Reversing Motor Circuits 341

“Reversing Motor Circuits” provides a comprehensive overview of the various means used to reverse electric motors. Motor control circuits use various control devices to change the direction in which a motor rotates. Reversing circuits typically use reversing starters, but they may also use drum switches, limit switches, and programmable logic controllers. To reverse a DC motor, the control device changes the direction of current flow in the motor’s armature. To reverse an AC motor, the control device interchanges two of the motor’s power lines.

Many applications require motors to run in reverse, either to change the direction of operation or to brake and stop the motor. After taking this class, users will understand the basic principles behind reversing circuits for motors and be familiar with the various control devices they use. This will prepare users for designing, working with, and selecting control devices for various types of motor reversing circuits.

Class Details

Class Name:
Reversing Motor Circuits 341
Version:
2.0
Difficulty:
Advanced
Number of Lessons:
16
Related 1.0 Class:
Reversing Motor Circuits 310

Class Outline

  • Reversing Motor Applications
  • Motor Control Circuits
  • Motor Starters
  • Reversing Starters
  • Interlock
  • Reversing Circuits and Motor Starters Review
  • Reversing DC Motors
  • DC Motor Reversing Circuits
  • Reversing AC Motors
  • AC Motor Reversing Circuits
  • Reversing Starter Circuits Review
  • Drum Switches
  • Drum Switch Reversing Circuits
  • Limit Switches
  • Programmable Logic Controllers
  • Final Review

Objectives

  • Explain the purpose of reversing motors.
  • Describe motor control circuits.
  • Describe motor starters.
  • Describe reversing starters.
  • Describe interlock.
  • Describe reversing circuits for DC motors.
  • Identify reversing circuit diagrams for DC motors.
  • Describe reversing circuits for AC motors.
  • Identify reversing circuit diagrams for AC motors.
  • Describe drum switches for reversing motors.
  • Describe limit switches for reversing motors.
  • Describe PLCs for reversing motors.

