Device and principle of operation of the thermal relay
- Principle of operation of thermal relays
- Time-current characteristics of the thermal relay and the protected object
- Time-current characteristics of the thermal relay
- Choice of thermal relays
- Influence of ambient temperature on operation of the thermal relay
- Design of thermal relays
Principle of operation of thermal relays
Thermal relays are the electric devices intended for protection of electric motors against the current overload. The most widespread types of thermal relays: TRP, TRILLION, RTL and RTT. The durability of the power equipment substantially depends on overloads to which it is exposed during the operating time.
For any object it is possible to find dependence of duration of course of current on its size at which reliable and long operation of the equipment is provided. This dependence is presented on the drawing (the curve 1). At rated current the admissible duration of its course is equal to infinity. Course of current, bigger, than rated, leads to additional temperature increase and additional aging of isolation. Therefore the more the overload, the more quickly it is admissible. The curve 1 in the drawing is established proceeding from required life expectancy of the equipment. The shorter it life, the big overloads are admissible.
Time-current characteristics of the thermal relay and the protected object
At ideal protection of the object the dependence of tcp (I) for the thermal relay has to go a little below than the curve for the object. For overload protection, thermal relays with the bimetallic plate most were widely adopted.
The bimetallic plate of the thermal relay consists of 2 plates, one of which has the bigger temperature coefficient of expansion, another — smaller. In the place of the prileganiye to each other plates are rigidly fastened or due to rolling in the hot state, or due to welding. If to fix not movably such plate and to heat, then there will be the plate bend towards material to smaller. This phenomenon is used in thermal relays.
In thermal relays materials the invar (small value a) and non-magnetic or chromonickel steel (great value of a) were widely adopted.
Heating of the bimetallic element of the thermal relay can be made due to heat generated in the plate by loading current. Very often heating of bimetal is made from the special heater on which loading current proceeds. The best characteristics turn out at the combined heating when the plate heats up also due to heat generated by the current passing through bimetal and due to heat generated by the special heater, also streamline current of loading. Caving in, the bimetallic plate the free end influences the contact system of the thermal relay.
Time-current characteristics of the thermal relay
The main characteristic of the thermal relay is the dependence of response time on loading current. Generally prior to the beginning of the overload via the relay current of Io which heats the plate to qo temperature proceeds.
At verification of vremyatokovy characteristics of thermal relays it is necessary to consider from what state (cold or superheated) there is the operation of the relay.
When checking thermal relays it must be kept in mind that heating elements of thermal relays are thermally unstable at short-circuit currents.
Choice of thermal relays
The rated current of the thermal relay is chosen proceeding from the rated load of the electric motor. The chosen current of the thermal relay makes (1,2-1,3) rated values of current of the electric motor (loading current), i.e. the thermal relay works at 20-30% to the overload within 20 minutes.
The constant of time of heating of the electric motor depends on duration of the current overload.
At the short-term overload only the winding of the electric motor and the constant of heating of 5-10 minutes participates in heating.
At the long overload all mass of the electric motor participates in heating and it is constant heating of 40-60 minutes. Therefore use of thermal relays is reasonable only when duration of inclusion is more than 30 minutes.
Influence of ambient temperature on operation of the thermal relay
Heating of the bimetallic plate of the thermal relay depends on the ambient temperature therefore with growth of ambient temperature current of operation of the relay decreases.
At the temperature very different from rated, it is necessary or to carry out additional (smooth) adjustment of the thermal relay, or to select the heating element taking into account actual ambient temperature.
In order that ambient temperature influenced current of operation of the thermal relay less, it is necessary that temperature of operation was chosen perhaps more.
For the correct work of thermal protection of the relay it is desirable to have in the same room, as the protected object. It is impossible to have the relay near the concentrated heat sources: heating furnaces, systems of heating, etc. Now relays with temperature compensation (TRILLION series) are issued.
Design of thermal relays
The deflection of the bimetallic plate occurs slowly. If directly to connect with the plate the mobile contact, then small speed of its movement, will not be able to provide arc extinction, the chain arising at shutdown. Therefore the plate affects contact through the accelerating device. The most perfect is the "jumping" contact.
In the deenergized state the spring 1 creates the moment concerning the point 0 closing contacts 2. The bimetallic plate 3 when heating is bent to the right, the provision of the spring changes. She creates the moment disconnecting contacts 2 during the time providing reliable arc extinction. Modern contactors and actuators are completed with the TRP thermal relays (single-phase) and TRILLION (two-phase).
TRP thermal relays
Thermal current single-pole relays of the TRP series with rated currents of thermal elements from 1 to 600 And are intended for protection against inadmissible overloads of the three-phase asynchronous electric motors working from network with rated voltage up to 500 V with the frequency of 50 and 60 Hz. The TRP thermal relays on currents to 150 A apply in direct current mains with the rated voltage up to 440 V.
Device of the thermal TRP relay
The bimetallic plate of the TRP thermal relay has the combined system of heating. The plate 1 heats up both at the expense of the heater 5, and at the expense of the current flow through the plate. At the deflection the end of the bimetallic plate influences the jumping contact bridge 3.
The TRP thermal relay allows to have slide control of current of operation in limits (±25% of rated current of the setting). This adjustment is carried out by the handle 2 changing initial deformation of the plate.
Such adjustment allows to lower number of required versions of the heater sharply. Return of the TRP relay to the starting position after operation is made by the button 4. Execution and with self-return after bimetal cooling is possible.
High temperature of operation (above 200 °C) reduces dependence of operation of the relay on ambient temperature.
The setting of the TRP thermal relay changes for 5% at change of ambient temperature for KUS. High udaro-and vibration resistance of the TRP thermal relay allow to use it in the most severe conditions.
RTL thermal relays
The relay thermal RTL is intended for ensuring protection of electric motors against current overloads of inadmissible duration. They also provide protection against lack of symmetry of currents in phases and from loss of one of phases.
The RTL electrothermal relays with the range of current from 0,1 to 86 are issued And.
The RTL thermal relays can be installed both directly on PML actuators, and separately from actuators (in the latter case they have to be supplied with terminal blocks KRL).
The RTL relay and terminal blocks KRL which have degree of protection _P20 are developed and are issued and can be installed on the standard rail. The rated current of contacts is equal to 10 A.
RTT thermal relays
Relays thermal RTT are intended for protection of three-phase asynchronous electric motors with the short-circuited rotor from the overloads of inadmissible duration including arising at loss of one of phases and also from asymmetry in phases.
RTT relays are intended for application as components in schemes of control of electric drives and also for integration in magnetic actuators of the PMA series for the purpose of alternating current by voltage of 660 V with a frequency of 50 or 60 Hz, in direct-current circuits of 440 V.Top