RU2424098C1 - Method of determining contact welding machine power factor - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: invention is intended for automatic control over contact welding machines. thyristor cutin state duration X at preset cutin angle is defined. Welding machine power factor cosφ is calculated for two current periods using relationship cosφ=A₀+A₁λ₂. Factors A₀ and A₁ are set discrete for every a proceeding from condition of minimisation of cosφ calculation error. During first period, thyristors are cut in with angle of α₁=90 electric degrees and preliminary value of cosφ₁ is calculated. In second period, thyristors are cutin with minimum angle α₂ satisfying the condition α₂>φ₁. Now, refined cosφ is calculated.

EFFECT: simplified determination of power factor, reduced measurement and computation errors.

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Description

The invention relates to the field of resistance welding, in particular to determining the power factor (cosφ) of welding machines, and can be applied in the automatic control and management of resistance spot welding machines.

Setting up resistance welding machines to optimal modes, building load characteristics and selecting phase adjustment parameters requires knowledge of the power factor of the welding machine, i.e. the shift angle between the instantaneous voltage values and the steady-state current in the secondary circuit during full-phase switching.

A known method for determining the power factor of a contact welding machine, which provides for the measurement of electrical welding parameters (welding current I _sv , short circuit current I _kz , welding voltage U _sv and open-circuit voltage U _xx ), calculates the power factor cosφ by the formula

,

,

, [Авторское свидетельство СССР №1743767, кл. В23К 11/24, 1992].Where

, [USSR author's certificate No. 1743767, cl. B23K 11/24, 1992].

This method does not require special equipment and allows you to determine the power factor of the welding machine by measuring the main electrical parameters, however, its implementation involves preliminary measurements in the secondary circuit using current and voltage meters, which increases the complexity and cost of the method.

A known method of measuring the power factor of a single-phase contact welding machine, in which measure the angle α of the inclusion of power thyristors and the angle λ, during which the pulse of the welding current flows, and determine the power factor of the welding machine by the formula

cosφ = cos (A ₀ + A ₁ λ + A ₂ α + A ₃ α ² ),

where the constant coefficients A ₀ ... A _{3 are} determined by numerical methods for the given ranges of variation of α and λ [USSR Author's Certificate No. 1310149, class. B23K 11/24, 1987].

This method does not impose stringent requirements on a fixed value of the angle α of turning on the thyristors and does not require a permanent storage device of large capacity, because it is necessary to store in memory only the coefficients A ₀ ... A ₃ , while the measurement of cosφ can be carried out directly in the welding process. The disadvantage of this method is that it does not take into account the features of the function cosφ = f (α, λ), which in the general case is not linear and its approximation with the help of the given four-term term does not allow achieving the specified accuracy of calculations in advance. It should also be taken into account that the measurement of λ and the subsequent calculation of cosφ has some error, depending on the method of measuring λ and the accuracy of the equipment used, while the error in measuring λ increases with increasing α.

Closest to the invention according to the technical solution, there is a method for determining the power factor of a full-phase current switching on for spot welding using single-phase current, which consists in determining cosφ from the welding current turning on time T _{λ and} the welding current delay time T _α relative to the mains voltage using the dependence cosφ = С ₀ + С ₁ · (ωТ _λ ), where ω is the circular frequency of the mains voltage; С ₀ and С ₁ are the coefficients determined depending on ωТ _α as polynomials, the degree of which is determined by the necessary accuracy of approximation to the actual value of cosφ [USSR Author's Certificate No. 1281358, cl. BK 11/24, 1985].

This method, taken as a prototype, allows the simple calculation of power factor values for use in automatic current stabilization systems, while the required accuracy of the calculation of cosφ can be obtained by increasing the degree of polynomials defining C ₀ and C ₁ . However, when implementing the method, it is not possible to reduce the error associated with the measurement error λ, which increases significantly with deep phase adjustment. It is also necessary to take into account that expanding the range of possible values of cosφ leads to a decrease in the accuracy of calculations from the given approximating dependences.

The problem to which the invention is directed is to increase the accuracy of determining the power factor cosφ of a contact welding machine by reducing the measurement and computational errors that occur when measuring the angle λ during which the welding current pulse flows, and calculating the value of cosφ from approximating dependencies.

