GB2061032A - Full wave frequency converter - Google Patents

Abstract

A three-phase full wave bridge type rectifier (42) utilizing silicon controlled rectifiers (52-62) is connected to receive a three-phase input electrical signal and a switching network utilizing four silicon controlled rectifiers (82-88) is connected to receive and gate full wave rectified three-phase signals from the three-phase rectifier to provide desired positive and negative output signals. A transformer (50) having a single primary winding or multiple separate windings connected in parallel or series receives the output of the switching network, and a timing unit (46) is provided for timing the rectifier and switching network in synchronism to produce alternate positive and negative full wave rectified three-phase signals at a selected frequency from the three- phase electrical energy input and gate them to the transformer primary. <IMAGE>

Description

SPECIFICATION Full wave frequency converter contactor structure and method BACKGROUND OF THE INVENTION Field of the invention The invention relates to full wave frequency converter contactor structures and methods, and refers more specifically to structure for and a method of providing a transformer primary with an alternating electrical signal having substantially direct current electrical characteristics within the individual alternations thereof at selected frequencies, and the method of providing the selected frequency alternating signal.

Description of the prior art In the past, frequency converter contactors have generally taken the form of ignitron tubes connected in the three separate phases of multi-part transformer primary windings, to produce half-wave rectified signals through the separate parts of the primary windings. One such prior art frequency converter contactor 10 is shown in Figure 1.

The frequency converter contactor 10 is utilized in conjunction with or is part of the input circuit to the transformer 12. The frequency converter contactor 10 includes the inverse parallel connected pairs of ignitron tubes 14, 16 and 18 shown as switches in Figure 1 for simplicity, connected between the conductors 20, 22 and 24 of a three-phase electrical power supply input line and the parts 26,28 and 30 of the primary winding 32 of transformer 12. The secondary winding 34 of the transformer 12 is then connected as shown in Figure 1 across the load 36.

With such structure, to generate a pulse of positive current in the secondary winding 34 of the transformer 12, the switch 14 is closed for a period during the time when the voltage between the conductors 20 and 22 is positive. Switch 14 opens when the voltage between conductors 20 and 22 is no longer positive. Switch 16 is next closed while the voltage between the conductors 22 and 24 is positive and is opened when the voltage is no longer positive.

Simiiarly, switch 18 is closed last while the voltage between conductors 24 and 20 is positive.

With such structure, the selecting of only the positive half cycles of the voltage across each phase of the three-phase power line 25 can continue until the transformer core saturates. With such structure the current of the secondary of the transformer will be the half-wave of the three-phase primary current.

Such direct current wave form is useful where there is considerable inductance in the secondary circuit of the transformer. The direct current wavelike form produced by the frequency converter contactor 10 is less affected by the inductance and allows more efficient use of the primary power.

However, the three interconnected windings of the transformer primary, that is, parts 26, 28 and 30 thereof, present a problem to the contactor in the form of high voltage across the switches or ignitrons. Normally, with a 480 volt alternating current line 25, the peak voltage that a contactor has to withstand is approximately 680 volts. However, with the cross coupling effect of the three primary windings 26, 28 and 30, the peak voltage, that is, across the ignitron tubes or switches 14, 16 and 18, is approximately 1270 volts.

Large voltage transients are generated by current cutoff at the end of each impulse due to the inductive nature of the transformer and its load. Likewise, interruptions in the current due to weld blowouts in the case where transformer 12 is a welding transformer and similar problems generate voltage transients across the primary windings 26, 28 and 30 of the transformer 12. These voltage transients can reach amplitudes of 1000 volts and more Because of the combined problems indicated above, any contactor used for frequency converter service on a 480 volt line must be capable of withstanding, that is, have a withstanding voltage of at least 1800 volts. Experience has shown that a withstanding voltage substantially higher than this, that is, about 2500 volts, is needed for truly reliable operation of a frequency converter.

