Recent Trends in Electric Machines:
Recent Trends in Electric Machines are neural networks, Artificial Intelligence, expert system, fibre communications and integrated electronics, hot superconductors and other new ceramic conducting and dielectric materials, magnetic levitation etc. should help young electrical engineers to develop newer, cheaper and more effective electrical energy converters and their controllers.
Among all the various forms of energy systems, electrical energy offers the most flexible, economic and efficient mode for generation, transmission and utilization and has become the backbone of modem civilization. It provides for lighting, heating, transportation, communications and practically all industrial processes. Most of the power required for human activities round the globe continue to come from electrical machines from the very large generators installed in power stations to the very small motors in automatic control systems.
The need for keeping the magnetic flux within identical paths led to a large scale use of silicon-iron electrical steel in most electrical devices from large transformers and generators to small transducers. Hadfield and his colleagues were the first to demonstrate in 1900 that addition of small amount of silicon or aluminium to the iron would reduce iron losses by a factor of 4.
Modem commercial electrical steels can be grouped under non-oriented, grain-oriented and rapidly quenched alloy types, of which the first two dominate the applications.
Because of the limitations on the shape of the magnetic path, grain-oriented types are used predominantly in large power and distribution transformers, while non-oriented in rotating machines and small apparatus. The silicon content is critical to the performance of electrical steels because it increases resistivity and reduces the anisotropy and magnetostriction but decreases the saturation magnetisation and permeability and makes the material brittle. In non-oriented type this content varies between 0% and 3% while for oriented type between 2.9% and 3.2%.
The continuing development of high permeability silicon-steel, metallic glass, ferrites, aluminium ceramics, and high temperature insulating materials, permanent magnetic materials like Neomax (Nd-Fe-B) and rare-earth cobalts has been influencing the construction and design of many large and small machines and apparatus from large generators to small step motors. Not only the cost of material per kVA has come down in many cases, more reliable and highly efficient machines have been successfully designed.
Areas which have revolutionised the growth of research in electrical machines could be grouped as under:
- Availability of solid-state devices of large current/voltage handling capacities and consequent growth of power electronics.
- The availability of cheap microprocessor based systems for operation and control.
- The development of artificial intelligence (AI) tools, and expert systems for machine design, operation and control.
- The development of CAD/CAM techniques.
Today single thyristor with maximum-voltage rating of 12 kV and maximum current rating of 6000 A are commercially available and devices with even larger ratings are on the anvil. Converter and inverter grade thyristors, gate turn off devices, triacs, asymmetric thyristors, power MOSFETS, power IGBTs have opened up new vistas for the electronic switching and control of energy converting devices. AC/DC converters, cycloconverter and AC/DC voltage regulators using SCRs are of naturally commutated type while the inverter, both the VSI and CSI types use force commutation. High voltage DC transmission, variable frequency operation of machines, voltage control of DC machines, switched mode regulators and power supplies, offer a wide variety of application for converter engineers to develop new and reliable configurations. The most common application is the variable speed drives using DC and AC machines. Power electronics appear to have shifted the emphasis of electrical engineers from the design of special types of variable speed machines to the use of special electronic circuits to make an existing machine give the desired variable speed characteristics.
To make the squirrel cage induction motor run as a conventional separately excited dc motor has remained the dream of generations of electrical engineers. Even the latest researches in vector control/field oriented control have not quite bridged the gap between the dream and the reality. Further, direct torque control (DTC) has simplified the control circuitry to a great extent. The availability of cheap p.p-based systems have further revolutionised the control of electrical devices of all forms for obtaining the desired steady-state, transient as well as dynamic characteristics from the existing drive machine with the help of suitably designed solid state control circuits. Advances in AI and development of expert systems are already influencing the design of devices and systems as well as the health monitoring of operating machines and taking advance action against failures.
One of the latest developments in ac motor research has been in the direction of field-oriented control or making ac motors perform like dc motors with highly accurate torque and power control.
Another recent development in ac motor drives is a system called Direct Torque Control (DTC) and Direct Self Control (DSC). There is no modulator and no need for an encoder to feed back information about motor shaft speed and position. The DTC control (sensorless type) incorporates fast digital signal processsing hardware, resulting in a torque response that is said to be ten times faster than any ac or dc drive. One of the reasons ac technology will continue to make inroads into dc dominance is reduced power consumption. Brushless dc will survive eventually but it is not dc at all but a completely different technology. It is called dc because of the concept of external commutation.
Now there is increased levels of customer support. With the emphasis on just-in-time production, downtime is unacceptable. With today’s systems, if the machine goes down one can via modem, have a technician at a remote site use system diagnostics software to troubleshoot the entire system from anywhere in the world.
If we could erase history and start out with a clean slate, there is no doubt that ac would be the choice. But the fact is that there are billions of dollars of dc investment running, converting operations throughout the world. Some day all motors and drives may be ac, but the conversion will progress at a rate dicted purely by economics.