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Introduction
To understand the operating principles of an electronic drive, it is necessary to understand the basic principles of electricity, electronics, and mechanical devices. In this section, we will review the required basic concepts, and relate them to AC- and DC-drive systems. This section is by no means a complete digest of electronics and mechanics but will provide for a baseline of understanding. Consult the Appendix when more information is needed. This will be a review for some readers and a basic introduction into the electrical and mechanical world for others.
Electrical Principles
Resistance, Voltage, and Current
Electricity comes in two forms: alternating current (AC) and direct current (DC). We will first consider the effects of DC on various electrical components and identify the three main characteristics of any electrical circuit.
FIG. 1 illustrates the basic atomic structure-where it all begins.
When we take a close look at nature, we find that all matter is composed of atoms. In the basic structure of an atom, we find that the nucleus is at the center, surrounded by one or more orbiting electrons. This structure is replicated many times for any material. If the material is an insulator, the orbiting electrons do not move from place to place or from atom to atom.
For the purposes of discussion, we will consider a conductor as an atom with three or less orbiting electrons in the outer shell. An atom with five or more orbiting electrons will be considered an insulator. An atom with four orbiting electrons is considered a semiconductor and will be discussed later in this section.
Electrons in the outer orbiting rings find it easy to move from atom to atom whenever they are forced to do so. The force that tends to move electrons is called voltage (electrical pressure in a circuit). Voltage is basically the force that causes electrons to travel from atom to atom. As you would expect, the higher the voltage, the more force that is available to move electrons. Some textbooks use the term electromotive force (EMF) when describing voltage. FIG. 2 shows how voltage "forces" electrons to move from atom to atom.
FIG. 1. Basic atomic structure
As shown in FIG. 2, electrons move from atom to atom to take up a spot vacated by the previous electron. Electrons flow in an orderly manner through a conductor. A typical comparison is to compare water flow in a pipe with that of electron flow in a conductor (FIG. 3).
FIG. 2. The movement of electrons in a conductor
FIG. 3. Water flow in a pipe vs. electron flow in a conductor
When you turn on a water faucet, a certain amount of water pressure forces water through the pipe and out the end of the faucet. The exact same phenomena holds true for electricity.
When you turn on a light, you allow voltage (force) to push electrons (current) through the wire and cause the light to illuminate. At this point, the obvious question is, Why is it necessary to move electrons in the first place? The reason is fundamental: every electrical user (light bulb, TV, toaster, etc.) has resistance, measured in ohms (?). The user of electricity is called an electrical load. FIG. 4 shows a simple fluid and electrical circuit and illustrates the relationship of voltage, current, and resistance.
As seen in the figure, a simple fluid circuit consists of a pump to supply the source of water pressure. The water fountain is considered the load. The pipes provide the path for the water to flow and also provide a certain amount of resistance to flow.
A simple electrical circuit consists of a source of electrons (battery), a load (light bulb), and conductors (wires) to complete the circuit.
Several devices have been added to measure what is happening in the fluid and electrical circuits. In the fluid circuit, a flow meter is measuring how many gallons per minute are being pumped. A pressure meter is also used to measure water pressure is in the system.
FIG. 4. Fluid circuit vs. a simple electrical circuit
In the electrical circuit, an ammeter is used to measure the rate of electron flow (ampere flow per second or how many electrons are used). A voltmeter is used to measure the electrical pressure available in the circuit. The basic principle is that it takes voltage (electrical pressure) to force current (electrons) to overcome resistance. Essentially, the more the restriction in the water nozzle of the fountain, the more water pressure is needed to overcome the resistance. Similarly, the more electrical resistance in the circuit, the more electrical pressure (voltage) is needed to overcome the resistance and light the bulb.
With this general background, we will now look at a DC voltage waveform and review magnetic properties in a circuit. Later in this section, we will review the principles of alternating current.
FIG. 5 shows a DC waveform from the battery shown in FIG. 4.
FIG. 5. DC and a simple circuit
DC Waveform
A typical means of demonstrating the characteristics of a circuit is by showing voltage or current versus time. In reviewing the circuit in FIG. 4, we find that voltage would be flowing continuously unless there is a way of breaking the circuit. The pushbutton switch in FIG. 5 allows the circuit to be broken when the button is not pressed.
