GB322011A - Improved system for regulating the speed of asynchronous motors - Google Patents

Abstract

322,011. British Thomson-Houston Co., Ltd., (Compagnie Francaise pour l'Exploitation des ProcÚdÚs Thomson-Houston). Sept. 5, 1928. Alternating-current induction machines.-In a system for controlling the speed and power factor of an induction motor by means of a cascaded commutator machine excited by a frequency changer, the rotor and stator windings of which respectively equilibrate the ohmic and inductive drops in the excitation windings, the frequency changer is fed from an alternator driven by a synchronous motor running on the main network and the exciting current from the frequency changer may be amplified by a commutator generator running at a speed substantially proportional to the slip of the induction motor. Means may also be provided to vary the speed of the induction motor automatically in accordance with the load. In the form shown in Fig. 1, the cascaded machine B of the induction motor A is excited at E from a frequency changer F which is fed through slip-rings f from an alternator D. This alternator is driven by a synchronous motor C running on the mains and is excited from a continuous-current source H by two coils, G', G<2> in quadrature, the field currents being controlled by rheostats I<1>, I<2>. The stator windings K of the frequency changer supply the inductive excitation drop component and its rotor supplies the ohmic component and since the flux of the machine B depends ultimately on the excitation of the alternator D, the speed and power factor of the motor A may be controlled by means of the rheostats I<1>, I<2>. The influence of the angle between the axis of the flux through the armature and the axis of the induction pole of the synchonous motor C on the volts of the alternator D may be eliminated by separating the machine B from the group C, D and driving it by an independent synchronous or asynchronous motor M, Fig. 2, running on the network. Fig. 2 shows also the use of a current transformer T, with variable air gap and number of turns, in the supply to the main motor A for improving the power factor of this machine and causing its speed to vary with the load. The secondary side t<2> is connected in series with the supply to the frequency changer F from the alternator D. For adjusting the phase of the excitation voltage of the machine B, the stator and brushes of the frequency changer may be displaceable. Also, the machines C and D may be joined by a displaceable coupling or one of their stators may be displaceable. With such arrangements it is generally possible to fix the value of one of the resistances I', I<2> and leave the other variable for speed control. Fig. 3 shows an arrangement in which the excitation supplied to the fields E of the cascade machine B by the frequency changer F is amplified by a separately-driven commutator exciter N. This machine is driven by a motor P connected to the slip-rings of the main motor A and supplies a part of the inductive drop component of the excitation, the remainder being supplied by the stator of the machine F as before. The stator windings K of the frequency changer in this case are no longer essential, their place, if requisite, being entirely taken by the rotor of the machine N. A watt-responsive relay in the main supply circuit or other device and a servo-motor may be employed to control the resistances I<1>, I<2>, this arrangement being particularly suitable in fly-wheel systems where the main asynchronous machine is subject to sudden variations of load.