Surname 2
Application No.: 07/786,209
Filed: October 31, 1991
Thermal motor protectors
IEC 60730 applies to the partial evaluation of thermal motor protectors. In IEC
part of 60730-1 in the household is used for heating, air-conditioning, and related applications. A
thermal motor protector is an incorporated control that is dependent on correct mounting and
fixing in a motor, and it can be thoroughly tested in combination with the relevant motor. The
requirements for testing and combination of motor and thermal motor protector are given in
Annex aa for information and are included in requirements taken from other IEC standards. The
part 60730 in a thermal motor protector applies to using of thermistors that are NTC and PTC,
and they are additional requirements contained in Annex J of part 1. A thermal motor protector
that are not incorporated with motor winding to the motor and are covered under the scope of
part 2 and is considered incorporated controls on the requirements of this standard.
The working of thermal motor protectors
Thermal motor protector consists of three bimetal strips and a trip mechanism in
a housing made of insulating material. When the bimetal strips are heated by the motor current,
they bend activating the trip mechanism after certain travel, which in this case depends on the
current setting of the relay. The release mechanism then actuates an auxiliary switch that breaks
the coil circuit of the motor contactor. The bimetal pieces may be heated directly or otherwise. In
the former, current flows through the bimetal and indirectly through an insulated heating winding
around the strip. Heat flow is delayed by the insulation resulting in the inertia of indirectly
heated thermal relays that is greater at higher currents compared to the one that is directly heated.