As we see in FIG. 5, whenever the switch is closed, the voltage rises to a maximum level. The voltage remains constant until the battery loses its ability to supply the rated level (e.g., 6 volts, 12 volts). Notice that the horizontal line with seconds indication is actually labeled negative. This point could also be labeled as 0, but in electrical terms, the zero point is more negative than positive. The positive point gives us a reference point.
Therefore, if one point in a circuit is positive, then the other reference point will be negative.
As seen in the figure, electrons flow out of the negative terminal of the battery, through the load, and back to the positive terminal of the battery.
As we have previously seen, this electron flow is due to the electrical pres sure (voltage) that is present in the circuit. It should be noted that in a DC circuit, electrons flow in only one direction.
Magnetic Flux
A magnet is a material that attracts pieces of iron or items made of iron.
FIG. 6 is a common figure shown in many science books and shows the relationship between the north and south poles of a magnet, and the magnetic field, flux.
FIG. 6. Magnetic poles and flux
The invisible lines of force (flux) travel from the north to south poles of a magnet. We can also assume that the basic principles of magnetism apply: north poles attract south poles and south poles attract north poles. It should also be noted that a material such as iron tends to concentrate or conduct the magnetic flux. Magnetic flux can pass through most materials, even those not having magnetic properties (e.g., plastic, rubber, or glass). But as seen in FIG. 6, iron tends to strengthen the effects of magnetic flux. Typically, the more iron in a material, the better its magnetic attracting capabilities.
Electromagnetism
Electromagnetism is the production of a magnetic field though the use of a voltage and a coil. When an electric current passes through a conductor, a magnetic field is created around the conductor. FIG. 7 shows the principle of magnetic field created current in a conductor.
FIG. 7. Magnetic field generation in a conductor
The magnetic strength can be increased by adding a material such as iron.
A common practice is to manufacture electromagnetic products with some type of iron core around or inside the coil. It should be noted that changing the direction of current flow also changes the direction of the magnetic field. In addition, the direction of current flow in a coil dictates which end is north and which is south.
There are several important characteristics that are worth mentioning at this point:
A scientist named Michael Faraday discovered that a voltage was generated by motion between a magnetic field and a conductor. This voltage was called induced voltage because there was no physical contact between the conductor and the device generating the magnetic field. Faraday discovered that the faster the motion between the conductor and the magnetic field, the stronger the induced voltage. Today, we know this characteristic of induced voltage as Faraday's Law. Induced voltage due to magnetic flux crossing the path of a conductor is one of the key reasons why motors develop torque. The generation of rotor flux is a result of the rotor conductors cutting though stator flux. This characteristic is vitally important to the motor's ability to generate torque.
Lenz's Law indicates that a phenomenon occurs whereby an induced electromotive force (EMF) is caused in a direction as to oppose the effect that is causing it. That is, since the stator contains many interconnected inductors, EMF tries to keep the stator magnetic field at a constant state. (The inductor opposes changes or a drop in current. It also opposes the collapsing of the magnetic flux due to AC being applied.) This characteristic is of prime consideration when design engineers consider motor performance.
Another characteristic of magnetic fields and conductors lies with how the magnetic field is produced and which direction mechanical movement develops. A discovery called Fleming's Right Hand Rule indicates which direction a conductor (rotor) will move in relation to current flow and magnetic flux direction (stator flux direction). The right middle finger points in the direction of current flow in the conductor. The index finger points in the direction of the magnetic field. The thumb points in the direction the conduct will move, given the above flux and current flow direction.
Magnetic flux moves from the north to the south pole. If conventional current flow is the method of tracing current, then current travels from positive to negative. With this in mind, we can predict the direction of rotor movement, with an induced current and magnetic field in the stator.
Thus it is possible to predict the direction of rotation, given the wiring characteristics of the motor poles.
Note: The above "Right Hand Rule" is valid if the reader accepts current flow as conventional flow-from positive to negative. If electron flow is accepted by the reader, then current flow is from negative to positive, with the rule now being considered the Left Hand Rule. (The same finger designations apply.) We have now reviewed the basic principles of current, resistance, voltage, and magnetic fields. To understand how these principles apply to drive technology and rotating machines, we must review the other type of cur rent available in our power system-alternating current.
