1.  Field of the Invention
The present invention relates to thermal relays providing thermal protection for electrical machines and apparatus such as electric motors against overcurrent. 
2.  Description of the Prior Art
In electrical machines and apparatus, for example, electric motors, overcurrent, when occurring owing to an overload or the like, must be immediately detected to interrupt the motor winding circuit so that the motor can be protected against the overcurrent.  Thermal relays utilizing a bimetallic member have been conventionally employed for providing for such overcurrent protection. 
FIG.  6 illustrates a conventional thermal play of the type described above.  A disc-shaped bimetallic member 2 having two movable contacts 3 secured to the respective opposite ends thereof is provided in a housing 1 formed from an electrically insulating material.  The disc-shaped bimetallic member 2 is biased against the head 8a of an adjusting screw 8 by a coil spring 7 and held in position.  A terminal 5 provided with two fixed contacts 4 is secured to the housing 1.  The movable contacts 3 are adapted to move into and out of engagement with the fixed contacts 4, respectively.  A heater 6 is provided beneath the disc-shaped bimetallic member 2 in the housing 1. 
An overcurrent, when induced in the motor winding circuit, is drawn into the heater 6 and causes the temperature of the heater 6 to increase.  When the temperature of the heater 6 reaches a predetermined value, the disc-shaped bimetallic member 2 reverses its curvature with snap action so that the movable contacts 3 are made out of engagement with the respective fixed contacts 4, thereby interrupting the motor winding circuit.  Thereafter, when the temperature of the heater 6 is decreased below the predetermined value, the disc-shaped bimetallic member 2 reverses its curvature again so that the movable contacts 3 are made into engagements with the respective fixed contacts 4. 
According to the above-described conventional construction, however, unless the cause of the overcurrent occurrence in the motor winding circuit is eliminated, the disc-shaped bimetallic member 2 reiternatively reverses its curvature so that the movable contacts 3 are reiteratively made into and out of engagement with the respective fixed contacts 4.  In such conditions, the movable contacts 3 are at last caused to be welded to the respective fixed contacts 4 and accordingly, the disc-shaped bimetallic member 2 is prevented from reversing its curvature, whereby the motor winding circuit cannot be interrupted even in the occurrence of an overcurrent.  It will be considered that when the heater 6 is disconnected from the motor winding circuit owing to the overheating, the motor winding circuit may be interrupted.  However, when such disconnection of the heater 6 is not expected, the overcurrent keeps flowing through the motor winding circuit, which results in a critical condition that the motor is burned out.