This problem is solved in that in a method for determining the power factor of a contact welding machine, which provides for measuring the duration λ of the on state of the thyristors at a given angle α of their inclusion and calculating the power factor as cosφ = A ₀ + A ₁ λ, where the coefficients A ₀ and A ₁ specify proceeding from the condition of minimizing the error in calculating cosφ, the measurements are carried out for two periods of current. In this case, in the first period, thyristors with an angle α ₁ = 90 electrical degrees are turned on and the preliminary value cosφ _{1 is} calculated. Then, the minimum thyristor opening angle α _{2 is} established in the second period, based on the condition α ₂ > φ ₁ , the thyristors with the selected angle α ₂ are turned _on and the refined value cosφ is calculated, and the coefficients A ₀ and A _{1 are} set discretely for each α.

Measurement of the power factor for two current periods allows one to obtain a preliminary value of cosφ ₁ in the first period and taking into account this value, to establish the optimal thyristor turn-on angle α ₂ in the second period, at which the minimum measurement error in determining cosφ is ensured.

The inclusion of thyristors with an angle α ₁ = 90 electrical degrees in the first period of the current eliminates the possibility of switching to the half-wave rectification mode (when α ₁ <φ) for all possible cosφ and to obtain an acceptable value of the duration λ _{1 of} their on state (further increase in α ₁ leads to a decrease in the value of λ ₁ and, therefore, the accuracy of its measurement).

Setting the minimum opening angle α ₂ of the thyristors in the second period, based on the condition α ₂ > φ ₁ , eliminates the possibility of the thyristors switching to the half-wave rectification mode (when α ₂ <φ) and at the same time obtain the maximum possible value of the duration λ _{2 of} their on state, the measurement of which can be performed with a minimum error for the selected method.

Setting the coefficients A ₁ and A ₂ for the expression cosφ = A ₀ + A ₁ λ discretely for each α allows us to simplify the calculations in comparison with existing analogs and significantly improve the accuracy of determining cosφ.

The invention is illustrated by drawings, in which:

figure 1 - device that implements the proposed method for determining cosφ;

figure 2 - the absolute measurement error Δcosφ obtained for Δλ = 0.5;

figure 3 - the duration λ of the on state of the thyristors depending on the angle α of their inclusion at different values of cosφ;

figure 4 is the relative error in calculating cosφ by the proposed method;

figure 5 is a diagram of the instantaneous values of the mains voltage u _c and secondary current i ₂ when implementing the proposed method for determining cosφ on a contact machine MTPU-300.

The method is implemented on standard resistance welding machines operating from an industrial frequency alternating current network and containing (Fig. 1) a microprocessor control unit 1, which is based on a algorithm for determining the opening angle of welding thyristors α and received from the thyristor turn-on detector 3 through a programmer 2 information on the duration λ of the thyristor on state controls the operation of the thyristor contactor CT, which connects the welding transformer TC to the network.

The method for determining the power factor of a contact welding machine is as follows.

As you know, the power factor of the welding machine can be uniquely determined based on the measured duration λ of the on state of the thyristors for a given angle α of their opening by solving the equation

The absolute measurement error Δcosφ can be determined for a given measurement error Δλ depending on the angle α and power factor cosφ as

where λ is the result of a numerical solution of (2) with respect to λ for given values of α and φ;

f (α, λ + Δλ) are the values of cosφ as a result of the numerical solution of equation (1) with respect to φ for given α and (λ + Δλ).

On the range of values cosφ∈ [0,1; 0.9] the function cosφ = f (α, λ) can be approximated by a linear dependence of the form

where A ₀ and A ₁ are the coefficients given for each α, based on minimizing the error in calculating cosφ, which for fixed values of α and ranges of variation of cosφ can be determined by the least squares method using the dataset cosφ = f (λ), which is obtained by numerical solution equations (1). For the range of variation of the power factor cosφ∈ [0,1; 0.9] and fixed values of α, the calculated values of the coefficients A ₀ and A ₁ are given in table 1.