Alternating Current
As stated earlier, current flow has been described as electron flow through a conductor. You will recall the fact that DC (direct current) flows only in one direction (stated as electron flow from minus (-) to plus (+). The difference between DC and AC is that AC flows in one direction, reverses, and then flows in the opposite direction. In essence, the flow of electrons continuously changes direction. FIG. 8 shows this principle.
FIG. 8. AC vs. DC waveforms
As seen earlier in this section, DC rises to a specific maximum value and stays at that point, until the power source is removed or drops to zero (e.g., the battery goes dead). AC, on the other hand, goes positive to a maximum level, then to zero, and then to maximum level in the negative direction, and then back to zero.
Useful work is accomplished by AC waveforms as well as DC. In AC, you may think that the work done during the positive half of the waveform is erased by the negative half. However, electrons have the same effects on a load, no matter which direction they flow. Therefore, useful work is accomplished during both the positive and negative halves of the wave form.
Current and Voltage Waveforms (AC)
We are now at the point where we need to review the concepts of voltage and current. When showing AC waveforms, many times you will see both voltage and current shown on the same graph. The graph will be similar to the one in FIG. 9.
As seen in FIG. 9, the voltage waveform rises to a higher maximum positive and negative value than the current waveform. Also, both the voltage and current waveforms cross the zero point at the same time.
These waveforms are considered to be in-phase.
Single- and Three-Phase AC
Alternating current is divided into two forms: one-phase (1f) and three phase (3f). Single phase is a series of continuous single AC waveforms.
This type of AC consists of one voltage and current component that crosses the zero point at the same time (FIG. 10).
FIG. 9. AC voltage and current waveforms
FIG. 10. Single-phase AC waveform
Three-phase AC is a series of continuous overlapping single-phase AC waveforms. Essentially, three phase is single-phase waves that are offset from each other by 1/3 of a cycle (FIG. 11).
FIG. 11. Three-phase AC waveform
The reader is probably most familiar with single-phase power, since that is the type that is typical in residential environments to operate toasters, TVs, VCRs, etc. Single phase is most recognizable by two wires and a safety ground (three conductors), with typical voltages of 120 and 240 V.
Three phase is typically used in industrial environments to operate drill presses, packaging lines, machining centers, etc. Three phase is most recognizable by three wires and a safety ground (four conductors), with typical voltages of 240 and 460 V.
Commercial and industrial electrical users mainly use three phase in their processes. Three phase is more efficient to generate and use (less current or amperes draw per horsepower). Three phase is also fairly easy to transmit, though the initial wiring and installation cost is greater than single phase. In a simplistic explanation, three phase would have three times the average usable power, compared with single phase (three positive and negative half waveforms per unit of time). The drawback of using three phase for residential power is the initial installation cost of power and the availability of three-phase consumer electronic equipment. The most energy savings is achieved when rotating machinery of significant horse power is used in a multitude of industrial processes.
Frequency
A familiar term listed on any electrical device nameplate is hertz (Hz). In the United States, alternating current changes direction 60 times per sec ond-60 Hz. In European countries, AC changes 50 times per second, which translates to 50 Hz. Essentially, the AC waveforms in Figures 2-8 and 2-9 would indicate 1/60 of a second. Sixty of those waveforms would occur every second in U.S. power systems.
As you might expect, electrical equipment that is designed for 50-Hz operation may not effectively operate in the United States on 60-Hz power and vice versa. However, many of the AC- and DC-drive products of today are designed for dual frequency (Hz) operation.
In electrical generation terms, one complete rotation of the generator shaft is 360º. In single-phase generation, one complete cycle is 360 electrical degrees. This one cycle also equals ~16 ms of time (0.016 s). Three-phase waveforms are therefore 120º out of phase with each other-1/3 of 360º.
In later sections, we will discuss the effects of frequency on AC motor speed. A motor designed for, or operated at, 50 Hz will run at a slower speed than if operated at 60 Hz (e.g., 1500 rpm or 1750 rpm, respectively).
