Protection of electric motors, magnetic starters and other equipment from overheating loads is carried out by means of special thermal protection devices. In order to make the right choice of thermal protection model, you need to know how it works, its structure, as well as the basic criteria for selection.
Contents
Design and principle of operation
Thermal relay (TR) is designed to provide protection of electric motors from overheating and premature failure. During long-term starting, the electric motor is subjected to current overloads, because during the start-up it consumes seven times the current value, leading to heating of the windings. The rated current (In) is the current drawn by the motor during operation. In addition, TRs increase the service life of electrical equipment.
Thermal relay, the structure of which consists of the simplest elements:
- Heat-sensitive element.
- A self-resetting contact.
- Contacts.
- Spring.
- Bimetal conductor plate.
- Button.
- Setpoint current regulator.
The thermal sensing element is a temperature sensor that serves to transfer heat to a bimetallic plate or other thermal protection element. The self-resetting contact allows the electric consumer's power supply circuit to be instantly opened when heated to avoid overheating.
The plate consists of two types of metal (bimetal), one of which has a high thermal expansion coefficient (Kp). They are bonded together by welding or rolling at high temperatures. When heated, the heat shield plate bends toward the material with the lower Kp, and when cooled, the plate returns to its original position. Generally the plates are made of invar (lower Kp value) and non-magnetic or chromium-nickel steel (higher Kp value).
The button turns on the TR, the setpoint current regulator is necessary to set the optimum value of I for the consumer, and exceeding it will cause the TR to trip.
The principle of operation of the TR is based on the Joule-Lenz law. Current is the directional motion of charged particles which collide with the atoms in the crystal lattice of a conductor (this value is resistance and is denoted by R). This interaction causes the appearance of thermal energy derived from electrical energy. The dependence of the duration of flow on the temperature of the conductor is determined by the Joule-Lenz law.
The formulation of this law is as follows: when I flows through a conductor, the amount of heat Q released by the current in its interaction with the atoms of the conductor crystal lattice is directly proportional to the square of I, the value of R of the conductor and the exposure time of the current on the conductor. Mathematically it can be written as follows: Q = a * I * I * R * t, where a is the conversion factor, I is the current flowing through the desired conductor, R is the resistance value and t is the time of flow of I.
If the coefficient a = 1, the result of the calculation is measured in joules, and if a = 0.24, the result is measured in calories.
Heating bimetallic material occurs in two ways. In the first, I flows through the bimetal, and in the second, through the winding. The insulation of the winding slows the flow of thermal energy. The thermal relay heats up more at high I values than it does at contact with the thermosensitive element. This results in a delay in the activation of the contact signal. Modern TRs use both principles.
The bimetallic plate of the thermal protection device is heated while the load is connected. Combined heating allows to obtain a device with optimal characteristics. The plate is heated by the heat generated by I flowing through it and by a special heater at I load. During heating, the bimetallic plate deforms and acts on the self-returning contact.
Key Features
Each RTD has individual technical characteristics (TC). The relay must be selected according to the load characteristic and the condition of use with an electric motor or other power consumer:
- In value.
- Adjustment range I of operation.
- Voltage.
- Additional control of TP operation.
- Power.
- The limit of response.
- Sensitivity to phase imbalance.
- Class of tripping.
Rated value of current - value of I, for which the TR is designed. Selected by the value of In of the consumer to which it is directly connected. In addition, it is necessary to choose with a reserve on In and be guided by the following formula: Inr = 1.5 * Ind, where Inr - In TP, which must be greater than the rated motor current (Ind) by 1.5 times.
The adjustment limit I operation is one of the important parameters of the thermal protection device. The designation of this parameter is the adjustment range of the value of In. Voltage - the value of the power voltage for which the relay contacts are designed; if the permissible value is exceeded, the failure of the device will occur.
Some types of relays are equipped with separate contacts to control the operation of the device and consumer. Power - this is one of the main parameters of TR, which determines the output power of the connected consumer or group of consumers.
