1. Field of the Invention
The invention disclosed relates to trip mechanism for overload relays.
2. Discussion of the Related Art
Overload relays are intended to protect motors conductors against excessive heating due to prolonged motor overcurrents up to and including locked rotor currents. Overload relays are distinguished from circuit breakers, in that circuit breakers typically protect other types of branch-circuit components from higher currents acting over a shorter interval, due to short circuits or grounds.
Thermal overload relays sense prolonged motor overcurrent by converting this current to heat in a resistance element. The heat generated is used to open a normally closed contact in series with a starter coil causing the motor to be disconnected from the line.
Generally, there are three types of overload relays, the melting alloy thermal overload relay, the bimetallic thermal overload relay, and the solid state overload relay.
In melting alloy thermal overload relays, the motor current passes through a small heater winding. Under overload conditions, the heat causes a special solder to melt, tripping the relay and opening the normally closed contact in series with a starter coil causing the motor to be disconnected from the line.
Bimetallic thermal overload relays employ a bimetal strip associated with a current carrying heater coil. When an overload occurs, the heat will cause the bi-metal to deflect and trip the relay, opening the normally closed contact in series with a starter coil causing the motor to be disconnected from the line.
Solid state electronic overload relays do not require thermal units, instead use current transformers that respond directly to the motor current. Once an overload condition is reached, the electronic circuit of the overload relay trips, causing the contacts to open in a manner similar to the bimetallic thermal overload relay, opening the normally closed contact in series with a starter coil causing the motor to be disconnected from the line.
The normally closed contact in existing overload relays is typically driven by a mechanical bi-stable spring that is tripped by a complex sequence of levers that are difficult to manufacture because of the tolerances they require. Spring actuated bi-stable mechanisms can be difficult to dimension correctly making it difficult to guaranty consistent tripping positions and contact forces. What is needed is a simplified overload tripping mechanism the replaces the mechanical bi-stable spring with a mechanism that does not require difficult manufacturing steps.
The subject invention provides a simplified overload tripping mechanism for an overload relay, by replacing the mechanical bi-stable spring with two opposing magnets. The magnetically driven trip mechanism is relatively easy to manufacture and provides consistent tripping positions and contact forces in an overload relay. The invention comprises a tripping actuator having a first permanent magnet and a moveable contact carrier having a second permanent magnet mounted in an opposed orientation to the first permanent magnet. A moveable electrical contact on the moveable contact carrier is urged, by repulsion between the magnets, to make electrical connection with a stationary electrical contact, when the tripping actuator is in an ON position and the moveable contact carrier in a first stable position.
The overload relay may use an overcurrent sensing mechanism, such as a bimetallic thermal overload sensor that employs a bimetal strip associated with a current carrying heater coil. The heater coil may be connected in series with a power source and a motor. The bimetal strip is configured to deflect from heat produced by the heater coil when an overcurrent condition occurs. The bimetal strip is connected to the tripping actuator and when an overcurrent condition is sensed, it moves the tripping actuator.
When the tripping actuator is moved to an OFF position in response to an overcurrent condition being sensed by the bimetallic thermal overload sensor, the first permanent magnet passes the second permanent magnet in a first direction through an over-center tripping position. The proximity of the first and second permanent magnets causes them to repel each other and urge the moveable contact carrier and its moveable contact toward a second stable position, moving away from the stationary contact in an opposite, second direction, to break the normally closed electrical connection with the stationary electrical contact. The opposing magnets provide the over-center trip function and apply the proper force to open the contacts.
The invention may include an auto-reset mode to automatically restore the normally closed electrical connection with the stationary electrical contact, after an interval has passed since the overcurrent condition has subsided. When the overcurrent condition subsides and the heater coil cools, the bimetal strip is configured to reverse its deflection, thereby moving the tripping actuator in the second direction, back through the over-center tripping position. The first and second magnets repel each other, to thereby urge the moveable contact carrier and its moveable contact to return toward the first stable position, to make the normally closed electrical connection with the stationary electrical contact. In the auto-reset mode, the contact carrier is blocked in a position so that it cannot move to the full off position, so that when the tripping actuator returns, it can cause the reset automatically. Without the contact carrier blocked, it moves to a position where the tripping actuator cannot move far enough to cause auto reset and a reset button may then be used
The invention may include an adjustable mount supporting the first magnet, to enable changing the location of the over-center tripping position by adjusting the orientation of the magnet, to thereby change the set point and sensitivity of the mechanism.
