JPS6348194A - Method for controlling motor - Google Patents

Abstract

PURPOSE:To improve the start performance of a machine having great statical friction torque, by feeding proper frequency and voltage to a motor with an inverter, even if there is the fluctuation of load torque and power voltage. CONSTITUTION:An inverter 4 is controlled via a PWM control circuit 11 through a control circuit 10, and variable voltage/variable frequency alternating current is fed to an induction motor 5. In this case, by the control circuit 10, the increment of frequency is stopped on the way of start, and during the period, output voltage is increased, and current before the change of the output voltage is compared with that after the change, and when the current is reduced, then the frequency is increased.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a control method for an induction motor, and particularly to a control method suitable for improving the starting performance of a motor used in a machine having a large static friction torque.

[Conventional technology]

In the conventional technology, when starting a mode using an inverter, the voltage at low frequency is increased, or the relationship between voltage and frequency is determined to be constant as described in Japanese Patent Application Laid-Open No. 60-35973. For this reason, sufficient consideration was not given to variations in load torque and fluctuations in power supply voltage.

[Problem that the invention seeks to solve]

The above conventional technology does not give sufficient consideration to variations in load torque and fluctuations in power supply voltage, and when starting a load with high static friction torque, the inverter is activated before the starting torque of the motor reaches the friction torque of the load. There was a problem in that the output current exceeded the allowable current of the inverter elements, causing the inverter to stop.

The purpose of the present invention is to solve the problem of using a suitable frequency, even in such a case.
The purpose is to also supply voltage and make it possible to start the electric motor.

[Means for solving problems]

The above purpose is to
This is achieved by supplying the electric motor with an appropriate frequency and appropriate voltage using an inverter.

As a means for this purpose, an appropriate frequency is set based on the characteristics of the inverter and the motor, and an appropriate voltage is set by detecting the current of the motor.

[Effect]

When 1-lux of the load is friction torque, since static friction 1-lux is greater than dynamic friction torque, 1-lux decreases as the load rotates. −・The current of the induction motor is the number of rotations.
Since it has a drooping characteristic with respect to the shoulder, it will drop as it rotates. By utilizing this phenomenon, it is possible to determine whether the motor has rotated by detecting the current.

As expressed by the following equation (1), the lower the frequency of the same secondary current, the higher the starting torque.

T s t = m1(I z s t)' r, zg
X w s = m1 (I z s t) 2r, (k g-m) −(
1) 2πf−g ==m 1 (I s t)' r, (k
g-m) - (ko) 12πf-g T s t: Starting torque mji phase number ■zst: Secondary N flow at starting r2: Secondary resistance g: Gravitational constant ws: Synchronous angular velocity f: Frequency Ist: Starting Current Normally, the secondary current (I Z S t) at the time of starting can be considered to be the same as the starting current (I S t), so equation (C) is expressed as (
1) The following equation is obtained. .

When starting a motor with an inverter, the voltage and frequency ratio are kept constant so that the magnetic flux density becomes constant, and the motor is often started at a lower frequency. In this case, since the reactance (xl+x2) is small at low frequencies, the reactance can be ignored, so (2) below.

As shown in equation (3), both the starting current and starting torque are proportional to the frequency.

JCr 1 + r 2 ) "[(x 1 + x 2
)”= ki'
-(2) (r ko + d 2) T s t = m] (1 s t,)' rj2πf-g ""k'm+r2f 2π ・g (rl+r2) V1 knee voltage r1 knee resistance r2: secondary Resistance x I knee = next soliactance 2 = second order reactance: constant (V = kf) The starting current is small at low frequency, and it is necessary to increase the voltage to increase the h, but as the voltage increases, the magnetic flux density increases. (starting current) = (
Secondary current at the time of starting) is established and 5 disappears.

With respect to this viscosity and inverter supply current, the lowest frequency of the generated starting torque (1 = torque) is not necessarily the highest, but there is a frequency at which the starting torque is highest when the same current flows at 1 and 7. According to the present invention, when a starting command is input to the electric motor, the increase in the -1 frequency is stopped near the above-mentioned frequency where the starting performance is the best.
By increasing only the voltage and detecting the current, it is possible to check the rotation of the motor and thereby accelerate the motor.

