What is a Contactor? | Working Principles | Summary and Q&A

TL;DR

This video explains the purpose of contactors, how they function, and the differences between contactors and relays.

Key Insights

• 🤔 A contactor is a device used to make and break electrical power circuits, typically to control the turning on and off of motors.
• 🧠 Connecting a motor directly to a PLC can damage the PLC cards due to potential electrical surges from the motor side, so a contactor is used as a safe and indirect connection.
• ⚡️ Contactors have a low voltage coil that, when energized, creates an electromagnetic field that closes the contacts, allowing the three-phase power to reach the motor and turn it on.
• 🔄 Contactors and relays function in a similar way, but contactors are used for larger devices with higher current and voltage rates, while relays are used for smaller devices with lower current and voltage rates.
• ⚙️ A contactor has wire terminals for both the coil and the power/device connections. The A1 and A2 terminals are for the 24V DC power to energize the coil, and the L1, L2, L3, T1, T2, T3 terminals are for power/device connections.
• 📣 Contactors often have an auxiliary or feedback contact, which is a normally open contact used to send a signal to the PLC input indicating the health of the contactor.
• 🔌 To control a motor using a PLC through a contactor, the PLC output is connected to the coil, the three-phase power supply is connected to the L1, L2, L3 terminals, and the motor is connected to the T1, T2, T3 terminals.
• 💡 A feedback contact and start/stop switches are also connected to the PLC, allowing for proper monitoring and control of the contactor and motor.

Transcript

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Questions & Answers

Q: How does a contactor protect a PLC from damage?

A contactor protects a PLC by acting as an intermediary between the PLC and the high voltage motor, preventing electrical surges from damaging the PLC cards.

Q: Can a relay be used instead of a contactor to control a high voltage motor?

No, relays are typically used for smaller devices with lower current and voltage ratings, while contactors are designed to handle the higher currents and voltages required for larger devices.

Q: How can you determine the voltage requirement for a contactor's coil?

Before connecting the wires to the coil, it is necessary to check the coil voltage specified by the contactor. Most contactors, like the one shown in the video, work with a 24 volt DC power supply.

Q: What is the purpose of the auxiliary or feedback contact on a contactor?

The auxiliary or feedback contact on a contactor is used to send a signal to the PLC input to monitor the health of the contactor. It provides a way to detect if the contactor is broken.

Q: How do you wire a contactor to control a motor using a PLC?

To control a motor using a PLC through a contactor, you need to connect the PLC output to the coil of the contactor, and then connect the three-phase power supply to the L1, L2, and L3 terminals. From the other end, you connect the T1, T2, and T3 terminals to the motor. Additionally, an auxiliary or feedback contact should be connected to the PLC input to monitor the contactor's health, and a start/stop switch should be connected to the PLC input for control.

Summary & Key Takeaways

• A contactor is a device used to make and break electrical power circuits, often used to control high voltage motors.

• Contactors work by using a low voltage coil that, when energized, creates an electromagnetic field that closes the contacts and allows power to reach the motor.

• Contactors differ from relays in that they are designed to handle higher currents and voltages, making them suitable for larger devices.