INDUSTRIAL SUPPLIES,

INVERTERS, AND CONVERTERSBy

Muh. Sainal Abidin20213032

7.1 OVERVIEW

Solid state electronics have become integrated into all aspects of industrial power supplies, converters, inverters, and choppers. Industrial power supplies are used in applications where a variety of voltages is required, such as power of PLC processors and their analog modules and other specialty modules. Power supplies are also used in all types of digital display in cathode ray tube (CRT) color display. Any equipment that has electronic circuit in it must have a DC supply voltage available.

7.2 INDUSTRIAL RECTIFIER CIRCUITS: AC – TO – DC CONVERSION

7.2.1 Single-Phase Rectifiers

Single-phase rectifier circuits have been used since the advent of vacuum-tube diodes. When vacuum tubes were first introduced to control voltage and current, they required a variety of DC power supplies. Since the DC power supplies originated from AC voltage, vacuum-tube diode rectifiers were used to converted AC voltage to DC voltage. When solid-state devices were developed, the first uses for solid-state diodes were to provide rectification of AC voltages to the necessary DC voltages.

Figure 7-1 shows an example of a power supply that uses a single solid-state diode rectifier. From this figure you can see that this type of power supply that uses a transformer to increase or decrease the voltage from the 110-volt AC supply voltage.

Figure 7-1 electrical circuit diagram of a single-diode rectifier power supply. The waveforms show AC voltage supply and half-wave DC at the load resistor.

7.2.2 Two-Diode Full-Wave Single-Phase Rectifiers

A drawback of the single-diode half-wave rectifier is that it produces only a half-wave dc output. If a second diode is added to this circuit and a center-tapped transformer is used, half-wave when the supply voltage is between 1800 and 3600. Figure 7-2 shows the electrical diagram of the two-diode full-wave bridge circuit. This diagram also shows the sine wave for the single-phase input voltage and the waveform of the two positive half-wave for the output.

7.2.3 Four-Diode Full-Wave Bridge Rectifiers

Another circuit that provides a full-wave output uses four diodes and a regular transformer without the center tap. This circuit uses two diodes at a time to rectify each half if the sine wave.

From the top circuit in figure 7-4 you can see that the positive half-cycle of the AC is shaded, and the first half-wave is shaded to indicate the output for this part of the circuit. The bottom circuit shows the negative half of the sine wave being rectified. The path the electron would travel thought the bridge is also shown. Notice that electron flow is always against the arrows of the diodes.

7.2.4 Three-Phase Full-Wave Rectifies

Most industrial power supplies for motor driver and welding application use three-phase AC voltage. This means that the rectifier for these circuits must use a three-phase bridge, which has six diodes to provide full-wave rectification (two diode for each line of the three phase).

Figure 7-6 shows the electrical diagram for a three-phase bridge rectifier. From this diagram you can see that the secondary winding of a three-phase transformer is shown connected to the diode rectifier

7.2.5 Other Types of Three-Phase Rectifiers

Several other variations of the three-phase rectifier are used in some industrial power supplies because they provide an advantage of less power being converted by each individual diode, which means smaller diodes can be used to provide the same voltage and current as another rectifier circuit. Two of the more usable types of alternative rectifier circuits are show in figure 7-7 and 7-8.

In this diagram you can see that the secondary windings of the transformer consist of six separate windings. All six of the windings are connected at one end to form a center point for the start configuration, which is actually a type of wye-connected transformer. The cathode of each diode in this rectifier are connected to provide the positive terminal of the DC power supply. This circuit is used where it is important that all of the diode in the circuit have a common connection for their cathodes.

7.2.6 Six-Phase Full-Wave Bridge Circuits

In the 1970s and 1980s power supplies required higher currents and voltages than the individual diodes could provide in the basic four-diode bridge. If the amount of current or voltage the power supply requires is larger than the individual diode can provide, the diodes can be connected in parallel to provide the extra current, and they can be connected in series to meet the higher voltage specification. Two types of circuits are generally used to provide these configurations.

Figure 7-9a shows an example using 12 diodes that are connected in parallel as a six-phase full-wave bridge to provide extra current. Figure 7-9b shows examples of 12 diodes connected in series to allow the bridge to be used in a circuit where the system voltage is higher than the specification for any of the individual diodes.