Table 1

cosφ [0,1; 0.9] [0,1; 0.3] [0.3; 0.5] [0.5; 0.7] [0.7; 0.9] α, el. hail. 90 90 73 61 46 A ₀ -0.873 -0.775 -0.774 -0.757 -0.583 A ₁ 2,680 2,394 2,772 2,862 2,533

From figure 2 it follows that the minimum measurement error cosφ is achieved with a minimum value of α, the value of which in order to exclude cases of half-wave rectification should be α> φ.

From figure 3 it follows that the maximum possible value of λ and, therefore, the minimum error of its measurement can be obtained with the minimum allowable angle α of turning on the thyristors for given values of cosφ, which for cosφ = 0.3; 0.5; 0.7; 0.9 is α = 73; 60; 46 and 26 electrical degrees, respectively.

In the first period of the current, thyristors with an angle α ₁ = 90 electrical degrees are turned on, the duration λ _{1 of} their on state is measured and the preliminary value cosφ _{1 is} calculated according to (3), where the coefficients A ₀ and A ₁ are taken based on the condition of minimizing the calculation error for cosφ ∈ [0,1; 0.9] and laid in the microprocessor unit through the programmer. Based on the preliminary value of cosφ _1, subject to the condition α ₂ > φ ₁ , the angle α _{2 is set} as the smallest of the fixed values stored in the memory of the microprocessor unit through the programmer, and thyristors with the selected angle α _{2 are} turned _on . During the flow of the second period of the current, the duration of the thyristor on state λ _{2 is} measured and the updated value of cosφ is calculated according to (3), where the coefficients A ₀ and A _{1 are} set discretely for each α ₂ , based on the condition for minimizing the calculation error cosφ, and put into the microprocessor block through the programmer.

The relative error in the calculation of cosφ by the proposed method is usually not more than 1.5% (figure 4), thus, the accuracy of the calculations is significantly higher than that of analogues.

Example. The power factor of a contact welding machine MTPU-300 was determined in the short circuit mode. When you turn on the thyristor contactor with a preliminary angle α ₁ = 90 electrical degrees, the measured value of the duration of the inclusion of thyristors was λ ₁ = 139 electrical degrees (figure 5). Calculated according to (3), where for the region cosφ∈ [0,1; 0.9] in accordance with Table 1, the values of the coefficients A ₀ = -0.873 and A ₁ = 2.680 were adopted, the preliminary value of the power factor was cosφ ₁ = 0.562, which made it possible to choose from table 1 the smallest angle of thyristor switching in the second current period α ₂ = 61 electrical degrees, satisfying the condition α ₂ > φ ₁ . When a thyristor contactor with an angle of α ₂ = 61 electrical degrees was turned on, the measured value of the thyristor on-time was λ ₂ = 174 electrical degrees. Calculated according to (3), where for the region cosφ∈ [0.5; 0.7] in accordance with Table 1, the coefficients A ₀ = -0.757 and A ₁ = 2.862 were adopted, the updated value of the power factor was cosφ = 0.563. At the same time, the solution of equation (1) for the obtained values of α ₂ and λ _{2 by} numerical methods allows us to obtain the exact value of the power factor equal to cosφ = 0.570, on the basis of which we can conclude that the error in determining cosφ in the given example was 1, 2%

Thus, the proposed method for determining the power factor of a contact welding machine allows the simple calculation of power factor values and reduce measurement and computational errors.

Claims (1)

A method for determining the power factor of a contact welding machine, which provides for measuring the duration λ of the on state of the thyristors at a given angle α of their inclusion and calculating the power factor as cosφ = A ₀ + A ₁ λ, where the coefficients A ₀ and A _{1 are} set based on the condition of minimizing the calculation error cosφ, characterized in that the measurements are carried out for two periods of current, while in the first period, thyristors with an angle α ₁ = 90 electrical degrees are turned on and the preliminary value of cosφ _{1 is} calculated, in the second In the first period, thyristors are turned on with a minimum angle α ₂ satisfying the condition α ₂ > φ ₁ , and the refined value cosφ is calculated, and the coefficients A ₀ and A _{1 are} set discretely for each α.

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