JPS62233092A - Compensating circuit for low-frequency characteristic of detection transformer for ac motor - Google Patents

Abstract

PURPOSE:To compensate the transfer characteristic of low efficiency for a detection transformer, by providing a compensating circuit substantial reciprocal to the detection transformer with respect to a transfer function. CONSTITUTION:ny By the voltage DELTAE to the secondary voltage E20 of a detection transformer 100 through a compensator 200 having a transfer function of a characteristic reciprocal to that of the detection transformer 100, an unerroneous detected value E d e t is obtained against the input voltage E. In constituting this compensating circuit, if the compensator 200 cannot be composed of a perfect integrator, a compensator 210 is composed of the first order lag elements with sufficient long time constant Tc to form a circuit.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a circuit for compensating the low frequency characteristics of a transformer for detecting the terminal voltage vector of an AC motor operated by a variable frequency power supply, particularly an induction motor. Regarding.

[Conventional technology]

In recent years, a vector control method has been adopted in order to obtain the characteristics of a DC motor with good controllability using a mechanically reliable induction motor. This vector control method controls the primary current and slip frequency of the induction motor, making it possible to obtain torque accuracy and response equivalent to that of a DC motor.

In this vector control method for an induction motor, it is necessary to detect the slip frequency.

Conventionally, slip frequency was commonly detected using a tachometer generator or pulse generator, but in order to simplify the structure and reduce costs, research is being conducted to calculate slip frequency without these detectors. is being carried out.

An example of slip frequency calculation will be explained.

As shown in the following equation (1), the slip angular frequency ω is expressed by the secondary magnetic flux (amplitude) φ2 and the secondary current (amplitude) tz.

ωs ”Rzji /φt ・・・・・・・・・f
l+where R1 is the secondary resistance.

The secondary magnetic flux φ2 and the secondary current it are obtained by detecting the secondary current and terminal voltage (vector) of the motor and calculating them.

A conventional method for detecting this terminal voltage vector is to use a detection transformer to electrically isolate it from the motor side.

FIG. 4 is a block diagram showing an example of a slip frequency calculation device using a detection transformer.

In the figure, 1 is a power supply, 2 is an induction motor, 3.4 is a current detector,
5 is a terminal voltage detection transformer according to the present invention, 6.7 is a 3-phase to 2-phase converter, 8 to 15 are multipliers, 16 is an absolute value circuit,
17-19 are dividers, 20.21 are square rooters, 22-25
26 is an amplifier, 26 is a setting device, and 27 to 32 are adders/subtractors.

This calculation device performs the following calculation to obtain the secondary magnetic flux φ2.
Find the secondary current L2 and slip frequency ω.

LP, -R,L," 11=--□ ・・・・・・・・・・・・・・・(2)
M.V.

ω5=Rzzz/φz ・−・−・−−−(xl
(Reposted) Here, L is the sum of mutual inductance M and total leakage inductance l in the equivalent circuit of the induction motor, P
, is the motor-order input, R is the −-order resistance value, J is the amplitude of the excitation current, vl is the primary voltage, 11 is the −-order current, P, ,
is the reactive power of the motor, and ω1 is the primary angular frequency.

In FIG. 4, the motor terminal voltage (phase voltage) detected by the transformer 5 is converted into two-phase alternating current voltages V and Vβ by a three-phase to two-phase converter 7.

The primary currents detected by the current detectors 3 and 4 are converted into two-phase alternating currents tα and lβ by a three-phase to two-phase converter 6.

Next, when calculating the secondary magnetic flux φ2, first the reactive power p
Find ra.

P,,=v 仄しβ−vp t@・・・・・・・・・
...(4) Also, the negative order current 11 is determined by the following equation.

j , ” x j 侃8+shi〆 ・・・・・・・
・・・・・・・・・+51P+ =V(K ta +vp
J, p −−・−・−−−−−−−(61 Through these calculations, (21, demand.

[Problem that the invention seeks to solve]

The equivalent circuit of the transformer can be represented as shown in FIG. 3(a).

At this time, the quadratic (
The relationship between the detected) voltage E2° (vector) is expressed by equation (7).

E. -PTL・E,. (l tenp T &)...
......(7) However, T L -L +. /
R.I. (Transformer-order time constant).

p-d/dt (differential operator), L+e=Lt* (transmission ratio 1:1) (see Figure 3, mountain)).

Here, if the transfer function of the secondary voltage E2° to the input voltage E1° is denoted by G(jω), this transfer function is expressed by equation (8).

l+jωT.

As is clear from representing this in a Bode diagram, when the frequency becomes low, the phase of the detection voltage E8° advances with respect to the input voltage E1°, and the amplitude also attenuates. Therefore, a phase error and an i-width error occur in the detected voltage with respect to the actual voltage.

As described above, in the method of detecting a terminal voltage vector using a transformer, there is a problem in that when the operating frequency becomes low, the transfer characteristics of the transformer, especially the phase characteristics deteriorate, and satisfactory detection cannot be performed.

SUMMARY OF THE INVENTION In view of these conventional problems, it is an object of the present invention to detect a terminal voltage vector with good phase and amplitude accuracy up to a low frequency range.

[Means for Solving the Problems] In order to achieve this object, the present invention converts the terminal voltage vector of an AC motor operated by a variable frequency power supply or the magnetic flux vector calculated from the vector into a voltage transformer for output. The detection device is characterized in that a circuit whose transfer function has a substantially reciprocal relationship with the detection transformer is connected to the detection transformer.

[Effect]

FIG. 1 is a block diagram of the present invention. In the figure, 100 is the aforementioned detection transformer, and 200 is a low frequency compensator.

By adding the voltage ΔE via the low frequency compensator 200 to the transformer secondary voltage E2°, the error-free detected value E61 is obtained for the input voltage E1°. This can be expressed as equation (9).

=E16 ・・・・・・・・・・・・・・・・・・
(91) In this way, the detection transformer 100
A compensator 200 having a transfer function with characteristics opposite to the transfer function of
By adding, the detected value E41 and the input voltage E,
. In addition, detection can be performed without phase shifts or amplitude changes.

When actually configuring this compensation circuit, the compensator 2
00 cannot be constructed with a perfect integrator as shown in FIG. 1, it is also possible to construct the compensator 210 with a first-order lag element having a sufficiently long time constant Tc, as shown in FIG. Even in this case, the low frequency characteristics can be substantially improved sufficiently.

〔Effect of the invention〕

As described above, in order to compensate for the low frequency transfer characteristics of the detection transformer, the present invention provides a compensation circuit whose transfer function is substantially the reciprocal of that of the transformer. For this reason,
It is possible to detect a terminal voltage vector with good phase and amplitude accuracy up to a low frequency range, and it is possible to realize good motor control based on this detection signal.

[Brief explanation of drawings]

Figures 1 and 2 are block diagrams of a compensation circuit for low frequency detection characteristics according to the present invention, Figure 3 is an equivalent circuit diagram of a potential transformer and a block diagram showing its transfer characteristics, and Figure 4 is a block diagram of the slip frequency. FIG. 2 is a block diagram showing an example of an arithmetic device. 100: Detection transformer

Claims (1)

[Claims] 1. A terminal voltage vector of an AC motor operated by a variable frequency power supply or a magnetic flux vector calculated from the vector,
A detection device for detecting via a detection transformer, characterized in that a circuit having a substantially reciprocal relationship with the detection transformer in terms of transfer function is connected to the detection transformer. Low frequency characteristic compensation circuit for detection transformer.