Electrical Drives and Controls
Electrical Drives and Controls
An electric motor is a device which converts electrical energy to mechanical energy. It can also be viewed as a device which transfers energy from an electrical source to a mechanical load. The system in which the motor is located and makes it spin is called the drive, and is also referred to as the electric drive or motor drive. In general, the device which controls the motor is called a drive. Drives designed for electric motors are also called electric drives.
Motor control is needed in large number of industrial applications such as transportation systems, rolling mills, machine tools, fans, and pumps etc. Systems used for motion control are called drives and can use any of the prime movers. The electric drive system involves controlling electrical motors in the steady state and in dynamic operations, taking into account the characteristics of the mechanical loads and behaviours of power electronic converters.
Electric drives have become popular because of their simplicity, reliability, cleanliness, easiness, and smooth control. Both AC (alternating current) and DC (direct current) motors are used as electric drives. However, the AC system is preferred since (i) it is cheaper, (ii) it can be easily transmitted with low-line losses, (iii) it can be easy to maintain the voltage at consumer premises within prescribed limits, and (iv) it is possible to increase or decrease the voltage without appreciable loss of power. In spite of the advantages of AC motor, sometimes DC motor is used since (i) in some processes, such as electro-chemical and battery charging, DC is the only type of power which is suitable, (ii) the speed control of DC motors is easy rather than AC motors, hence, for variable speed applications, the DC motors are preferred, and (iii) DC series motor is suited for traction work because of high starting torque.
Basically, an electric variable speed drive is a device which controls the speed of a motor by varying the magnitude of one of its controllable variables such as voltage, current, or frequency. Clearly, the technique used to vary the speed largely depends on the load type of the drive. The function of the motor drive is to draw electrical energy from the electrical source and supply it to the motor in such a state, so that the desired mechanical output is achieved. Typically, this is the speed of the motor, torque, and the position of the motor shaft.
Diverse applications of electrical motors in industry need speed control capabilities, not infrequently in a broad range. Present-day motor drives used in industry can reach considerable rated powers, with several megawatts (MW) or more. This makes it even more important to use solutions which allows a person to precisely control the speed of the system and to properly manage the industrial process.
Electric drives are used to control the speed of motor which can be AC motor or DC motor. Several applications need variable speed motors in the industry. Hence, drives have become an integral part of the industry. Electric drive can control both voltage and frequency input to the motor. If only voltage input to the motor is controlled by drive, then speed of motor is controlled. If both voltage and frequency inputs are controlled by drive then torque of motor is controlled. Drives can do more than control speed. The drive can also runs the motor clockwise, counter clockwise or at a certain torque.
The classification of electrical drives can be done depending upon the various components of the drive system. As per the design, the drives can be classified into three types such as single-motor drive, group-motor drive, and multi-motor drive. The single-motor drive is the very basic type of drive. The group-motor drive is used in the industry because of different complexities. Multi-motor drive is used in the heavy industry or where multiple motor units are needed. As per another point of view, the drives can be classified into two types namely (i) reversible types drives, and (ii) non-reversible types drives. This depends mainly on the capability of the drive system to alter the direction of the flux generated.
Motors drives can be normally classified by the type of motors, mainly in two categories namely (i) DC drives, and (ii) AC drives. There are several terms used to describe devices which control motor speed. While the acronyms are frequently used inter-changeably, the terms have different meanings. In the past, because of the convenience of the torque and speed control, the DC motors have been used widely with adjustable speed drive (ASD). AC drives can be divided into synchronous motor drives and induction motor drives. AC drives are also further divided into (i) variable frequency drives (VFD), and (II) variable speed drives (VSD).
A drive system consists of a drive and auxiliary electrical apparatus. ASD is a more generic term which applies to both mechanical and electrical means of controlling speed of a mechanical load coupled to a motor. As per the IEEE standard 1566, the term, ASD is defined as an inter-connected combination of equipment which provides means of adjusting the speed of a mechanical load coupled to a motor. VFD uses power electronics to vary the frequency of input power to the motor, hence controlling the motor speed. VSD is the more generic term which applies to devices which control the speed of either the motor or the equipment driven by the motor. This device can be either electronic or mechanical.
Basically, VSD is a device which controls the speed of a motor by varying the magnitude of one of its controllable variables such as voltage, current, or frequency. Obviously, the technique used to vary the speed largely depends on the load type of the drive. VSDs have a wide variety of potential applications in electric drives. This is because of the possibility to drive different types of loads which include constant power loads, constant torque loads, and variable torque loads.