It is important to compare all of different types of rectifier circuits. you will find each of these different types of circuit when you troubleshoot modem electronic control such as AC and DC motor drives, welding power supplies, uninterruptible power supplies (UPS), and other industrial system that require DC voltage.

7.2.8 Using Capacitors and Inductors as Filters for Power Supplies

Most power supplies found in industrial electronics circuit have capacitors and inductors used as filter. A filter on the power supply circuit will reduce the amount of ripple to a point where the output DC voltage is nearly a straight line, or pure DC. It is important in some circuit where the DC voltage is converter back to AC voltage that all traces of the original frequency of the input voltage is removed.

7.2.7 A comparison of the Different Types of Rectifier Circuits

Figure 7.11 shows a diagram of a typical capacitor and inductor in the power supply circuit. The capacitor is connected in parallel with the load, and the inductor is connected in series with the DC voltage terminals.

7.2.9 Using a Zener Diode for Voltage Regulation

Most industrial power supplies require the DC output voltage to have some type of regulation to keep the output voltage level constant when the input voltage fluctuates. The AC voltage that supplies power to industry today will fluctuate up to 10% of the supply voltage specification. This mean that in the hottest days of summer it may not be uncommon for the three-phase 208 volts that is supplied to a machine to drop below 200 volts. When this occurs, the DC output voltage of all of the rectifier cicruit will also drop. When the DC voltage drops, the circuit may become unreliable, so a zener diode is generally used in the output section if the power supply to provide voltage regulation. Figure 7.12 shows a circuit with the zener diode connected In parallel with the load.

The zener diode must be rated for the same voltage the DC voltage requires. For example, if the DC load needs 20 volts DC, the zener will be rated for 20 volts.

7.2.10 Surge Protection for Rectifier CircuitsThe rectifier circuits in power supplies are subject to a large variety of resistant voltage from lighting or from inductive lads such as motor and coils that produce a spike when they are de-energized. When these transients occur, they may have voltage levels that are two to five times the original supply voltage. The fuses in a circuit are designed to protect a circuit against overcurrent, but they cannot detect overvoltage.

7.2.11 Crowbar Protection Against OvervoltageAnother way to protect power supply circuits against voltage conditions is with a crowbar circuit.

Figure 7-14 shows two example of a crowbar circuit. In these circuits, an SCR is used to sense the overvoltage condition and go into conduction. The SCR is strategically located in the circuit to cause a short circuit of sufficient size to cause the fuse or circuit breaker to open and protect the circuit.

7.3 APPLICATONS FOR INDUSTRIAL POWER SUPPLIES

The rectifier sections are used in all types of industrial application. Virtually every piece of industrial equipment that has electronic circuit must have a means of converting AC voltage to DC voltage. These system use the rectifier circuit discussed earlier to provide the DC power.

7.3.1 Power Supply for a Variable-Frequency Motor DriveYou can get a better idea of how the diodes in the rectifier and

the devices in the filter and regulator section of a circuit all work together when you see a complete electronic circuit for an operational system. Figure 7-15 Shows the electrical diagram of a variable frequency motor drive that is commonly used in industrial applications. From this diagram you can see that the drive circuit uses three-phase supply voltage, so a three-phase full-wave bridge rectifier is used.

A picture of this type of bridge is shown in chapter 4 and you should remember that it is encapsulated so you do not see each of the diodes. Rather you would find the terminals for the three input terminals and the two DC output terminals.

7.3.2 Welding Power Supply

The next application of industrial equipment that uses a large power supply is a welding system. The welding system is specifically for DC arc welding. This means that it will get its supply power from an AC power source that must be converted to DC voltage. Figure 7-16 shows the diagram of the power supply for this welding system.

You can see that the supply voltage in this system is single-phase (two wires) AC voltage, which can be 220, 380, or 440 volts. The main transformer has multiple taps to accommodate each of these supply voltages. All the technician needs to do when supply power is connected during the installation process is to measure the supply voltage and connect the lines to the appropriate terminals on the main transformer.

7.3.3 Uninterruptible Power Supplies

Another popular type of power supply used in industrial application is called an uninterruptible power supply (UPS). The UPS has become important in industrial and commercial power supplies because it provides a means of supplying power to computers and programmable logic controller (PLCs) in application where a power failure cannot be tolerated.

It most parts of the United State, weather conditions such as lighting storms and ice storm may cause the power company to lose power for a period of time. The amount of time the power is disrupted may last from 10 seconds to several hours. If the power outage occurs while a computer or PLC is running, it will cease the system to be restarted, which may take additional time. The UPS combines a power supply with a battery to provide a circuit that can provide output power while the power company’s incoming power is down.

