GB740623A - Improvements in or relating to electric motor control systems - Google Patents

Abstract

740,623. Control of several A.C. motors. ELECTRIC & MUSICAL INDUSTRIES, Ltd. Nov. 18, 1953 [Dec. 2, 1952], No. 30480/52. Class 38(3) In Fig. 1, two identical three-phase synchronous motors 10, 10<SP>1</SP>, have deltaconnected stator windings 11, 11<SP>1</SP>, and unwqund rotors 14, 14<SP>1</SP>, respectively, the motors being mechanically coupled in driving relation to two devices to be operated in synchronism. The stator windings of the motors are connected in parallel through leads 22, 23, 24, so that the motors are started and operated in synchronism by connecting these leads directly to a three-phase supply through contacts 17, 20, of a three-position switch 16. To stop the motors, contacts 20 are engaged with contacts 19, the motors being supplied with reverse-current for braking. Impedances 25 limit the braking-current to a value below that required to start the motors from rest so that reversal does not take place after the motors have been brought to a standstill. The impedances 25 may be adjustable to vary the value of the braking-current. The permanent connections 22, 23, 24, are used to convey synchronizing current between the motors should they tend to fall out of step during periods of acceleration and deceleration. In Fig. 2, the motors may have deltawound rotors, two rotor terminals 31, 32, and 31<SP>1</SP>, 32<SP>1</SP>, on each motor being interconnected at 34, 34<SP>1</SP>, and leads 33, 35, connecting the rotor windings to a source of direct-current. If it is desired to operate the motors at sub-synchronous speeds, the stator and rotor windings are supplied with currents at different frequencies, the leads 33, 35, then being connected to a single-phase supply. The wound rotors of the motors may be supplied with both alternating and direct currents, the leads 34, 34<SP>1</SP>, being replaced by the centre-tapped secondary windings of a transformer, Fig. 3 (not shown).