JPS5910150B2 - frequency converter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a frequency conversion device that directly extracts single-phase outputs with different frequencies from three-phase currents, and particularly relates to an output voltage control method.

Frequency conversion methods include methods that first convert AC power into DC power and extract AC power at a predetermined frequency using an inverter, and methods that use a cyclone converter (direct frequency converter), but the former has a lower conversion rate than the latter. They are less efficient, and the latter have a limited range of frequencies that can be converted.

An object of the present invention is to provide a frequency converter that can easily control the output voltage while preventing phase-to-phase short circuits of the converter in a direct frequency converter that outputs a frequency-converted single-phase output directly from a three-phase power source. .

FIG. 1 shows an embodiment of the main circuit according to the present invention.

The transformer Tl has a three-phase 50H2 secondary output U4, V,, W1
A phase difference of 30 degrees is provided between U2, V2, and W2. The three-phase bridges Bf and Bb having a thyristor configuration are connected in antiparallel to each other to form a three-phase 50H2 power supply (U1, Vl, W
, ) are given, and the three-phase bridges Af and Ab are connected in antiparallel to each other to give a three-phase 50H2 power supply (U2, V2, W2). Then, the three-phase bridge Af, Ab (5Bf
, Bb are connected in series in a mesh connection, and one end of the three-phase bridge Af, Ab and one end of Bf, Bb are connected to a single-phase 6
0H2 output end, single phase 60H2 through transformer T2
It is configured to take out. Note that the main thyristors 51 to 524 constituting each three-phase bridge are not each made into one element, but a plurality of elements are connected in parallel according to the control capacity. FIG. 2 is a waveform diagram for explaining a control method in the main circuit configuration of FIG. 1.

In FIG. 2, U, V1, U2V2, W, and V4 each indicate interphase voltage, and V indicates single-phase output voltage. Now, the main thyristor 51 | 57, 55 | 511 | 513 in FIG.
1519, 51|, 523 are firing, the output voltage V becomes as shown in the figure in the range θ<θ1, and when the main thyristors 53, 59 fire at the point θ4, the output voltage V changes from U, phase to W,
Even in the range of θ≧θ1, the state continues as shown in the figure, resulting in a positive half cycle of 60H2. Thus, in the positive half cycle of the voltage, the △ side of the transformer Tl, that is, the 3
Commutation is carried out in the bridges Bf and Bb on the side that are delayed by 0 degrees, and in the negative half cycle, the bridges Af and Bb on the side that are delayed by 30 degrees are
Commutation is carried out at Ab, but of course the thyristors 51, 56, 513, 517 through which current is actually flowing, as well as the thyristors S7, Sl,, S, , S, which are connected in antiparallel thereto.
23 is also constantly given a gate pulse. This is so that even a harmonic current, that is, a load current whose direction changes in a fast cycle, can be energized. Here, in order to prevent a short circuit between phases due to applying a gate signal to both the inverted sequence thyristors, the commutation point is appropriately selected.

FIG. 3 shows the case of commutation from ULV to Wll. In the figure, in order to commutate from UlV to WlVl, the main thyristors Sl and S of bridges Bf and Bb in Figure 1 are required.
It is necessary to switch the gate signal from 7 to S3,S, but if W,l〉Ull at the switching time θ, then W1〉
U1, and at this time, if 1L of load current is flowing through the main thyristors Sl and S6 (θ1>power factor angle θL),
It is a positive current (same polarity as the voltage), and after turning off the gates of the main thyristors Sl and S at θ, the main thyristor S
3. If S, is ignited, the current changes from S, to S at the time θ,
Naturally translocates to 3. This is because the voltage of W and U is applied as a reverse voltage to S, which turns it off, and since it is a positive current, no current flows through S. If the main thyristor S,,
If the load current 1L is flowing through S, , Sll at the time θ1 when the gate signal is switched from S7 to S6, S, (θ1
θ and θ, ), negative current (opposite polarity to voltage), S3,
When S9 is ignited at θ1, a short circuit occurs between the lines through W, -S, -S, and -U1. Generally, in the case of a positive current, natural commutation is possible from a low potential to a high potential, and in the case of a negative current, on the contrary, natural commutation is possible from a high potential to a low potential. If this is not the case, commutation will fail and a short circuit will occur between the lines. Now, when there is a natural commutation from U,, to Wl, the third
With the load power factor as shown in the figure, natural commutation is possible in the range of θ<θ2. With the load power factor of FIG. 4, natural commutation is possible in the range of θ2 - θ<θ2.

However, at a load power factor as shown in FIG. 5, there is no region where natural commutation is possible. In this case, short circuits between the lines can be prevented by igniting the thyristors W and I after confirming that the current has become zero and the thyristors have naturally extinguished. In the present invention, as shown in Fig. 6, after confirming that the current has spontaneously disappeared to zero regardless of the load power factor, the next thyristor to be fired is fired, thereby preventing short circuits. The output voltage is controlled by controlling the zero section Δθ of the current. As described above, the present invention has the effect of making it possible to control the output voltage while preventing line-to-line short circuits.

In particular, the commutation point is set close to the peak value of the output voltage so that it is less susceptible to the influence of the load power factor, and at the same time, the current zero point is detected and the ignition phase is controlled based on it, so it is stable regardless of the load power factor. This has the effect of enabling commutation.

[Brief explanation of drawings]

FIG. 1 is a main circuit configuration diagram showing an embodiment of a frequency converter according to the present invention, and FIGS. 2 to 6 are waveform diagrams for explaining the operation of the present invention. Af, Ab, Bf, Bb...3-phase bridge, T
,,T2......Transformer.

Claims (1)

[Claims] 1 Two sets of three-phase currents with phase differences are applied to three-phase bridges connected in antiparallel, and commutation is performed alternately in the three-phase bridge every half cycle, resulting in a single-phase output that converts the frequency from the three-phase power supply. In a frequency converter that directly obtains the current, the current flowing through the rectifying element of the three-phase bridge or the load current is detected, and from the point when the positive (negative) half-wave current reaches the zero point, the next negative (positive) half-wave current is detected. A frequency conversion device characterized by controlling a single-phase output voltage by controlling the time until the current starts flowing near the peak value of the output voltage waveform.