What Is Wire Load?

In China's national standard GB / T7260-3, there is a clear definition of linear load: "3.2.6 linear load linear load When a variable sinusoidal voltage is applied, the load impedance parameter (Z) is constant and constant. " ^[1]

In AC circuits, the load element has a resistance R,

Linear load and

Linear load and non-linear load are the two root loads in the circuit. These two loads are often encountered in UPS equipment and circuits. Therefore, the characteristics and differences of these two loads should be understood.

1 Definition and characteristics of linear load

The definition of linear load in the national standard GB / T7260-3 of Chinese UPS: "3.2.6 linear load linear load When a variable sinusoidal voltage is applied, the load impedance parameter (Z) is stabilized to a constant load . "

In the communication circuit, there are three types of load components: resistance R, inductance L, and capacitance C. Their effects in the circuit are different.

In a purely resistive circuit, when a sinusoidal voltage U is applied to a resistor R, the inrush current I is also sinusoidal, and the phase of the current I and the voltage U are the same.

If the voltage u = Umsint, then i = Imsint; the useful value of the current I = U / R. The current is heated through the resistor, and the electrical energy is changed to thermal energy, that is, P = UI = I2R.

In a purely inductive circuit, a sinusoidal voltage is applied to an inductor coil L. Because the current is alternating, an induced potential is generated in the coil, so that although the current is still sinusoidal, the phase lags the voltage by 90 ° (TV point Is / 2).

If the voltage u = Umsint, then i = Imsin (t- / 2). The useful value of the current I = U / (2f L) = U / XL; XL = 2f L is called the inductive reactance. The current moves in the circuit, bringing the power of the power source into the coil, changing it to magnetic energy, and then changing the magnetic energy back to the power source. So there is no power consumption in the circuit, and the uniform power is zero. Reactive power Q = UI = I2XL.

In a purely capacitive circuit, a sinusoidal voltage is applied to a capacitor with a capacitance of C. Because the current is accumulated on the capacitor's plate with a charge, the capacitor voltage occurs, so that although the current is still sinusoidal, it is 90% ahead of the phase ° (TV point is / 2).

If the voltage u = Umsint, then i = Imsin (t + / 2); the current useful value I = 2fCU = U / XC; XC = 1 / (2fC). Call it capacitive reactance. The current moves in the circuit, bringing the power of the power source into the capacitor, changing it into electric field energy, and then changing the electric field energy into electric energy back to the power source. So there is no power consumption in the circuit, and the uniform power is zero. Reactive power Q = UI = I2XC. Inductive and capacitive reactances are generally referred to as reactances.

When a sinusoidal voltage is applied to a linear load that usually has a resistance R, an inductance L, and a capacitance C, the current is still sinusoidal, but the phase connection between the current and the voltage is neither the same phase nor a 90 ° difference, but They differ by an angle of .

If the voltage u = Umsint, then i = Imsin (t ± ). The useful current value is I = U / Z. Z is impedance, and its connection with resistance and reactance is: Z2 = R2 + X2. The reactance is an induction value of inductive reactance XL and capacitive reactance XC. The phase difference angle is selected by the R, L, and C parameters in the load. is positive when it appears as rational, and is negative when it is capacitive. tg = X / R. The impedance Z, the reactance X, and the resistance R constitute an impedance right triangle. Apparent power S = UI, active power P = UIcos, reactive power Q = UIsin, S2 = P2 + Q2, the three constitute the power triangle.

It should be clarified here that the choice of load characteristics is not only the size of the load impedance, but also the size of the power factor. In summary, in linear loads, there are purely resistive (power factor is 1) and rational (power factor is less than 1), capacitive (power factor is less than 1), and pure rational and pure capacitive (both power factor is 0) ). These loads are all classified as linear loads. You cannot think that a purely resistive load with a power factor of 1 is linear, and other loads with a power factor other than 1 are not linear. This is what this article wants to emphasize.

2 Definitions and characteristics of nonlinear loads

In the national standard GB / T7260-3 of China UPS, there is also a definition of non-linear load: "3.2.7 non-linear load non-linear load load impedance parameter (Z) is not always a stable constant, such as voltage or time, etc. Other parameters. "

The variety of non-linear loads is complex. Most of the loads powered by UPS are rectifier and filter type, and the input of UPS is also rectifier and filter type. Therefore, a reference non-linear load is drafted in the IEC specification and is included in the specification as an appendix to the specification. Use this benchmark non-linear load to check the talent of UPS with non-linear load. In the national standard GB / T7260-3 of UPS, this reference non-linear load circuit is also given in Appendix E.

The reason why this circuit is a non-linear load is that when a sinusoidal voltage u is applied to the input, when the instantaneous value of the voltage is greater than the DC voltage on the capacitor, the power supply supplies power to the load R1 and charges the capacitor. When the instantaneous value of the voltage is less than the DC voltage on the capacitor, the power is no longer supplied due to the blocking effect of the diode, and the load is discharged by the capacitor to make the load adhere to the continuity of the current. So the impedance of this load with respect to the power source changes with the size of the instantaneous voltage value.

One of the first characteristics of a non-linear load is that when a sinusoidal voltage is applied to the load, the current is not sinusoidal. The communication current of the load circuit of Fig. 1 is successive and spiked. Figure 2 is a waveform diagram of the voltage and current of this non-linear load. From this we can see that the current is a peak shape.

To analyze and calculate the current and power in a non-linear circuit, the method used is to use the Fourier function analysis method to replace the non-sinusoidal quantity with an equivalent sinusoidal quantity. In this detailed circuit:

Power input voltage u = u1 + u3 + u5 + u7 +, where u1 is the fundamental voltage weight, because the communication input power can be considered sinusoidal, so there is no higher harmonic weight, then u = u1.

Here the communication current i = i1 + i3 + i5 + i7 + i9 + i11 .

Each harmonic current is sinusoidal, and they all have their own amplitude, useful value (I1, I3, I5 ...) and the phase difference between the current and the voltage of the same frequency (1, 3, 5, 7 ...).

The equivalent sinusoidal current is used to replace the non-sinusoidal current, and the square of its useful value is equal to the sum of the squares of the useful values of each harmonic weight, that is: I2 = I12 + I32 + I52 + I72 + .

In this circuit, the instantaneous power value p = ui = u1 (i1 + i3 + i5 + i7 + i9 + i11 ...).

Uniform power P = U1I1cos1 = UI1cos1, also called active power.

As with linear circuits, let the apparent power in the circuit be S, S = UI.

The same reactive power is Q, the connection between the three powers is still S2 = P2 + Q2.

The ratio of active power to apparent power is the power factor in the circuit: PF = P / S = UI1cos1 / UI = I1cos1 / I = cos1. The coefficient = I1 / I <1.

The power factor PF value is smaller than the power factor cos1 of the phase difference of the fundamental wave. The greater the share of higher harmonics in harmonics, the smaller the , and the smaller the power factor. In this way, a non-linear load can be converted into a linear load for calculation and analysis.

In many loads, non-linear loads are very messy, and there are many varieties of current waveforms. There are spikes, double peaks, etc. It is still impossible to clarify with the magnitude of the current. In order to clarify the degree of divergence between nonlinear and linear currents, a parameter is used to indicate this, which is the crest factor. It is said in the GB / T7260-3 specification: "3.3.29 The ratio of the peak value of the peak factor period to the root-mean-square value of the crest factor."