FI90700C - Procedure for controlling a frequency converter's bridge - Google Patents

Description

90700

METHOD OF CONTROLLING A DRIVE NETWORK BRIDGE

The invention relates to a method for controlling the network bridge 5 of a frequency converter when supplying power back to the network via semiconductor switches connected in parallel with the diodes of the frequency converter.

Many applications require a -10 vaM rectifier bridge to be used as a DC source so that the DC source is capable of both input and output. Such a need exists, for example, in a constant voltage inverter controlled by an AC motor, if the motor load is capable of supplying power. The elevator represents a typical load with the ability to supply power. Suitable application areas for bidirectional power flow are, for example, transistor and thyristor inverters in AC motor drives.

Today, circulating and non-circulating thyristor bridges are often used as a bidirectional DC voltage source between 20 AC voltage networks and DC voltage circuits, where the power supplied by the load is supplied to the AC voltage network. The connection implemented with a non-circulating thyristor bridge is based on the fact that one of the two bridges is conductive depending on the power flow direction. 25 The changeover time between bridges is relatively long. The circulating current-free connection can also be realized by means of a diode and a thyristor bridge, so that a storage transformer is used to avoid circulating power. In this case, the reversible thyristor bridge can be considered conductive at all times. In a circulating thyristor bridge-30 connection, both bridges can also be kept conductive at all times. In this case, it is not necessary to monitor the direction of energy flow separately. A disadvantage of thyristor connections is the complexity of the thyristor control connections. In addition, the time required for the thyristors to turn on and off is relatively long.

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As a rectifier source between the three-phase network and the equalizer circuit, a diode bridge according to Fig. 1 is also used, which rectifies the three-phase voltage as a rectifier as the power flows from the three-phase network to the equalizer circuit. In order to convert the rectifier i-2 90700 to three-phase voltage when power flows from the DC circuit to the three-phase network, transistors are connected in parallel with the diodes of the diode bridge. The control voltages of the transistors are generated by diodes from a three-phase network so that each transistor conducts during the conduction of a diode adjacent to the transistor. The conduction times of the transistors are shortened to reduce the circulating power by connecting a Zener diode in series with the controller of each transistor.

10 When the motor brakes, the voltage of the DC link circuit of the frequency converter tends to rise, whereby the inverting transistors in the rectifier bridge are controlled to conduct, whereby current flows from the DC link to the mains. When the control voltages are formed from a three-phase network with diodes. The disadvantage of this method 15 is that when the control angle is constant, a circulating current is always generated through a capacitor in the DC circuit. In addition, the control circuit must be changed in networks with different voltages, because the control angle is obtained with Zener diodes. Variation in the mains voltage also changes the steering angle.

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Finnish patent 78202 discloses a method for generating control pulses based on measuring phase voltage. In the present invention, the reverse semiconductor switches connected in parallel with the rectifier means of the rectifier-reverse bridge connected to the alternating voltage network are controlled by comparing a voltage proportional to the phase voltage to a reference voltage. In this case, the control angle of the semiconductor switches 30 of the mains bridge is automatically adjusted to the correct one. The steering angle is myds the minimum required, resulting in low mains disturbances. However, the above-mentioned method has the disadvantage that it always requires phase voltages in the network, in which case an artificial zero must be formed in the non-zero network with 35 separate transformers. To eliminate this disadvantage, reference is made to 1. to the characterizing part of the claim.

The present invention provides control of transistors 3 907Q0 without zero, because only mains voltages are used. As a result, there is no need to use a separate transformer and, in addition, the invention does not incur significant additional costs.

The invention will now be described in more detail, by way of example, with reference to the accompanying drawings, in which Figure 1 shows a frequency converter with a constant voltage DC circuit for use in supplying an elevator hoist motor.

Fig. 2 shows how a voltage Uac2 proportional to the absolute value of the peak values of the mains voltages and a voltage Uac1 proportional to the absolute value of the maximum values of the main voltages are formed from the network voltages.

Fig. 3 shows a drawing of the voltages shown above.

Figure 4 shows a drawing for selecting the correct pair of transistors-20 to conduct.

Figure 1 shows transistors Q1 ... Q6 and they form a network bridge. They are controlled to conduct when the motor brakes and the energy returned to the grid. The present invention provides control of transistors without zero because only mains voltages are used.

