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Glossary of Electrical Terms

 

Alternator
A synchronous machine used to convert mechanical power into alternating current electric power.
Ambient Temperature
The temperature of the surrounding cooling medium. Commonly known as room temperature when the air is the cooling medium in contact with the equipment.
Base Line
A vibration reading taken when a machine is in good operating condition that is used as a reference for monitoring and analysis.
Breakdown Torque
The maximum torque that an AC motor will develop with rated voltage applied at rated frequency without an abrupt drop in speed. Also termed pull-out torque or maximum torque.
Code Letter
A letter which appears on the nameplates of AC motors to show their locked-rotor kilovolt-amperes per horsepower at rated voltage and frequency.
Constant Horsepower Motor
A term used to describe a multi-speed motor in which the rated horsepower is the same for all operating speeds. When applied to a solid state drive unit, it refers to the ability to deliver constant horsepower over a predetermined speed range.
Constant  Torque Motor
A multi-speed motor for which the rated horsepower varies in direct ratio to the synchronous
speeds. The output torque is essentially the same at all speeds.
Delta Connection
A three-phase winding connection in which the phases are connected in series to form a closed circuit.
Design
NEMA design letters A, B and C define certain starting and running characteristics of three phase squirrel cage induction motors. These characteristics include locked-rotor torque, locked-rotor current, pull-up torque, breakdown torque, slip at rated load, and the ability to withstand full-voltage starting.
Duty
A continuous or short-time rating of a machine. Continuous-duty machines reach an equilibrium temperature within the temperature limits of the insulation system. Machines which do not, or cannot, reach an equilibrium temperature have a short-time or intermittent-duty rating. Short-time ratings are usually one hour or less for motors.
Efficiency
The ratio between useful work performed and the energy expended in producing it. It is the ratio of output power divided by the input power.
Foot-Pound
The amount of work, in the English system, required to raise a one pound weight a distance of one foot.
Frequency
The number of cycles in a time period (usually one second). Alternating current frequency is expressed in cycles per second, termed Hertz (Hz).
Full-Load Current
The current required for any electrical machine to produce its rated output or perform its rated function.
Full-Load Speed

The speed at which any rotating machine produces its rated output.
 

Full-Load Torque

The torque required to produce rated power at full-load speed.
 

Harmonic

A multiple of the fundamental electrical frequency. Harmonics are present whenever the
electrical power waveforms (voltage and current) are not pure sine waves.

Hertz (HZ)

The preferred terminology for cycles per second (frequency).

Horsepower

A unit for measuring the power of motors or the rate of doing work. One horsepower equals
33,000 foot-pounds of work per minute (550 ft-lbs per second) or 746 watts.

IEC

International Electrotechnical Commission.

IEEE

Institute of Electrical and Electronics Engineers.

Insulation

Non-conducting materials separating the current-carrying parts of an electric machine from
each other or from adjacent conducting material at a different potential.

Insulation Class

A letter or number that designates the temperature rating of an insulation material or system with respect to thermal endurance.

Kilowatt

A unit of electrical power. Also, the output rating of motors manufactured and used off the North American continent.

Locked-Rotor Current

Steady-state current taken from the line with the rotor of a motor at standstill and at rated voltage and frequency.

Locked-Rotor Torque

The minimum torque that a motor will develop at standstill for all angular positions of the rotor, with rated voltage applied at rated frequency.

Megohmmeter

An instrument for measuring insulation resistance.

Motor

A rotating machine that converts electrical power (either alternating current or direct current) into mechanical power.

NEC

National Electrical Code.

NEMA

National Electrical Manufacturers Association.

Newton-Meter

Unit of torque, in the metric system, that is a force of one Newton, applied at a radius of one meter and in a direction perpendicular to the radius arm.

Part-Winding
Starting

A part-winding start three-phase motor is one arranged for starting by first energizing part of its primary winding and, subsequently, energizing the remainder of the primary winding.  The leads are normally numbered 1,2,3 (starting) and 7,8,9 (remaining).

Poles

The magnetic poles set up inside an electric machine by the placement and connection of the windings.

Pound-Foot

Unit of torque, in the English system, that is a force of one pound, applied at a radius of one foot, and in a direction perpendicular to the radius arm.

Power Factor

The ratio of watts to volt-amperes of an AC electric circuit.

Rated
Temperature
Rise

The permissible rise in temperature above ambient for an electric machine operating under load.

 

Resistance
Temperature
Detector (RTD)

A device used for temperature sensing consisting of a wire coil or deposited film of pure metal for which the change in resistance is a known function of temperature. The most common type is nickel, with other types being copper, platinum, and nickel-iron.
 

Rotor

The rotating element of any motor or generator.

Service Factor

A multiplier which, when applied to rated power, indicates a permissible power loading that may
be carried under the conditions specified for the service factor.

Slip

The difference between synchronous and operating speeds, compared to synchronous speed, expressed as a percentage. Also the difference between synchronous and operating speeds, expressed in rpm.

Starting Torque 

The torque produced by a motor at rest when power is applied. For an AC machine, this is the locked-rotor torque.

Stator

The stationary part of a rotating electric machine. Commonly used to describe the stationary part of an AC machine that contains the primary windings.

Synchronous
Speed

The speed of the rotating magnetic field created by the primary winding of a rotating electric machine. When the speed of the rotating element matches the speed of the rotating magnetic field, it is said to be rotating at synchronous speed.

Synchronous speed = Frequency x 120 / Number of poles

Thermistor

A resistive device used for temperature sensing that is composed of metal oxides formed into a bead and encapsulated in epoxy or glass. A typical thermistor has a positive temperature coefficient; that is, resistance increases dramatically and non-linearly with temperature. Though less common, there are negative temperature coefficient thermistors.

Torque

The rotating force produced by a motor. The units of torque may be expressed as pound-foot, pound-inch (English system), or Newton-meter (metric system).

Trending

Analysis of the change in measured data over at least three data measurement intervals.

Variable
Torque Motor

A multi-speed motor in which the rated horsepower varies as the square of the synchronous speeds.

Wye Connection

A three-phase winding connection formed by joining one end of each phase to make a "Y" point. The other ends of each phase are connected to the line. Also termed a star connection.

Wye-Delta Starting

Wye-delta is a connection which is used to reduce the inrush current and torque of a three-phase motor. A wye (star) start, delta run motor is one arranged for starting by connecting to the line with the winding initially connected wye (star). The winding is then reconnected to run in delta after a predetermined time. The lead numbers for a single run voltage are normally 1 ,2,3,4,5 and 6.