Patent GB-1201353-A (GB)
Title: A drive system for an electric truck
IPC: B60L11/18
Filed: 1967-11-20
Abstract: 1,201,353. Control of D.C. motors. EATON YALE & TOWNE Inc. 20 Nov., 1967 [22 March, 1967], No. 52618/67. Heading H2J. In an electric truck, a D.C. motor is supplied from a battery through a semi-conductor switch with unidirectional current pulses of uniform amplitude. The pulses are generated at a substantially fixed frequency, their durations being adjustable over a predetermined range. The rate of increase of the pulse-duration is limited for a predetermined time interval after the motor is initially energized to limit acceleration of the truck. The rate-limiting means comprises a capacitor whose charge is varied as a predetermined function of time after initial energization of the motor, the pulse-duration being limited in accordance with the charge on the capacitor. The pulse-duration is adjusted by a multi vibrator circuit including a variable resistor, the rate-limiter having a transistor for variably shunting the resistor. The field windings F1, F2, are connectible in series with armature A1 for rotation in opposite directions, and are adapted to be arranged in series or parallel with each other. The duty cycle of power transistors Q1, Q2, is controlled by the setting of rheostat R24 which is operated by an accelerator and forms a timing element of a pulse-width controlling multivibrator circuit 13. At the end of the range of speed control provided by the rheostat R24, the transistors Q1, Q2, are by-passed and full battery voltage is applied to the motor. The field windings are then placed in parallel rather than in series for further increase in speed of the traction motor. An inverter power supply 11 comprises transformer T1 with feedback windings W3, W4, interconnected with transistors Q7, Q8, in an oscillator circuit. Rectifier QS1 is triggered into conduction only when the oscillations of the inverter have reached full amplitude, so that contactors K1W, K2W, are not energized to complete the motor circuit until proper bias voltages are applied to the transistor circuits. The power transistors Q1, Q2, and driver transistors Q3, Q4, are cut off until bias voltages are available. The multivibrator circuit 13 is supplied with triggering signals of constant frequency from winding W7, rheostat 24 being shunted by rheostat 24A for preselecting the maximum pulse width and the speed. A thermistor TH1 reduces the pulse width provided by the circuit 13 when the transistors Q1 Q2, are over-heated due to stalling of the motor. Prior to initial energization of the motor, transistors Q21, Q20, are respectively non-conductive and conductive so that capacitor C20 is charged. Transistor Q12 shunts the rheostat R24 whereby the pulses operated by the circuit 13 are of short duration. When one of the contactors K1W, K2W, is energized, transistor Q21 is turned-on to discharge capacitor C20 through transistor Q12 so that the limiting valve of the pulse duration is increased at a timed rate to allow acceleration of the track. When the respective contactor K1W or K2W is deenergized, the transistor Q21 is turned-off and transistor Q22 conducts to remove the triggering signal input to the circuit 13. This ensures that the transistors Q1, Q2, do not pass the full pulse current at the moment when the contacts of the contactor are opened. At the end of the timed acceleration period, capacitor C20 is fully discharged and biasing current is applied to the emitter of unijunction transistor Q23 to trigger rectifier QS2 in series with relay K6W. Thus, full battery voltage is not applied to the motor until the period of timed acceleration is completed. When the field connections of the motor are reversed for braking, the motor re-generates through the battery and diodes D1, D2, provided that the armature voltage exceeds the battery voltage. In the range over which the generated voltage is less than that of the battery and diode D2 is inoperative, a signal representing the armature voltage is applied to transistor Q25 to charge capacitor C20 through transistor Q20, and transistor Q12 conducts to limit the energization of the motor, the pulse duration being an inverse function of the armature voltage. During normal pulsed operation of the motor, capacitor C25 is charged through transistor Q25 and then discharged by transistor Q26. The threshold of trigger diode DT1 is not reached and rectifier QS3 is non-conductive. If however, one of the transistors Q1, Q2, becomes shortcircuited, the battery voltage is applied to the motor for longer than the normal period between pulses so that the capacitor C25 is charged sufficiently to trigger rectifier QS3 which turns-off transistor QP. This de-energizes the motor, all the power transistors sharing the motor current during the period in which the respective contactor releases its contacts so that the short-circuited transistor does not carry the whole motor current between pulses. The operation of the protection circuit 15 is prevented before the motor is initially energized, when it is being operated at full battery voltage, and during the brief interval after one of the contactors K1W, K2W, is energized and before the respective contacts operate.