Zero cross relay actuation method and system implementing same

Details Zero cross relay actuation method and system implementing same Zero cross relay actuation method and system implementing same

1999-09-01

2001-05-15

Sherry, Michael J. (Department: 2836)

Electricity: electrical systems and devices

Control circuits for electromagnetic devices

For relays or solenoids

C361S152000, C361S187000, C361S002000

Reexamination Certificate

active

06233132

ABSTRACT:

FIELD OF THE INVENTION
The instant invention relates to relay switching circuits, and more particularly to relay timing and control circuits for ensuring zero cross switching of a relay.
BACKGROUND OF THE INVENTION
The switching of electric power has long been a requirement for the operation and control of various systems. These systems include everything from the simple flipping of a light switch to turn on a light or the resetting of a circuit breaker switch which has automatically tripped due to a circuit overload, to the very complex and sophisticated computer controlled switching and load shedding of electric power on the space shuttle. While manually operated electrical switches are adequate for many of these applications, increasingly electronic control is being utilized to effectuate the switching of electric power. Even modern room lighting systems utilize electronic motion sensors to control electrically actuated switches to turn on and off lights within a room.
While small control electronics are well suited for processing the required inputs and performing the required logic to control the switching of the electric power, many of these electronic components operate on digital voltages and currents and are not suitable for the switching of the greater amounts of electric power needed to operate most electrical equipment. While there have been many advances in the development and manufacture of high power switching electronic circuitry, the cost and cooling requirements of these devices, such as IGBTs, MCTs, and MOSFETs, preclude their application in many electric power switching applications. In many of these applications, ranging everywhere from consumer appliances, to electronic wall-mounted hand dryers, to large computer controlled factory equipment, the use of the electronically controlled electromechanical relay provides the required function at a cost and with a reliability which is acceptable.
A typical electromechanical relay, such as that illustrated in
FIG. 5
, typically comprises at least one, and possibly two drive coils
10
. In the case of a single coil relay, the coil
10
is energized to create a magnetic field which pulls a moveable contact electrode
12
into physical contact with a stationary contact electrode
14
to complete the electrical circuit between the two power terminals
16
,
18
for a normally open relay. If the relay is of the normally closed type, the energization of the drive coil
10
will create a magnetic field which separates the physical contact of the two contact electrodes
12
,
22
thereby breaking the electrical circuit between the two power terminals
18
,
20
. These single coil relays also typically include a bias spring (not shown) to hold the moveable contact electrode into its quiescent state, i.e. away from the stationary contact electrode
14
for a normally open relay, and in contact with the stationary electrode
22
in the normally closed type relay. Various other designs are available for relays depending upon the particular application requirements. More sophisticated electromechanical relay designs include both a drive open and a drive close coil, requiring the application of an electrical drive signal to both open and close the relay. Other designs include latching type relays which allow the coil current to be switched off once the relay has transitioned, as well as coil cutthroat mechanisms which ensure that both the open and close drive coils are not energized at the same time. Other relay designs provide both normally opened and normally closed contacts, and many provide auxiliary contacts for relay position sensing for feedback control.
Regardless of the particular construction of the actual relay switching element, its reliability will be determined by the number of cycles it will withstand in its lifetime. As one skilled in the art will recognize, the mechanical simplicity and robustness of a typical relay design does not provide the limiting factor which determines the relays life. Instead, the typical limiting factor in a relay's life is a purely electrical phenomenon occurring in most relays upon the opening and closing of the contact electrodes. Specifically, the opening and closing of the contact electrodes results in an electrical arc forming across the contacts for a small period of time. The period of time during which an arc flows is determined by many factors including the mechanical bounce of the contacts upon closure, the distance between the contact electrodes, the magnitude of current flowing, as well as the level of ionization of the air in the gap between the contact electrodes. This electrical arc will also be extinguished, in the case where an AC current is being switched, when the voltage between the contacts traverses through zero and the cycle changes from positive to negative or negative to positive.
The electrical arc between the contact electrodes of an electromechanical relay limit the life of the relay in essentially two ways. First, the electrical arcing leaves carbon deposits on each of the contact electrodes which, over time, build up to form a high resistance contact between the contact electrodes. This high contact resistance results in increased heat dissipation within the electromechanical relay, as well as reduced voltage available at the relay output. Eventually, the material build up on the contact electrode surfaces will result in intermittent contact of the contact electrodes. This intermittent contact results in the electrical circuit not being completed when the relay is energized due to the insulating properties of the build up material which prevent a physical contact of the conductive material of the contact electrodes.
A second way in which the life of an electromechanical relay is shortened by the electrical arc formed between the contacts during opening and closure thereof is a result of the extreme heat of an electrical arc. Specifically, as an electrical arc is drawn between the two contact electrodes, a small portion of the contact electrode material will be melted or vaporized off of the surface. The amount of material burned away during each cycle during which an arc is formed is a function of the voltage and current which the relay is attempting to switch. The higher the current flow between the electrical contact electrodes, the hotter the electrical arc, and thus the more contact material that is burned away. A second factor is the amount of contact material on the surface of the contact electrode. While gold provides a very high fidelity electrical contact, its expense requires that it be plated onto the surface of the contact electrode in relatively thin layers. These gold plated contacts are particularly susceptible to failure from electrical arcs drawn during the switching operation due in part to the small amount of gold which is present and in part because of the softness of gold itself.
An alternate failure mode of electromechanical relays due to the arc generated, primarily during closure of the contacts, is the welding together of the contact electrodes. Specifically, as the contact electrodes come into contact, the force with which they are brought together typically results in a slight mechanical bounce of the two contact electrodes, resulting in multiple contact and separation events in a very short period of time. Each of these bounce events results in the generation of an electrical arc which tends to greatly increase the temperature of the contact electrode surfaces. This particular failure mode is generated when the surface material on the contact electrodes is heated to a sufficient degree to liquify, to some degree, the surface material. If both electrical contact surface materials are liquefied and the contact electrodes are brought into physical contact, these two electrodes will be welded together. Such an event is a latent failure, the existence of which is not known until it is desired to break the electrical contact to de-energize the load to which the relay is connected. At that point it is realized that the relay h

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