GB1422331A - - Google Patents

Abstract

1422331 Control of AC motors D B WATSON 2 Feb 1973 [19 Feb 1972] 7793/72 Heading H2J An induction motor having a linear, cylindrical or disc type rotor is operated at a variable speed by forming the winding arrangement of its stator as a number of identical windings or sets of windings connected in series between two terminals receiving a single phase AC or chopped DC supply, the junctions between consecutive windings or sets being connected through respective capacitors to a common one of the terminals, each capacitor forming with the associated winding or set an elementary circuit whereby the winding arrangement comprises a series of such circuits representing in lumped circuit component form the evenly distributed inductance and capacitance characteristics of a notional transmission line, the travelling magnetic field produced by the A.C. or D.C. supply having a desired velocity determined by selecting simultaneously the capacitance and/or inductance value of each of the elementary circuits. As shown in Figure 1, each elementary circuit comprises a winding 3 and a variable capacitor 4, all of the latter being simultaneously adjustable together with the final capacitor 5, which is shunted by a load 6 whose impedance corresponds to the characteristic impedance Z 0 of the notional transmission line formed by the series of circuits. Each capacitor 4 has a capacitance C regarded as the sum of two parallel capacitances C/2 respectively associated with the adjacent windings 3, the capacitor 5 having a capacitance C/2. Application of an A.C. or chopped D.C. supply to terminals 1, 2 produces a travelling magnetic field, since the current in adjacent windings 3 is substantially equal in magnitude but differs in phase by an angle Œ#= # #LC from one winding to the next. The field velocity is 1/#LC units per second, where a unit is the distance between the respective centres of two adjacent windings, whose resistance is negligible. The speed may be varied by adjusting capacitors 4, 5 and/or variable inductors (not shown) in series with the windings 3 and/or taps (not shown) on the windings 3. The mutual inductance between adjacent windings 3 may be minimized by connecting each in series with a compensating coil 7, Fig. 2 (not shown). In another arrangement for a cylindrical motor, each winding 3 comprises two parts (3A, 3A<SP>1</SP>; 3B, 3B<SP>1</SP>... ), Fig. 3A (not shown), respectively located at opposite ends of a stator diameter, Fig. 3B (not shown), to produce a substantially symmetrical flux in the rotor and hence a polyphase effect.