Introduction to Electric Motors 301

AC

Alternating current. Electricity that reverses direction at regularly recurring intervals of time. AC switches direction 60 times per second, or 60 hertz (Hz), in the US.

alternating current

AC. Electricity that reverses direction at regularly recurring intervals of time. Alternating current switches direction 60 times per second, or 60 hertz (Hz), in the US.

armature

The part of a motor in which a 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.

bearings

A friction-reducing device that allows one moving part to glide past or rotate within another moving part. Bearings operate using a sliding or rolling mechanism.

brushes

Sliding electrical contacts used to provide a connection between the armature and the external circuit of a DC motor. Brushes contact the commutator to maintain DC.

commutator

The rotating switch that contacts the brushes of a DC motor to maintain constant polarity. The commutator maintains DC when the rotation of the armature switches the polarity of the conductor.

compound motor

A DC motor that has both series and shunt field windings. Compound motors combine the advantages of the shunt and series motors.

conductor

A material that allows electricity to flow easily. Conductors are typically metals.

constant speed motor

A motor that provides good speed regulation. A constant speed motor maintains a steady rate of rpm from no load to full load.

copper loss

Power lost when electrical energy converts into heat. Copper loss occurs when current flows through wire.

counter electromotive force

Counter EMF. Voltage induced in the armature of a DC motor that opposes the applied voltage and limits armature current. Counter electromotive force is also known as generator action.

counter EMF

Counter electromotive force. Voltage induced in the armature of a DC motor that opposes the applied voltage and limits armature current. Counter EMF is also known as generator action.

current

I. 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.

efficiency

A measure of a system’s work output compared to the total work input. Efficiency indicates how much supplied energy is converted into the intended useful work.

electromagnets

A magnet that gains an attractive force only when current passes through it. Electromagnets in motors are formed by pole pieces and windings.

electromotive force

EMF. The electrical pressure or potential that pushes electrons through a conductor. Electromotive force is measured in volts (V) and is also called voltage.

electrons

A negatively charged particle that orbits the nucleus of an atom. Electrons flowing between atoms causes electricity.

energy

The ability to do work. Energy comes in many forms, including electrical, mechanical, thermal, and chemical energy.

field windings

A coil of conducting wire connected to the armature that energizes the pole pieces. Field windings are connected in series or parallel.

foot-pounds

ft-lb. A unit used to measure power in the English system and sometimes mistakenly used instead of pound-foot to measure torque. Foot-pounds' metric equivalent is joules.

frequency

The number of complete AC cycles that occurs in one second. Frequency is measured in hertz (Hz).

friction

A force that resists motion between two objects that are in contact with each other. Friction generates heat and increases wear.

generator action

Voltage induced in the armature of a DC motor that opposes the applied voltage and limits armature current. Generator action is another name for counter electromotive force.

generators

A device that converts mechanical energy into electrical energy. Generators use magnetic induction to create electricity.

horsepower

hp. A unit used to measure power in large electrical devices such as motors. One horsepower equals 33,000 ft-lbs (44,748 J) of work per minute, or 746 watts.

housing

A protective cover designed to contain or support a component such as an electric motor. The housing of a motor is called the stator.

induction motors

An AC motor in which power is connected only to the stator. In induction motors, the current in the stator generates current in the rotor and creates an electromagnetic field that produces rotation.

left-hand flux rule

A method used to determine the direction of magnetic flux in relation to the direction of current flow in a conductor. The left-hand flux rule uses the thumb to show the direction of current flow and the fingers to show the direction of flux.

load

The opposition to applied force, such as a weight, to be carried or moved. A load provides resistance to a motor.

load

The opposition to applied force. A load provides resistance to a motor.

lockout/tagout

A method of protecting employees from accidental machine startup through proper locking and labeling of machines that are undergoing maintenance. Lockout/tagout renders a machine inoperable to ensure that the machine is not accidentally energized.

machine guard

A rigid shield or cover that encloses hazardous areas on a machine. Machine guards prevent accidental machine contact with body parts and prevent debris, such as chips, from exiting the machine.

magnet

A metallic object or substance that possesses a force that attracts or repels other metals. Magnets attract opposite charges and repel like charges.

magnetic field

The area in and around a magnet in which a magnetic force exists. Magnetic fields exhibit the powers of attraction and repulsion.

magnetic flux

The force that surrounds a magnet and exhibits the powers of attraction and repulsion. Magnetic flux is described as imaginary lines of force that exit the magnet's north pole and return to its south pole.

magnetic induction

The use of magnets to cause voltage in a conductor. Magnetic induction occurs whenever a conductor passes through magnetic lines of flux.

magnetism

The power of attraction and repulsion that exists in materials. Magnetism most often occurs between metals.

mechanical power variables

The properties of mechanical energy that vary for specific machines and applications. Mechanical power variables for motors include speed, torque, and power.

motor

A machine that converts one form of energy, such as electricity, into mechanical energy or motion. Motors operate on the principle of magnetic induction.

multispeed motor

An AC motor with windings that may be reconnected to form different numbers of poles. Multispeed motors are designed to allow for motor speed control.