Capacitance and Inductance
Capacitance
All electrical circuits have a certain amount of resistance. As we have learned, resistance is an opposition to current flow. Resistance has primarily the same effects on an AC or DC circuit. Capacitance, on the other hand, is the ability to block DC, but appears to allow AC to flow in a circuit. The device that accomplishes this task is known as the capacitor and is shown in FIG. 12.
FIG. 12. Capacitors and their effects on a circuit
If the power applied to a capacitor is DC, then the capacitor tends to charge to whatever voltage is applied. It should also be noted that the DC voltage level remains on a capacitor for a period of time (up to several hours or days for some large capacitor values-50 µF or higher). The capacitor will slowly discharge into the atmosphere over time. It will discharge rapidly when connected to a load, such as a resistor that will quickly absorb the energy.
If the power applied to the capacitor is AC, it appears that AC is flowing through. In reality, the capacitor charges and discharges so rapidly that it is common practice to refer to AC as "flowing through a capacitor." Drive manufacturers install what are called bleeder resistors across large capacitor circuits to bring the voltage down to a safe level after power down (e.g., discharge 680 VDC down to less than 50 VDC in 1 minute). The ability of a capacitor to store and discharge energy allows improvement in DC drive output voltage regulation (consistency).
In an AC drive, this charging effect also comes in quite handy. The capacitor circuit charges and discharges, keeping the flow of voltage constant and improving the quality of the AC output waveform.
The main purpose of a capacitor is to oppose any change in voltage. As expected, the more capacitance in a circuit, the longer the time required for charging and discharging to occur. FIG. 13 shows the effects of higher capacitance on a rectified AC waveform.
FIG. 13. Effects of capacitance on an AC waveform
Note: For illustration purposes, a half-wave rectifier waveform is shown in FIG. 13. A typical AC drive rectifier output would be "full-wave," which would double the number of positive half waves and increase the DC voltage output.
Inductance
Inductance is the ability to block AC but allow DC to flow in a circuit. The device that accomplishes this, is known as an inductor and is shown electrically in FIG. 14.
FIG. 14. Inductance and the effects on a circuit
As shown earlier, an inductor produces a definite polarity when connected to DC voltage. The inductor will be an electromagnet with a specific north and south pole.
The main purpose of an inductor is to oppose any change in current. As you recall, any coil of wire will generate a magnetic field. The inductor works by controlling the expanding and collapsing of the magnetic field.
When there is a presence of DC voltage, the magnetic field expands. When DC voltage is removed, the magnetic field collapses and creates a surge of energy. It would not be uncommon for an inductor to produce a short burst of 70 volts, after removal from a 6 volt battery. With this principle in mind, it is easy to see why there is an electrical arc at the contacts of any circuit, whenever a voltage is removed from an inductor.
The magnetic field strength is stronger, with larger amounts of inductance, commonly referred to as henries. Typical values would be µh (microhenries) or mh (millihenries). FIG. 15 shows the effects of higher inductance on an AC waveform.
FIG. 15. Effects of inductance on an AC waveform
As to be expected, the more inductance in a circuit, the more time that is needed to expand and contract the magnetic field around the inductor. In addition, inductors have higher amounts of resistance to AC voltage as compared with DC. An inductor may only have 15 ? of resistance to DC, but 1000-ohm resistance to AC. AC resistance is call impedance and is signified by the letter Z.
Inductors are used in the DC bus
Bus circuit of some AC drives to reduce the amount of AC voltage in that circuit. This tends to "purify" the DC, which in turn provides a cleaner output waveform from the drive.
Because of the process of reducing or blocking AC, inductors are some times called chokes.
Power Factor
By strict definition, power factor is a measure of the time phase difference between voltage and current in an AC circuit. When an inductor is used in an AC power system, the current waveform tends to lag behind the volt age waveform. FIG. 16 shows in-phase and out-of-phase voltage and current waveforms.
FIG. 16. Power factor-voltage and current waveforms
In a purely resistive electrical circuit, the voltage and current waveforms would be synchronized or in-phase. In-phase voltage and current has a unity power factor of 1.0. Unity power is transmitted to customers by the utility. However, inductive loads such as motors cause the current to lag behind the voltage waveform. Once this occurs, the current being consumed is out-of-phase with the voltage waveform.
