Patent GB-1222205-A (GB)
Title: Improvements in control systems for cranes
Inventor: DAVEY ANTHONEY WALTER
IPC: B66C13/26
Filed: 1967-07-21
Abstract: 1,222,205. Crane or hoist motor control systems. HERBERT MORRIS Ltd. 12 July, 1968 [21 July, 1967], No. 33622/67. Heading H2J. [Also in Division G3]. The lifting and lowering speed of an A.C. crane or hoist motor A.B.C. e.g. a squirrel cage or slip ring motor is controlled by supplying a sawtooth voltage waveform to the triggers of supply thyristors D1, D4, D7. The firing angle is controlled by deriving a square wave of fixed amplitude from the phase terminals P, Q, R by means of a Zener diode D19 and ballast resistor R10, a Zener diode D29 and ballast resistor R11, and a Zener diode D34 and ballast resistor R12  in the respective phases. The square waves charge respective capacitors C6, C13, C18 via resistors R25, RV1 and RV2 to produce sawtooth waveforms which are applied to the bases of transistors TR3, TR6, TR9. The transistors TR3, TR6, TR9, conduct when their respective base voltages exceed the common emitter bias voltage 100. When the transistors TR3, TR6, TR9 conduct they drive transistors TR2, TR5, TR8 to provide an output across collector loads R19, R31 and R100. This voltage drives blocking oscillators so that their transistors TR1, TR4, TR7 conduct to produce a pulse in the primary of transformers T2, T3, T4 which induces a voltage in the secondary to fire the gates of thyristors D1, D4, D7. The feedback windings of transformers T2, T3, T4, have a voltage induced therein which charges capacitors C3, C9, C14 until transistors TR1, TR4, TR7 are cut off. When the pulse ceases capacitors C3, C9, C14 discharge through resistors R17, R18, R29, R30, R39, R40 until transistors TR1, TR4 and TR7 again conduct. Resistors RV1, RV2 are adjustable to ensure that the rise of the sawteeth of all three phases are identical. For hoisting all loads and lowering light loads bias 100 is set to zero so that the thyristors are turned on as soon as the sawteeth commence. When set speed is reached the output of tachogenerator G causes transistors TR13, TR14 to conduct to raise the bias 10O until just sufficient power is applied to the motor to maintain the set speed. For lowering heavy loads, where the motor is used as a brake, the bias 100 is set to a level above the peak of the sawteeth so that the load will descend under gravity until the set speed is reached whereupon tachogenerator G will cause transistor TR15 to conduct and lower the bias 100 causing the thyristors to conduct and brake the motor. Depending on whether a raising or lowering notch is selected a rectifier D52 or D53 (Fig. 2A) is inserted in the tachogenerator circuit to oppose its output. Should for example a lowering motion take place when on a hoisting notch, rectifier D52 will conduct operating relay RL2 (Fig. 2B) to de-energize contactor A (Fig. 2A) and apply brake D (Fig. 2A). If the load is too light to descend under gravity when brake D is released a circuit comprising transistor TR10, relay RL3, transistors TR11, TR22 and relay RL4 will after a short delay reverse the motor and tachogenerator connection for wind down at the correct speed. When a slip ring motor is used contactors J, K, L (Fig. 2A) are operated at the correct motor speeds to switch their associated rotor resistances R1-R9 into and out of circuit. The contactor K is controlled by transistors TR18, TR19 and relay RL6, the contactor J by transistors TR16, TR17, relay RL5 and the contactor L by transistors TR20, TR21, and relay RL7, each circuit being supplied by tachogenerator G from separate potential dividers. If on lowering, the maximum set speed is exceeded, transistor TR23, Zener diode D54 and relays RL1, RL2 ensure that the Main Contactor is de-energized to cut off power to the motor until reset button X is operated (Fig. 2A) and brake D is energized. In Figs. 4A, 4B (not shown) an A.C. tachogenerator is used instead of a D.C. one.