PLC Programming Level 2 Quiz

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Questions and Answers

What happens to the feedback contact when the contactor is energized and functioning properly?

It remains energized without closing.
It intermittently opens and closes.
It closes and sends a signal to the PLC. (correct)
It remains open and sends no signal.

Why is a feedback contact necessary in a PLC-controlled system with a contactor?

To reduce wear on the contactor.
To verify if the contactor is functioning correctly. (correct)
To directly control the PLC outputs.
To ensure the motor receives three-phase power.

What primary function does a contactor serve in a PLC system?

To enhance the signal strength to the PLC.
To connect the PLC directly to the motor.
To turn on and off high voltage devices indirectly. (correct)
To convert DC voltage to AC voltage.

What is one of the key differences between a contactor and a relay?

Contactors are intended for heavy high voltage devices. (A) Signup and view all the answers

What must be done to energize the contactor coil and start the motor?

Press the start switch. (B) Signup and view all the answers

When pressing the stop switch, what occurs in the PLC-controlled motor system?

The contact opens, de-energizing the coil and stopping the motor. (B) Signup and view all the answers

How is a three-phase power supply connected in a system utilizing a contactor?

Connected to L1, L2, and L3 at one end and T1, T2, and T3 to the motor at the other. (D) Signup and view all the answers

What should the PLC input connect to in addition to the feedback contact to operate a motor?

A start and stop switch. (A) Signup and view all the answers

What is the primary purpose of a contactor?

To make and break an electrical power circuit (D) Signup and view all the answers

Why is it unsafe to directly connect a high voltage motor to a PLC?

It can damage the PLC cards due to electrical surges (A) Signup and view all the answers

How is a contactor different from a relay?

A contactor is built for higher current capacities compared to a relay (D) Signup and view all the answers

What initiates the operation of a contactor?

An electromagnetic field created by an energized coil (A) Signup and view all the answers

What signal is commonly used to energize the coil of a contactor?

24 volts DC (C) Signup and view all the answers

What happens to the contacts of a contactor when the coil is not energized?

They open and stop the flow of current (C) Signup and view all the answers

Which of the following statements is NOT true regarding contactors?

They can be used interchangeably with relays for any application (D) Signup and view all the answers

What type of current do contactors generally control?

High voltage AC (B) Signup and view all the answers

What is the primary function of a relay compared to a contactor?

Relays are used for smaller devices with lower current and voltage. (C) Signup and view all the answers

What are the voltage options that a contactor's coil could operate on?

Commonly 12 volt DC, 24 volt DC, or 220 volt AC. (C) Signup and view all the answers

What do the wire terminals labeled L1, L2, and L3 on a contactor represent?

They connect the power wires to the contactor. (B) Signup and view all the answers

Which terminals on a contactor would be used to connect device wires?

T1 and T2. (D) Signup and view all the answers

When the coil in a contactor is de-energized, what happens to the normally open contacts?

They remain open and do not supply power to the device. (C) Signup and view all the answers

What is the purpose of the normally open (NO) contact on a contactor?

To send a signal about the contactor's health to the PLC. (B) Signup and view all the answers

How is the coil energized in a contactor?

By connecting both positive and negative voltages to A1 and A2. (D) Signup and view all the answers

What must be verified before connecting wires to a contactor's coil?

The voltage rating of the coil. (D) Signup and view all the answers

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Study Notes

Understanding Relays and Contactors

A contactor is a device designed to make or break an electrical power circuit, primarily used for controlling heavy-duty electrical equipment like motors.
Direct connection of a high voltage motor to a PLC is discouraged due to risk of damaging the PLC from electrical surges.
Contactors provide an indirect and safer interface between the PLC and high voltage devices.

How Contactors Work

Contactors operate with a low voltage coil, usually energized by a 24V DC signal from the PLC.
Energizing the coil generates an electromagnetic field that closes the contactor's three main contacts, allowing three-phase power to flow to the motor.
When the coil is de-energized, contacts open and stop power from reaching the motor.

Feedback Contacts

Contactors include auxiliary or feedback contacts that signal the PLC whether the contactor is functioning correctly.
If a contactor malfunctions, the feedback contact will not close, preventing a signal from reaching the PLC and indicating the issue.

Wiring a Contactor

To connect a contactor:
- Wire the PLC output to the coil using terminals A1 (positive) and A2 (negative).
- Connect L1, L2, and L3 terminals to a three-phase power supply. T1, T2, and T3 then connect to the motor.
Ensure the coil voltage is suitable, as it can vary: common voltages include 12V DC, 24V DC, and 120V/220V AC.

Difference Between Contactors and Relays

Contactors are designed for high current and voltage applications (e.g., motors), while relays are typically used for lower voltage equipment.
Both devices share a similar operational principle, but their specifications differ significantly.

Operational Summary

To operate a motor:
- Press the start button to energize the coil, closing the contacts and starting the motor.
- Press the stop button to de-energize the coil, opening the contacts and stopping the motor.
Always include feedback contacts in your circuit design to ensure proper operation and to receive alerts on contactor health.