The power factor is calculated by taking the ratio of true power divided by the apparent power in a circuit. True power is the actual power converted to another form of energy by a circuit, and is expressed in watts (W). Apparent power is the power delivered to an AC circuit and is usually expressed in kilovoltamperes (KVA).
Apparent power is the power obtained by taking volts × amperes. FIG. 17 shows the formula for power factor and unity versus 50% power factor.
FIG. 17. Power factor calculation
The deviation between the voltage and current waveform is called the phase angle or displacement angle. If voltage and current were 90º out-of phase, the result would be a power factor of zero.
The utility generates power that is 100% or unity. If the operating equipment in a factory, such as AC motors, causes less than 100% power factor, the factory will be assessed a penalty by the utility. Essentially, the factory uses devices that cause current to lag voltage, meaning that the factory is wasting energy generated by the utility. The utility must generate more energy to make up for the energy wasted by the factory. If we use this analogy, the graph to the right in FIG. 17 would indicate wasting 50% of the utility's energy.
In recent years, the utilities have promoted the use of high-efficiency motors. A high-efficiency motor wastes less energy. Typically, high efficiency motors have a higher power factor compared with motors of standard efficiency. However, manufacturers have to make trade-offs between high efficiency and high power factor because of magnetic and electrical characteristics of the motor.
Utilities require customers to correct for poor power factor, or at the very least, assess a penalty for inadequate power factor ratings. The power factor of motors can be improved by installing devices such as power factor correction capacitors. Capacitance counteracts the effects of inductance.
With capacitors used in connection with AC motors, the results are a higher power factor and less waste of power. The voltage and current waveform are approaching the in-phase power generated by the utility.
Electrical/Electronic Devices
Introduction
In this section, we will review the major components that comprise an AC- or DC-drive unit. Drive units are built using components in addition to those discussed here.
Inductors, Relays, Transformers and Contactors
Inductors
As discussed in an earlier section, inductance is the ability to block AC and allow DC to flow in a circuit. The main device that accomplishes this phenomenon is the inductor with the electrical symbol shown in FIG. 18.
FIG. 18. Inductor electrical symbol
As seen earlier, an inductor is basically an iron core, with a coil of wire wrapped around it. Inductors are found in many sizes and, as in resistors, the higher the value of an inductor, the greater the opposition to AC flow.
Inductors are rated in units called henries, with typical values in millihenries (mH) due to the large unit of measure. (Note: a millihenry is 1/1000 of a henry.) Inductors are found in various types of drives, both AC and DC, and will be covered in a later section. When inductors are placed in front of drives, they act as a protective device. In the discussion of electromagnetism, it was stated that Lenz's Law applies to a conductor moving through a magnetic field. An EMF is generated that tries to oppose change in the induced voltage that created it. An inductor would therefore oppose any surge (instantaneous energy increase) if it would occur on the input of a drive.
As shown in FIG. 19, the three-phase inductor reduces any surge cur rent that could enter the input section of the drive.
FIG. 19. AC inductor (line reactor)
DC drives use power components called thyristors (SCRs- Silicon Con trolled Rectifiers). If an output short to ground condition exists, tremendous inrush current into the drive would result. This inrush current would result in severe damage to the drive's power conversion and logic sections.
When inductors are placed ahead of the drive unit, the input and protective sections of the drive see a limit and delay of short-circuit current. The delay allows time for incoming line fuses to clear and protect the expensive power components.
Relays
Relays are considered an electromagnetic device. Basically, a magnetic field is used to energize a coil, which in turn causes an electrical contact to close. Relays consist of two separate circuits-a control circuit and a power circuit. The control circuit contains the magnetic coil and can be operated by DC or AC voltage, depending on the coil make-up. The power circuit is where the electrical connection is made that allows current to flow. Power circuits can make single contact (SPST-single pole single throw) or are multi-contact (e.g., DPDT-double pole double throw). Power circuits are sometimes considered Form C because of the nature of their operation, which look like the letter C. They are also considered "dry" contacts, since they do not source voltage themselves (FIG. 20).
FIG. 20a shows the relay is energized. FIG. 20b shows the relay power circuit when "de-energized."
