Inversion From DC Voltage
Two of the fastest growing inverter applications are not as well served by the SCR. These are circuits for independent backup power and circuits for control of AC motors. A variety of modern commercial units are shown below. The most straightforward inverter circuits use timing information from the ac voltage source to control their operation. Backup circuits and motor controllers do not have access to this sort of time reference information. Without such timing information, inverter control can be complicated. Motor control and backup applications were difficult to build from electronic circuits until relatively recently. One early example was the Stir-Lec I, and experimental electric vehicle built by General Motors in 1968. This car used SCRs in an innovated but complicated arrangement to convert DC power from batteries for an AC motor.
Both backup power and AC motor control systems were considered to be classical applications of motor-generator sets prior to the semiconductor revolution. Diesel-driven generator systems remain the standard choice for large backup power sources. Battery backup is common in DC applications, such as telephone networks and communication equipment. Batteries are becoming more common in small backup applications. Equipment rated for the 0-1 kW range is now readily available.
The growth in low-power battery backup inverters can be attributed in part to developments in transistor technology. In power electronics applications, bipolar transistors (BJTs), field-effect transistors (FETs), and more recent insulated-gate bipolar transistors (IGBTs) are in wide use. Power BJTs were first developed for the U.S. space program. By the late 1960s, power handling capacities had reached 1kW. These ratings were well-matched to small solar panel units. Ratings quickly reached 5kW early in the 1970s – sufficient to meet the needs of small computers and other light commercial backup applications. By the early 1980s, an experimental device with 1MW power handling level had been constructed on a single 7.5 cm wafer.
As power BJT technology was developed, improvements in FETs and advances in SCRs began to make inroads in these applications. FETs have certain advantages over BJTs. They are more convenient to use in many circuits. After the power FET was introduced commercially in 1975, it rapidly came into use for low-level conversion applications. This device offers an interesting contrast to other power semiconductors. While power BJTs, diodes, and SCRs are individual components with large geometries, or even full single-wafer devices, power FETs are formed using MOSFET technology as parallel combinations of thousands or even millions of devices. Recent FETs have power handling levels well beyond 10 kW, and have been being applied to inverters for computer power, and even for heavier loads such as AM broadcast transmitters and inverters for AC motors up to about 50 HP.
The IGBT, introduced in the mid-1980s, acts in many ways as a combination device. In practice, its behavior is like that of a BJT, with a power FET to supply the base current. This combination allows the IGBT to approach power handling levels of BJTs, while retaining the simplified operating characteristics of FETs. Recent IGBTs have power handling capability above 500 kW. They dominate motor control inverters.
The figure below shows just a few of the range of devices and packages used in power electronics today.
Motor control and motor drives are often considered a separate field related to power electronics. The most important long-temp goal of engineers designing motor controls has been to supplant DC motors with AC machines. In a typical commercial AC motor controller, the incoming AC power is rectified to create a DC source. This DC voltage supplies an inverter virtually identical to that used in a backup power application. Control of AC motors has been an important technological objective since Tesla introduced the polyphase induction motor in the late 1880s. DC motors are common in control applications, because their speed can be altered simply by adjusting the input DC voltage level, and their output torque can be manipulated through control of their main winding current. They have major disadvantages in cost and reliability: a true DC motor has brushes and a mechanical communicator which mush be maintained. AC motors, and especially induction motors, are inherently cheaper to build and maintain that DC machines. They have better power-to-weight ration than DC machines, and can operate at higher speeds. Moving parts are few, and only the bearings themselves require upkeep in motor ratings are observes. However, the speed of an AC machine is tied to the input frequency, and the torque is adjusted by altering the magnetic field levels in the device.
The difficult challenge of providing adjustable magnetic field and input frequency makes AC motors hard to control. Before about 1980, the extra cost of control equipment far exceeded the cost disadvantage of DC machines, and DC systems were common when control of motor speed or torque was needed in an application. In a few cases, the reliability advantages of AC machines were critically important. Motor-generator sets provided adjustable frequency for these applications. Examples include the Scherbius combination, a combined AC motor-AC generator-AC motor, and the Ward-Leonard combination, a DC or AC motor-DC generator-DC motor combination.
The SCR can be used with some difficulty in electronic converters for AC motor controls, as mentioned above in the example of the 1968 electric car drive. One of the first examples was the so-called static Scherbius system, in which combinations of SCRs substituted for the functions of the Scherbius motor-generator arrangement. The SCR is hard to use in such a system because its gate controls only the turn-on behavior. It is possible to alter an SCR so that current can be turned off by means of a negative gate signal. This device is called a gate turn-off SCR or GTO, is used in some large motor control circuits today, particularly those on AC locomotives.
More recent electronic circuits built from power FETs or IGBTs meet the basic functional requirements of AC motor control with inverters. In the early 1990s, the cost of these electronic drives began to drop so dramatically that now the combination of a power electronic circuit and an AC motor is cheaper than the cost of an equivalent DC motor system. This development is bringing extraordinarily rapid change in manufacturing and industrial processing. Advanced AC motion control equipment has reached the cost and performance level at which almost any automation application can be addresses. Another widely anticipated device, the MOSFET-controlled GTO, or MCT, will make circuits for 100 kW and beyond more convenient.