GB1384001A - Magnet controller - Google Patents

Abstract

1384001 Control systems for electromagnets SQUARE D CO 10 Jan 1973 [19 Jan 1972] 1299/73 Heading H2H A control system for energizing and demagnetizing an electromagnet or other inductive device, comprises switching means for connecting the electromagnet to the source of power and reverse switching means to cause a reverse current to flow on cessation of the charging or lift current, the reverse current being ceased when it reaches a predetermined relationship to the lift current. In Fig. 2, closing of switch 36 operates relay 15 to close contacts 15a, 15b and 15c which apply lift current through magnet 52. In the "drop" position, contact 37 closes and for a short overlap period, both switches 36 and 27 are closed, operating relay 17 .to close contacts 17a, 17b and 17c. This places resistor 21, and resistors 26, 27 in parallel with the magnet. The potential across resistor 27, proportional to lift current, is applied via lead 41, resistor 107, diode 99 to capacitor 101 which, via emitter followers 90, 91, maintains junction 94 at a potential representative of lift current. When switch 36 opens, contacts 15a, 15b, 15c open and reverse polarity is applied to the magnet. Eventually the current reverses, resulting in a potential across resistor 26 which is applied via resistors 120, 122 to the base of transistor 86; when this potential reaches the required level relative to the emitter (which is still at the potential representative of the lift current) transistor 86 conducts, rendering transistor 80 conductive to charge capacitor 76 and trigger unijunction transistor 71 which pulses thyristor 64 to conduct. This thyristor 64 bypasses relay 17 which releases to open contacts 17a, 17b and terminate the reverse current through the magnet. A further circuit means is provided (resistor 74) to terminate the reverse current after a predetermined time period, even if the reverse current has not reached the required level for automatic control.