〔Example〕

EMBODIMENT OF THE INVENTION Hereinafter, the electric motor control method according to the present invention will be explained in detail using illustrated embodiments.

FIG. 3 shows an example of a circuit implementing the method of the present invention, where 1 is an inverter, 2 is a forward conversion section, 3 is a capacitor, 4 is an inverse conversion section,
5 is an induction motor (hereinafter referred to as IM) 6 is a current detection unit;
7 is a rectifier, 8 is a low-pass filter, 9 is an analog-to-digital converter (hereinafter referred to as A/D), 10 is a control circuit using a microcomputer, and 11 is a PWM control circuit.

Inverter 1, capacitor 3 and PWM control circuit] -
A well-known voltage type PWM inverter equipped with 1,■
It serves to supply variable voltage, variable frequency, two-phase power to 45.

The control circuit 10 is composed of a lamp circuit 12, an arithmetic unit 13, and a memory 14, as shown in FIG.

The starting command for the IM 5 is taken into the arithmetic unit 13□ via the lamp circuit 12 in the control circuit 10. On the other hand, the current of the IM 5 is taken into the computing unit 13 as digital data of the average current via the current detector 6, the rectifier 7, the low-pass filter 8, and the A/D 9. The control circuit 10 receives the current of the IM5 as feedback data in response to the starting command of the IM5, and gives voltage and frequency commands to the PWM control circuit 11 in response to the change in the current of the IM5, thereby adjusting the output voltage of the inverter 1. , which functions to control the output frequency.

Next, the operation of the control circuit 10 according to this embodiment will be explained with reference to the flowchart of FIG. 1 and the characteristic diagram of FIG. 4.

As mentioned above, the control circuit 10 includes a microcomputer, which allows the process shown in FIG. 3 to be performed at a predetermined constant cycle.
For example, it is executed every 0.1 seconds.

When the control circuit 1o enters the process shown in FIG. 1, as the process of S (step) 1, the current In
Incorporate. Here, the subscript n of 1n represents the current when this process is currently started. Therefore, the current at the time of the previous process is current In-1. This is the same for the output voltage V and output frequency f of inverter 1; the current value is Vn, fn, and the next value is Vn.
Hail +1 f n+1.

After Sl, the process proceeds to Sl and compares the current current value In with the maximum allowable current Imax of the inverter. The maximum allowable current Imax is for preserving the elements of the inverter 1, and is a value stored in the memory 14 in advance. When the current current value In is larger than the allowable maximum current, that is, when Sl is NO, the inverter stops.

If the result in Sl is YES, proceed to processing in S3.

The current current value In and the previous current value In-1 are compared to determine whether the current is increasing or decreasing.

If the current tends to increase, that is, if S3 is YES, 1M5 is in a restrained state and the process proceeds to S4.

In the process of S4, the current frequency fn is compared with a preset frequency fO. Since the current frequency fn is determined by the frequency command given from the control circuit 10 to the PWM control circuit 11, the control circuit 10 naturally knows the current frequency fn. The preset frequency fO is stored in the memory] 4, and is determined by the voltage, frequency characteristics, and IM5 characteristics of the inverter 1. It is a low frequency of about ~20%.

When the current frequency fn has not reached the set frequency fo, that is, when S4 is YES, the process of S5 is entered.

At this time, the next voltage Vn+1 and frequency f n + 1 are set as △ for the current voltage Vn and frequency fn, respectively.
■1, voltage and frequency command to add △f to inverter 1
and then proceed to the next process. Note that the current voltage Vn is also based on a voltage command from the control circuit 10, and the control circuit]0 naturally knows the current voltage.

As the above process is repeated, the current frequency fn reaches the set frequency fo, and the process in S4 becomes NO.

At this time, the next frequency fn+1 is left as the current frequency fn, only the voltage is set as the next voltage Vn+1, and the voltage and frequency commands obtained by adding △v2 to the current voltage Vn are given to the inverter 1, and the next process begins. .

