Abstract:
In a monitor for an inductive load recycling by a thermal cut-off sensor is prevented by maintaining an off condition with a delay. The delay has an adjustable time in excess of the cut-off sensor&#39;s reset time.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to protective monitors or controls, and particularly to monitors or controls for protecting compressors in air conditioning units.  
         BACKGROUND OF THE INVENTION  
         [0002]    Loads such as motors in air conditioning units, especially air conditioning compressors, may draw excessive currents and overheat during operation. To protect the compressor and the unit from the effects of overheating, a thermal protective device in the unit may cut off further flow to the compressor. The protective sensor device later allows current to the unit to resume. However, the thermal protective sensor often allows current to restart before the unit has actually recovered from its overloaded condition. Then the thermal protective device again cuts off current. This cut off and resumption of current may recycle many times before system operates normally. Repeated cycling shortens the life of the sensor contacts as well as the compressor.  
           [0003]    An object of the invention is to overcome these difficulties.  
         SUMMARY OF EMBODIMENTS OF THE INVENTION  
         [0004]    According to an embodiment of the invention, a thermal protector opens in response to excess current or heat and a delay defers resumption of current flow through the thermal protector by a period that extends beyond the expected current resumption.  
           [0005]    According to an embodiment a reset relay responds to the delay to maintain the off condition during an adjustable period.  
           [0006]    According to an embodiment this is accomplished by an arrangement external to the unit. 
       
    
    
       [0007]    The various features that characterize the invention are pointed out in the claims forming a part of this specification. Other objects and advantages of the invention will become evident from the following detailed description when read in light of the following drawings.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a schematic drawing illustrating an embodiment of the invention.  
         [0009]    [0009]FIG. 2 is a schematic drawing illustrating other details of the embodiment of FIG. 1  
         [0010]    [0010]FIG. 3 is a cross-sectional view of a monitor such as that of FIGS.  1  or  2  in a casing with connectors. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0011]    In FIG. 1, a monitor MO 1  embodying the invention appears between a power source SO 1  and an inductive load LO 1  such as an industrial air conditioning unit. In the un-energized state, suitable means (not shown) bias an armature AR 1  of a single-pole double-throw relay RE 1  upward as illustrated in the drawing and into connection with a contact K 1 - 1  of the relay RE 1 . This permits current flow from the source SO 1  through an input lead L 1 IN, through the armature AR 1 , and through a normally closed thermal protection heat sensor SE 1  to an output lead L 1 IN and the load LO 1 . The current returns through lines L 20 UT, LT 2 IN to the source SO 1 . Normal current can thus flow from source to load.  
         [0012]    In case of an overload and current rising above a predetermine value, the thermal protection heat sensor SE 1  heats up, senses the overload, and opens. This cuts power from the source SO 1  to the load LO 1 . This closes a path from the source SO 1  through the coil CO 1  of the relay RE 1  via a normally closed contact K 2  of a single-pole single-throw relay RE 2 , and to the load LO 1 . This path has sufficiently high impedance to limit current to less than 5 amperes. This low current switches the armature AR 1  of the relay RE 1  to the contact K 1 - 2 . As a result current starts to flow through an adjustable normally-conductive time delay DE 1 , through a coil CO 2  of a relay RE 2 . The term normally-conductive means conductive when un-energized or energized below a predetermined current value.  
         [0013]    The start of the current immediately pulses the delay DE 1  and renders the latter substantially non-conductive for an adjustable period such as 6 to 300 seconds. The term non-conductive in this instant means insufficiently conductive to actuate the coil CO 2  of the relay RE 2 . More specifically the rating of the relay coil CO 2  is such as to prevent the initial current therethrough and the current of the non-conductive delay DE 1  from actuating the relay RE 2 . Hence the normally closed contact K 2  keeps coil CO 1  energized, which pulls the armature AR 1  of the relay RE 1  downwardly (in the drawing) against the contact K 1 - 2 . This cuts off, and continues to cut off, current between the source SO 1  and load LO 1  through the thermal sensor SE 1 .  
