Patent GB-597032-A (GB)
Title: Alternating-current motor control systems
IPC: H02P27/02
Filed: 1944-07-06
Abstract: 597,032. Control of A.C. motors. ROBINETTE, W. C. July 6, 1944, No. 12971. Convention date, Feb. 16, 1943. [Class 38(iii)] [Also in Group XXXVIII] A control system for an induction motor comprises means for supplying a voltage to one phase. winding of said motor, means coupled thereto for deriving a voltage from said first means for application to a second phase winding of said motor, said second means including a coupling transformer, an amplifier having its input circuit connected to the secondary winding of said transformer and its output circuit coupled to second phase winding, and a phase shift' circuit intermediate said transformer and said amplifier, a device mechanically coupled to said motor and actuated thereby to cause said system to reverse the torque of the motor and means for effecting said reverse of torque in advance of the motor reaching its true balance-point. The motor may be made to reverse in advance of the true balance-point by an amount proportional to. motor-speed. An induction motor of the two-phase or single-phase type with a starting winding has a squirrel cage rotor and two stator windings disposed in quadrature. The winding designated as Phase A is fed directly from an alternating current source through an impedance 17, and Phase B is fed from the secondary of a transformer 21, the primary winding of which is in the output circuit of a power amplifier 23, the grid excitation of which is determined according to the current flowing in Phase A. The power amplifier consists of two thermionic valves 25 and 27, connected in push-pull. The primary winding of the transformer 21 is centre-tapped and connected to an electronic rectifier circuit consisting of two half-wave rectifier tubes 45 and 46 developing a direct voltage across condensers 47. The voltage to be amplified is applied across the grid leak resistances 37 and 39 in series. A transformer 51 has a primary winding 53 connected in series with the Phase A winding, and a variable resistor 54 affords manual adjustment of the phase displacement of the primary current. The transformer 51 is designed to operate near the bend of its magnetic saturation characteristic so as to provide considerable distortion of the waveform of the transformer secondary voltage, the amount of distortion being controllable through an adjustable resistor 56. The secondary of the transformer 51 is connected across a high resistance 61, and this circuit is tuned by means of series condensers 63 to a frequency at which higher odd harmonics will predominate and will lag the fundamental. A shift in phase of the input voltage to the power amplifier may be effected by varying the resistance between the grid 33 and the end 69 of the resistance 61. such change causing alteration of the phase relationship of the voltages in the Phases A and B of the motor, which may be sufficient to reverse the motor. A photo-electric cell, or two contacts with varying pressure applied thereto may be connected between the grid 33 and the point 69. In the arrangement shown, the motor is connected through gearing to a shaft carrying a slip-ring 75 which is connected through a brush to the point 69. A shaft 79 is independently rotatable and is provided with a slip-ring which is connected through a brush to the grid 33. The shafts carry radially supported contacts 85 and 87. When the motor is started, the contacts will meet and when the resistance there-between becomes sufficiently low the grid potential of the push-pull amplifier is reversed, together with the Phase B winding of the motor. Reversal of Phase B rapidly decelerates the motor and reverses its rotation. The motor will, in effect, come to a stop with imperceptible oscillation of pressure between the contacts sufficient to change the contact-resistance over a range necessary to effect a continuous repetition of reversal of the motor. Rotation of the shaft 79 causes the motor to rotate until the contacts 85.and 87 again meet. Means are provided to prevent hunting of the contacts, comprising a gear train 115, Fig. 2, similar to that shown previously except that the contact 117 is flexible and is mounted on a hollow shaft 119. The other contact 118 is rigid. A yoke 121 is connected to a shaft 123 which terminates in a magnet 125. An aluminium cup 129 is affixed to an idler gear 127 and a soft iron ring is adapted to be drifted in and out of the plane of the magnet. As the motor rotates the gear train, the aluminium cup rotates in accordance with the speed of the motor and drags the magnet with it. The shaft 123 transmits its motion to the yoke 121 which engages the contact 117 and causes it to flex. The contact 117 assumes a position in advance of that it would normally occupy so as to prevent over-shooting of the balance-point,