What are reversing and sequence controls?

In most digital applications involving electronic circuits and advanced control systems, the control of electronic components such as motors, on/off switches, mechanical valves, and so on is not controlled by analog means, but is controlled digitally. The control operations involve binary variables of either two values, 0 and 1. These values mean either ON and OFF, or open and closed. The electronic circuit and the components operating through binary operations form a control system. These control systems control the components through the input signals and operating conditions, which are a function of time. These systems are preferably known as a sequence or logic control systems using various sensors. For instance, considering the production and assembly line in the manufacturing industry, all the operations and sequences of operations like parts scanning, change of tool, machining operations on the workpiece, finishing operations, etc. all are controlled logically using sequence control systems.

In large-scale AC-controlled motors, especially three-phase motors, it is possible to operate the motor both in the forward and reverse direction. It is achieved simply by interchanging any two lead wirings that power the motor. A motor starter is mainly responsible for reversing the motor rotations. The motor starter is a small circuit that changes the magnetic field direction, which creates a reverse torque in the motor armature. It is also sometimes known as reversing the motor phase rotation. However, due to incorrect connection in electrical circuitry, a problem of phase reversing occurs that can damage the electric component. It occurs mainly when the phase sequence of any three-phase system is wired incorrectly. The system functions in the reverse direction than the desired direction. The term is primarily known as phase reversing, caused due to incorrect phase sequence.

Reversing magnetic starters

A magnetic starter is an electromechanical device with a switching mechanism operated by an overload relay and a contactor. It provides a safe operation of an electric motor. On detecting an overload, the relay opens up the control voltage to the starter coil.
The reversing magnetic starters are also called forward/reverse magnetic starters. These help to change the magnetic field inside the motor frequently and can be used to reverse the direction of rotation of three-phase motors. These starters have two three-pole contactors unlike single pole contactors found in conventional starters. These contactors are generally horsepower rated. The single-pole contactors are used for crankcase heaters where a shunt circuit energizes a component continuously. The two horsepower-rated contactors power the motor creating different phase rotations.
Both the contactors are not allowed to energize simultaneously as it may result in a short circuit. To do the task, the contactors are electrically and mechanically interlocked.

Electrical interlock

The electrical interlocks prevent one coil from getting energized if the other is active. It is achieved by making a series connection of the relay that provides forward direction with the reverse direction relay and vice versa. When the forward relay is active, pushing a reverse push button will ensure no power goes to the reverse relay.

Mechanical interlock

It is generally used as the last step for protection when the electrical interlock fails to perform as desired. The mechanical interlock system has an additional coil. If somehow, both the coils get energized, the adjacent coil of one of the relays is made active. Such coil prevents the armature of the relay to make contact with the real output port. It simply acts as a physical barrier.

Forward/Reverse sequence control circuit

A reverse sequence control circuit is a motor controller circuit that can safely control the motor rotation both in the forward and reverse direction. A standard three-wire circuit is used with a normally open pushbutton and a holding contact branch to design a reverse sequence control circuit for controlling the second relay coil. Both electrical and mechanical interlocking is provided for the coils. Two pushbuttons are connected parallelly to the circuit, namely forward and reverse pushbuttons. It can be referred from the schematic diagram below.

In this schematic diagram, FPB and RPB represent the forward and reverse pushbuttons respectively, whereas F and R represent the forward and reverse relays respectively. The forward relay has straight motor wiring to ensure forward motor rotation, while the reverse relay has interchanged motor wiring which ensures reverse motor rotation. NC IN 1 and NC IN 2 represents normally closed electrical interlocks 1 and 2 respectively. The symbols '+' and '-' signify power source. The dotted line ML represents mechanical interlock.
When the forward pushbutton is pressed, the current flows into the circuit and energizes the forward relay, F, and all associated contact with F is activated. Due to ML, the NC IN 2 gets opened and the current cannot energize the reverse relay, hence the motor rotates in the forward direction. Even if the reverse pushbutton is pressed keeping the forward coil engaged, the current does not flow through NC IN 2 and nothing happens. To make the motor rotate in the reverse direction, the switch is pressed that cuts off the circuit making the forward relay de-energized. The reverse pushbutton is pressed that energizes the reverse relay and cuts off the NC IN 1 due to ML, and so the motor rotates in the reverse direction.

Programmable Logic Controllers (PLC)

These are the devices that control electrical devices especially sensors and motors logically. These are specially used in transfer lines, robotics, process control, and especially in various automation industries. PLCs are known as special-purpose real-time computers that use microprocessors. They operate through PLC programs written on PLC software. The major benefit of such devices is that they can control any motor, and any specially designed reverse/forward sequence circuit is not needed additionally.

The following are uses of PLC with respect to industrial automation.

• Monitor and control input sensors and motors
• Execute logic, sequencing, and timing functions
• Driving actuators and indicators
• Communication with other logic controllers and computers

Some of the advantages of PLC are:

• Easy assembly and easy to use module.
• Easy expansion of input and output modules.
• Programming language is simple with easy to implement logic.
• Highly robust and flexible.

Context and Applications

This topic is taught in many undergraduate and postgraduate degree courses like:

• Bachelors of Technology (Electrical Engineering, Electronics Engineering, Mechanical Engineering, Instrumentation Engineering)
• Master of Technology (Electrical Engineering, Electronics Engineering, Mechanical Engineering, Instrumentation Engineering)

Practice Problems

Q 1. In which of the following case a single point contactor is used?
a. When a component is to be energized continuously
b. Operating crankcase heaters
c. Single-phase motors
d. Both a and b

Answer: Option d

Explanation: A single point contactor is primarily used when an electrical component such as a crankcase heater, needs to be energized continuously or to have power supplied to it continuously.

Q 2. Why are both the starter coils not allowed to energize simultaneously?
a. It may result in short circuit.
b. It may result in current overflow.
c. It may result in heating.
d. None of these

Answer: Option a

Explanation: Both the relay coils are not allowed to energize simultaneously as this may result in a short circuit.

Q 3. Which of the following is a large-scale motor used commonly?
a. NEMA stepper motors
b. Three-phase AC motors
c. Servo motors
d. DC motors

Answer: Option b

Explanation: The three-phase AC motors are the large-scale motors used for most electrical applications.

Q 4. What is the requirement of sequence logic circuits?
a. It allows forward rotation of motors.
b. It allows reverse rotation of motors.
c. It allows both reverse and forward rotations of motors.
d. None of these

Answer: Option c

Explanation: Sequence logic circuits are those circuits that are used to control reverse and forward rotations logically and safely.

Q 5. How do the sequence logic circuits turn the motor in the reverse direction?
a. It uses relays and switches that create a reverse magnetic field.
b. It makes use of both forward and reverses relays.
c. Both a and b
d. None of these

Answer: Option c

Explanation: A sequence circuit logically controls a motor rotation by using forward and reverse relays with switches, they create a reverse magnetic field which energizes the motor armature creating an opposite torque.