Until a few year 2500 volt silicon controller rectifiers of any size were unavailable. The technology to make them had not yet been developed. Even now, 2500 volt silicon controlled rectifiers with sufficient current handling capacity to control a large frequency converter transformer are prohibitively expensive when compared to ignitron tubes.

Further, with silicon controlled rectifiers in a traditional frequency converter contactor as disclosed in Figure 1, there is a tendency for them to turn on spuriously when a fast rise time voltage is applied across them. This is unacceptable since if more than one silicon controlled rectifier is turned on at any given time, then a short circuit exists between two of the three phases in a three-phase power line through the transformer. The resultant high primary current, which may reach as high as 10,000 amperes, causes extreme stresses in a power transformer which will greatly shorten its life as a result of insulation failure.

SUMMARY OF THE INVENTION In accordance with the invention, a full wave frequency converter contactor, constructed as a three-phase full wave bridge rectifier utilizing six silicon controlled rectifiers in conjunction with a switching network utilizing four additional silicon controlled rectifiers is provided between a threephase electrical power supply line connected to the three-phase rectifier and a transformer having a single primary winding or multiple separate primary portions connected with each other connected across the output of the switching network.Timing structure is also provided in accordance with the invention for turning the silicon controlled rectifiers on and off at selected times to provide full wave rectified electrical energy from the three-phase rectifier to the switching network and to gate the desired polarity pulses of rectified electrical energy through the switching network of the transformer secondary winding.

In the full wave frequency conversion method of the invention, a three-phase alternating current signal is full wave rectified and the full wave rectified signal is gated to a utilizing transformer in pulses of selected polarity at selected times. The resuiting positive and negative substantially direct current pulses are fed into a single or multiple part transformer primary winding.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram of prior art frequency converter contactor structure utilizing conventional ignitron tubes.

Figure 2 is a schematic diagram of the full wave frequency converter contactor structure utilizing silicon control led rectifiers constructed in accordance with the invention for performing the method ofthe invention.

Figure 3 is a diagram of the wave form of the signal from the full wave frequency converter contactor structure illustrated in Figure 2, useful in describing the structure and method of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in Figure 2, the full wave frequency converter contactor structure 40 of the invention includes a three-phase full wave rectifier bridge circuit 42, a switching network 44 and a digital timer 46 connected between a three-phase power line 48 and a welding transformer 50.

Electrical energy from the three-phase electrical energy supply line 48 is connected to the threephase rectifier 42 over the conductors 104, 106 and 108, as shown.

The three-phase rectifier 42 includes silicon controlled rectifiers 52, 54, 56, 58, 60 and 62. The silicon controlled rectifiers are connected, as shown, in a full wave bridge rectifier circuit and have control electrodes 64, 66, 68, 70, 72 and 74 connected to separate ones of the leads 76 from the digital timer 46. The silicon controlled rectifiers 52 through 62 are caused to conduct in pairs of signals through leads 76 from the timer 48 to provide rectified three-phase alternating current of the polarity shown on the output conductors 78 and 80, as desired.

The switching network 44 includes the silicon controlled rectifiers 82, 84, 86 and 88 connected in a full wave bridge-like circuit, as shown, having an input from the conductors 78 and 80 from the rectifier 42. The silicon controlled rectifiers 82, 84, 86 and 88 have control electrodes 90, 92, 94 and 96 connected to separate ones of the leads 98 from the digital timer 46. The silicon controlled rectifiers 82, 84, 86 and 88 are caused to conduct in pairs by signals over conductors 98 from the timer 48. The switching network 44 is operable to gate pulses of selected polarity from the rectifier 42 to the transformer 50 over the output conductors 100 and 102.

The transformer 50, which may be a welding transformer, is polarized positive as shown by the dots, and includes the primary winding circuit 110 including the separate parts 112, 114 and 116 and the secondary winding circuit 118. Parts 112, 114and 116 of the primary winding circuit 110 are connected in parallel with each other and receive the output of the switching network 44 over the conductors 100 and 102. It will be understood that the primary winding circuit 110 may be a single part winding or a multiple part winding with the parts connected in series as well as the multiple part winding 110 with the parts 112,114 and 116 connected in parallel shown in Figure 2.

