The history of electric motors dates back to 1820, when Hans Christian Oster discovered the magnetic effect of electric current, and a year later Michael Faraday discovered electromagnetic rotation and built the first primitive DC motor. Faraday discovered electromagnetic induction in 1831, but it wasn’t until 1883 that Tesla invented the induction (asynchronous) motor. Today, the main types of electric machines remain the same, DC, induction (asynchronous) and synchronous, all based on theories developed and discovered by Alstead, Faraday and Tesla over a hundred years ago.
Since the invention of the induction motor, it has become the most widely used motor today due to the advantages of induction motor over other motors. The main advantage is that induction motors do not require an electrical connection between the stationary and rotating parts of the motor, therefore, they do not require any mechanical commutators (brushes) and they are maintenance free motors. Induction motors also have the characteristics of light weight, low inertia, high efficiency, and strong overload capacity. As a result, they are cheaper, stronger, and don’t fail at high speeds. In addition, the motor can work in an explosive atmosphere without sparking.
Considering all the above advantages, induction motors are considered perfect electromechanical energy converters, however, mechanical energy is often required at variable speeds, where speed control systems are not a trivial matter. The only effective way to generate stepless speed change is to provide three-phase voltage with variable frequency and amplitude for the asynchronous motor. The rotor speed depends on the speed of the rotating magnetic field provided by the stator, so frequency conversion is required. Variable voltage is required, the motor impedance is reduced at low frequencies, and the current must be limited by reducing the supply voltage.
Before the advent of power electronics, speed-limiting control of induction motors was achieved by switching the three stator windings from a delta to a star connection, which reduced the voltage across the motor windings. Induction motors also have more than three stator windings to allow varying the number of pole pairs. However, a motor with multiple windings is more expensive because the motor requires more than three connection ports and only specific discrete speeds are available. Another alternative method of speed control can be achieved with a wound rotor induction motor, where the rotor winding ends are brought onto slip rings. However, this approach apparently removes most of the advantages of induction motors, while also introducing additional losses, which can result in poor performance by placing resistors or reactances in series across the stator windings of an induction motor.
At the time, the above methods were the only ones available to control the speed of induction motors, and DC motors already existed with infinitely variable speed drives that not only allowed operation in four quadrants, but also covered a wide power range. They are very efficient and have suitable control and even a good dynamic response, however, its main disadvantage is the mandatory requirement for brushes.
in conclusion
In the past 20 years, semiconductor technology has made tremendous progress, providing the necessary conditions for the development of suitable induction motor drive systems. These conditions fall into two main categories:
(1) Cost reduction and performance improvement of power electronic switching devices.
(2) Possibility to implement complex algorithms in new microprocessors.
However, a prerequisite must be made to develop suitable methods to control the speed of induction motors whose complexity, in contrast to their mechanical simplicity, is particularly important with regard to their mathematical structure (multivariate and nonlinear).
Post time: Aug-05-2022