What is a thermal relay, what does it serve? What is the principle of operation of the device based on, and what characteristics does it have? What to consider 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 schemes.
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 (engines) and batteries from overheating during current overloads. Also, relays of this type are present in electrical circuits that monitor the temperature at the stage of various technological operations in the production and heating element circuits.
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 the electrical contacts of the protection. An electrothermal relay usually 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 current passing through the load exceeds the rated operating current of the device. On the property of the current to heat 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 the movable lever, which, in turn, acts on the protective contact that opens the circuit. In fact, the current at which the circuit was opened is the trip current. Its value is equivalent to temperature, exceeding which can lead to physical destruction of electrical appliances.
Modern TRs have a standard group of contacts, one pair of which is normally closed - 95, 96; the other is normally open - 97, 98. The first is for connecting a starter, the second is for alarm circuits. The thermal relay for the electric motor can operate in two modes. Automatic provides for the independent inclusion of starter contacts when cooling the plates. In manual mode, the contact returns to the initial state by pressing the "reset" button. You can also adjust the threshold of the device by rotating the tuning screw.
Another function of the protective device is to turn off the motor during a phase failure. 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 not able to protect the motor, a circuit breaker must be included in the circuit.
Types of Thermal Relays
The following device modifications are available: RTL, TRN, PTT and TRP.
- Features TRP-relay. A device of this type is suitable for use in conditions of increased mechanical load. It has a shockproof housing and vibration resistant mechanism. The sensitivity of the automation element does not depend on the temperature of the surrounding space, since the response point lies outside the limit of 200 degrees Celsius. They are mainly used with three-phase power asynchronous motors (current limit is 600 amperes and power is up to 500 volts) and in direct current circuits up to 440 volts. The relay circuit provides a special heating element for transferring heat to the plate, as well as smooth adjustment of the bend of the latter. Due to this, you can change the limit of the mechanism to 5%.
- Features of the RTL relay. The mechanism of the device is designed in such a way that allows you to protect the load of the motor from current overload, as well as in cases where a phase failure has occurred, and phase asymmetry has occurred. The current operating range lies between 0.10-86.00 amperes. There are models that are combined with starters or not.
- Features of the PTT relay. The purpose is to protect asynchronous motors, where the rotor is short-circuited, from current surges, as well as in cases of phase mismatch. They are built into magnetic starters and in circuits controlled by electric drives.
Specifications
The most important characteristic of a thermal relay for an electric motor is the dependence of the contact disconnection speed on the current value. 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 a working plate. The current at which bimetal is able to deform in the working limit without irreversible disturbances.
- The limits of the current setpoint adjustment. The current range in which the relay will operate, performing a protective function.
How to connect a relay to the circuit
Most often, the TR is connected to the load (motor) not directly, but through the starter. In the classical connection scheme, KK1.1 is used as a control contact, which is closed in the initial state. The power group (electricity is supplied to the engine through it) is represented by a KK1 contact.
At the moment when the circuit breaker supplies the phase supplying the circuit through the stop button, it passes to the "start" button (3 contact). When you press the last one, the winding of the starter receives power, and he, 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 begins to exceed the nominal, the protection trips and de-energizes the starter.
The following circuit is very similar to the one described above with the only difference being that the 1.1-contact (95-96 on the housing) is included in the starter winding zero. This is a more simplified version, which is widely used. With a reversible motor connection circuit, two starters are present in the circuit. They can be controlled using a thermal relay only when the latter is included in the gap of the neutral wire, which is common to both starters.
Relay selection
The main parameter by which the thermal relay for the electric motor is selected is the rated current. This indicator is calculated based on the magnitude of the working (rated) current of the electric motor. Ideally, when the response current of the device is higher than the working one by 0.2-0.3 times with a duration of overload of one third of an hour.
It is necessary to distinguish between short-term overload, where only the wire of the winding of the electric machine is heated, from long-term overload, which is accompanied by heating of the entire body. In the latter case, heating lasts up to an hour, and, therefore, only in this case it is advisable to use TP. The choice of a thermal relay is also influenced by external operating factors, namely the ambient temperature and its stability. With constant temperature jumps, it is necessary that the relay circuit has a built-in temperature compensation of the TRH type.
What to consider when installing a relay
It is important to remember that a bimetallic plate can heat up not only from the passing current, but also from the ambient temperature. This primarily affects the response speed, although there may not be overcurrents. Another option is when the motor protection relay enters the forced cooling zone. In this case, on the contrary, the engine may experience thermal overload, and the protection device may not work.
To avoid such situations, you should adhere to the following installation rules:
- Choose a relay with a permissible higher operating temperature without sacrificing load.
- Install the protective device in the room where the engine is located.
- Avoid places of increased heat radiation or the proximity of air conditioners.
- Use models that have built-in thermal compensation.
- Use the adjustment of the response of the plate, adjust in accordance with the actual temperature at the installation site.
Conclusion
All electrical work on connecting the relay and other high-voltage equipment must be performed by a qualified specialist who has a permit and a specialized education. The independent conduct 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 you purchase it, you need to require a printout of the circuit, which usually comes with the product.