Patent GB-860431-A (GB)
Title: Improvements in and relating to electrically driven hoists, winders and the like
Inventor: ABRAM LEO
IPC: B66B1/06
Filed: 1956-03-06
Abstract: 860,431. Control of D.C. motors. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. Feb. 26, 1957 [March 6, 1956], No. 7021/56. Class 38 (3). [Also in Group XXXV] In an arrangement for controlling the speed of a D.C. hoist or winder motor M, Fig. 1, fed from an A.C. supply through a converter R of the mercury arc rectifier type and reversing switches R 1 , R 2 , F 1 , F 2 , the firing of the converter is determined by the difference between the actual speed and a reference quantity and means are provided whereby the armature connections are reversed and the converter is arranged to act as an inverter and thus provide regenerative braking when the motor speed is above the desired value, the motor current being thus controlled to increase the motor torque in response to increase in error in either sense in the motor speed from the desired value. The voltage difference between the output of tachometer generator TG and the proportion of the reversible D.C. reference voltage, applied to resistance RES5, Fig. 4, taken off by the wiper thereon which is controlled by the driver's lever, is connected through reversing switches to control the phase of grid pulses supplied by peaking units PUT1-PUT3, Fig. 2, by means of transductors TD2-TD4 and TD5-TD7, the control windings CW of the last-mentioned being connected in series with a limiter resistance RES16 across the output of self-saturating type transductor TD1. Acceleration control is provided by a transductor TD3 having its output winding, which is saturated and therefore of low impedance during constant speed running connected in series with the output winding of speed-responsive transductor TD2, the transductor TD3 being controlled by a limiting potential applied across resistance RES9 and the potential across condenser CP4 fed through a resistance RES8 from the motor brushes. In this arrangement different limits for acceleration and retardation are obtained by including in the limiting potential supply a resistor RES14 which is short-circuited during retardation. Current limitation is provided by a similar circuit including a D.C. transformer TD8 developing across a resistor RES11 a potential proportional to current which is used in conjunction with a limiting voltage to control transductor TD4. This transductor is also normally saturated. Each of the three controlling circuits includes a further resistor across which a derivative is applied to prevent hunting. Change-over from driving to braking is effected by a relay RER, Fig. 5, by the reversal in potential of points c, b in the speed comparator circuit, Fig. 4, i.e. when the potential of point c is below that of b. The relay is operated through rectifiers RC6 and RC7 for forward and reverse running, respectively and magnetic amplifiers TD9, TD10, a resistor RES20 being included in the biasing circuit of the latter and being short-circuited when the relay is operated, to prevent chatter. Operation of the driving lever to close switch MCF operates forward direction-selective relay FDS which operates the self-holding relay P to apply the grid potentials to converter 1 from a grid control transformer GCT and to operate forward relay F. This latter relay completes the motor armature circuit and the hoist, in moving, operates speed-detector switch SDS to provide by means of relay FD a holding circuit around the driving switch MCF. The motor field MF is supplied from a constant source of D.C. and electrical and mechanical interlocks are provided. In the event of overspeed, inverter relay I is energized after relays P and F have released and relay R is in consequence energized. The change-over of the motor connections by the release of relay F and the operation of relay R together with the phase-displacement of the grid pulses by approximately 60 degrees by contacts I 1-6  provides controlled regenerative braking until the desired speed is attained. In a modification described with reference to Fig. 6 (not shown), contacts of relays P and I are included in the primary circuit of transformer GCT.