JPS62171493A - Detecting method for current of ac motor - Google Patents

Abstract

PURPOSE:To accurately detect the titled current without influence of the waveform distortion of an output voltage by detecting the effective and reactive components of an output current by calculating a sinusoidal value and a cosine value corresponding to the output current and the output voltage phase. CONSTITUTION:A current component detector 7 obtains reactive and effective current detection signals from a 3-phase primary current signal detected by a current detector 6 and a 2-phase sinusoidal wave output signal of a 2-phase oscillator 10. A slip frequency calculator 8 obtains a signal proportional to the slip frequency from the detection signal. An adder 9 adds the slip frequency signal and a speed command signal to obtain a frequency command signal. The oscillator 10 generates a 2-phase sinusoidal wave signal having a frequency proportional to the frequency command signal.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for detecting current of an AC motor driven by a voltage type frequency converter that outputs alternating current of variable voltage and variable frequency.

[Conventional technology]

When an AC motor is driven by a frequency converter that outputs variable voltage/variable frequency AC, the motor's rotational speed N
is given by the following equation.

N=-(flfs)...(1)O Here, po: number of pole pairs, f: drive frequency, fs
: Slip frequency.

Since the slippage frequency js in equation (1) changes in proportion to the torque, when the drive frequency f1 is a constant value, the rotational speed N changes in accordance with the torque. For this reason, accurate speed control cannot be achieved simply by keeping the drive frequency f1 constant.On the other hand, a speed detector is attached to the end of the motor shaft, and the motor is driven according to the deviation between the speed command signal and the detection signal of the speed detector. High accuracy can be obtained by controlling the frequency f1. However, this method requires a speed detector, is troublesome to install, complicates the structure around the motor, and requires a long distance cable for transmitting the speed signal.

Therefore, the primary voltage v1 of the induction motor and the primary current A method is known in which the excitation current component f8 is calculated by calculation, and the frequency f8 is calculated from the ratio of the excitation current component f8. In this way, when calculating the sub-frequency, it is necessary to detect the primary current component with high precision. This is described, for example, in Japanese Unexamined Patent Publication No. 11413/1983. However, this method is easily affected by voltage waveform distortion due to harmonics contained in the output voltage of the thyristor converter, especially when the rotation speed of the motor is low (
This effect is noticeable when the induced electromotive force of the motor is small) or when using a voltage type thyristor converter whose output voltage waveform is a square wave, making it difficult to detect the slip frequency fs with high accuracy.

The above explained how to find the slip frequency, but
There is a strong demand to accurately detect the effective current and reactive current of the primary current flowing through an induction motor.

SUMMARY OF THE INVENTION An object of the present invention is to provide a current detection method for an AC motor that can accurately detect the active current and reactive current of the primary current supplied to the AC motor from a voltage frequency converter.

[Means for solving problems]

The active or reactive current is determined by calculating the actual value of the output current of the voltage type frequency converter according to the invention and the sine and cosine values corresponding to the voltage phase of one output phase of the frequency converter.

[Effect]

A voltage type frequency converter can control the magnitude and phase of the fundamental wave component of its output voltage according to a voltage command signal. Therefore, the voltage phase command signal of the voltage command signal is multiplied by the output current of the voltage type frequency converter, and according to the Fourier transform principle, the component (effective component) of the output current that is in phase with the output voltage and the 90 degree phase difference with respect to the output voltage are Detect each of the components (ineffective components).

〔Example〕

FIG. 1 shows an embodiment of the present invention.

FIG. 1 shows an example in which the present invention is applied to an induction motor that performs speed control without a flank detector.

In Figure 1, 1 is a diode rectifier that rectifies the commercial AC voltage, 2 is a smoothing capacitor for smoothing the output voltage of rectifier 1, 3 is a voltage-type PWM inverter using a GTO thyristor as a switching element, and 4 is an inductive 5 is a speed command circuit; 6 is a current detector that detects the primary current of the motor 4; 7 is a torque acting current component (active current) Iz# and an exciting current component (reactive current) Its of the primary current.
8 is the effective current■1
A slip frequency calculation circuit that outputs a slip frequency signal that calculates the slip frequency based on β and reactive current It (9) is a frequency that adds the slip frequency signal and the speed command signal and commands the output frequency of the PWM inverter 3. an adder that outputs a command signal; 10 a two-phase oscillator that generates a two-shiatsu sinusoidal wave signal (voltage phase command signal) with a frequency proportional to the frequency command signal of the adder 9; 11 a two-phase oscillator that commands the amount of magnetic flux of the motor 4; A magnetic flux command circuit, 12 is a multiplier that multiplies the magnetic flux command signal and the frequency command signal and outputs a voltage command signal, 13. 14 is an oscillator 10 and a multiplier that multiplies the output signal of the multiplier 12. 15
is a phase number converter that converts a 2-phase inversion code into a 3-phase inversion code, and 16 is a P
ON/OFF of the GTO thyristor that constitutes the WM inverter 3
an oscillator that generates a carrier signal that controls the off frequency;
A comparator 17 compares the output signals of the phase number converter 15 and the oscillator 16 and outputs a pulse width variable M signal (PWM signal).