This may sound like a strange way to provide an AC voltage output if AC voltage is the original supply, but in the case of the variable-frequency motor drive, the frequency of the supply voltage will be 50 or 60 Hz and the output AC voltage needs the possibility of frequencies between 1 and 120 Hz. In the case of the UPS, the AC supply voltage needs to be change to DC so it can be stored in a battery for later use of the power supply is interrupted. Since the voltage is changed to DC and is stored in a battery, it must be changed back to AC to be usable. In the UPS, the output frequency will be a constant 60 Hz.

7.4.1 Single-Phase Inverters

7.4 INVERTERS: CHANGING DC VOLTAGE TO AC VOLTAGE

The simplest inverter to understand in the single-phase inverter, which takes a DC input voltage and converters it to single-phase AC voltage. The main components of the inverter can be either four silicon controlled rectifier (SCRs) or for transistors.

7.4.2 Using transistors for a Six-Step InverterFigure 7-20 shows the electrical diagram of an inverter that uses

four transistors instead of four SCRs. Since the transistors can be biased to any voltage between saturation and zero, the waveform of this type of inverter can be more complex to look more like the traditional AC sine wave. The waveform shown in this figure is a six-step AC sine wave. Two of the transistor will be used to produce the top (positive) part of the sine wave, and the remaining two transistors will be used to produce the bottom (negative) part of the sine wave.

7.4.3 Three-Phase Inverter Three-phase inverters are much more efficient for industrial

applications where large amounts of voltage and current are required. The basic circuit and theory of operation are similar to the single-phase transistor inverter.

7.4.4 Variable-Voltage Inverters (VVIs)

A variable-voltage inverter (VVI) is basically a six-step, single-phase or three-phase inverter. The need to vary the amount of voltage to the load became necessary when these inverter circuits were used in AC variable-frequency motor drives and welding circuits. Originally these circuits provide a limit voltage and limited variable-frequency adjustments because oscillators were used to control the biasing circuit. Also many of the early VVI inverters used thyristor technology, which mean that groups of SCRs were used with chopper circuits to create the six-step waveform. After microprocessors became inexpensive and widely used, they were used to control the biasing circuit for transistor-type inverters to give these six-step inverter circuits the ability to adjust the amount of voltage and the frequency through a much wider range.

Motors needed the adjustable frequency to increase or decrease their speeds from their rating that was determined by the number of poles the motor has when it is manufactured. The voltage of the drive needed to be constantly adjusted as the frequency was adjusted so that the motor received a constant ratio of voltage to hertz to keep the torque constant. This became a problem at very low speeds where motors tended to loose torque.

7.4.5 Pulse-Width Modulation Inverters

Another method of providing variable-voltage and variable-frequency control for inverters is to use pulse-width modulation (PWM) control. This type of control uses transistors that are turned on and off at variety of frequencies. This provides a unique waveform that makes multiple square wave cycles that are turned on and off specific times to give the overall appearance of a sine wave. The outline of the waveform actually looks very similar to the six-step inverter signal.

7.4.6 Current-Source Input (CSI) InverterThe current input (CSI) inverter produce a voltage waveform that looks more like an AC sine wave and current waveform that looks similar to the original on/off square wave of the earliest inverters that cycled SCRs on and off in sequence. This type of inverter uses transistors to control the output voltage and current. The on-time and off-time of the transistors are adjusted to create a change in frequency for the inverter. The amplitude of each wave can also be adjusted to change the amount of voltage at the output. This means that the CSI inverter like the previous inverter can adjust voltage and frequency usable in variable-frequency motor drive application or other application that require variable voltage and frequency.

7.4.7 CycloconvertersA cycloconverter is a circuit designed to convert the frequency of

AC voltage directly to another frequency of AC voltage without first converting the voltage to DC voltage.

The history of this circuit dates back to the 1930s when mercury arc rectifiers were used to control the frequency of railroad engines in Germany. The supply voltage for these original circuits was a fixed 50 Hz AC sine wave common in Europe. The train engines used low frequency (16.6 Hz) so their electric motors would turn slowly, creating a tremendous amount of toque. These earliest rectifiers were rather large tube thyristors. The input circuit for the cycloconverter used a large transformer, and the output section used the thyristors to adjust the timing of the output stage, which allowed frequency to be changed.

