Double Stator Induction Machine for Variable Speed And Variable Torque Applications

Abstract

Conventional Induction machine has only one drawback of not having a smooth speed control if compared with DC Machines. But because of recent developments in technology such as semiconductors, drives, this drawback can be ruled out. A conventional Induction machine has a single speed-torque characteristic. The application of a respective machine depends on the load characteristics. Many applications in the industry need variable torque and variable speed. A double stator induction machine is so designed that there are two different stator windings housed in the same stator core, which then gives the best of both machines in a single machine. Using a simple controller, a wide range of speed and torque is available using a single machine. Such machines will find applications in the Textile, EV, Food industries where variable torques and speeds are required. The windings housed in the stator core can be for the same/ different.number of poles [1]. As per the rotor construction, the conventional induction machines are classified as squirrel cage and Split wound. Both these constructions have their own merits and demerits. Typically, Squirrel cage induction motors find applications in vivid areas due to simple and rugged construction and negligible maintenance. This type of Induction machine has versatile applications in various fields, covering almost 90% share in the electrical machines market. Speed -torque characteristics of such machines are almost constant. The variation in torque can be achieved by changing resistance in the rotor circuit. However, in squirrel cage induction machines, this method is not feasible. The only way to vary torque or speed is the stator side control. Stator side control methods are change of poles, change of supply frequency, change of supply voltage, change of the ratio of voltage/frequency, and adding an additional resistance/impedance in stator circuit. The common methods to vary the speed as discussed above are not flexible enough to achieve the required Torque-Speed characteristics of the motor suitable to the load requirement pertaining to a particular application. In addition to this, these methods also have a problem that the electrical frequency changes with the mechanical speed (n = (2f/p)(1.s)). Hence, smooth control over Torque-speed is not feasible. Voltage Source Inverter (VSI) fed Induction machine does not face this issue. But this induces harmonics in the system and makes the machine less reliable and also costlier. Variable Frequency Drives (VFD) require filters to limit the rate of change of voltage (dV/dt) up to 6 kV/ μS, which otherwise may cause insulation failure. As per the above discussion, it is concluded that a Double Stator Induction Machine (DSIM) can provide a better option for variable speed applications.