The rated current of the 7.5KW motor is about 14.2A. Use a T45 thermal relay with a setting adjustment range of 16~27A.

The current of the motor I＝P/U/1.732/cosφ＝7500/380/1.732/0.8≈14.2A

The setting current of the thermal relay is the rated current 1.1~1.25 times, take 1.2 times. The set current is 14.2×1.2=17A

Thermal relay selection

The thermal relay is mainly used for overload protection of the motor, so it is necessary to understand the working environment, starting conditions, load properties, and operation of the motor limit and allowable overload capacity. The ampere-second characteristic of the thermal relay should be located below the overload characteristic of the motor and as close as possible so that the overload capability of the motor can be fully utilized without affecting the short-term overload and starting moment of the motor.

The selection of thermal relay is closely related to the working system of the motor to be protected, which is described below:

⑴ When working for a long time or intermittent long-term working system:

1) To ensure that the thermal relay does not malfunction during the motor starting process. Select a thermal relay with a return time of 0.5~0.7 for the 6IN thermal relay. The action time under 6IN can be found on the ampere-second characteristics of the thermal relay.

2) The middle value of the thermal relay’s setting current range is the rated current of the motor. When in use, the thermal relay setting current knob should be adjusted to the rated value, otherwise the protection will not be achieved.

3) Selection of thermal relay for motor phase failure protection. The choice of thermal relay for motor phase failure protection is directly related to the wiring form of the motor stator winding.

When the stator winding of the motor is Y-connected, the three-phase thermal relay with or without phase failure protection is connected in the phase line. When a three-phase uniform overload, uneven overload, or a one-phase thermal relay occurs, When a phase is broken, because the current flowing through the thermal relay is the current flowing through the motor winding, the thermal relay can truthfully reflect the motor overload condition, and they can all realize motor phase failure protection.

When the stator winding of the motor is connected in Δ connection, in order to achieve phase failure protection, the thermal relay with phase failure protection and without phase failure protection have different ways of connecting to the motor stator circuit.

For a thermal relay without phase failure protection, if it is still connected to the stator phase line, if a phase disconnection occurs before the motor is started, the current flowing through the thermal relay will be 4.5 to 6 times The rated current of the motor is enough to make the thermal relay operate; if the motor is running and one phase is disconnected under full load, the current in the phase winding with the largest current reaches 2.4~2.5 times the phase current, and flows through the thermal relay. The line current reaches 2 times the rated current, and the thermal relay can still operate. Therefore, in the above two cases, the thermal relay is connected to the phase line and can protect the motor from phase failure. However, most motors experience disconnection when running below full load. According to the pre vious analysis, if a disconnection occurs when the motor is running at a rated current of 0.58, the most severe phase current in one phase winding can reach 1.15 times the rated phase. Current, this phase winding is already in an overload state, but because the thermal relay is connected to the phase line, the thermal relay cannot be activated, and the phase failure protection of the motor cannot be realized. Therefore, if a three-phase thermal relay without phase failure is used to achieve phase failure protection, the three heating elements should be connected in series to the phase circuit of the three-phase winding of the motor, but this wiring method will bring some inconvenience. For this reason, the three-phase motor with Δ-shaped wiring should not choose a three-phase thermal relay without phase failure protection, but should choose a three-phase thermal relay with phase failure protection, and it can be connected in series in the motor line circuit. The differential structure can realize phase failure protection.

⑵ When the thermal relay is used in repeated short-time working systems:

When the thermal relay is used in a motor with repeated short-time working systems, the allowable operating frequency of the thermal relay should first be considered. When the motor starting current is 6 IN, the starting time is 1 second, the motor is working at full load, and the power-on duration is 60%, the maximum number of operations allowed per hour is no more than 40.

For motors with forward and reverse rotation and intensive on-off operation, it is not suitable to use thermal relay protection. Temperature relays or thermistors buried in the motor windings can be used for protection.

5. There are many brands and types of thermal relays. The technical parameters of the commonly used T series thermal relays are:

A 7.5kw motor uses a contactor of about 40a and a thermal relay of about 16a. The rated current of the 7.5kw motor is about 15a, and the circuit breaker capacity should be greater than about 1.4 times the rated current of the motor, pre ferably 25a; the contactor is 40a, the thermal relay set current is 16a; 2.5 square meters of copper core cable.
Notes on selecting thermal relays for motors:
1. In principle, the ampere-second characteristics of the thermal relay should be as close as possible to or even coincide with the overload characteristics of the motor, or below the overload characteristics of the motor, and at the same time The thermal relay should not be affected (not act) during short-term overload and startup.
2. When the thermal relay is used to protect a motor with long-term working system or intermittent long-term working system, it is generally selected according to the rated current of the motor. For example, the setting value of the thermal relay can be equal to 0.95 to 1.05 times the rated current of the motor, or the median value of the setting current of the thermal relay can be equal to the rated current of the motor, and then adjusted.
3. When the thermal relay is used to protect a motor that operates repeatedly for short periods of time, the thermal relay has only a certain range of adaptability. If there are many operations in a short period of time, a thermal relay with a speed saturation current transformer should be used.