Figure 2 shows how the network voltages Urs, are formed from the phase voltages Ur, Us and Ut of the network by amplifiers A1-A3; '30 Ust and Utr, which form a voltage Uac2 proportional to the absolute value of the peak values of the mains voltages and a voltage Uac1 proportional to the absolute value of the instantaneous maximum values of the voltages. In other words, Uacl is directly rectified. . mains voltage with 300Hz ripple and Uac2 is additionally 3’5 filtered. The differential voltage Udiff is formed in such a way that it indicates the difference between the mains voltage and the voltage Ude of the direct current circuit. When the engine brakes, Ude tends to grow, as does Udiff. The reference voltage Uref is formed from Uac2 and 4 90700 and Udiff so that when no current flows to the motor and thus Udiff is zero, the reference voltage Uref = Uac2. When the engine brakes, Uref decreases sitS the more Ude increases. Al ... A3 are differential amplifiers which attenuate 5 mains voltages and form voltages Urs, Ust and Utr proportional to the mains voltages. A4 ... A6 are inverter-amplifiers with exactly one gain, so they provide voltages proportional to the voltages in opposite directions. Diodes D1 ... D6 form a selector 10 with which the largest absolute value of the network voltage, i.e. Uac1, is selected. A7, D7 and C1 form a peak holding circuit which provides a DC voltage Uac2 proportional to the peak value of the mains voltage. A8 is a differential amplifier whose attenuation gives the voltage 15 Ude of the drive's DC voltage circuit. It is fed together with Uac2 to the differential amplifier A9, from the source of which the difference Udiff of the DC voltage and the network peak voltage is thus obtained. When the engine brakes, Ude tends to grow, as does my5s Udiff. The reference voltage Uref is formed from Uac2 and Udiff so that when no current flows to the motor and thus Udiff is zero, the reference voltage Uref = Uac2. When the engine brakes, Uref decreases the more Ude grows. The comparator Compi compares the reference voltage Uref and the voltage Uacl, so that the output of the comparator always changes state at the peak value of the mains voltage of the network. 2 $ Thus, the output of the comparator forms a pulse during which the bridge Q1 ... Q6 is controlled.

Figure 3 shows some of the voltages shown above. Differential amplifier A10 generates a reference voltage Uref ‘30 such that when Udiff is zero, Uref is equal to the peak value of Uac1. Now, when the reference voltage and the Uacl are compared in the comparator COMP1, its state does not change in this situation. If the voltage of the DC link now rises, which occurs when the motor is braked, the lower the reference voltage decreases, the more the motor brakes.

li 5 90700

Figure 4 shows a buffer amplifier All, which supplies a resistor divider, from which a voltage Uac3 is obtained, which is a slightly attenuated voltage Uac1. The comparators COMP2 ... 7 compare this voltage Uac3 and each pVoltage in the network and the inversion of the voltage.

5 In this case, at the peak value of each voltage, one of the comparators gives the signal "1", the others are "O", ie it is possible to know which of the voltages is the largest. The outputs of the comparators are then applied to OR circuits which select the transistors to be connected. Thus, for example, if Urs is 10 largest, the output of COMP2 is high and the OR circuits corresponding to Q1 and Q4 are myds high, allowing Q1 and Q4 to be conducted when pulse Up is in the "1" state. In this case, the current phase R phase ... Q1..Cdc.-Q4..S phase is formed and the braking energy stored in the circuit capacitor Cdc is discharged into the network. After selecting the component 15, there is still an AND circuit with which the component is set to conduct only if the voltage of the DC circuit has actually risen, conducting a pulse voltage to another AND input terminal. Thus, the output of the OR circuits selects the conducting transistors and the pulse voltage Up pulse width, which is zero if the DC voltage 20 has not risen, i.e. the motor does not brake. Of the comparator pairs, the one whose voltage is greater than Uac3 changes state, indicating a pair of transistors to be connected as conductive. The control pulses for the transistors are obtained by AND circuits, one input of which determines the length of the control pulse and the other the selected transistor. It is clear that the circuit is shown in a simplified manner here and that a fault connection must be added so that only one of the comparators 2 ... 7 can be on at the same time.

It will be apparent to those skilled in the art that the invention is not limited to the examples described above but may vary to the extent set forth in the claims.

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Claims (6)

1. A method for controlling a drive when returning energy to the grid via semiconductor switches which are connected in parallel over the diodes of the drive's rectifier, characterized by the control pulses of the semiconductor switches (Q1 ... Q6) being generated from the main , U-st, Ut) and the DC voltage (Ude) so that the length of the control pulse 10 is determined by means of the main voltages (Urs, Ust, Ut) and the voltage of the direct current circuit (Ude) and the controlled electric switch is selected using the main voltages. 2. A method according to claim 1, characterized in that a reference voltage (Uref) is formed by a ratio of voltage (Uca2) to the peak voltage of the main voltage and a difference of the voltage of the main voltage to the DC voltage so that the reference voltage The inlet reaches the motor with no current equal to the peak voltage of the main voltage (Uac2) and decreases with increasing voltage (Ude) in the DC circuit. 3. A method according to claim 2, characterized in that it is proportional to the peak voltage of the main voltage. (Uca2) is formed with a mover (D1 ... D6) which selects the largest of the main stresses (Uacl) of the net, and a holding circuit (A7, D7, C1) for the peak care. 4. A method according to claims 1 ... 3, characterized in that the signal for selection of the rat-two semiconductor couplers is formed with the comparators (COMP2 ... COMP7), which compare each main voltage in the night and each inverted main voltage with the voltage. (Uac3) which is slightly smaller than the relative voltage (Uacl) proportional to the absolute peak of the instantaneous peak care (Uacl), partly OR gates (21) which are connected to the comparators' outputs. 5. The drive according to claims 1 ... 4, characterized by S 90700, in that the signal determining the length of the control pulse is formed with the comparator (COMP1) which compares the main voltage (Uacl) with the reference voltage (Uref) to the absolute value. 5 6. The method according to claims 1 to 5, characterized in that the first and second signals are fed to the AND gates (31) of the spring's output determine the control pulses of the semiconductor switches. 10 15 20. - 25 30 - • 35