FIG. 20. Relay symbol and electrical connections
As shown, the relay coil is operated from a 120-VAC power source. Typical relay coil voltages are 120 VAC, 240 VAC, 24 VDC, and 12 VDC. The power circuit contacts are typically rated in both DC and AC voltages and currents. The relay may indicate a maximum switching voltage of 300 VDC/250 VAC. It would also indicate a maximum switching current rating (e.g., 8A @ 24 VDC, 0.4A @ 250 VDC, or 2000 VA @ 250 VAC).
As the power circuit contacts open, a small amount of arcing occurs. To help preserve and increase the life of the contacts, some type of noise suppression is normally used. A MOV (metal oxide varistor) or RC (resistor/ capacitor) network is installed in series with the incoming power circuit.
As shown in FIG. 20, suppression would be installed if the incoming power circuit is controlling the coil of a main contactor (discussed in the next section).
Contactors
FIG. 21. Contactor control and power circuits
Contactors are essentially higher power relays. They use the same two circuits that a relay uses: the control circuit and power circuit. It is a common practice to indicate the contactor control circuit (coil) separate from the power circuit. FIG. 21 shows the contactor coil symbol and the separate power circuit.
As with relays, the coil (control circuit) is designed for various voltages.
Coil voltages of 115 VAC, 230 VAC, and 460 VAC, single phase are common because of their easy connection to incoming line power. The power circuit is designed to handle DC or AC voltage, at a specific current rating.
In the case of DC drives, a main DC Loop contactor is often used to energize the armature circuit of the DC motor. When control-coil voltage is removed, a positive disconnect is accomplished, completely disengaging DC voltage from the motor.
Transformers
Transformers are essentially two inductors separated by an iron core. The iron core increases the electromagnetic characteristics and efficiency of the transformer.
The transformer transfers energy from the primary side (input) to the secondary side (output). FIG. 22 shows the construction of a transformer and its symbol.
FIG. 22. Transformer and symbol
There is no physical connection between the coils. Therefore, the trans former has an inherent capability of isolating the primary side from the secondary side. A transformer that merely transfers the same voltage from the primary to the secondary (e.g., 460 VAC to 460 VAC) is termed an isolation transformer. Because of the design, the transformer isolates the secondary from the primary, in the event of a short circuit or line interference. In this way, a shock hazard will not exist-protecting the equipment and personnel.
Another feature of the isolation transformer is its ability to step up or step down voltage. For example, common three-phase industrial voltages are 240 VAC or 460 VAC. When an isolation transformer is used, the secondary can step down voltages to 208 VAC or 415 VAC.
One note of importance should be stated here. If voltage is stepped down, the available current is stepped up. For example, if the input voltage is stepped down two times, then the output current will be stepped up two times. The reverse is also true. If the voltage is stepped up two times, then the output current will be stepped down two times. This is because of the fact that power in = power out minus any losses.
The transformer is a very efficient device, with losses due only to heat.
Note: P = I × E, where power (watts), I = intensity of current, and E = electromotive force (voltage). If power in = power out, then input I × E = output I × E.
If the voltage is doubled in the output, then the current must be half, in order for the equation to be equal.
Transformers are rated in kVA (kilovolt amps). A transformer with a large kVA or current rating (200 kVA) will be physically larger than one of a smaller rating (2.0 kVA). For example, on a 150-HP, 460-VAC DC drive, a 175-kVA input isolation transformer would be required. A 7-1/2 HP DC drive at 460 VAC would require only an 11 kVA input isolation trans former.
Current Transformers
Another type of transformer used in conjunction with drive technology is termed a current transformer. Basically, this transformer is a sensing circuit that identifies the amount of current flowing through an incoming power line. The basic characteristics of a transformer apply. Current flowing through a conductor induces a corresponding current into the coil around the conductor. FIG. 23 shows a current transformer (CT) used to sense input current to an AC drive.
If an extremely high amount of current is detected by the CT and corresponding protective circuitry, the drive is shut down to protect itself and the motor. Drive diagnostics and protective features will be explored in the AC- and DC-drive sections.
FIG. 23. Current transformer use
Capacitors
As discussed in an earlier section, capacitance is the ability to block DC and allow AC to flow in a circuit. The main device that accomplishes this phenomenon is the capacitor with the electrical symbol shown in FIG. 24.
