Alternating Current (AC)

Rapid and interrupted current that flows in one direction and then in the opposite direction.


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.


The number of amperes measured in an electrical current.


the basic unit of electric current in the SI system.

Automatic Transfer Switch (ATS)

transfer switch is an electrical switch that switches a load between two sources. Some transfer switches are manual, in that an operator initiates the transfer by throwing a circuit breaker, while others are automatic and switch when they sense one of the sources has lost or gained power. Automatic Transfer Switches communicate to the generator controller to start the machine and begin producing power.


There are two types of charge: positive (+, proton) and negative (-, electron). An atom with electrons missing is unbalanced; it has more protons than electrons and is therefore positively charged. The same analysis applies to an atom having more electrons than it should; it is negatively charged. Like charges repel each other. Unlike charges attract each other.

Circuit Breaker

A device that can automatically stop the flow of electricity in a circuit if there is too much current to operate safely.

Continuous Standby

The rating at which a generator set may be operated for the duration of a power outage. No overload capacity is guaranteed.


Current is a flow of electricity. DC flows from negative to positive. AC alternates in direction. The standard symbol for current is “I” and it is measured in Amperes (Amps).

Delta Connection

A three-phase winding connection in which the phases are connected in series to form a closed circuit.


NEMA design letters A, B, C and D 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.

Direct Current (DC)

A constant, even-flowing current that travels in one direction only.

Distribution Panel or Panelboard

An electric switchboard used to distribute power within a building. It’s enclosed in a metal box, which includes circuit breakers, fuses and switches.


A continuous or short-time rating of a machine. Continuous-duty machines reach 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.


Synchronous AC generators require DC field excitation current. Most such generators today are furnished with exciters which are AC generators having rectified output.



The number of cycles in a time period (usually one second). Alternating current frequency is expressed in cycles per second, termed Hertz (Hz).


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

Harmonic Distortion

Harmonic distortion in electrical circuits causes waves to change shape and deform as they move through the system. This results in voltage fluctuations that can damage sensitive equipment.

Hertz (Hz)

The preferred terminology for cycles per second (frequency)


A unit of 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.


International Electrotechnical Commission


Institute of Electrical and Electronics Engineers


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 designated the temperature rating of an insulation material or system with respect to thermal endurance.

lsochronous Governor

A governor that maintains constant engine speed from no-load to full load. It is a zero-droop governor. Typical accuracy is ±.25% of rated speed.

Kilovolt-Amperes (Symbol kVA)

In AC circuits, kVA is the measure of the apparent power flowing in the circuit. To find the true or actual power (kW), the kVA must be multiplied by the power factor (expressed as a decimal).


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


National Electrical Code


National Electrical Manufacturers Association

NEMA 1 Enclosure

This is commonly used to describe an enclosure that is rated for indoor use only. “Type 1” is a newer term that replaces “NEMA 1” in some literature.

NEMA 3R Enclosure

Commonly used to describe an enclosure that is rated for outdoor use, but it may also be used indoors. “Type 3” is a newer term that replaces “NEMA 3R” in some literature.

Parallel Operation

Units to be paralleled must have the same frequency, the same number of phases, the same voltage, and the same phase rotation. The latter merely means that the voltages across the terminals must reach their maximum and minimum values in the same order. Otherwise, the magnetic forces would try to turn the rotors in opposite directions.


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


DC power is always the product of Volts times Amps and is expressed in Watts.
 Watts = Volts x Amps (P = E x I)
AC output of a generator is the apparent power and is equal to the Volts times Amps, as measured at the generator.

Power Conditioner

A device that removes undesirable transients and distortion from a power source.

Power Factor

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

Prime Power

The rating at which a generator may be operated continuously as a sole source of power, with intermittent overloads up to the standby rating.

Rated Temperature Rise

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


Voltage regulation is defined as the rise in voltage, (field current and speed remaining constant), when full load is thrown off the generator.
% Voltage Regulation =   (voltage at no load – voltage at full load) x 1 00 voltage at full load
Speed regulation is similar.
% Speed Regulation =  (no load rpm – full load rpm) x 100 full load rpm


A method of electric power transmission in which the voltage is taken from one phase of a three-phase source. Most household loads are single-phase.

Standby generator power

A temporary source of electrical power that comes on automatically when the power goes out due to a weather-related power failure or local utility breakdown.

Star Connection

See Wye Connection.

Starting kVA (Kilovolt-Amperes)

Induction motors demand more kVA to start than is required for steady state operation. “Starting kVA” is used to define the condition of this extra demand, which normally lasts for a brief period of seconds or less. It is a transient effect, but of great importance. Standard motors have a code letter indicating starting kVA per hp.

Temperature Rating

A generator with a temperature rise rating of 221° F (105°C) is one in which the manufacturer guarantees that the temperature of the generator will not rise more than 122° F (50°C) above an ambient temperature of 104°F (40°C), when carrying full rated load continuously, at an altitude not exceeding 3300 ft. (1006 m) above sea-level. The term “rated load” implies that the voltage and power factor are as called for by the nameplate of the generator. The same generator is permitted (by NEMA MG1-16.40) to have a 266° F (130° C) temperature rise at a standby rating.


A method of electrical power transmission that makes use of three wires to deliver three independent alternating electrical currents.

Uninterruptible Power Supply (UPS)

A system designed to provide power without delay or transients during any period that the normal power supply is incapable of performing acceptably. To avoid a brief (10-20 second) interruption of power, a UPS can be used on critical equipment to provide continuous power during the seconds between a utility outage and the restoration of back-up power provided by a standby generator.


Voltage Dip

The momentary drop of generator output voltage that occurs whenever a load is added to the system. There is a momentary increase in output voltage whenever a load is removed from the system. This is called “Voltage Rise.” “Voltage Rise” is seldom of concern with an adequate voltage regulator.


This instrument when connected across the line will indicate the potential difference in volts.

Volt (Symbol V or E)

The unit for measuring electric pressure or electromotive force required to force an electric current to flow. Voltage actually shows the difference in electromotive force between two points in a circuit. One volt is required to force one ampere through one ohm of resistance. The usual AC voltmeter generally measures effective volts, and unless otherwise specified, voltage values are always given as effective volts.


The rate at which energy is drawn from a source that produces a flow of electricity through a circuit.


See kilowatt.

Wave Form

See kilowatt.The shape of the voltage wave that a generator produces is largely under the control of the designer, although most machines are designed to produce waves that closely approximate the true sine wave. Such factors as hysteresis, rotor and stator slotting and armature reactance prevent a perfect sine from being generated.


Electric power is measured by means of a wattmeter. Because electric power is a function of current and voltage, a wattmeter must have two elements, one for current and the other for voltage. The power indicated by a wattmeter is a result of the voltage across the load, the current through the load and the power factor on the load.  In effect, the wattmeter multiplies the voltage, current and power factor to indicate the true power. When using a wattmeter, take all precautions mentioned for ammeters and voltmeters. In addition, make sure that neither the current nor voltage exceeds the wattmeter capacity. Test the circuit with a voltmeter and ammeter before connecting a wattmeter. The wattmeter scale deflection does not indicate whether the meter is overloaded or not. The voltage may be low and the current high and still indicate a true power-within the meter scale limit, but the current element may be overloaded.

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.