PLC and Motor Control Circuit Worksheet

Which of the following input devices would most commonly be connected to a PLC to detect physical conditions?

• Thermocouple

• Relay

• Switch

Answer explanation

A thermocouple is commonly used to measure temperature, making it ideal for detecting physical conditions. Relays and switches are more for control rather than direct measurement, which is why thermocouples are the correct choice.

A technician needs to troubleshoot an electrical circuit and identify which physical wires connect to specific components. Which diagram should they use?

• Schematic Diagram

• Block Diagram

• Wiring Diagram

Answer explanation

A Wiring Diagram is essential for identifying physical connections between wires and components in an electrical circuit, making it the best choice for troubleshooting. Schematic and Block Diagrams do not provide this level of detail.

Use the diagram provided to respond. A relay contact connected directly in series with the reverse start button of a reversing motor control circuit would affect the:

• Starting and stopping in both directions

• Starting but not the stopping in the reverse direction

• Starting but not the running in both directions

Answer explanation

The relay contact in series with the reverse start button allows starting the motor in reverse but does not affect stopping, as the stopping mechanism is independent of the start button. Thus, the correct choice is 'Starting but not the stopping in the reverse direction'.

What is the main purpose of a contactor in a three-phase motor control circuit?

• To connect and disconnect the motor from the power supply

• To reverse the direction of the motor

• To protect the motor from overload

Answer explanation

The main purpose of a contactor in a three-phase motor control circuit is to connect and disconnect the motor from the power supply, allowing for safe operation and control of the motor.

In a circuit, you encounter a symbol of a rectangle containing diagonal stripes and a letter 'G'. What does this represent?

• Ground fault circuit interrupter

• Grounding terminal

Answer explanation

The rectangle with diagonal stripes and the letter 'G' represents a grounding terminal, which is used to connect electrical circuits to the ground for safety. The other option, ground fault circuit interrupter, is represented differently.

You are troubleshooting a motor control circuit in an industrial application where the motor jogs intermittently for positioning. After reviewing the control wiring, you notice that the motor stops once the jogging button is released, and the holding circuit is no longer activated. Which of the following best explains the problem?

• The motor is improperly wired for continuous operation

• The motor control circuit is configured for negative jogging

Answer explanation

The motor control circuit is configured for negative jogging, which means it only operates while the jogging button is pressed. Releasing the button deactivates the holding circuit, causing the motor to stop.

A jogging circuit needs to be designed to operate a conveyor system for precise alignment of products. What would be the best way to implement the jogging function for accuracy?

• Using a double acting push button to control the motor movement

• Using a double acting push button to control the motor's movement

Answer explanation

Using a double acting push button allows for precise control of the motor's movement, enabling accurate jogging of the conveyor system for product alignment. This method ensures responsiveness and accuracy in operation.

A liquid level transmitter is calibrated for a range of 0 to 1000 liters, with a 4-20 mA output. If the output reads 14 mA, what is the liquid level in the tank?

• 500 liters

• 625 liters

Answer explanation

The output of 14 mA corresponds to a level between 4 mA (0 liters) and 20 mA (1000 liters). Using the formula: Level = (Output - 4) / (20 - 4) * 1000, we find Level = (14 - 4) / 16 * 1000 = 625 liters.

During dynamic braking, the electrical energy generated by the motor is dissipated as heat. Which component is responsible for this energy dissipation?

• Braking resistor

• Motor windings

Answer explanation

The braking resistor is responsible for dissipating the electrical energy generated during dynamic braking as heat, preventing damage to the motor and ensuring safe operation.

While testing a jogging circuit in an industrial setting, you observe that the motor does not stop immediately after releasing the jogging button. What is the most likely cause of this problem?

• The push button is defective

• The control relay is not de-energizing

Answer explanation

The motor continues running because the control relay is not de-energizing after the jogging button is released, indicating a potential issue with the relay itself rather than the push button.