Newton-meter

Nm. The unit used to measure torque in the metric system. The Newton-meter's English equivalent is the pound-foot.

output shaft

The rotating part of a motor that transmits torque to the load. The output shaft turns loads such as pumps, circular saws, and drill presses.

parallel

An electrical route that has multiple loads and multiple paths. Parallel windings consist of many turns of thin wires.

percent slip

The speed regulation of an AC induction motor. Percent slip compares a motor's synchronous speed to the speed of its rotation.

permanent magnets

A magnet that retains its attractive force after it is removed from a magnetic field. Permanent magnets have high residual magnetism.

personal protective equipment

PPE. Any example of various safety equipment that employees wear or use to prevent injury in the workplace. Personal protective equipment includes safety glasses or goggles, gloves, and footwear.

polarity

Having two oppositely charged poles, one positive and one negative. Polarity determines the direction in which current flows.

pole pieces

A metallic object used to create a magnetic field inside a motor. Pole pieces are used with windings to form electromagnets.

poles

The oppositely charged ends of a magnet. The opposition of the poles creates magnetic flux.

pound-feet

lb-ft. The unit used to measure torque in the English system. Pound-feet's metric equivalent is Newton-meters.

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.

resistance

R. The opposition to current flow. Resistance is measured in ohms (Ω).

revolutions per minute

rpm. A unit of measurement that indicates the number of times a machine component rotates in one minute. Revolutions per minute is a measurement of speed.

right-hand motor rule

A method used to determine the direction of conductor movement in a magnetic field. The right-hand motor rule uses the middle finger to show the direction of current, the index finger to show the direction of flux, and the thumb to show the direction of conductor movement.

rotor

The rotating part of a motor. The rotor connects to an output shaft that drives the load.

rpm

Revolutions per minute. A unit of measurement that indicates the number of times a machine component rotates in one minute. Revolutions per minute is a measurement of speed.

series

An electrical route that may have multiple loads but has only one path. Series windings consist of a few turns of thick wire.

series field

A winding designed to be connected in series with the armature of a DC motor or generator. Series fields consist of few turns of thick wire.

series motor

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.

shunt field

A winding designed to be connected in parallel with the armature of a DC motor or generator. 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

Alternating current power that consists of only one voltage. Single-phase power is used for simple residential applications.

slip

The difference between the synchronous speed and the rotor speed of an induction motor. Slip is used to calculate the speed regulation of induction motors.

speed

The rate at which an object travels in a given period of time. In motors, speed reflects rotational movement and is measured in revolutions per minute (rpm).

speed control

The external means of changing a motor's speed. Speed control is easier to accomplish on DC motors than AC motors.

speed regulation

The ability of a motor to maintain its speed when a load is applied. Speed regulation compares a motor's no-load speed to its full-load speed.

squirrel cage motor

An AC induction motor with a rotor that consists of metal bars connected at either end. The squirrel cage motor is the most common AC motor.

stator

The stationary part of a motor. The stator houses the motor's rotor and windings.

synchronous motor

An AC motor that requires external DC excitation to energize the rotor. A synchronous motor does not use induction to operate.

synchronous speed

The speed of the rotating magnetic field of an AC induction motor. Synchronous speed is always faster than the speed at which the rotor turns.

three-phase

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.

torque

A force that produces rotation. Torque is measured in pound-feet in the English system and Newton-meters in the metric system.

variable frequency drive

A device that converts incoming 60 Hz AC power into other desired frequencies. Variable frequency drives can be used to control the speed of AC motors.

voltage

E. The electrical pressure or potential that pushes electrons through a conductor. Voltage is measured in volts (V) and is also called electromotive force.

watts

W. A unit used to measure electrical power. The watt is the product of current and voltage.

windings

Wire that is wrapped around a core or into a coil and used to conduct current. Windings connect to pole pieces to form electromagnets and magnetic fields in motors.

work

The result of a force applied to an object and the distance through which the force is applied. Work occurs when electricity flows through some type of resistance.

wound rotor motor

An AC induction motor with a rotor that contains windings. Wound rotor motors produce high startup torque.