When the starting torque generated by 1M5 becomes larger than the static friction torque of the load, IM5 starts rotating and the load torque becomes dynamic friction torque, and the load torque becomes smaller.
The current in M5 decreases. When this state is entered, the current current In becomes smaller than the previous current value n-1, and the processing in S3 becomes No.

At this time, the process of S7 is performed. Although it is not necessary to carry out this process, it is carried out for the purpose of preventing malfunction and ensuring sufficient time for starting. N represents the number of times the step S7 is continuously repeated.

Until the specified number of times (3 times in this example) is repeated, S7 becomes No and the voltage 1 is applied to the inverter 1 while the primary voltage V n + 1 and frequency f n + 1 remain the current voltage Vn and frequency fn.
Give the frequency command and start the next process.

At this time, it is safer to set a larger current value as the previous current value, so it is better to leave the previous current value used for the next current comparison as it is.

Once the above processing is completed, IM startup will be completed, so S
7 becomes YES, this process is completed, and the frequency is accelerated up to the frequency input from the outside.

FIG. 4 shows each characteristic of this processing state. T is the generated torque of 1M5, ■ is the output voltage of inverter 1, f is the output frequency of inverter 1, f is the current of IM5, N is the rotation speed of IM5, Ts is the static friction torque of the load, and 6t is time. . Torque characteristics of the load connected to IM5 and IM5
The current characteristics are as shown in FIG. TL is the load torque, and N is the rotation speed.

During the time from when the start command is input until the frequency fn reaches the set frequency fO, the processing in S5 is performed, and the time is shown as A in Fig. 5.
corresponds to between When the frequency fn reaches the set frequency fo, the process of S6 is performed until the IM5 starts rotating. This period corresponds to period B in FIG. 1M5
When started, processing 87 is performed, and this period corresponds to period C in FIG. When starting is completed, acceleration is executed based on the normal speed command.

According to this embodiment, the starting performance of the inverter is improved because voltage compensation is automatically performed for variations in load torque and fluctuations in power supply voltage.

Note that the set frequency fO does not need to be stored in the memory 14 in advance, and may be the actual starting frequency of the inverter 1 (usually 2 to 3 Hz), and if the current In of IM 5 is less than or equal to the maximum allowable current Imax. The frequency fn when the current reaches a certain value (for example, 60% of the maximum allowable current) may be set. However, settings must be made so that the starting current and no-load current of IM5 are not equal, that is, so that IM5 is not over-excited.

〔Effect of the invention〕

The method of the present invention has the following effects.

Since voltage compensation is performed for fluctuations in the single-load torque and power supply voltage, starting characteristics are improved.

Even if the secondary resistance of the two electric motors is small, it becomes easy to generate the necessary starting torque, improving efficiency during operation.

[Brief explanation of drawings]

FIG. 1 is a flowchart for explaining the operation of an embodiment of the method of the present invention.
Chart 4 FIG. 2 is a block diagram of the control device. FIG. 3 is a block diagram showing an example of a control circuit implementing the method of the present invention, FIG. 4 is a characteristic diagram for explaining operation, and FIG. 5 is a characteristic diagram of loads and modes. 1... Inverter, 2... Forward conversion section, 3... Capacitor, 4... Inverse conversion section, 5... Induction motor (IM
), 6... Current detector, 7, Rectifier, 8, Low pass filter, 9... Analog digital converter, 10...
・Control circuit, 11...PWM control circuit, 12...Lamp circuit section, 13...Computer unit, 14...Memory part size I-1++ Do ← ≧← Top powder H

Claims (1)

[Claims] 1. In a control method for an AC motor driven by a variable voltage, variable frequency AC power supply, the increase in frequency is stopped during startup, and during this period the output voltage is increased by a predetermined value at predetermined intervals, and before and after this change in output voltage. A motor control method characterized in that the currents of the AC motor are compared, a change in them is detected, and when the current decreases, the frequency is increased.