VSD classification can be made based on voltage and power ranges of the drive. The output voltage of a LV (low voltage) drive is typically less than 750 V (volt). LV drives are typically used for powers lower than around 375 kW (kilowatt). Medium voltage (MV) drives cover power ratings from 0.4 MW to 40 MW at the medium-voltage level of 2.3 kV (kilo-volt) to 13.8 kV. The power rating can be extended to 100 MW, where synchronous motor drives with load commutated inverters are frequently used. However, the majority of the installed MV drives are in the 1 MW to 4 MW range, with the voltage ratings from 3.3 kV to 6.6 kV.
Typical electric VSD systems consist of three basic components namely electrical motor, power converter, and control system. Topologies of high-power converters for industrial medium-voltage drives fall within two major categories namely direct converters and indirect converters. Direct AC/AC converter (cyclo-converter) type of VSD makes a direct power conversion from constant frequency and constant voltage to variable frequency and variable voltage. The conversion is performed in one step, without resorting to an intermediate DC link for energy storage. There are different forms of cyclo-conversion such as AC/AC matrix converters and high frequency AC/AC converters and these use self-controlled switches. Main advantages of the matrix converters come (i) from their compact design (because of the elimination of the DC-link reactive element), (ii) possibility of simultaneous power factor correction at the input and vector control at the output, and (iii) high-quality wave-forms on both sides. The main disadvantages of the matrix converters are (i) high switching losses, (ii) complexity of control, (iii) lack of energy storage (leading to practically no ride-through capacity), (iv) direct transfer of harmonic distortion, and (v) imbalance in the side-1 voltage and side-2 current to the side-2 voltage and side-1 current.
Indirect converters are classified into current-source converter (CSC) and voltage-source converter (VSC) topologies, depending on the DC-link energy-storage component. Depending on system needs and the type of the converters used, the line-side and the motor-side filters are optional. A phase-shifting transformer with multiple secondary windings is frequently used, mainly for reduction of line-current distortion.
Medium-voltage VSDs are connected to the grid through 2-winding or 3-winding transformers. For the 3-winding transformers, two lower windings are used for connecting the rectifier system. For reducing unfavourable effects of the converters (rectifier and inverter), filter systems can be used on the AC system or on the AC motor (the line-side AC filter and the motor-side AC filter respectively). Some VSDs can have a regeneration capability. In that case, depending on the mode of operation being ‘motoring’ or ‘regenerative braking’, one of the two back-to-back converters assumes the role of a ‘rectifier’, and the other converter acts as an inverter.
Rectifier (AC/DC) converters can be classified by topology as working with low switching frequency (line commutated) and other circuits which operate with high switching frequency (internally commutated).
With the development of power electronics technology, the AC motor drive system such as the induction motor and the synchronous motor driven by a variable voltage variable frequency (VVVF) inverter is being used widely. The inverter can replace the commutator and brush of DC motor, which need regular maintenance and are the weak points of a DC motor.
Whenever the term electric motor is used, people tend to think that the speed of rotation of these motors is totally controlled only by the applied voltage and frequency of the source electric current. But the speed of rotation of the electrical motor can be controlled precisely also by implementing the concept of drive. The main advantage of this concept is that the motion control of the motor is easily optimized with the help of drive. In very simple words, the systems which control the motion of the electrical motors, are known as electrical drives. A typical drive system is assembled with an electric motor (can be several) and a sophisticated control system which controls the rotation of the motor shaft. In the present-day scenario, this control can be done easily with the help of software. So, the controlling becomes more and more accurate and this concept of drive also provides the ease of use.
The main purpose of the drive is to continuously control the speed of the motor from zero to several times the nominal speed. But drives can control power, speed, current, voltage, torque, acceleration, deceleration, position, frequency, and different other parameters. They can perform several functions, such as controlling rotation direction, maintaining constant speed under varying loads, adjusting torque for different tasks, and ensuring precise positioning in automated systems. Each function needs sensors or feed-back mechanisms to provide real-time data for accurate control.
A modern electrical drive system has six main functional blocks (Fig 1). These are a mechanical load, a motor, a converter, a power source, a sensor, and a controller. The power source provides the energy the drive system needs. The main function of a converter is to transform the wave-form of a power source to that needed by the electric motor in order to achieve the desired performance. Majority of the converters provide adjustable voltage, current, and / or frequency to control the speed. The converter interfaces the motor with adjustable voltage, current, and / or frequency. The controller supervises the operation of the entire system to improve overall system performance and stability.