In operation of the full wave frequency converter contactor structure 40 in accordance with the method of the invention, the three-phase electrical power signal from the electrical power line 48 is passed to the rectifier circuit 42 over the conductors 104, 106 and 108 and the rectifiers 52 through 62 are energized sequentially in pairs by signais from the digital timer 46 over conductors 76 to produce full wave rectification of the three-phase signal. A three phase rectified, substantially direct current electrical signal polarized as shown is produced on the conductors 78 and 80.

With reference to Figure 3, wherein the electrical current from the conductors 104, 106 and 108 is indicated as wave forms 122,124 and 126, respectively, the order of turning on the silicon controlled rectifiers to rectify positive half-cycles is as follows: For phase one, 122, silicon controlled rectifiers 54 and 56 are turned on. For phase two, silicon controlled rectifiers 58 and 60 will be turned on. For phase 3, 126, silicon controlled rectifiers 62 and 52 will be turned on.

For negative half-cycle rectification, shown in Figure 3, for phase one, 122 of the input electrical energy, rectifiers 52 and 58 would be turned on. For phase 2, 124, rectifiers 56 and 62 would be turned on.

For phase three, 126, rectifiers 60 and 54 would be turned on.

Synchronized with the switching of the threephase full wave bridge rectifier 42 by the digital timer 46 to produce pulses of electrical energy on conductors 100 and 102, the switching network 44 is energized to apply voltage of the desired polarity at desired times to the transformer primary winding 110. Thus, if it is desired to pass a positive pulse to the transformer 50, the silicon controlled rectifiers 92 and 94 are energized, while if it is desired to pass a negative pulse through the transformer 50, the silicon controlled rectifiers 90 and 96 are energized.

It will be noted that the silicon controlled rectifiers are turned off for a short period of time indicated 134 in Figure 3 between the production of the positive and negative pulses gated to the transformer 50 to permit the rectifiers to stabilize.

The digital timer 46 may be any of a number of known digital controllers capable of putting out pulses on the conductors 76 and 78 at selected times to turn the silicon controlled rectifiers on in pairs as required. Since such digital timers are within the skill of the art to build or select or program in accordance with the requirements of the invention, the digital timer 46 will not be considered in further detail herein.

Thus, in operation,thethree-phase bridge circuit is phase controlled to produce a substantially direct current voltage across the output conductors 78 and 80. The switching network circuit is used to select which way the welding transformer 50 is connected to the positive and negative output conductors 78 and 80.

In the full wave frequency converter contactor 40, the silicon controlled rectifiers may be 1400 volt units, which are relatively inexpensive and quite available. The reason that 1400 volt units are sufficient is that in the three-phase bridge circuit 42 there are two silicon control led rectifiers in series for each current path through the bridge. Thus, the withstanding voltage is the sum of the two silicon controlled rectifiers withstanding voltages, or 2800 volts.

The silicon controlled rectifiers in the switching network circuit 44 see the full 480 volt line voltage across them, that is, 680 volts peak. However, they are not subject to any greater voltages than peak line voltage because there is no cross coupling effect between the primary parts of the transformer since the three parts of the primary winding 110 of the transformer 50 are connected in parallel as shown.

Further, the transients caused by inductance of the transformer and its load on turn-off are also dealt with effectively by the structure and method of the invention. Any voltage transients that appear across the transformer as its primary current is cut off are applied to at least two silicon controlled rectifiers in series, one of which will be reverse biased in accordance with the present invention. Therefore, there is at least 2800 volts of withstanding capacity to withstand such transients.

In addition, any voltage transients generated in the transformer circuit do not get coupled to the power line, because in order to do so the resulting current has to break down four silicon controlled rectifiers in series before such coupling can occur. The total withstanding voltage of the four silicon controlled rectifiers in series is approximately 5600 volts.

