Type and dimensional synthesis of spring mechanics for circuit breakers

Abstract

Circuit breakers are crucial elements in high?voltage electrical substations, used to make and break circuits manually or automatically. They protect high?value equipment in the switchyard, such as transformers, by automatically opening the contacts during faults. The mechanism that drives the contacts is the heart of the circuit breaker. This research investigates design improvements of the opening mechanism, closing mechanism, and charging mechanism of a circuit breaker. The structural synthesis of the opening mechanism, considering higher?pair joints, results in several possible mechanism combinations. The focus of this work is on designing opening mechanisms that achieve high opening velocity using minimum opening?spring energy. Two six?bar linkages are selected for detailed evaluation-one based on Watt抯 mechanism and the other on a toggle linkage. A toggle linkage with the same link lengths, mass, and inertia as the Watt抯 mechanism exhibited a 30% increase in opening speed for the same opening?spring energy. A further 20% enhancement is achieved after optimization. The optimized toggle mechanism delivers 50% higher opening speed than Watt抯 mechanism for the same opening?spring energy. The closing mechanism is based on a cam with an oscillating?follower linkage. The cam is spring?driven, resulting in non?constant angular velocity during the closing operation. The desired time杁isplacement relationship for the contact during closing is non?linear. Therefore, conventional cam?design methods, which assume constant angular velocity, are not applicable. The kinetic energy of the contact and the potential energy of the opening spring are known. The maximum potential energy of the closing spring is calculated considering friction in the mechanism. The cam抯 angular displacement with respect to time is obtained by solving a non?linear energy?balance equation, from which the follower rotation is derived. The cam profile is then designed using this derived relationship. The ratchet?pawl closing?spring charging mechanism is an important subsystem of the circuit breaker. It transfers energy from the motor to the closing spring. Key challenges in designing the ratchet?pawl mechanism include avoiding failures such as motor burnout, pawl?tip and ratchet?tooth wear, pawl?stopper wear, and eccentric?shaft breakage. To mitigate these failures, this thesis focuses on minimizing motor driving torque by controlling friction, eccentricity, and pressure angle. The mechanism is designed for four critical conditions-two load?handover positions and two extreme pawl conditions-with corresponding mathematical relations established. Three major factors are examined: (a) effect of friction between pawl and eccentric shaft on driving torque, (b) effect of friction between pawl tip and ratchet wheel, (c) effect of eccentricity, backlash, and pressure angle on driving torque. Multibody dynamic analysis shows an exponential increase in driving torque with the coefficient of friction between the pawl and eccentric shaft. Similarly, unequal backlash between pawls leads to higher impact loads from the ratchet wheel compared to equal backlash. In a third case study, undesirable rubbing of pawl tips with the non?toothed region of the ratchet wheel during closing is avoided through an innovative design that disengages the pawls after the charging operation. The enhanced designs resulted in a 100�0% improvement in fatigue life, verified experimentally.