Fig 1 Functional blocks of the electrical drive system
The diagram which shows the basic circuit design and components of a drive, also shows that, drives have some fixed parts such as, load, motor, power processor, control unit, and source. These equipments are termed as parts of drive system. Power processor is an electronic converter which controls the power flow to motor to get variable speed. It performs several functions such as (i) it processes flow of power from the source to the motor and impart speed−torque characteristics needed by the load, (ii) it regulates source and motor currents within permissible values, such as starting, braking, and speed reversal conditions, (iii) it selects the mode of operation of motor, i.e., motoring or braking, and (iv) It converts source energy in the form suitable to the motor.
Control unit controls the function of power processor. The nature of control unit for a particular drive depends on the type of power processor used. When semi-conductor converters are used, the control unit consists of firing circuits. Micro-processors are also used when sophisticated control is needed. Sensing unit consists of speed sensor or current sensor. The sensing of speed is needed for the implementation of closed loop speed control schemes. Speed is normally sensed using tachometers coupled to the motor shaft. Current sensing is needed for the implementation of current limit control.
AC-DC, AC-AC, DC-AC and DC-DC converters are some electronic converters used in drives. In AC-DC converter (Fig 2a), the AC wave form is converted to DC with adjustable magnitude. The input can be a single or multi-phase source. This type of converter is used in DC drives. In AC-AC converter (Fig 2b), the input wave-form is typically AC with fixed magnitude and frequency. The output is an AC with variable frequency, magnitude or both. In DC-AC converter (Fig 2c), the DC wave-form of the power source is converted to a single-phase, or multi-phase AC wave-form. The output frequency, current, and / or voltage can be adjusted as per the application. This type of converter is suitable for AC motors. The DC-DC converter (Fig 2d) is also known as ‘chopper’. This converter is used to convert the constant input DC wave form to a DC wave-form with variable magnitude. The typical application of this converter is in DC motor drives.
Fig 2 Types of converters
Sensor senses the speed of motor and sends signal to controller. A well design controller has several functions. The most basic function is to monitor system variables, compare with some desired values, and then readjust the converter output until the system achieves a desired performance. This feature is used in such applications as speed or position control. In case of rectifier converters, the rectifier converts the utility supply voltage to a DC voltage with a fixed or adjustable magnitude. The normally used rectifier topologies include multi-pulse diode rectifiers, thyristor rectifiers, and pulse-width modulated (PWM) rectifiers. Controller controls the power output of power processor. The power processor sends controlled output voltage to motor. Micro-controller and micro-processor are the normally used controllers.
For the design of the electrical drive system, several other things including the electric motor is to be considered. While designing the electrical drive system, the same system performance can be achieved in different ways. The final criterion for the best design is not only the economic reasons such as initial investment, and running cost etc., but also non-economic reasons such as environmental friendliness, ethics, and regulations. Recently, because of the concern of engineering to the social responsibility, the non-economic reasons have gained importance.
The thyristor DC drive remains an important speed-controlled industrial drive, especially where the higher maintenance cost associated with the DC motor brushes is tolerable. The controlled (thyristor) rectifier provides a low-impedance adjustable DC voltage for the motor armature, thereby providing speed control.
Until the 1960s, the only really satisfactory way of obtaining the variable-voltage DC supply needed for speed control of an industrial DC motor was to generate it with a DC generator. The generator was driven at fixed speed by an induction motor, and the field of the generator was varied in order to vary the generated voltage. The motor / generator (MG) set could be sited remote from the DC motor, and multi-drive sites (e.g., rolling mill in a steel plant) had large rooms full of MG sets, one for each variable-speed motor installed in the rolling mill. Three machines (all of the same power rating) were needed for each of these ‘Ward Leonard’ drives. For a brief period in the 1950s they were superseded by grid-controlled mercury arc rectifiers, but these were soon replaced by thyristor converters which offered cheaper first cost, higher efficiency (typically over 95 %), smaller size, reduced maintenance, and faster response to the changes in set speed. The disadvantages of rectified supplies were (i) the wave-forms are not pure DC, (ii) the overload capacity of the converter is very limited, and (iii) a single converter is not capable of regeneration.
