Alternating Current (AC)
Rapid and interrupted current that flows in one direction and then in the opposite direction.
A device that converts mechanical energy to electrical energy in the form of alternating current.
The temperature of the air surrounding the equipment.
The strength of an electric current measured in amperes.
A basic unit of measurement of electric current.
Apparent Power (Symbol kVA, VA)
When the current and voltage are not in phase, i.e., voltage and current do not reach corresponding values at the same instant, the resultant product of voltage and current is apparent power instead of actual power. Apparent power is measured in volt-amperes or kilo-volt-amperes. Actual power (kW) is the product of kVA and the power factor.
Automatic Transfer Switch (ATS)
The brains behind your home generator system that continuously monitors the power coming from your utility. When the power goes out, the ATS automatically transfers power from the utility to your generator. When utility power is restored, the ATS turns off your generator and returns your home’s power to the utility grid.
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.
A special switch used to protect electrical circuits. It’s designed to open or break the circuit when an abnormal condition, such as an overload, occurs.
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).
So named because it resembles the Greek letter. To make a delta connection, the finish end of the first winding is connected to the start of the second winding, the finish of the second winding is connected to the start of the third winding and the finish of the third winding is connected to the start of the first winding. Modern generators are normally connected in a wye or, star pattern rather than delta for several reasons. The-delta-connected generator has no advantages over the wye-connected machine; the wye machine has the advantage of being able to bring out the neutral wire. In the delta-connected machine, it is difficult to design the generator to keep the circulating currents low in magnitude. Normally the wye-connected machine will give better waveform characteristics than the delta-connected generator. In the wye machine, the harmonics tend to cancel each other out when line-to-line voltage is checked between two legs or phases. In the delta-connected machine, the line-to-line voltage is across one coil or set of coils and there is no cancellation of harmonic effects, except that the third harmonic and its multiples are shorted out and do not appear in the output. Delta-connected generators are used to supply 120/240-Volt, three-phase/single-phase, 4-wire systems.
National Electrical Manufacturers Association (NEMA) standards provide that each generator of 250 watts output or more be given the following high potential factory test to check generator insulation:
Stator Windings - apply two times the normal voltage plus 1000-Volts.
Field Windings - apply ten times the exciter voltage, but in no case less than 1500-Volts.
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.
Input times efficiency equals output, or efficiency equals output divided by the output plus losses. Efficiencies of generators are commonly given at 4/4, 3/4 and 1/2 load. Unless otherwise stated, the efficiency of the generator is always based on the kVA and power factor at which it is rated.
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 per second the current alternates.
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.
A plan whereby an electric utility, elects to interrupt service to a specific customer at any time. Special rates are often available to customers under such agreements.
Insulating materials are used in all electrical machinery to isolate and maintain the flow of current through the conductors.
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 measure of electric power equal to 1000 watts.
The lines of force due to the proper alignment of the atoms are called magnetic flux lines and make up the magnetic field. By convention, the lines begin at the north pole and end at the south pole.
Nameplate load/nameplate rating
The normal maximum operating rating applied to a piece of electrical equipment in Volts, Amps, horsepower or kW.
National Electrical Code (NEC)
The National Electrical Code is a volume of standard electrical rules prepared by the National Fire Protection Association. The code contains basic minimum provisions considered necessary for safety. These minimum rules are modified, expanded and interpreted by local electrical safety governing bodies. Local electrical and building inspectors in a particular community should be consulted for answers to specific questions and interpretation of the local codes covering a particular installation.
National Electrical Manufacturers Association (NEMA)
This is an organization of electrical manufacturers set up to provide limited information pertaining to certain types of electrical equipment. A main function of the organization is to establish uniform nomenclature throughout the industry and to promote manufacturing economics.
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.
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.
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.
A device that removes undesirable transients and distortion from a power source.
The rating at which a generator may be operated continuously as a sole source of power, with intermittent overloads up to the standby rating.
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.
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.
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.
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.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.
In a wye or star-connected generator the three start ends of each single-phase winding are connected together to a common neutral point, and the opposite or finish ends are connected to the line terminals. When both low-voltage, single-phase loads and higher-voltage, three-phase loads are encountered, a line to the neutral or common point will serve as a common return circuit for all three phases, i.e., 120/208-Volt, 3-phase, 4-wire machine. In a wye-connected machine the voltage from line to line is equal to the product of 1.73 and the line to neutral voltage. With a 4-wire, 120/208-Volt generator, motors can be operated on the 3-phase, 208-Volt leads, and 120-Volt lighting loads can be connected anywhere in the circuit between the various lines and the neutral. If this is done it is important to balance the 120-Volt, single-phase load as much as possible so that the entire added lighting load is not connected to one single-phase leg or coil.