Tripping limit or threshold is a coefficient that depends on the rated current. In general, its value is in the range from 1.1 to 1.5.
The sensitivity to phase imbalance (phase asymmetry) indicates the percentage ratio of the imbalanced phase to the phase through which the rated current of the required value flows.
Tripping class - a parameter that represents the average response time of the TR depending on the multiplicity of the setpoint current.
The main characteristic, by which it is necessary to choose a TR, is the dependence of the response time from the load current.
Wiring diagram
Wiring diagrams of thermal relays in a circuit can vary significantly depending on the device. However, TRs are connected in series with the motor winding or magnetic starter coil to a normally open contact, because this kind of connection helps to protect the device from overloads. If the current consumption is exceeded, the TR disconnects the unit from the mains supply.
Most circuits use a permanently open contact that operates when connected in series with the stop button on the control panel. Basically this contact is marked with the letters NC or H3.
The normally closed contact can be used when a protection alarm is connected. Also, in more complex circuits this contact is used to implement software control of the emergency stop of the device using microprocessors and microcontrollers.
The thermostat is simple enough to connect. To do this, you need to be guided by the following principle: TP is placed after the contactors of the starter, but before the motor, and the permanently closed contact is included in series with a stop button.
Types of thermal relays
There are many types into which thermal relays are divided:
- Bimetallic - PTLs (ksd, lrf, lrd, lr, iek and ptlr).
- Solid state.
- Relay to control the temperature mode of the device. The basic designations are as follows: RTK, NR, TF, ERB and DU.
- Alloy melting relays.
Bimetallic TRs have a primitive design and are simple devices.
The principle of operation of a solid state type thermal relay is quite different from a bimetallic type. Solid state relay is an electronic device, also called a snapper, which is made on radio elements without mechanical contacts.
They include RTR and RTI IEK, which calculates the average temperature of the electric motor by monitoring its starting and In. The main feature of these relays is the ability to resist sparks, i.e. they can be used in explosive environments. This type of relay is faster in tripping time and easier to adjust.
RTCs are designed to monitor the temperature condition of an electric motor or other device using a thermistor or a thermal resistor (probe). When the temperature rises to a critical condition, its resistance increases sharply. According to Ohm's law, when R increases, the current decreases and the consumer is switched off because its value is not sufficient for normal consumer operation. This type of relay is used in refrigerators and freezers.
The design of the fusion thermal relay differs significantly from other models and consists of the following elements:
- Heater winding.
- Alloy having a low melting point (eutectic).
- Circuit Breaker Mechanism.
The eutectic alloy melts at a low temperature and protects the consumer's power circuit by breaking the contact. This relay is built into the device and is used in washing machines and automotive applications.
The selection of the thermal relay is made by analyzing the TC and operating conditions of the device to be protected from overheating.
How to choose the thermal relay
Without complex calculations you can choose the suitable rating of the thermal relay for the motor by power (table of specifications of thermal protection devices).
The basic formula for calculating the rated current of the thermal relay:
Intr = 1.5 * Ind.
For example, it is necessary to calculate In TR for an asynchronous electric motor of 1.5 kW, powered from a three-phase AC voltage network with a value of 380 V.
This is quite easy to do. To calculate the value of rated motor current, use the power formula:
P = I * U.
Hence, Ind = P / U = 1500 / 380 ≈ 3.95 A. The value of nominal current of TP is calculated as follows: Intr = 1.5 * 3.95 ≈ 6 A.
On the basis of calculations, select the RTL-1014-2 type TP with an adjustable range of the setpoint current from 7 to 10 A.
If the ambient temperature is high, the setpoint value should be set to the minimum. At low ambient temperatures, the increasing load on the motor stator windings must be taken into account and, if possible, not switched on. If circumstances require the use of the motor under adverse conditions, it is necessary to start the setting with a low setpoint current and then increase it to the required value.
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