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.
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)
A 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.
A device that can automatically stop the flow of electricity in a circuit if there is too much current to operate safely.
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).
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)
Distribution Panel or Panelboard
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.
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.
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.
A letter or number that designated the temperature rating of an insulation material or system with respect to thermal endurance.
Kilovolt-Amperes (Symbol kVA)
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
NEMA 3R Enclosure
The magnetic poles set up inside an electric machine by the placement and connection of the windings.
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.
Standby generator power
Starting kVA (Kilovolt-Amperes)
Uninterruptible Power Supply (UPS)
Volt (Symbol V or E)
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.
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 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.