Though no longer pre-eminent, knowledge of the DC drive is valuable for several reasons namely (i) the structure and operation of the DC drive are reflected in almost all other drives, and lessons are learned from the knowledge of the DC drive, (ii) the DC drive tends to remain the yard-stick by which other drives are judged, and (iii) under constant-flux conditions the behaviour is governed by a relatively simple set of linear equations, so predicting both steady-state and transient behaviour is not difficult. In case of the successors of the DC drive, notably the induction motor drive, it is found that things are much more complex, and that in order to overcome the poor transient behaviour, the strategies adopted are based on emulating the DC drive.
For motors up to a few kWs the armature converter can be supplied from either single-phase or three-phase mains, but for larger motors three-phase is always used. A separate thyristor or diode rectifier is used to supply the field of the motor, the power is much less than the armature power, so the supply is frequently single-phase. The arrangement is typical of the majority of the DC drives and provides for closed-loop speed control.
The main power circuit consists of a six-thyristor bridge circuit, which rectifies the incoming AC supply to produce a DC supply to the motor armature. The assembly of thyristors, mounted on a heat-sink, is normally referred to as the ‘stack’. By altering the firing angle of the thyristors, the mean value of the rectified voltage can be varied, thereby allowing the motor speed to be controlled. The controlled rectifier produces a crude form of DC with a pronounced ripple in the output voltage. This ripple component gives rise to pulsating currents and fluxes in the motor, and in order to avoid excessive eddy-current losses and commutation problems, the poles and frame are to be of laminated construction. It is accepted practice for motors supplied for use with thyristor drives to have laminated construction, but older motors frequently have solid poles and / or frames, and these do not always work satisfactorily with a rectifier supply. It is also the norm for drive motors to be supplied with an attached ‘blower’ motor as standard. This provides continuous through ventilation and allows the motor to operate continuously at full torque even down to the lowest speeds without overheating.
Low power control circuits are used for monitoring the principal variables of interest (normally motor current and speed), and for generating appropriate firing pulses so that the motor maintains constant speed despite variations in the load. The ‘speed reference’ is typically an analog voltage varying from 0 V to 10 V, and achieved from a manual speed-setting potentiometer or from elsewhere in the plant. The combination of power, control, and protective circuits constitutes the converter. Standard modular converters are available as off-the-shelf items in sizes from 0.5 kW up to several hundred kW, while larger drives are to be tailored to individual needs. Individual converters can be mounted in enclosures with isolators, and fuses etc., or groups of converters can be mounted together to form a multi-motor drive.
By no stretch of imagination, the wave-forms of armature voltage can be thought of as good DC, and it is not unreasonable to question the wisdom of feeding such an unpleasant looking waveform to a DC motor. In fact, it turns out that the motor works almost as well as it works if fed with pure DC, for two main reasons. Firstly, the armature inductance of the motor causes the wave-form of armature current to be much smoother than the wave-form of armature voltage, which in turn means that the torque ripple is much less than might have been feared, and secondly, the inertia of the armature is sufficiently large for the speed to remain almost steady despite the torque ripple. It is indeed fortunate that such a simple arrangement works so well, since any attempt to smooth-out the voltage wave-form (perhaps by adding smoothing capacitors) proves to be prohibitively expensive in the power ranges of interest.
The ripple voltage causes a ripple current to flow in the armature, but because of the armature inductance, the amplitude of the ripple current is small. In other words, the armature presents a high impedance to AC voltages. Because of the smoothing effect of the armature inductance, the current ripple is relatively small in comparison with the corresponding voltage ripple. The average value of the ripple current is of course zero, so it has no effect on the average torque of the motor. There is however a variation in torque every half-cycle of the mains, but since it is of small amplitude and high frequency the variation in speed (and hence back emf) is not normally noticeable. The current at the end of each pulse is the same as at the beginning, so it follows that the average voltage across the armature inductance is zero. Hence, the average applied voltage can be equated to the sum of the back emf (electromotive force), which is exactly the same as for operation from a pure DC supply. This is very important, since it underlines the fact that the mean motor voltage can be controlled, and hence the speed, simply by varying the converter delay angle.
