What is a thermal relay, what is it for? What is the principle of operation of the device based on, and what characteristics does it have? What should be considered when choosing a relay and installing it? You will find answers to these and other questions in our article. We will also consider the basic relay connection diagrams.
What is a thermal relay for an electric motor
A device called a thermal relay (TR) is a series of devices designed to protect electromechanical machines (motors) and batteries from overheating during current overloads. Also, relays of this type are present in electrical circuits that control the temperature regime at the stage of performing various technological operations in production and circuits of heating elements.
The basic component built into the thermal relay is a group of metal plates, parts of which have different coefficients of thermal expansion (bimetal). The mechanical part is represented by a movable system associated with electrical protection contacts. Electrothermal relayusually comes with a magnetic starter and a circuit breaker.
The principle of operation of the device
Thermal overloads in motors and other electrical devices occur when the amount of current passing through the load exceeds the rated operating current of the apparatus. On the property of the current to heat up the conductor during the passage, and built TR. The bimetallic plates built into it are designed for a certain current load, the excess of which leads to their strong deformation (bending).
The plates press on a movable lever, which, in turn, acts on a protective contact that opens the circuit. In fact, the current at which the circuit opened is the trip current. Its value is equivalent to a temperature, the excess of which can lead to the physical destruction of electrical appliances.
Modern TRs have a standard group of contacts, one pair of which is normally closed - 95, 96; the other - normally open - 97, 98. The first is designed to connect the starter, the second - for signaling circuits. The thermal relay for the electric motor is capable of operating in two modes. Automatic provides for independent switching on of the starter contacts when the plates are cooled. In manual mode, the operator returns the contacts to their original state by pressing the "reset" button. You can also adjust the trigger threshold of the device by turning the tuning screw.
Another function of the protective device is to turn off the engine when thephases. In this case, the motor also overheats, consuming more current, and, accordingly, the relay plates break the circuit. To prevent the effects of short-circuit currents, from which the TR is unable to protect the motor, a circuit breaker must be included in the circuit.
Types of thermal relays
The following device modifications exist - RTL, TRN, PTT and TRP.
Features of the TRP relay. This type of device is suitable for applications with increased mechanical stress. It has a shock-resistant body and a vibration-resistant mechanism. The sensitivity of the automation element does not depend on the temperature of the surrounding space, since the trigger point lies beyond the limit of 200 degrees Celsius. They are mainly used with motors of asynchronous type of three-phase power supply (current limit - 600 amperes and power supply - up to 500 volts) and in DC circuits up to 440 volts. The relay circuit provides a special heating element for heat transfer to the plate, as well as smooth adjustment of the bend of the latter. Due to this, it is possible to change the limit of operation of the mechanism up to 5%
- Features of the RTL relay. The mechanism of the device is designed in such a way that it allows you to protect the load of the electric motor from overcurrent, as well as in cases where a phase failure has occurred and a phase asymmetry has occurred. The current operating range lies within 0.10-86.00 amperes. There are models combined with starters or not.
- Features of the PTT relay. The purpose is to protect asynchronous motors, where the rotor is shortclosed, against current surges, as well as in cases of phase mismatch. They are built into magnetic starters and into circuits controlled by electric drives.
Specifications
The most important characteristic of a thermal relay for an electric motor is the dependence of the speed of contact disconnection on the magnitude of the current. It shows the performance of the device during overloads and is called the time-current indicator.
The main characteristics include:
- Rated current. This is the operating current for which the device is designed to operate.
- Rated current of the working plate. The current at which the bimetal is able to deform within the operating limit without irreversible damage.
- Limits of current setting adjustment. The current range in which the relay will operate, performing a protective function.
How to connect a relay to a circuit
Most often, TR is connected to the load (motor) not directly, but through a starter. In the classical connection scheme, KK1.1 is used as a control contact, which is closed in the initial state. The power group (through which electricity flows to the engine) is represented by the KK1 contact.
At the moment when the circuit breaker supplies the phase that feeds the circuit through the stop button, it passes to the "start" button (3rd contact). When the latter is pressed, the starter winding receives power, and it, in turn, connects the load. The phases entering the motor also pass through the bimetallic relay plates. As soon as the magnitude of the passing current beginsexceed the rated value, the protection trips and de-energizes the starter.
The following circuit is very similar to the one described above with the only difference that the KK1.1 contact (95-96 on the case) is included in the zero of the starter winding. This is a more simplified version, which is widely used. With a reversible motor connection scheme, there are two starters in the circuit. Controlling them with a thermal relay is only possible when the latter is included in the neutral wire break, which is common to both starters.
Relay selection
The main parameter by which a thermal relay for an electric motor is selected is the rated current. This indicator is calculated based on the value of the operating (rated) current of the electric motor. Ideally, when the operating current of the device is 0.2-0.3 times higher than the operating current with an overload duration of a third of an hour.
It is necessary to distinguish between a short-term overload, where only the wire of the winding of the electric machine is heated, from a long-term overload, which is accompanied by the heating of the entire body. In the latter variant, heating lasts up to an hour, and, therefore, only in this case it is advisable to use TP. The choice of thermal relay is also influenced by external operating factors, namely the ambient temperature and its stability. With constant temperature fluctuations, it is necessary that the relay circuit has a built-in temperature compensation type TPH.
Things to consider when installing relays
It is important to remember that a bimetallic plate can heat up not only from the passing current, but also fromambient temperature. This primarily affects the response speed, although there may not be overcurrent. Another option is when the engine protection relay enters the forced cooling zone. In this case, on the contrary, the motor may experience thermal overload, and the protection device will not work.
To avoid such situations, you should follow these installation rules:
- Choose a relay with a higher operating temperature without damaging the load.
- Install a protective device in the room where the engine itself is located.
- Avoid high heat radiation or near air conditioners.
- Use models with built-in temperature compensation.
- Use plate actuation adjustment, adjust according to the actual temperature at the installation site.
Conclusion
All electrical work to connect relays and other high-voltage equipment must be carried out by a qualified person with a permit and specialized education. Independent carrying out of such work is associated with a danger to the life and performance of electrical devices. If you still need to figure out how to connect the relay, when buying it, you need to require a printout of the circuit, which usually comes with the product.