FIG. 24. Capacitor symbol and use
As seen earlier, a capacitor is basically two plates of conducting material, separated by an insulator. The insulator is usually plastic, mica, or even air, and is called a dielectric. Capacitors are found in many sizes and just as in resistors, the higher the value of a capacitor, the greater the opposition to DC flow. Capacitors are rated in units called farads, with typical values in microfarads (µF) due to the large unit of measure. (Note: a µF is 1/1,000,000 of a farad.)
Capacitors also have another rating-working voltage. WVDC (Working Voltage DC) is the maximum voltage a capacitor can handle without being damaged. In drive power circuits, 800 or 1000 WVDC are typical values.
As mentioned earlier, one of the main uses of capacitors in drive circuitry is to filter out AC waveforms in a DC circuit. FIG. 24 shows this function, using an electrolytic capacitor in the DC bus circuit. The capacitor's charging and discharging capability make it a perfect candidate for smoothing out the DC waveform, which in turn, creates a purer output voltage waveform.
Semiconductors
To understand the theory of regulation and power conversion in adjust able speed drives, it is essential that semiconductor basics be discussed.
Semiconductors are poor conductors under normal conditions. However, once impurity elements are added to semiconductor materials, their atomic structure changes. Semiconductor materials can be P (positive) or N (negative), depending on the impurity added at the time of manufacture.
Silicon or germanium provide the base material for electronic components such as SCRs (silicon-controlled rectifiers), GTOs (gate turn-off devices), and ICs (integrated circuits). They also form the base for many of the recently developed logic components such as ASICs (application-specific integrated circuits) and DSPs (digital signal processors).
In an earlier section of this section we discussed basic electrical principles, with electrons flowing from negative to positive. This is a common description when discussing the basics of electricity. However, because of the nature of semiconductor circuits, it is conventional to trace current flow from positive to negative. This is termed conventional current flow and will allow for easier understanding of circuit operation. Therefore, for the remainder of this guide, we will consider the current flowing in a drive circuit from positive to negative.
A continuing concept throughout discussions of electronics is that electrons tend to seek a point of zero potential. This is similar to the concept that water will tend to seek its lowest point. Voltage will cause current to flow from the highest potential (supply) down to the lowest potential (ground). Voltage will continue to force current flow to ground until no difference of potential exists or until the supply is cut off.
Diodes
Diodes may be considered the simplest semiconductor device. As shown in FIG. 25, current flow follows in the direction of the triangle.
FIG. 25. Diode symbol and circuit operation
As shown in the figure, current flow would be from positive to negative. A diode allows current flow only in one direction, much like a check valve would allow water to flow in only one direction. If the direction of current flow is from negative to positive, no current will flow. This condition would exist when the power source is AC, which changes direction 60 times/second. A diode has the effect of cutting off the bottom portion of the AC waveform. In doing so, a diode can basically change AC to DC, as shown in FIG. 26. The circuit on the right (FIG. 26) is commonly referred to as a switch mode power supply.
FIG. 26. Changing AC to DC, using diodes
Many modern three-phase AC drives use a six-diode bridge rectifier circuit that changes AC to DC, which is supplied to the DC bus circuit.
This type of rectifier circuit is the most common in use by today's drive manufacturers and is shown in FIG. 27.
FIG. 27. Three-phase diode bridge rectifier circuit
This diode bridge circuit works continuously to change AC to DC. There is no control of the output voltage value. As shown in the FIG. 27, a 460-VAC 3-phase input will yield a 650-VDC output (line voltage × 1.414).
If the drive input voltage is 230 VAC, the diode bridge output would be half (e.g., 325 VDC).
Thyristors (SCRs-Silicon-Controlled Rectifiers)
Diodes provide continuous, fixed-voltage output. However, there are many times when a variable-voltage output is needed. When this is the case, SCRs are typically used. SCRs are essentially two diodes connected back-to-back, with a controlling element called a gate. FIG. 28 shows the schematic symbol for an SCR.
When the gate voltage is positive, with respect to the cathode, current is allowed to flow from the anode (+) to the cathode (-). This operation is commonly referred to as gating on the SCR or in other terms, firing the SCR, full-on. The circuitry associated with SCR gating is called a pulse generator.