Example embodiments of the invention are depicted in the accompanying drawings that are briefly described as follows:
In the example embodiment, the tripping actuator 12 is mechanically coupled in this manner to the bimetallic thermal overload sensor 16. The tripping actuator 12 has a pivoted end mounted on a pivot 14 on a base 10 in the housing 10′. The tripping actuator 12 is shown in
The tripping actuator 12 has a first permanent magnet 18 mounted on an end opposite to the pivoted end, with north-south poles of the first permanent magnet 18 oriented in a substantially radial direction with respect to the pivot 14. The first permanent magnet 18 moves in the first direction 11 when the bimetallic thermal overload sensor 16 causes the tripping actuator 12 to move in the first direction 11 in response to an overcurrent condition being sensed by the bimetallic thermal overload sensor 16. When an overload occurs, the heat will cause the bimetal strip 16A to deflect and move the tripping actuator 12 in the first direction 11.
A moveable contact carrier 20 is slideably mounted on the base 10. The moveable contact carrier 20 includes a moveable electrical contact 22 of the overload relay. The moveable electrical contact 22 may be on or actuate the contact carrier 20. The moveable electrical contact 22 may be located near the end of a flexible conductor wire 22′. The moveable electrical contact 22 is in a normally closed electrical connection with a stationary electrical contact 24 of the overload relay, as shown in
The first permanent magnet 18 passes through the over-center tripping (T) position 32 when the first permanent magnet 18 moves in the first direction 11 past the second permanent magnet 28. Their proximity causes the first permanent magnet 18 and the second permanent magnet 28 to repel each other and urge the moveable contact carrier 20 and its moveable electrical contact 22 to slide in the second direction 13 toward a second stable position 26 away from the stationary electrical contact 24, as shown in
As the tripping actuator 12 returns to rest in the ON position 15 in
An adjustable mounting 30 on the tripping actuator 12 supports the first magnet 18. The degree of repulsion between the first permanent magnet 18 and the second permanent magnet 28 may be adjusted by rotating the adjustable mounting 30 to change the orientation of the first magnet 18 in the adjustable mounting 30, thereby changing a location of the over-center tripping (T) position 32, and the set point and sensitivity of the mechanism.
A manual reset button 27′ (
In the auto-reset embodiment shown in the figures, there is also an automatic reset (A) position of
In an alternate example embodiment of the invention, the moveable contact carrier 20 may further include a second moveable electrical contact (not shown) on or actuated by the moveable contact carrier 20. The second moveable electrical contact may be configured to be urged, by the repulsion between the first and second permanent magnets 18 and 28, to remain disconnected in a normally open electrical connection with a second stationary electrical contact (not shown), when the tripping actuator 12 is in the ON position 15 and the moveable contact carrier 20 in the first stable position 26′. The second moveable electrical contact may be configured to make a connection with the second stationary electrical contact in the normally open electrical connection, when the first permanent magnet 18 passes the second permanent magnet 28 in the first direction 11 through the over-center tripping (T) position 32. This occurs when the tripping actuator 12 is moved to the OFF position 23 and the moveable contact carrier 20 is in the second stable position 26 in response to the overcurrent condition being sensed by the overcurrent sensing mechanism 16. The second moveable electrical contact may be configured to break the connection with the second stationary electrical contact in the normally open electrical connection, when the first permanent magnet 18 passes the second permanent magnet 28 in the second direction 13 through the over-center tripping (T) position 32. This occurs when the tripping actuator 12 is moved to the ON position 15 and the moveable contact carrier 20 is in the first stable position 26′, in response to the overcurrent condition being sensed to subside, by the overcurrent sensing mechanism 16.
The overcurrent sensing mechanism of the present invention might use any one of a melting alloy thermal overload sensor, a bimetallic thermal overload sensor, or a solid state overload sensor.
Although specific example embodiments of the invention have been disclosed, persons of skill in the art will appreciate that changes may be made to the details described for the specific example embodiments, without departing from the spirit and the scope of the invention.
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Extended European Search Report for Application No. 15186544.1-1808 dated Mar. 17, 2016. |
Number | Date | Country | |
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20160126039 A1 | May 2016 | US |