Connecting a Three-Phase Motor

Connecting a three-phase motor to switch

Connecting a three-phase motor requires following safety rules and understanding the connection diagram. The basic steps are:

• Check motor parameters: Before starting the connection, check carefully motor nameplate. It contains key information such as the supply voltage (e.g. 400V), power, rated current and the connection diagram (star or delta).

• Selecting the right switch: Switch must be customized to motor current rating. Circuit breakers (CT/Fuses ) or motor circuit breakers are usually used, which offer additional protection against overload and short circuits.

• Cables preparation: Use cables with a proper cross-section, suited motor current and the length of the installation. The ends of the cables should be properly prepared (e.g. crimped with ferrules).

• Identification of motor terminals: A three-phase motor usually has six terminals marked U1, V1, W1 and U2, V2, W2. Depending on the power supply configuration (star or delta), these terminals are connected in a specific way.

• Star (Y) connection:
  – Connect terminals U2, V2, W2 together (this is the neutral point of the star).
  – Connect phase cables L1, L2, L3 to terminals U1, V1, W1 respectively.

• Delta connection (Δ):
  – Connect terminal U2 to V1.
  – Connect terminal V2 to W1.
  – Connect terminal W2 to U1.
  – Connect phase cables L1, L2, L3 to connected pairs of terminals (e.g. L1 to U1 and W2, L2 to V1 and U2, L3 to W1 and V2).

• Connection to switch:
  – Connect the phase cables L1, L2, L3 from the power source to switch input.
  – Connect cables leading to the motor from switch output motor terminals (U1, V1, W1).

• Protective conductor (PE) connection: do not forget to connect the protective conductor (PE) to the marked terminal on the motor and to the proper terminals of switch. Safety first!

• Check Connections: Before turning on power, check all connections, make sure they are tight screwed and there are no loose wires.

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Important safety notes

• Work involving installation or modification of electrical circuits should be performed by qualified personnel with the appropriate authorizations.

• Always disconnect the power supply before starting any work.

• Use appropriate tools and personal protective equipment.

• Before starting the engine for the first time after changing connections, it is worth checking for any disturbing sounds or vibrations.

Summary

Correct connecting of three-phase motor to a switch and know-how of changing direction of rotation are essential skills in the electrical industry. Understanding wiring diagrams, following safety rules and accuracy are key to ensuring proper and safe operation of the motor. Remember, when in doubt, it is always worth consulting an experienced electrician.

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