Job Roles

Certifications

Glossary

Vocabulary Term Definition
AC Alternating current. Electricity that regularly reverses the direction of its flow. AC switches direction 60 times per second, or 60 hertz (Hz), in the US.
alternating current AC. Electricity that regularly reverses the direction of its flow. Alternating current switches direction 60 times per second, or 60 hertz (Hz), in the US.
armature The part of a motor in which current is induced to create a magnetic field. The armature usually consists of a series of coils or groups of insulated conductors surrounding a core of iron.
auxiliary contact A motor starter contact used to provide memory to control circuits. Auxiliary contacts close when a motor starter is energized, and they stay closed until another signal opens them.
auxiliary contact interlock Electrical isolation provided by auxiliary contacts in a magnetic reversing starter to prevent both sets of contacts from closing at the same time. Auxiliary contact interlock is also called electrical interlock and is used in addition to mechanical interlock.
ball screw A long, threaded device with reciprocating ball bearings that rotates to move the worktable of a CNC mill in a linear direction. A ball screw is driven by a motor and may use limit switches to stop automatically.
braking Slowing and eventually stopping motion. Braking an electric motor can be accomplished by reversing the direction of the motor's rotation.
coils Multiple loops of conducting wire used to create a magnetic field when current passes through it. Coils are often wrapped continuously around magnetic cores made of iron or steel.
compound motor A DC motor that has both series and shunt field windings. Compound motors combine the advantages of the shunt and series motors.
computer numerical control mill CNC mill. A machine tool that uses computer numerical data to control cutting operations on flat, square, or rectangular workpieces. Computer numerical control mills often use limit switches to reverse ball screws.
contactor A type of relay that uses a small control current to operate contacts and energize or de-energize a load. Contactors can handle high amounts of current and are combined with overload relays to create motor starters.
contacts A conductive device that connects to other conductive components to allow electricity to flow between them. Contacts connect points between conductors to create a circuit.
control circuit The part of a motor control circuit that determines when and how a motor is energized or de-energized. Control circuits consist of control devices and usually carry lower voltages than power circuits.
control devices An input component that controls the flow of current in a circuit. Control devices determine when loads are energized or de-energized.
control relays An electrically controlled mechanical device that controls one circuit by opening and closing contacts in another circuit. Control relays often use electromagnetic coils to open and close contacts.
control transformer An electromagnetic device that decreases voltage to usable levels for the motor control circuit. Control transformers also provide electrical isolation.
conveyor belts A movable device used in industry to transport materials over distances. Conveyor belts often use limit switches to stop and reverse direction.
crane A machine for lifting and moving extremely heavy loads. A crane provides both vertical and horizontal movement of heavy and oversized loads.
current The flow of electricity. Current strength is called amperage and is measured in amperes (A).
DC Direct current. Electricity that flows in one direction. DC does not reverse the direction of flow.
direct current DC. Electricity that flows in one direction. Direct current does not reverse the direction of flow.
drum switch A control device that uses a handle to control moving contacts mounted on a rotating shaft. Drum switches allow operators to manually reverse a motor circuit without having to look at the controls.
electrical interlock Electrical isolation provided by auxiliary contacts in a magnetic reversing starter to prevent both sets of contacts from closing at the same time. Electrical interlock is also called auxiliary contact interlock and is used in addition to mechanical interlock.
enclosures A physical barrier designed to provide mechanical or electrical protection for components used in a system. Enclosures for manual motor starters come in a variety of styles.
field The part of a motor that induces current in the armature. The field consists of conducting wires that form electromagnets when energized.
hard-wired Having wires that are physically connected to other devices or wires in a circuit so as to consistently function the same way. Hard-wired circuits are being replaced with PLCs for motor control.
hoist A lifting device that exerts vertical forces for lifting and lowering. A hoist may be part of a crane.
interlock A feature of reversing starters that prevents both sets of contacts from closing at the same time. Interlock prevents short circuits and motor failure.
isolation A barrier or measure put in place to prevent an uncontrolled release of energy. Isolation between a motor's control circuit and power circuit is typically accomplished with a control transformer.
ladder logic A graphical programming language based on line diagrams. Ladder logic is the most common programming language for PLCs.
limit switches A mechanical switch that is triggered when it detects the physical presence or absence of an object. Limit switches are used in pairs to automatically reverse or stop motors at specific points.
line diagrams An electrical print that shows the logic of an electrical circuit or system using standard symbols. Line diagrams are also known as ladder diagrams because they demonstrate ladder logic.
logic The reasoning behind the function of an electrical circuit or system. The logic of a circuit comprises all the principles required to understand electrical circuitry.
magnetic reversing starter A reversing starter made by wiring two magnetic motor starters to a motor. Magnetic reversing starters usually have mechanical interlock as a safety feature and may also use auxiliary contact interlock.
magnetic starter A motor starter that uses a solenoid as a magnetic switch to provide control of a motor. Magnetic starters can operate automatically and be controlled remotely.
manual reversing starter A reversing starter made by connecting two manual motor starters. Manual reversing starters use mechanical interlock as a safety measure.
manual starter A motor starter that uses a manual switch directly on the starter to provide control of a motor. Manual starters are relatively small and inexpensive.
mechanical interlock A physical arrangement of the forward and reverse contacts of a reversing starter that make it physically impossible for both sets of contacts to close at the same time. Mechanical interlock is a safety feature used with both manual and magnetic reversing starters.
memory circuit A circuit that stores a signal to keep a load energized even after the signal is removed. Memory circuits use auxiliary contacts.
motor control circuit A circuit designed to provide power and control to an electric motor. A motor control circuit consists of a power circuit and a control circuit.
motor starters An electrically operated switch that starts a motor when actuated. Motor starters use magnetic induction to provide the startup current for a motor.
National Electrical Manufacturers Association NEMA. An organization that sets standards for electrical equipment used in the United States. The National Electrical Manufacturers Association provides ratings and standards for various types of motor designs and operations.
NO Normally open. An electrical contact that typically does not connect to another conductor. NO contacts generally do not allow electricity to flow.
normally open NO. An electrical contact that typically does not connect to another conductor. Normally open contacts generally do not allow electricity to flow.
overload relay A device that protects a motor from overheating due to overload conditions in the machinery. Overload relays connect to contacts that open when overload conditions are detected.
permanent magnet direct current PMDC. A DC motor that uses permanent magnets instead of field windings to create a magnetic field. Permanent magnet direct current motors have a simple design and are used for low-horsepower applications.
power circuit The part of a motor control circuit that carries power to the motor. Power circuits often carry high voltages from the main incoming power to the motor and motor starter.
prime movers The device that introduces energy into a system and converts energy into the appropriate form. Prime movers include electric motors and diesel engines.
program A computer-based series of commands that contains all pertinent instructions and information for a given operation. Programs for PLCs are based on line diagrams and ladder logic.
programmable logic controller PLC. A processor-driven device that uses logic-based software to provide electrical control to machines. Programmable logic controllers have been replacing hard-wired circuits for motor control.
pushbuttons A manual control device that opens or closes a circuit when pressed. Pushbuttons are used to control manual motor starters.
reversing starter A device used to start a motor in forward or reverse. Reversing starters are made by connecting two manual motor starters or two magnetic motor starters.
rotor The rotating part of a motor. The rotor connects to an output shaft that drives the load.
running winding The primary winding of a single-phase AC motor that receives current during operation. Running windings consist of heavy, insulated copper wire.
series field A winding connected in series with the armature of a DC motor. Series fields consist of few turns of thick wire.
series motors A DC motor that has field windings connected in series with the armature. Series motors provide very high startup torque but must never be run without a load.
short circuit A situation in which current takes a shorter, unintended path between two conductors. Short circuits may occur if both sets of contacts in a reversing starter close at the same time.
shunt field A winding connected in parallel with the armature of a DC motor. Shunt fields consist of many turns of thin wire.
shunt motor A DC motor that has field windings connected in parallel with the armature. Shunt motors are commonly used because of their excellent speed regulation.
single-phase 1Φ. Alternating current power that consists of only one voltage. Single-phase power is used for simple residential applications.
single-phase motor An AC motor that operates on power that has only one voltage. Single-phase motors are often used in residential appliances like washing machines and air conditioners.
software The coded instructions, formulas, and operations that structure and control a computer's hardware functions and operations. Software in PLCs can be used to replace hard-wired circuits for motor control.
solenoid principle The practice of using a coil to convert electrical energy into mechanical energy via magnetic fields. The solenoid principle describes how magnetic motor starters open and close contacts.
source The device that provides electrical energy to a circuit. Sources include batteries, generators, and other devices.
starting winding An auxiliary winding of a single-phase AC motor that receives current during startup and then disconnects. Starting windings consist of fine, insulated copper.
terminals A connecting point in a circuit where a wire can be attached to connect a component. Terminals in a motor receive current from the input power lines.
three-phase 3Φ. Alternating current power that consists of three overlapping voltages. Three-phase power is used for all large AC motors and is the standard power supply that enters homes and factories.
three-phase motor An AC motor that operates on power with a continuous series of three overlapping AC voltages. Three-phase motors are used for all large AC motor applications.
voltage The electrical pressure or potential that pushes current through a conductor. Voltage is measured in volts (V) and is also called electromotive force.
windings Wire that is wrapped around a core or into a coil and used to conduct current. Windings form electromagnetic fields in motors.