Patent GB-803365-A (GB)
Title: Improvements in and relating to electric motor control systems
Inventor: PROWSE DONALD WINDSOR
IPC: B66B1/12
Filed: 1955-09-15
Abstract: 803,365. Automatic speed control systems. BRITISH THOMSON-HOUSTON CO., Ltd. Sept. 13, 1956 [Sept. 15, 1955], No. 26476/55. Class 38 (4). [Also in Group XXXVII] An induction motor 1 has a tachometer generator 2 coupled to it, the voltage of this generator being compared with a reversible reference voltage VR and the difference applied to a field winding A of an amplidyne generator 3 which supplies a pilot motor 4. A cam-operated controller 5 is driven by the pilot motor for determining the speed of the motor 1 in accordance with the value to which the reference voltage is adjusted. When the motor 1 is braked so that the tachometer generator is stationary, the field winding A is so excited that the pilot motor would normally operate to decrease the rotor resistance of the motor 1. In order to select a torque-value to be immediately available when the brakes are released, however, the rotor resistance is adjusted to a desired value while the brakes are still applied. The controller 5 comprises main contactors C2 ... C5 for regulating the rotor resistance of the motor 1, contact bars W, W<SP>1</SP> ... Z, Z<SP>1</SP> being arranged to close contacts Cw ... Cz when moved by the pilot motor in a selected direction. The driver's lever can be adjusted to select one of three values of torque before releasing the brakes, and, if the lever is moved to a first position in the forward direction, a pilot relay PC2 is energized to close contacts PC2<SP>1</SP> and actuate contactor C2. The controller is now driven forward until contacts Cx close and pilot relay; PC3, is energized. The closure of contacts PC3<SP>1</SP> does not immediately energize the main contactor C3, since contact TL2<SP>1</SP> is not closed in this position of the driver's lever, and contact T2<SP>1</SP> is associated with a relay T2 which delays the closure of the contact for a period after its coil has been shortcircuited by contacts PC3<SP>11</SP>. The opening of contacts PC3<SP>111</SP> de-energizes contactor TLD whereby contacts TLD<SP>1</SP> open to introduce resistor RL in series with field winding A. The field winding B of the amplidyne thus becomes effective to move the controller in the opposite direction until contacts Cx reopen to de-energize relay PC3. As long as the first torque-value is selected, this procedure will continue but the main contactor C3 is never energized, the hunting-action being undertaken by the pilot relay PC3. If the second torque-position is selected, contacts STL2<SP>1</SP> are closed, thereby energizing relay TL2 to close contacts TL2<SP>1</SP>, TL2<SP>11</SP>. As soon as contact PC3<SP>1</SP> closes, the contactor C3 is actuated and the controller continues to creep forward until contact-bar Y closes Cy. As before, the automatic introduction of resistance in the circuit of field coil A prevents energization of contactor C4 and oscillatory action will affect only the pilot relay PC4. If the third torquevalue is selected relay TL3 is energized and the subsequent sequence of operations is similar to that described. The field winding B is supplied from a magnetic amplifier 7, a control coil of which is excited in accordance with the stator current of the induction motor. When the brakes are released, the field winding B provides torque-limit control, the resistor RL being permanently short-circuited. The release of the brakes de-energizes relays T2 ... T4 so that the time-delay action of the contactors C2 ... C5 is eliminated thereafter. In a modification the time-delay relays are replaced by a single relay having three contacts, each in series with one of the contactors C3, C4, or C5, Fig. 4 (not shown).