Patent GB-903892-A (GB)
Title: Speed control circuits for electric motors
IPC: H04N5/04
Filed: 1961-01-24
Abstract: 903,892. Automatic speed control systems; dynamo-electric machines; electrostatic generators. MINNESOTA MINING & MANUFACTURING CO. Jan. 24, 1961 [Feb. 2, 1960], No. 2828/61. Classes 35 and 38 (4). A speed control system for an electric motor 170 comprises a tachometer generator 10 coupled to the motor and providing a signal, of frequency related to the speed of the motor, to a cathode follower 11 coupled through transistor 25 to a trigger circuit comprising transistors 28, 35. The trigger circuit converts the sinusoidal signal from the transistor 25 to a square wave at the instantaneous frequency of the tachometer signal, the square wave output being amplified by transistor 40 and fed to the primary winding of a differentiating transformer 45. The secondary windings 47, 48 form part of phase detectors 51, 50, for comparing the differentiated signal with a reference signal from oscillator 70. The signal from the oscillator is introduced to a phase-shifting network 71 providing two signals which are phase displaced by 90 degrees. The lagging sine signal is supplied to the phase detector 50, whilst the leading cosine signal is fed to the detector 51. The output from the detector 50 is a series of pulses spaced at a repetition rate equal to the instantaneous frequency of the tachometer signal, and varying in amplitude at a frequency which is the difference between those of the tachometer signal and the lagging reference signal. The beat frequency signals are applied to a capacitor 87, the signal across which is of substantially sinusoidal form. The detector 51 operates in a similar way, the capacitor 86 being similar to the capacitor 87. The signal from the capacitor 86 is coupled through transistors 85, 95 to transformer 98 and a phase comparator 100 functioning as a switch operative at the peaks of the cosine input signal through the transformer 98. The other input to the phase comparator, 100 is derived from the sinusoidal beat signal provided by the detector 50. The signal across the capacitor 87 is coupled through transistor 90 and transistor 120, having smoothing capacitor 119, to differentiating circuit 111 converting the sine error signal to a cosine error signal. When the motor speed is greater than normal, the two cosine signals (one from the transformer 98 and the other from the circuit 111) introduced to the phase comparator 100 are in phase; when the speed is smaller, the two signals are out of phase. The output of the comparator 100 is applied to storage capacitor 110 coupled through transistors 112, 116, to Zener diode 131 associated with transistor 135 which also receives the sine beat frequency signal modified by the network 125. The signal at the base of the transistor 130 is an error signal at the beat frequency adjusted to compensate for variation of the motor characteristics with speed. The output from the transistor 135 is coupled through transistors 140, 145 to control transistors 171, 172, in the motor circuit. A relay winding 162 is energized once during each cycle of the cosine beat frequency, the relay having contacts in circuit with indicator 178. The repetition rate of relay operation provides an indication of the magnitude of the motor speed variation. Dynamo-electric machines.-The motor 170 has a thin disc armature with copper conductors printed on each of its faces, a number of permanent magnets being positioned adjacent the armature. The tachometer generator 10 comprises a toothed rotor on the motor shaft coupled capacitively to a toothed stator ring, Figs. 3, 4 (not shown).