The smoothing effect of the armature inductance is important in achieving successful motor operation. The armature acts as a low-pass filter, blocking most of the ripple, and leading to a more or less constant armature current. For the smoothing to be effective, the armature time-constant needs to be long compared with the pulse duration (half a cycle with a 2-pulse drive, but only one sixth of a cycle in a 6-pulse drive). This condition is met in all 6-pulse drives, and in several 2-pulse drives. Overall, the motor then behaves much as it is if it has been supplied from an ideal DC source.
Industrial electronics and electric drives technology have made considerable developments after several decades of the dynamic evolution of power semiconductor devices, converters, pulse width modulation (PWM) techniques, and advanced control and simulation techniques. Recently its applications have been fast expanding in the industry because of the reduction in cost and size and improvements in performance.
In the beginning of the 20th century, since the price of the electric motor and its associated control system was very expensive, a large electric motor was used in the whole plant, and the mechanical power from the motor was distributed to every mechanical equipment where the mechanical power is needed through gears and belts. Because of the reduction of the price of the electric motor and the control system, a separate electric motor is being used for each mechanical equipment, which has several motions, and still the mechanical power from the motor is transmitted and converted to an appropriate form at each point of the motion in the equipment.
Recently, even in a single mechanical equipment, multiple electric motors are used at each motion point. The motion needed at that point can be achieved by the motor directly without speed or torque conversion from the motor. In this way, the efficiency of the system can be improved. Also, the motion control performance can be improved by eliminating all non-linear effects and losses such as backlash, torsional oscillation, and friction. In the future, this tendency can be continued and the custom designed motor can be used widely at each moving part. For example, for high-speed operation, the high-speed motor can be used without amplification of the speed through gears. For linear motion, a linear motor can be used without a ball screw mechanism. For high-torque low-speed traction drive, the direct drive motor can be used for reducing the size and loss of the system.
The control method of the motor drive system has been developed from manual operation to automatic control system. Recently, intelligent control techniques have been used and the control system itself can operate the system at optimal operating conditions without human intervention. Also, in the early stages of automatic control of the motor drive system, the simple supervisory control has been implemented, and the control unit has transferred the operating command set by the user to the motor drive system. Through the direct digital control, right now, distributed intelligent control techniques are used widely in the up-to-date motion control system.
In the late 1950s, with the invention of the thyristor, power electronics has been introduced. The power semi-conductor has been the key of the power electronics. With the rapid improvement of performance against cost of the power semi-conductors, the power electronics technology has improved in a revolutionary way. The original thyristors of the 1950s and 1960s can only be turned on by an external signal to the gate but is to be turned off by the external circuits. And it needs a complicated forced commutating circuit. In the 1970s, the gate turn-off (GTO) thyristor has been commercialized. And the GTO thyristor can be not only turned on but also turned off by external signal to the gate of the semi-conductor. In the late 1970s, the bipolar power transistor opened a new horizon of the control of power because of its relatively simple on and off capabilities.
Traditional drives, which use asynchronous cage motors or DC motors, are unsuitable for sufficiently precise and energy-efficient control of motors in dynamic and static operating conditions. Because of the development of new, highly efficient and characterized with short response IGBT (insulated gate bi-polar transistors), a marked rise in the number of VSD systems can be observed in the recent years. High interest in industrial applications of the VSD systems is because of their advantages which include controlled starting current, reduced harmful disturbances in the power grid, lower power requirement of the drive at start-up, controlled value and characteristics of accelerations, smooth regulation of motor speed (measured in revolution per minute, rpm), controlled torque, fully controlled drive deceleration, electricity savings, power recuperation, easy motor reverse, and elimination of additional mechanical parts.
With the transistor, general-purpose VVVF (variable voltage variable frequency) inverters has been commercialized and are being used in several ASD applications. Recently, with the introduction of the integrated gate-controlled thyristor (IGCT) and the fifth-generation IGBT, the performance of the electric motor drive system has been dramatically improved in the sense of output power of the system and the control bandwidth of the motion of the drive system. However, still, all the power semi-conductors have been fabricated based on silicon, and its junction temperature has been limited up to 150 deg C in the majority of the cases. Recently, the power semi-conductor based on silicon carbide (SiC) has been introduced, and the operating temperature and operating voltage of the power semi-conductor can be increased several-fold. With this material, the semi-conductor operating at above 300 deg C and at several thousand-voltage can conduct several hundred amperes within one-tenth of the wafer size of the device made by silicon. In particular, the Schottky diode and field effect transistor (FET) based on SiC have been the first devices in the field, and extraordinary performances of the devices have been reported.