7.4.8 Applications for InverterInverters are seldom found as state-alone circuits. You will

normally find them used in conjunction with other circuits such as rectifiers and filter circuit filter is power supplies that will provide a source for the DC voltage the inverters needs. You may also find the inverter as in integral part of the DC-to-DC converter circuitry used in many types of DC power supplies. The major of inverters in industry today is for variable-frequency AC motor drives and high-frequency power supplies for welding application.

7.5 DC-TO-DC CONTROL (CONVERTERS AND CHOPPERS)

7.5.1 Overview of DC-to-DC Voltage Conversion

Today DC-to-DC voltage conversion is more widely used in power supply circuits because every piece of equipment that has an electronic board in it requires a wide variety of DC voltage supplies. Each voltage must be supplied from a power supply. This means that the computers, PLCs, and all other electronic equipment require to DC power supply. This means that computers, PLCs, and all other electronic equipment require a DC power supply.

7.5.2 Linear Power SuppliesLinear power supplies have been popular since the beginning of

vacuum-tube electronics. Their operation is simple, but their efficiency is quite poor in the range 30% to 40%. Figure 7-25 shows the electronic diagram for a typical linear power supply. From this diagram you can see that the first part of the power supply is exactly like the rectifier section presented earlier in this chapter.

7.5.3 Switching Power Supplies

Switch-mode power supplies (SMPS), also called switching power supplies, have become more popular than linear power supplies in the past ten years because they provide a regulated voltage with more efficiency and they not require the larger transformers and filtering devices that the linear power supplies require.

7.5.4 The Buck Converter The buck converter circuit is the basis for several other similar circuits called forward converters. The buck converter circuit and the input and output voltages for this circuit are shows in figure 7-27. This circuit would be connected directly after the power transformer.

7.5.5 The Boost RegulatorThe boost regulator is a second type of fundamental regulator

circuit for the switch-mode power supply. The electronic diagram and waveform for this type of converter are shown in figure 7-28.

7.5.6 The buck-Boost Regulator

A combination of the buck regulator and boost regulator is shown in figure 7-29. This combination circuit is called buck-boost regulator and it utilized the strong point of both of the previous regulators.

7.5.7 The Forward ConverterThe forward converter is basically a buck converter with a

transformer and a second diode added to allow energy to be delivered directly to the output through the indicator during the transistor on-time. Figure 7-30 shows the electronic diagram and waveform for the forward converter. In this diagram you can see that the transistor is connected in series with the primary of the additional transformer. The second transformer provides a phase shift that causes the polarity of its voltage to be such that it will flow to the output while the transistor is in conduction.

7.5.8 The Push-Pull ConverterAs the switch-mode power supply has evolved , additional

adjustments to the original circuits have been made to get more power from smaller components. This means that the efficiency for the system must be increased. One simple way to do is to use a center-tapped transformer that utilizes both the top and bottom half cycle.

7.5.9 The Half-Bridge Converter

Ones of the problem with the push-pull converter is that the flux in the two section of the center tapped transformer primary and secondary windings can become unbalanced and cause heating problems. Another problem is that each transistor must block twice the amount of voltage that other converter. The half-bridge converter provides several advantages over the push-pull converter. This circuit still uses two transistors and two sets of diodes like the push-pull circuit. The main different of the half-bridge converter is that it utilizes two large bulk capacitors (C1 and C2). These capacitors are connected so that each one in series with one of the transistors. That means that power can be transferred to the output during to on time for each transistor, which increases efficiencies to the 90% range.

7.5.10 The Full-Bridge ConverterThe full-bridge converter adds two additional transistors to the half-bridge converter. This means that four transistors are available to provide power to the output section, so this type of converter is used in power is excess of 1000 W.

7.6 WHAT YOU MAY FIND WHEN YOU WORK ON A POWER SUPPLY

When you are asked to work on a system that uses a power supply, you should remember that the power supply is require to provide one or more of the following function: Ac-to-DC voltage conversion, AC-to-DC to AC voltage conversion with battery backup, voltage conversion with a variety of Dc voltage available, or AC voltage conversion with variable frequency. The equipment and circuit for each of these functions will be similar in that they will have converter circuits that include variety of single-phase or three-phase rectifiers, and they may have inverter circuits that provide DC- to AC voltage conversion.

TERIMA KASIH