Thus, in accordance with the present structure and method, silicon controlled rectifiers have been made available for frequency converter service. In addition, the full wave frequency converter contactor of the invention has other advantages.

Because the transformer has full wave rectified direct current applied to its primaries, greater secondarywelding currents are possible because any inductance in the primary windings will no longer offer impedance to the applied direct current primary current. In addition, primary resistance of a multiple-winding transformer is reduced by the parallel connection of the three windings.

Also, the wave form of the current is much smoother, particularly at lower phase settings, because the current wave form is full wave, not half wave rectified direct current. The ripple frequency of 360 hz is very effectively filtered by secondary inductance, so that a standard strip chart recorder reading the voltage across a load shows almost no ripple, even at the lowest heat settings.

The power factor reflected back to the incoming power line is improved because there is no net direct current component to the current drawn. As a load, the contactor of Figure 2 much resembles a threephase motor load.

When the current is off, the primary windings of the transformer are completely disconnected from the incoming power line. No voltage exists between the primary windings and ground. This has favorable safety implications as well as isolating the transformer from the effects of very high voltage line surges due to lightning and other catastrophic events.

Less expensive frequency converter transformers are possible with the contactor arrangement of the invention because they will only need one primary winding. Reliability and longevity of the transformer will likely be better because of greater simplicity.

While one embodiment of the present invention has been considered in detail, it will be understood that other embodiments and modifications thereof are contemplated by the inventor. It is the intention to include all embodiments and modifications as are defined by the appended claims within the scope of the invention.

Claims (10)

1. Full wave frequency converter contactor structure comprising a three-phase rectifier adapted to be connected to a source of three-phase electrical energy for supplying of full wave rectified substantially direct current energy to a switching network, a switching network connected to the three-phase rectifier to receive the rectified direct current energy for gating the direct current energy in pulses to a transformer primary winding in accordance with desired polarity and timing means connected to the three-phase rectifier and switching network for synchronizing the operation of the three-phase rectifier and the switching network to produce alternate polarity pulses of substantially direct current electrical energy from the switching network at a selected frequency.

2. Structure as set forth in Claim 1, wherein the three-phase rectifier includes six silicon controlled rectifiers connected in a full wave three-phase bridge rectifier circuit and the silicon controlled rectifiers are connected to the timing means so that the silicon controlled rectifiers are turned on in pairs to provide full wave rectification of the three-phase electrical energy from the three-phase electrical energy source.

3. Structure as set forth in Claim 1, wherein the switching network includes four silicon controlled rectifiers connected in a full wave single-phase bridge-like switching network circuit and the silicon controlled rectifiers are connected to the timing means so that the silicon controlled rectifiers are turned on in pairs to gate the direct current energy from the three-phase full-wave rectifier through the switching network of the transformer primary winding with the desired polarity.

4. Structure as set forth in Claim 1, and further including the transformer primary winding and wherein the transformer primary winding is one of a single coil or multiple coils connected to behave as a single coil.

5. Structure as set forth in Claim 1, and further including the transformer primary winding and wherein the transformer primary winding has three separate coils and the three separate windings are connected in parallel with each other and across the switching network.

6. Structure as set forth in Claim 1, wherein the timing means includes means for turning offthe rectifier and switching network for a brief period between each alternate positive and negative-pulse.

7. A full wave frequency conversion method comprising converting three-phase alternating electrical energy into full wave rectified direct current energy and gating the direct current electrical energy in pulses of desired polarity to a transformer primary winding at a desired frequency.

8. The method as set forth in Claim 7, wherein the three-phase electrical energy is rectified in a full wave bridge type rectifier including six silicon controlled rectifiers by alternatively energizing selected pairs of the silicon controlled rectifiers.

9. The method as setforth in Claim 7, wherein the gating is accomplished by a switching network including four silicon controlled rectifiers by alternately energizing pairs of the rectifiers.

10. The method as set forth in Claim 7, wherein the converting of the three-phase alternating energy and the gating of the direct current electrical energy in pulses is briefly halted between pulses of positive and negative polarity.