In the early days of research and development, the control signal for the power semi-conductors came from analog electronics circuits consisting of transistors, diodes, and ‘R’ (resistor), ‘L’ (inductor), ‘C’ (capacitor) passive components. With the development of electronics technology, especially integrated circuit technology, the mixed digital and analog circuit consisting of operational amplifiers and TTL (transistor-transistor logic) circuit has been used. Recently, except for high-frequency switching power supplies, the major part of the power electronics system, especially the electric motor drive system, is controlled digitally by one or a few digital signal processors (DSP).
In power-factor correction (PFC) converters, by applying semi-conductors such as IGBT, GTO, IGCT, a silicon-controlled rectifiers (SCR) allows to reduce harmonics and to improve power factor. Rectifiers of active front end (AFE) type can operate with high power factor or any active-reactive power combination. These rectifiers can be classified as voltage-source rectifiers (VSRs) and current-source rectifiers (CSRs). AFE drives are inherently ‘four-quadrant’ ones (i.e. they can drive and brake in both directions of rotation with any excess of kinetic energy during braking returned to the supply).
For meeting the motor-side challenges, a variety of inverter topologies can be adopted for the MV drive. The most used inverters are conventional two-level inverter, three-level neutral-point clamped (NPC) inverter, seven-level cascaded H-bridge inverter, and four-level flying-capacitor inverter. Either IGBT or IGCT can be employed in these inverters as switching devices. Current-source inverter (CSI) technology has been widely accepted in the drive industry. The most frequently used inverters are load-commutated inverter (LCI), pulse width modulation (PWM) CSI, and parallel PWM CSI.
The SCR-based LCI is particularly suitable for very large synchronous motor drives, while the PWM CSI is a preferred choice for most industrial applications. The parallel PWM CSI is composed of two or more single-bridge inverters connected in parallel for super-high-power applications. Symmetrical IGCTs are typically used in the PWM current source inverters. CSI technology is well suited for high-power drives. The current-source converters feature a simple converter structure, low switch count, low switching dV/dt, and reliable over-current / short-circuit protection. The main drawback lies in its limited dynamic performance because of the use of a large DC choke.
Traditional two-level voltage-source inverters (VSIs) (2L-VSIs) have been limited to low-power or medium-power applications because of the power semi-conductor voltage limits. Series connection of switching devices has enabled high power 2L-VSIs. The well-known 2L-VSI is also used in medium-power and high-power traction and industrial high-power drives. The 2L-VSI inverter is a simple converter topology and has an easy PWM modulation pattern. However, the inverter produces high dV/dt and high THD (total harmonic distortion) in its output voltage and, hence, frequently needs a large-size LC (inductor-capacitor) filter installed at its output terminals.
The three-level NPC-VSI (3L-VSI) has been successfully used in the industry in past years. The main features of the NPC (neutral point clamped) inverters include reduced dV/dt and low THD in its AC output voltages in comparison to the 2L-VSI topology. The 3L-VSI can be used in MV drives to reach a certain voltage level without switching devices in a series connection. Hence, the efficiency levels can reach 99 %. It is to be noted that, in terms of efficiency, the VSIs and CSIs are attractive for non-regenerative low dynamic requirement drives. For regenerative applications, the three-level NPC VSI achieves higher efficiency in comparison to the CSI converter. For very high-power applications, the thyristor-based current-source topology offers considerably higher performance because of the low-voltage drop of the semi-conductors used.
The primary adverse outcome of a VSD for a power system is the effect of harmonics generated by the VSD. There are two mechanisms by which the VSD generates harmonic currents. The first mechanism is the converter operation which injects harmonic currents into the supply system by an electronic switching process. The second mechanism is the inverter operation. The magnitude of the harmonics generated by the VSD is determined by (i) topology of the drive (number of pulses, and rectifier type), (ii) percentage of the total power system capacity which the VSD represents, (iii) stiffness or short circuit capacity of the power system supplying the VSD, (iv) whether or not the VSD is electrically isolated from other sources of harmonics, (v) installation practices for the VSD, and (vi) rating of electrical load of the VSD.