Current flow is limited only by the resistance of the circuit, external to the SCR. As with a diode, an SCR does not allow current to flow in the reverse direction (from cathode to anode).
The SCR is made up of positive (P) and negative (N) junctions-actually two P-N junctions back-to-back. When no gate voltage is applied, at least one of the P-N junctions is reversed biased (polarity in the opposite direction). When a junction is reversed biased, no current will flow. When a signal is applied to the gate, both P-N junctions are forward biased (polarity in the direction of current flow). Current then flows in the forward direction.
When the positive gate signal is removed, the SCR still remains in a con ducting state. The SCR can be "turned off" by cutting off the flow of for ward current, by applying a negative polarity to the gate, or by reversing the polarity of the voltage on the anode. When polarity to the SCR is reversed, the SCR will block current flow from the anode to the cathode- basically shut off.
The ability of the SCR to handle high voltages, its small size, and adequate switching speed make it a prime candidate for use in DC as well as AC drives. Switching speed is the ability to quickly turn on and off the device.
SCRs can be "turned on" in 1 to 5 µs. However, it may take as much as 100 µs to turn off the SCR.
FIG. 28. Silicon-controlled rectifier (SCR)
The basic purpose of the SCR is to control current flow. FIG. 29 shows how that is accomplished.
FIG. 29. Gating of SCR's
The amount of current output from the SCR depends on when the device is gated on. If the SCR is gated on early in the AC cycle, a higher average output voltage is seen, compared with a late gating during the AC cycle.
Many DC drives use SCRs in the power section. The basic purpose of a DC drive is to generate DC output from an AC input. The DC output must be variable to allow the DC motor speed to change.
GTOs (Gate Turn-Off) Devices
A GTO is basically an SCR with a bit more capability. The GTO is a switching device that provides the latching function (turn-on) capability of an SCR and the controlling function (self turn-off) of a transistor. Turning a GTO on and off is accomplished by simply changing the polarity of the gate current. The GTOs switching time and high current capability is very similar to that of the normal SCR. The GTO features a good short-circuit overload rating, when used in conjunction with overload fuses. Figure 2 30 shows the schematic symbol for a GTO.
FIG. 30. GTO (gate turn off) device
GTOs have been successfully applied in AC-drive technology for many years. GTOs can withstand high voltages, between 600-800 V, and have high current capability. Some drive manufacturers still use the GTO for horsepower ranging from 5-1500 HP. The GTO has the ability to be switched on with a positive gate pulse and off with a negative gate pulse without the use of power commutating (switch-off) circuits. A disadvantage of GTOs over SCRs is that GTOs have two to three times the power loss because of the voltage drop across the device.
Transistor
A transistor is a connection of three sections of positive and negative semi conductor material. The P and N materials are connected so that one of the materials are joined or sandwiched between the other two sections.
There are two basis styles of transistors-NPN and PNP. The style is deter mined by which material is in the middle, between the other two materials. All transistors contain three leads: emitter, base, and collector. FIG. 31 shows the two basic styles of transistors and the schematic symbols for each.
FIG. 31. Transistor construction and symbols
How the transistor is biased will determine the amount of current flow.
The majority of the current flows through the emitter and base to the collector junction. Very little current (2-8%) flows through the base itself. In a transistor, the voltage applied to the base-emitter junction controls the amount of current in the collector. In this way, we can use the transistor as a switching device-allowing large amounts of current output to flow, with only a small amount of controlling voltage applied to the base. When the control current is removed from the base circuit, the transistor stops conducting and turns off.
Darlington Bipolar Transistors
A transistor, is a true switch-it can be turned on and turned off by use of a command signal. The power Darlington transistors are essentially two or more bipolar transistors, internally connected in one package. FIG. 32 shows the schematic symbol for the power Darlington transistor.
FIG. 32. Power Darlington transistor
The power Darlington transistor has the advantages of the bipolar transistor, but provides high gain circuitry, much higher than a normal bipolar transistor. The bipolar and power Darlington transistors both have an advantage over SCR and GTO devices. Circuitry that needed to turn on the transistors are much less intensive than that of SCRs and GTOs. In many cases, base driver circuits can be included on existing drive circuit cards, rather than require separate, more complex cards.