Variety and protection characteristics of thermal relay

Motor overload for a number of reasons, such as phase failure, mechanical overload (blocking), under voltage, high operating frequency and cooling channels blocked, etc., in fact, the fault of the fault phase is the majority. In the operation of the motor, the winding insulation has a certain temperature rise limit, that is to say, in order to ensure the normal operation of the motor, the winding insulation has an inverse time limit of allowable overload characteristics. For a series of motors, it is a permissible overload characteristic curve. The protection characteristics of thermal relay thermal relay should be lower in the characteristics of the belt should ensure insulation before reaching dangerous temperature and reliable action in winding, disconnect the motor.

The basic requirement of the thermal relay is that it can reliably protect the motor from burning due to overload, and at the same time, it can ensure the normal operation and starting of the motor, and give full play to the overload capacity of the motor. The following is a brief introduction to the variety, protection and selection of thermal relay.

(1) varieties and protection characteristics of China's current production of thermal relay series JR16, JR14, JR15, etc.. "The dish is the name of the Chinese phonetic alphabet of the thermal relay. At the end of "D, for three-phase break phase protection device.

JR16, JR14 series thermal relay is three-phase, mechanical action for reed type or spring type (JR14) instantaneous jumping mechanism, current specifications since 0.25160, there are 20 heating element. They can be used for overload protection of motor, but also can be equipped with phase failure protection device. JRt type thermal relay phase, phase protection device is not broken, the action mechanism for reed type instantaneous jumping mechanism, current specifications from O.25 150, there are 20 heating element. The hot relays are used in temperature compensation, temperature range of 25 to +40 DEG C in effect protective properties due to temperature change is very small, generally with a thermal relay normally closed and normally open contact control, normally closed contact breaking capacity is 380 V, 2 A, the largest closed the current normally open contact is 380 volts, 5. After the action of the thermal relay, the user can select the automatic reset or manual reset according to the requirements. Relay manufacturers

When ordering a thermal relay, the type of the thermal relay, current rating, and thermal component specifications shall be specified. Such as JR16-20 / 3D, 7.2 - 11, indicating that the JR16 series thermal relay, the safety level of 20, with a phase failure protection device, thermal component current is 7.2-l1.

The protective properties of thermal relays shall be in accordance with the requirements of the standard of JB2457 - 78 double metal thermal relay.

(2) correct selection

Correct and reasonable selection of thermal relay is very important for reliable protection of motor. The results show that there is a considerable part of the thermal relay in the use of improper selection, not to play a protective role. Many units simply by motor rated current matching. It is not enough to satisfy this condition. Such as a heavy duty starting motor, the starting time is longer. If a common type of thermal relay, which is only suitable for light load starting, is used to protect the motor, the thermal relay will act in the starting process even if it is consistent with the rated current of the motor. The choice of thermal relay should ascertain the motor type specifications and the mechanical load properties (work), under normal load starting time and starting current, and the use of occasions the importance of environment and working machinery etc.. If the motor will cause great loss of production parking. You should consider the combination of protection and other selected thermal relay signals, even in the possible thermal relay can shorten the service life of the motor when the trip (or more losses), only in the case that it really dangerous trip off motor. According to the different use of the motor can be selected according to the thermal relay:

A. for long-term or intermittent work of the motor, the motor should work according to the process flow and the actual load rate, the thermal relay setting current value is selected as 0.9 a 1.05Ie (Ie for motor rated current). For the Y connection of the motor, the use of ordinary three-phase thermal relay can meet the requirements. But for the thermal relay delta connection method must be chosen with three-phase motor phase failure protection device. Because the connection of the Delta motor open phase (Figure 1). The current distribution inside the motor windings will change a lot, such as the motor running at 58% load off of a phase, according to the theoretical calculation of line current will increase to 100%, but some have increased to 115% phase winding currents. Because of the thermal relay is connected in series with the main circuit of the motor, so the thermal relay is current flowing through the motor line current, ordinary thermal relay will not work, but some of the current continuous winding by 115% may be burned. The delta connection method of the motor should be used with phaseopen "3D" type thermal relay protection

B. can not use the thermal relay protection for the motor with frequent on-off and positive reverse operation. This point should be noted in the selection.

In order to ensure the protection of thermal relay reliability, thermal relay should be installed according to normal installation, connecting wires shall comply with the provisions, before putting into operation, should match the rated current check adjustment knob on the scale current value and whether the motor is protected, but also regular verification, thermal relay operation, as long as a reasonable selection of heat the relay, and the importance of inspection and maintenance, thermal relay is able to effectively protect the motor.