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Star-delta connection is a method used to start a three-phase asynchronous motor from a reduced voltage at power-up and then step up to rated voltage once the motor is running. A thermal relay is often used to protect the motor from overheating due to excessive current. Here is how to set a thermal relay in such a system:
Understand the motor specifications: Before setting the thermal relay, you must know the motor specifications, including the rated current (In) and the permissible overcurrent range.
Repair the thermal relay: Locate the thermal relay on the motor control panel. It is usually located near the engine control unit.
Identify the settings: The thermal relay generally has two main settings: the setting current (Ir) and the thermal trip time.
Setting the setting current (Ir): Set the setting current (Ir) of the thermal relay according to the rated current of the motor (In). The setting current must be slightly higher than the motor's rated current to avoid unwanted tripping. The setting will depend on the specific thermal relay you are using, so refer to the manufacturer's manual for instructions on how to make this setting. Some thermal relays have a rotary knob to adjust the current, while others use dip-switches or adjustment screws.
Thermal Trip Time Setting: The thermal relay usually has a setting for the thermal trip time, which determines how quickly the relay will respond to an overcurrent. This setting also depends on the engine specifications and manufacturer's recommendations. In general, it is best to set the trigger time slightly shorter than the motor curve, but again, refer to the manufacturer's manual for specific advice.
Test the setting: Once you have adjusted the thermal relay, perform a test to make sure it is working properly. You can simulate an overload by shorting two phases of the motor for a short time to see if the thermal relay trips as expected.
Powering up in star configuration: When starting the motor, make sure it is connected in star configuration first. This means that the motor windings are star (Y) connected, including the voltage applied at start-up.
Switching to delta configuration: Once the motor has reached its rated operating speed, you can switch from star configuration to delta (Δ) configuration to obtain full power.
Continuous monitoring: The thermal relay must be continuously monitored to ensure that it protects the motor in the event of overheating. If the thermal relay trips frequently, this may indicate a problem with the motor or relay setting that requires immediate attention.