         [0014]    After the thermal sensor senses a reduction of the temperature to its predetermined level, the sensor SE 1  closes its contacts. However the delay DE 1  still keeps the contact K 2  closed. The adjusted delay period of the delay DE 1  to its conductive condition is sufficiently long to outlast the resumption of the sensor SE 1  to its conductive condition.  
         [0015]    When the delay DE 1  has completed the specified period, it turns conductive and allows a sufficiently heavy current flow through the downwardly (in the drawing) pulled armature AR 1  and through the coil CO 2  of relay RE 2  to open the contact K 2 . This cuts off power to the coil CO 1  of relay RE 1 . Armature AR 1  snaps to its up (in the drawing) un-energized position and allows current to resume from the source SO 1  to the load LO 1  through the armature and the now conductive sensor SE 1 . Such current flows because the sensor SE 1  has reset itself from the thermally-activated non-conductive condition. As stated, the adjusted delay period of the time delay DE 1  is selected to exceed the expected time in which the sensor SE 1  resets itself. The closed contact K 1 - 1  and the conductive sensor SE 1  in effect short any current flow through the coil CO 1  and the contact K 2 . Operation of the load LO 1  then resumes.  
         [0016]    According to an embodiment, the time period of the delay DE 1  is adjustable from 6 to 300 seconds, although other delays are possible. The delay is set sufficiently large so as to exceed the recycle time of the thermal sensor SE 1 . This prevents the sensor from turning non-conductive in response to excessive current indicative of a power excess, turning conductive in response to reduced current flow, and then turning non-conductive again when the excess current in the load LO 1  resumes. Keeping the system off for additional time forestalls premature resumption of operation and subsequent on-off cycling.  
         [0017]    According to an embodiment of the invention the sensor SE 1  responds to excess current or temperature before any internal protective device in the air conditioner. For example an air conditioner protector may wait 16 seconds after reaching 135 F. and then cut off. The sensor SE 1  is the set to wait 6 to 15 seconds after reaching 120 F. Also the current path through the coil CO 1  may carry reverse current from the inductive load.  
         [0018]    [0018]FIG. 2 illustrates a variation of the embodiment in FIG. 1. Like parts are designated with like characters. FIG. 2 illustrates fuses FS 1  and FS 2  as well as a light indicator LT 1 .  
         [0019]    [0019]FIG. 3 is a perspective view of the monitor in a case CO 1  with connectors CN 1  and CN 2  (having lines L 1 IN, L 2 IN and L 1 OUT, L 2 OUT), a plug PL 1  and a jack JA 1 .  
         [0020]    The invention provides the benefit of preventing premature resumption of current through the load LO 1  and protecting the contacts of the sensor SE 1 . It inhibits wear-out of the sensor from the effects of repeated on-off cycles. It similarly protects the compressor or the air conditioner from the repeated on-off operations, and the consequence of early recurrences of current flow. The sensor, in one embodiment, includes a temperature sensitive bimetallic thermostat where the bimetal makes or breaks with a contact.  
         [0021]    According to an embodiment, the load LO 1  is the compressor of the air conditioning unit. In another, it includes the entire unit. The unit may have an internal thermal protector. Typically, the internal protector may cut out at 130 degrees F. and resets in 16 seconds, although other values exist. The thermal protective sensor SE 1  is current sensitive and heats; cuts out at a lower temperature, for example at 120 degrees F. although other temperatures may also be used and resets in 6 to 300 seconds. In an embodiment the sensor SE 1  cuts back in at 78 degrees F. and the delay DE 1  is long enough to prevent current return until after the temperature has fallen even further.  
         [0022]    While embodiments of the invention have been described in detail, it will be evident to those skilled in the art that the invention may be embodied otherwise.