Circuit for controlling an electromagnet
AssigneeLucas Industries Limited
InventorWilliam F. Hill
Life of this patent
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Abstract
A control circuit for an electromagnet includes a main current control element in series with the electromagnet winding and controlled so as to switch on and pass a "pull-in" current for an initial period and to control the current to a lower "hold" level during a subsequent period. A capacitor is connected in series with a diode and a transistor across the winding and the transistor is controlled to switch on during the change from "pull-in" to "hold" so that the capacitor is charged up during this change. A second transistor is connected in series with a second diode to permit reverse current flow from the capacitor to the winding during switch-off to hasten collapse of the flux in the winding and overcome the drop-out delaying effect of eddy currents in the electromagnet core.
Claims8
A circuit for controlling an electromagnet having a winding with first and second terminals, said circuit comprising:
- a main current control element in series with the electromagnet winding across a supply, said main current control element acting initially as a switch to allow "pull-in" current to pass through the winding from said first terminal to said second terminal, and subsequently as a current limiter to limit the current to a lower "hold" level;
- a capacitor;
- and
- current path means for connecting said capacitor across the winding, for conducting during changeover from "pull-in" to "hold", permitting current flow through said capacitor from said second terminal to said first terminal and providing a reverse charge on said capacitor, and for conducting again at switch-off, discharging said capacitor through the winding, reversing the winding current and providing a magnetizing force in opposition to that of eddy currents sustained in the electromagnet after said main current control element is switched off.
A circuit for controlling an electromagnet having a winding with first and second terminals, said circuit comprising:
- a main current control element in series with the electromagnet winding across a supply, said main current control element acting initially as a switch to allow "pull-in" current to pass through the winding from said first terminal to said second terminal, and subsequently as a current limiter to limit the current to a lower "hold" level, said main current control element comprising a main transistor;
- a capacitor;
- current path means for connecting said capacitor across the winding, for conducting during changeover from "pull-in" to "hold", permitting current flow through said capacitor from said second terminal to said first terminal and providing a reverse charge on said capacitor, and for conducting again at switch-off, discharging said capacitor through the winding, reversing the winding current and providing a magnetizing force in opposition to that of eddy currents sustained in the electromagnet after said main current control element is switched off;
- a control transistor, having an emitter and a base and having a collector operatively connected to said main transistor, for controlling said main transistor and being sensitive to the current in said winding;
- a current sensing resistor in series with the winding and connected to the emitter of the control transistor;
- and
- means, connected to the base of the control transistor, for applying a first voltage signal to the base of the control transistor during "pull-in" and a lower second voltage signal thereto during "hold".
Dependencies are inferred from the claim text.