Thermal relays are widely used in industry and in everyday life, protecting electric motors in many technical devices. The principle of operation of such relays is based on the property of bimetals to change their shape when heated, breaking the electrical circuit. The choice of a thermal relay is largely determined by its purpose and the specific features of the electrical equipment to be protected.
Instructions
Step 1
When choosing a specific device, proceed from the fact that one of the main requirements for thermal relays is the compliance of their rated current with the current of the equipment to be protected. In addition, thermal relays themselves often require short-circuit protection, for which fuses should be included in the relay connection diagrams.
Step 2
Understand for yourself the limits of applicability of thermal relays in order to correctly choose the type of protection. If in the protected system emergency modes of operation of the electric motor are possible, which are not associated with an increase in the consumed current, then the use of a thermal relay will not give the required protective effect. For such cases, a special thermal protection is built into the stator winding of the motor.
Step 3
If no special requirements are imposed on the thermal protection of the equipment, choose the thermal relay taking into account that the maximum operating current of the relay should not be lower than the rated current of the protected equipment. Another selection criterion is that the relay installation current should slightly exceed the motor rating (within 5%).
Step 4
Also pay attention to the fact that the selected thermal relay should provide a larger margin for adjusting the relay installation current in the direction of decreasing and increasing. Ideally, the adjustment margin should be very large, which will make the protection more reliable and manageable. On the relay setting scale, note the presence of one or two readings on either side of the handle position corresponding to the initial setting.
Step 5
When choosing thermal relays to protect asynchronous electric motors from overloads that can occur when one of the phases is broken, use a PTT or RTI type relay. Such devices can be built directly into magnetic starters, protecting the motor from rotor jamming and prolonged starting.
Step 6
When choosing a specific model, be guided by the presence of a built-in indicator that shows the state of the thermal relay at the moment.
