Reversing Motor Circuits 341

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.