequency fm becomes the 11 output frequency fm'.
becomes.

In this case, the structure is completely equivalent to that shown in FIG. 1, and general constant slip control is performed.

In the configuration of the above embodiment, the actual slip frequency fM8 of the induction motor 5 is expressed as follows.

That is, when fosc≧fm, It is.

The relationship between these equations (2) and (3) is illustrated in FIG. 6.

That is, the actual slip fM8 is smaller than fs at the initial stage when the motor is started.

As a result, the torque TM for the equivalent motor frequency fm,
The relationship between the power supply frequency f and the actual slip frequency fMs is the seventh
The result will be as shown in the figure.

As shown in FIG. 7, the actual slip frequency fM
Since the torque 8 is small at the beginning of startup, the torque TM is also small at the beginning of startup and exhibits a characteristic of gradually increasing.

Therefore, a soft start of the induction motor is realized.

As a result, the following effects are obtained. (1) The driven device can be started smoothly.

(2) Since the starting frequency can be set high, it is possible to avoid low-order resonance frequencies of mechanical systems, etc.

(3) When the load is not completely restrained, such as in the case of trains, startup is performed almost exclusively by control, so it can be expected to suppress slipping and improve re-adhesion.

Therefore, as shown in FIG. 8, it is advantageous to drive a large number of induction motors when they are connected in parallel or in series.

As described above, according to the present invention, it is possible to perform a soft start in which the starting torque of the induction motor is gradually increased.

[Brief explanation of drawings]

Fig. 1 is a block diagram of a general AC induction motor control device, Fig. 2 is a graph showing the relationship between slip frequency and torque of an induction motor, and Fig. 3 is an embodiment of an AC induction motor control device according to the present invention. 4 is a block diagram showing the specific configuration of the self-oscillation circuit used in the example,
FIG. 5 is a graph for explaining the operation of the self-oscillation circuit, FIGS. 6 and 7 are graphs for explaining the operation of the embodiment, and FIG. 8 is an explanatory diagram for explaining the effects of the embodiment. DESCRIPTION OF SYMBOLS 1... DC power supply, 4... Variable voltage variable frequency inverter, 5... Induction motor, 6... Pulse generator, 7
, 10... Adder, 8... Gate control circuit, 20...
...Self-oscillation circuit.

Claims (1)

[Claims] 1. In an induction motor control device that uses a variable voltage variable frequency inverter and performs variable speed control by feeding back the input current or voltage of the induction motor and the equivalent speed frequency, the equivalent speed frequency is A control device for an induction motor, comprising a self-oscillation circuit that increases the slip frequency when the speed frequency is below a certain value.