COMPARATIVE ANALYSIS OF A TRANSFER FIELD MACHINE AND AN INDUCTION MACHINE

This work presents a comparative analysis of a transfer field machine (TFM) and a polyp-phase induction machine (IM) . Although the two machines belong to two different classes of machine and quite different in physical configuration , yet both display similar torque – Slip characteristics. However, the synchronous speed of the transfer field machine is ωo/2, that is, one-half that of the induction machine. In their principle of operation, the induced electromotive force (e.m.f) as well as the frequency of this induced e.m.f in both the auxiliary winding of the transfer field machine and the rotor of an induction machine, is proportional to slip. The self inductance matrix of the two machines are derived and both shown to be independent of the rotor angular position. However, the mutual coupling inductance in both cases are dependent on rotor angular position. For the transfer field machine, in addition to rotor angle dependence, it also depends on the difference between the direct – and quadrature-axes reactances. Consequently, the machine produces reluctance torque as a result of the rotor pole –axis trying to align with the axis of the maximum flux. But that of induction machine is by alignment of fields, that is, the rotating magnetic field of the rotor trying to catch up with that of the stator. Under steady-state performance, the transfer field machine exhibited a lower pull out and starting torque as well as lower efficiency than the induction machine. In dynamic mode, the torque versus speed characteristic of both machines are very identical which is akin to what obtains in the steady – state simulation. Also the starting current of the transfer field machine is not high – a feature that makes it possible for the transfer field machine to tolerate a longer starting time without any major disturbance to the supply unlike the induction machine.

The theory of induction machine is old and well known. An induction machine consists essentially of two major parts, the stator and the rotor. When an a.c voltage is impressed on the terminals of the stator windings, a rotating magnetic field is set up. This rotating magnetic field produces an electromotive force (e.m.f) in the rotor by electromagnetic induction (transformer action) which in turn, circulate current in the rotor usually short-circuited. This current circulating in the short-circuited rotor, produces a rotating magnetic field which now interact with the rotating magnetic field already established in the stator. This interaction produces a torque which is responsible for the rotation of the machine.

Induction machine is also known as the asynchronous machine which derives from the fact that the rotor magnetic field is always lagging the stator magnetic field. The difference is called the slip, and it is a fundamental characteristic in the operation of an induction machine. An induction machine when it operates below synchronous speed, is a motor while it is a generator when it operates above the synchronous speed. In fact induction machines are mostly used as motors.