Reference numeral 18 denotes a gate amplifier that outputs a gate signal for controlling on/off of the GTO thyristor forming the PWM inverter 3.

FIG. 5 shows a detailed circuit diagram of the current component detector 7.

71 is an adder that adds or subtracts IU r l'V g IW at a predetermined ratio; 72 is a subtracter that subtracts iv from it;
This converts the three-phase current signal iu''iw into the two-phase current signal ia
, ib. Multipliers 73 to 76 multiply the output signals a and b of the two-phase oscillator 1o by the two-phase current signal ia+ib, and adder/subtractors 77 and 78 add and subtract the output signals of the multipliers 73 to 76.

Next, its operation will be explained. As is well known, the voltage type pulse width modulation inverter 3 is a GT
The output voltage can be varied by changing the on/off time of the thyristor. Specifically, the voltage command signal (sine wave) from the phase number converter 15 and the carrier wave signal (triangular wave) from the oscillator 16 are compared in the comparator 17, and the inverter 3 is The output voltage is controlled to be proportional to the voltage command signal by controlling the GTO thyristor on and off.

Next, in order to facilitate understanding of the operation shown in FIG. 1, the basic concept of the operation will be explained. The exciting current component 1xa and the torque acting current component 1β of the primary current of the motor 4 are given by the following equations.

Here, P: d/dt (operator) T2: Secondary time constant M' = Mutual inductance between primary and secondary φ2': Secondary linkage magnetic flux fs: Slip frequency, that is, slip frequency f, is expressed by the following formula It can be calculated according to

Or, on the other hand, the rotational frequency J of the electric motor 4 and the driving frequency f1 are fr=fs fs (in the case of a two-pole machine) −(
Because of the relationship 6), the very frequency 1. Make it larger by f L' = f 1 + f s
-(7) If fx' is the new driving frequency, then! r=ft'-fs=ft
”·(8) holds true. That is, the rotational frequency ! corresponds to the original drive frequency fx.

From this, the speed command signal is made to correspond to the command signal of the drive frequency fs, and a signal corresponding to the slip frequency Js is added thereto, thereby making the output frequency fl' of the inverter 3
By controlling the rotational speed f, it is possible to precisely match the rotational speed f with the speed command.

The above is the basic concept of operation.

Next, the operation of the embodiment shown in FIG. 1 will be explained. First, current detection, which is the main part of the present invention, will be explained.

The current component detector 7 detects the active current Isβ and the reactive current 11a as follows. First, the three-phase primary current signal (1U+I V viw) detected by the current detector 6 is converted into two-phase signals (ia,
ib). The content of the calculation is as shown below, and the phase relationship of the vectors is shown in FIG.

Here, k: proportional constant The two-phase signals ia, ib thus obtained and the sine wave output signals (a, b) of the two-phase oscillator 10 are multiplied by multipliers 73 to 76, and the reactive current E 1a, effective Detection signals i1d and i1β of current Itl are obtained. The contents of the calculation are as shown in the following equation, and the phase relationship of the vectors is shown in FIG.

1ta=-b-ia+a-ib='l5inθ 11β=a-ia+b-ib=k'・Icos(1・=(10)Here,k'
: Proportionality constant : Amplitude θ of primary current : Delayed phase angle of signals ia and ib relative to signals a and b Signals a and b of the two-phase oscillator and the primary voltage of the motor 4 match in phase as described below Therefore, the reactive current Ire and the active current 1β of the primary current are detected by the calculation of equation (10).

A detailed configuration diagram of the full frequency calculation circuit 8 is shown in FIG. It consists of a first-order delay circuit 81 and a divider 82, and performs calculations corresponding to equation (4) to obtain a signal proportional to the frequency fS.

The slip frequency signal obtained by the slip frequency calculation circuit 8 and the speed command signal given from the speed command circuit 5 are added by an adder 9, and a frequency command signal is taken out. This relationship is as described in equations (7), (8), etc.

A two-phase oscillator 1o generates two acupressure string wave signals a and b having frequencies proportional to this frequency command signal.