When planning the installation of VSDs in a power supply system, a choice has to be made between designing non-linear devices for low levels of wave-form distortion or installing harmonic compensation equipment at the terminals. The first solution is frequently possible by phase-shifting of the transformers and / or the control of converter bridges or by the use of switching devices with turn-off capability. Second solution of harmonic elimination is achieved by means of filters (external harmonic compensation). Passive filters comprise inductance, capacitance, and resistance elements configured and tuned to control harmonics characterizing operation of particular VSD system. These are normally used and are relatively inexpensive compared with other means for eliminating harmonic distortion. As these have disadvantage of potential adverse inter-action with the power system, it is important to verify all possible system inter-actions in the system planning and design stage.
The AC motor drive, as the name suggests, needs an AC input to the induction motor used to drive large industrial loads. An AC motor drive takes an AC energy source, rectifies it to a DC bus voltage and, implementing complex control algorithms, inverts the DC back to AC into the motor using complex control algorithms based on load demand.
The power converter topology used in the power stage is that of a three-phase inverter which transfers power in the range of kW to MW. Inverters convert DC power to AC power. Typical DC bus voltage levels are 600 V to 1,200 V. Considering the high power and voltage levels, the three-phase inverter uses six isolated gate drivers. Each phase uses a high-side and low-side insulated gate bi-polar transistor (IGBT) switch. Operating normally in the 20 kHz (kilohertz) to 30 kHz frequency range, each phase applies positive and negative high-voltage DC pulses to the motor windings in an alternating mode.
High power IGBT needs isolated gate drivers to control their operations. Each IGBT is driven by a single isolated gate driver. The isolation is galvanic between the high voltage output of the gate driver and the Low voltage control inputs which come from the controller. The emitter of the top IGBT floats, which necessitates using an isolated gate driver. In order to isolate a high voltage circuit with that of a low voltage control circuit, isolated gate drivers are used to control the bottom IGBTs.
Gate drivers convert the pulse-width modulation (PWM) signals from the controller into gate pulses for the FETs or IGBTs. Moreover, these gate drivers need to have integrated protection features such as desaturation, active Miller clamping, soft turn-off and so on. These isolated gate drivers normally suffer from low drive strength, especially when the drive current capability is below the 2A (ampere) range. Traditionally, these drive applications use discrete circuits to boost the drive current. Recently, there have been several gate driver ICs (integrated circuit) developed to replace the discrete solution. Fig 3 shows this trend.
Fig 3 Three-phase inverter topology with boost gate driver power supplies for IGBT gate drivers in the power stage
In order to take advantage of the low conduction losses in IGBTs, gate drivers need to operate at voltages much higher than their threshold voltage in the range of 15 V to 18 V. Also, an IGBT is a minority-carrier device with high input impedance and large bipolar current-carrying capability. The switching characteristics of an IGBT are similar to that of a power MOSFET (Metal-oxide-semiconductor field-effect transistor). For a given condition when turned on, the IGBT behaves much like to a power MOSFET, showing similar current rise and voltage fall times. However, the switching current during turn-off is different.
At the end of the switching event, the IGBT has a ‘tail current’ which does not exist for the MOSFET. This tail is caused by minority carriers trapped in the ‘base’ of the bipolar output section of the IGBT. This causes the IGBT to remain turned on. Unlike a bi-polar transistor, it is not possible to extract these carriers to speed up switching, as there is no external connection to the base section. Hence, the device remains turned on until the carriers recombine. This tail current increases the turn-off loss which needs an increase in the dead time between the conduction of two devices for a given phase of a half-bridge circuit.
Having a negative voltage (–5 V to –10 V) at the gate helps to reduce the turn-off time by helping to recombine the trapped carriers. When the IGBT is turned on the high dv/dt and parasitic capacitance between gate and emitter generates voltage spikes across the gate terminal. These spikes can cause a false turn-on of the bottom IGBT. Having a negative voltage at the gate helps to avoid this false turn-on trigger. Normally 15 V to 18 V is applied to the gate to turn-on the device and a negative voltage of –5 V to –8 V is applied to turn off the IGBT. This requirement is key to determine the power supply rating to the IGBT driver.
Typically, such a power supply is a PWM controller with a topology which has the ability to scale the output power while driving these high-power IGBTs. Typical inputs for these power supplies are regulated to 24 V. One example of a classic topology used for this power supply is the push-pull isolated converter. This topology is similar to a forward converter with two primary winding. The advantage which the push-pull converters have over fly-back and forward converters is that they can be scaled up to higher powers, in addition to higher efficiency.
