How one can select a thermal relay?

The thermal relay is a standard electrical device used to guard motors. It’s used to guard motors or other electrical devices and circuits from overload. It has the identical reverse-time operating characteristics because the permissible overload characteristics of the motors. Furthermore, it is especially utilized in three-phase AC motors. Overload protection and phase loss protection. Judging by the present situation, accidents still occur every so often that cause the engine to burn out as a consequence of improper selection and use of the thermal relay. So how do you select a thermal relay correctly? Let’s have a look together:

From a structural perspective, thermal relays are divided into two-pole type and three-pole type, and the three-pole type is split into two types with phase loss protection and without phase loss protection. And since thermal relays are mainly used to guard the motor from overload, when choosing, it’s vital to grasp the operating conditions of the motor, equivalent to the working environment, starting current, the character of the load, the working system, the permissible overload capability, etc.

1. In keeping with the overload characteristics

Basically, the ampere-second characteristic of the thermal relay needs to be as close as possible to and even overlapping with or below the motor overload characteristic, and at the identical time the thermal relay mustn’t be affected (non-functional) during a short-term overload and motor start. .

2. In keeping with the rated current

The rated current of the thermal relay needs to be barely greater than the rated current of the motor. When the starting current of the motor is 6 times the rated current and the starting time is lower than 5 seconds, the setting current of the thermal relay is adjusted to be equal to the rated current of the motor; when the motor starting time is long, the shock load is dragged or parking is just not allowed, the setting current of the thermal relay is ready to 1.1-1.15 times the motor rated current.

3. By type

Under normal circumstances, a two-phase design thermal relay may be chosen, but when the three-phase voltage balance is poor, the working environment is poor or the motor is unattended, a three-phase design thermal relay needs to be used. Within the case of delta-connected motors, thermal relays with phase loss protection devices needs to be used.

4. In keeping with the work situation

When a thermal relay is used to guard a motor under repeated short-term operation, the thermal relay only has a certain range of adaptability. For a lot of operations in a short while, a thermal relay with a fast saturation current transformer needs to be used. Relating to a motor for special applications, with frequent forward and reverse rotation and frequent switching on and off, the thermal relay mustn’t be used as an overload protection device, but a thermal relay or thermistor built into the motor winding needs to be used for defense.

5. By insulation class and construction

Because of the various level of insulation of the motors, their permissible temperature rise and overload capability also differ. Under the identical conditions, the upper the insulation level, the greater the overload capability. Even when the insulating materials used are the identical however the motor design is different, there needs to be differences when choosing thermal relays. For instance, the warmth dissipation of a closed motor is worse than that of an open motor, and its overload capability is lower than that of an open motor. The setting current of the thermal relay needs to be chosen as 60-80% of the rated motor current.

6. By role

If a thermal relay is used to guard against motor phase loss, consideration have to be given to how the motor is connected. Within the case of a Y-connected motor, when a certain phase is disconnected, the present of the remaining uninterrupted phase windings increases in proportion to the present flowing through the thermal relay. A general three-phase thermal relay, if the setting current is correctly regulated, can provide open-phase protection for a Y-connected motor. Within the case of a delta-connected motor, when the phase is disconnected, the ratio of the rise in the present flowing through the uninterrupted phase winding and the present flowing through the relay thermal is different. Because of this the present flowing through the thermal relay cannot reflect the winding overload current after the interruption of the commercial control phase. Due to this fact, the overall thermal relay, even whether it is three-phase type, can’t be the open phase of a three-phase asynchronous motor with delta connection. Provide adequate protection while working. On this case, JR20 or T series thermal relays with differential phase loss protection mechanisms needs to be used.

The above is an introduction to the choice of thermal relays. In summary, as a motor overload protection element, thermal relays are widely utilized in manufacturing as a consequence of their small size, easy structure and low price.