As the two-phase oscillator 10, for example, one consisting of a well-known integral type tooth wave oscillator and a function generator is used, and its output signal a,
b is a sine wave signal with a constant amplitude as shown in the following equation.

a = sin (2πftt) b = cos (2x f x t)
-(11) Multipliers 13 and 14 are two-phase signals a or b
is multiplied by the voltage command signal from the multiplier 12 to output signals c and d shown in the following equations.

c = Asin (2tc Jtt) d = Acos (2πftt) ... (
12) Here, A: Voltage command signal amplitude phase number converter 15 converts the two-phase signal C9d into three
Convert to Aishukugo. The output voltage of the PWM inverter 3 is controlled to be proportional to this three-phase signal as described above, but at this time, the output voltage Eu"Ew and the primary current factor υ~
The phase relationship of Is is as shown in FIG. At this time, 1
The reactive current any of the next current and the active current factor 1β are given by the following equation, and a proportional relationship holds with i 1g and i tl in equation (10).

I1a=I sinθ IIβ=Icosθ−(13
) The rotational speed of the electric motor 4 can be accurately controlled in proportion to the speed command signal. Furthermore, since the speed detector can be omitted, a great effect can be obtained especially in cases where its installation is difficult.

In the one shown in FIG. 1, a magnetic flux command circuit 11 and a multiplier 1
The amount of magnetic flux can be arbitrarily controlled by the function of 2.

However, when operating under conditions where the amount of magnetic flux is constant, the reactive current T1α is constant, so as is clear from equation (4), f
It can be considered that s=k"iiβ (k': proportionality constant). Therefore, in this case, the slip frequency calculation circuit 8 in FIG. 1 can be omitted.

Further, the slip frequency fs can also be determined according to equation (5). Therefore, the same effect can be obtained by calculating the full frequency fs by dividing the effective current 11β by the magnetic flux command signal and adding it to the adder 9 instead of using the full frequency calculation circuit 8.

FIG. 7 shows an example in which the present invention is applied to another control device.

The difference between FIG. 7 and FIG. 1 is that a speed control circuit is provided to improve the speed control response. In FIG. 7, 19 is a speed detection subtracter, 20 is a speed deviation amplifier, and 21 is a slip frequency deviation amplifier, and the other parts are the same as those shown in FIG. The subtracter 19 detects a speed signal from the difference between the input signal of the two-phase oscillator 10, which is a signal proportional to the drive frequency fi, and the output signal of the slip frequency calculation circuit 8, according to the relationship of equation (11). The amplifier 20 amplifies the deviation between the speed command and the speed signal, and its output signal is added to the amplifier 21 as a slip frequency command. Note that in order to prevent overload and overcurrent of the induction motor 4, the amplifier 2o is usually provided with a slip frequency limiter for limiting the amplitude of the slip frequency command and limiting the maximum value of the slip frequency. desirable. The amplifier 21 operates as an integrator, and the frequency command signal changes depending on the deviation between the slip frequency command and the slip frequency signal. Note that when the deviation is zero, the drive frequency f1 is maintained at a constant value.

In the embodiment shown in FIG. 7, the speed deviation is sufficiently amplified by the action of the speed deviation amplifier 20, and the slip frequency, which is proportional to the motor torque, is controlled in accordance with the deviation signal, resulting in excellent speed response. Performance can be obtained.

〔Effect of the invention〕

As explained above, the present invention detects the active component and the reactive component of the output current by calculating the output current of the voltage type frequency converter and the sine value and cosine value corresponding to the output voltage phase. Highly accurate detection is possible without being affected by waveform distortion of the output voltage of the type frequency converter.

[Brief explanation of drawings]

Figure 1 is a circuit configuration diagram showing one embodiment of the present invention, and Figures 2-4
The figures are diagrams for explaining the operation of the device in Figure 1, Figure 5,
FIG. 6 is a detailed circuit configuration diagram of the circuit components shown in FIG. 1, and FIG. 7 is a circuit configuration diagram showing another example of application of the present invention. 1... Diode rectifier, 2... Smoothing capacitor,
3... PWM inverter, 4... induction motor, 5...
... Speed command circuit, 6... Current detector, 7... Current component detector, 8... Slip frequency calculation circuit, 9...
Adder, 10... Two-phase oscillator, 11... Magnetic flux command circuit, 12... Multiplier, 13.14... Multiplier, 15...
...Phase number converter, 16... -r゛＼

Claims (1)

[Claims] 1. A voltage type frequency converter capable of varying the magnitude and frequency of an output voltage, an AC motor driven by the voltage type frequency converter, and detecting the output current of the voltage type frequency converter. It comprises a current detection means and a function signal generation means for outputting a sine value and a cosine value corresponding to the instantaneous voltage phase of at least one output phase from a signal controlling a switching element constituting the voltage type frequency converter. In the method for obtaining a current detection signal, the effective or invalid current of the output phase of the voltage type frequency converter is calculated by calculating the actual value of the output current of the voltage type frequency converter and a sine value and a cosine value corresponding to the voltage phase. A current detection method for an AC motor, characterized in that the current is determined. 2. In claim 1, the AC motor is characterized in that the AC motor is an induction motor, and the active current and reactive current obtained by calculation are used to calculate the slip frequency of the induction motor. Current detection method.