Silicon carbide (SiC) MOSFETs are gaining traction in the power stage for motor drives. IGBTs and SiC MOSFETs, are a good fit in high-power AC power stage applications because of their high voltage ratings and high current ratings. SiC MOSFETs however differ from IGBTs in the switching frequency requirement. IGBTs typically operate in the lower switching frequency range (5 kHz to 20 kHz) whereas SiC MOSFETs operate at a much higher switching frequency range (50 kHz to +300 kHz). The ability to switch at higher frequencies allows system benefits such as higher power density, higher efficiency, and lower heat dissipation. As switching frequency requirements increase, it is critical to select a gate driver with high CMTI (common-mode transient immunity) rating.
High power switches are able to change voltage and current within hundreds of nano-seconds (ns). This generates very large voltage transients, normally higher than 100 V/ns. The gate driver experiences these voltage swings at every switching instant, particularly when the driver is referenced to the switch node. It is important that the gate driver be able to withstand these large voltage transients for preventing noise coupling back to the primary side of the gate driver for preventing false triggers on the gate drive logic.
Fig 4 shows an off-line power supply which draws power from the three-phase universal AC line to a regulated 24-V DC output. Because of the low-power level (below 75W), power factor correction (PFC) is not needed. These off-line power supplies are typically flyback topology converter ICs which can be a controller with external MOSFET, or an integrated MOSFET controller or switcher. The choice of the power supply IC is flexible and is influenced by the power level, number of outputs, and accuracy of the regulation. This off-line power supply is normally a separate module. The 24 V DC output is the system power bus in the AC motor drive system which is input into the bias power supply for the power stage and non-isolated DC/DC converter. This non-isolated DC/DC regulator from the 24 V system provides power to the controller, communications and safety micro-controllers, interface transceivers, and data converters.
Fig 4 Block diagram for an AC motor drive
A BLDC (brush less DC) power stage is also an inverter similar to an AC motor drive, except that the input can be single or three-phases. DC rail voltages are typically 48 V to 600 V, depending on the power levels. The switch is normally a power MOSFET switching at around 100 kHz. Gate drivers are high-side, low-side or half-bridge drivers per inverter phase with no isolation requirement. Protection features are not as critical as those needed for the AC motor drive, except for dead-time control to avoid shoot-through since the high-side and low-side drivers are operating from one IC.
Bias power to the controller and gate drivers comes off a regulated power supply from the battery. A typical battery used in this space is the 18 V nominal Lithium-ion (Li-ion) five-cell battery. Since these are cordless tools, a wall charger is needed to charge the drill periodically. Typically, charging in the range of 50 W to 1,000 W is done using an isolated controller which is topology-specific, depending on the power level. Also, PFC (power factor correction) is normally not needed unless the power level is in the few hundred watts. Typical charging controllers are based off of a fly-back, inter-leaved fly-back, or push-pull topologies.
The electrical motor, the work-horse of the modern variable frequency AC drive, has gone through slow but sustained growth during the past century. The initiation of powerful digital computers, new and improved materials, coupled with extensive research, has resulted in higher power density, higher efficiency, and many performance improvements of motors. The progression of power semi-conductor devices, various converter topologies, advanced PWM techniques and improved control and estimation methods gradually brought high-performance AC drives of different types into use, pushing DC drives toward obsolescence.
It is interesting to note that recently, voltage-fed multi-level inverters are finding almost universal acceptance for large-power four-quadrant induction motor and synchronous motor drives, replacing the traditional thyristor-based cyclo-converters and current-fed inverters. Permanent magnet synchronous motors (PMSMs), particularly brushless DC drives with trapezoidal motors, are more popular in the lower end of power. Normally, PMSM drives are more expensive than cage-type motors, but have the advantages of higher efficiency (lower life-cycle cost) and lower rotor inertia. As the cost of high-energy NdFeB (neodymium iron boron) magnets decreases in future, PM (permanent magnet) motor drives are going to gradually find increased acceptance.
It is interesting to note that switched reluctance motor (SRM) drives are getting wide attention. The SRM is simple in construction, economical and robust, and is frequently compared with the induction motor, although it is the closest relative of the synchronous reluctance motor. However, the SRM drive has inherent pulsating torque and acoustic noise issues, and needs an absolute position encoder like a self-controlled PMSM (permanent magnet synchronous motor) drive, although extensive studies have mitigated some of these issues.
Motor drives are becoming more efficient as power electronic devices such as power switches (IGBTs and MOSFETs), gate drivers and bias supplies are being incorporated.
Leave a Comment