Generator Glossary: A-Z Diesel Generator Terminology

A comprehensive glossary of diesel generator terminology. This reference covers 40 essential terms across engine, alternator, control, emissions, and power systems — the vocabulary every generator professional and buyer needs to know.

Alternator

The component that converts mechanical energy from the engine into electrical energy. Also called a generator end or synchronous generator. Produces AC power through electromagnetic induction between a rotating rotor (DC field) and stationary stator (AC output).

ATS (Automatic Transfer Switch)

A device that automatically switches electrical load from utility to generator during an outage and back when utility returns. Must transfer within 10 seconds for NFPA 110 Level 1 systems.

AVR (Automatic Voltage Regulator)

Electronic device that maintains generator output voltage within ±1% by controlling the DC excitation current to the alternator rotor field. Responds to load changes in milliseconds.

Bi-Fuel

A generator that runs primarily on natural gas but uses diesel (5-10%) as the ignition source. If gas supply fails, runs on 100% diesel. Provides fuel redundancy for mission-critical applications.

Black Start

The ability to start a generator without any external power source. Requires a battery starter system. All diesel generators are black-start capable; gas turbines typically are not.

CAN Bus

Controller Area Network — a digital communication protocol used in modern generator controllers to share data between paralleled units for precise load sharing.

CHP (Combined Heat and Power)

Also called cogeneration. A system that captures waste heat from the engine cooling system and exhaust for building heating or industrial processes, achieving total efficiency of 80-90% vs 35-40% for power-only.

Continuous Power (COP)

Maximum power a generator can deliver continuously for an unlimited number of hours per year, with maintenance intervals as specified by the manufacturer. ISO 8528-1 rating.

dBA

A-weighted decibel — noise measurement that approximates human ear sensitivity. Generator noise is always measured in dBA. 100 dBA at 1m = rock concert; 65 dBA at 7m = normal conversation.

DeepSea Controller

A leading brand of generator control modules manufactured by Deep Sea Electronics (UK). Provides engine monitoring, protection, and automatic start/stop functions. Models: DSE 7320, DSE 8610.

Diesel Bug

Microbial growth (bacteria and fungi) at the diesel-water interface in storage tanks. Produces sludge that clogs filters and corrodes tanks. Prevented by keeping tanks full and using biocides.

Droop Governing

Engine speed control mode where speed decreases by 3-5% from no-load to full-load. Enables natural proportional load sharing between paralleled generators without communication.

EPA Tier 4

US Environmental Protection Agency emission standards requiring near-zero emissions for stationary diesel engines. Tier 4 Final (2015) requires PM <0.015 g/kWh and NOx <0.67 g/kWh for engines >560kW.

Excitation System

Provides DC current to the alternator rotor to create the magnetic field. Self-excited: uses residual magnetism. PMG-excited: uses permanent magnet generator for superior short-circuit capability.

FLA (Full Load Amps)

The current drawn by the generator at rated kW output. Used for cable sizing per NEC 445.13: conductors must be rated at 115% of FLA.

Governor

Controls engine speed by regulating fuel flow. Mechanical: flyweight mechanism. Electronic: actuator driven by controller based on speed sensor feedback. Electronic governors provide ±0.25% speed regulation.

Genset

Portmanteau of 'generator set' — the complete generator system comprising engine, alternator, control panel, cooling system, exhaust system, and base frame/skid.

Harmonic Distortion

Voltage waveform distortion caused by non-linear loads (UPS, VFD drives). Total Harmonic Distortion (THD) <5% is standard; <3% for critical applications. Excessive THD causes equipment overheating.

Hertz (Hz)

Unit of frequency — cycles per second. Generator output frequency: 50 Hz (most of world) or 60 Hz (Americas, parts of Asia). Engine speed: 1500 RPM = 50 Hz, 1800 RPM = 60 Hz (4-pole alternator).

IEC 60034

International standard for rotating electrical machines (alternators). Defines ratings, efficiency classes (IE1-IE4), temperature rise limits, and testing procedures.

Isochronous Governing

Engine speed control mode where speed remains constant (0% droop) regardless of load. Requires active load sharing communication between paralleled generators.

ISO 8528

International standard for reciprocating internal combustion engine driven AC generating sets. Part 1: ratings. Part 5: specifications. Part 12: emergency power supply to safety devices.

kVA (Kilovolt-Ampere)

Apparent power — the product of voltage and current without considering power factor. Generator ratings are in kVA. 500kVA at 0.8 PF = 400kW.

kW (Kilowatt)

Real/active power — the power that actually does work. kW = kVA x Power Factor. A 500kVA generator at 0.8 PF delivers 400kW.

Load Bank

A device that applies artificial electrical load to a generator for testing and exercising. Resistive (kW only), inductive (kW + kVAR), or capacitive. Prevents wet-stacking in lightly loaded standby generators.

Load Shedding

Selectively disconnecting non-critical loads to prevent generator overload. Managed by the ATS or generator controller. Typical priority: life safety > critical equipment > comfort loads.

N+1 Redundancy

A system design where one more unit than required is installed, so failure of any single unit does not cause system failure. 3x 500kW generators in N+1 provide 1000kW with one spare.

NFPA 110

US National Fire Protection Association standard for emergency and standby power systems. Level 1: life safety systems must be restored within 10 seconds. Level 2: less critical.

PMG (Permanent Magnet Generator)

An auxiliary generator that powers the AVR independently of the main alternator output. Provides superior short-circuit current capability (300% for 10 seconds) and immunity to load-induced voltage distortion.

Power Factor (PF)

The ratio of real power (kW) to apparent power (kVA). Standard generator PF is 0.8 lagging. kW = kVA x 0.8. A 1000kVA generator at 0.8 PF = 800kW.

Prime Power (PRP)

Maximum power a generator can deliver for an unlimited number of hours per year in variable load applications, with an average load not exceeding 70% of the PRP rating. ISO 8528-1.

Radiator

Air-to-water heat exchanger that rejects engine heat. Typically mounted on the generator skid with engine-driven fan. Must be sized for worst-case ambient temperature (40-50°C).

SCA (Supplemental Coolant Additive)

Chemical additive package that forms a protective film on cylinder liners to prevent cavitation erosion. Essential for wet-sleeve diesel engines. Tested with test strips; maintained at 1.5-3.0 units/mL.

Soft Loading

Gradually ramping load onto a generator instead of applying full load in one step. Reduces voltage and frequency dips. Used in closed-transition ATS and utility-parallel applications.

Sound Pressure Level (SPL)

Noise level at a specific distance, measured in dBA. Generator SPL is typically specified at 1m or 7m. Every doubling of distance reduces SPL by 6 dBA in free-field conditions.

Standby Power (ESP)

Maximum power a generator can deliver for up to 200 hours per year in the event of a utility outage, with the average load not exceeding 70% of the ESP rating. ISO 8528-1. The most common rating for backup generators.

Synchronization

The process of matching voltage, frequency, phase angle, and phase rotation of a generator to the bus before closing its circuit breaker. Modern digital controllers complete synchronization in 10-15 seconds.

THD (Total Harmonic Distortion)

Measurement of voltage waveform distortion from a perfect sine wave. Standard: <5%. Hospitals and data centers: <3%. Excessive THD causes equipment malfunction and overheating.

Turbocharger

Uses exhaust gas energy to compress intake air, increasing engine power density by 30-50% vs naturally aspirated. Water-cooled turbochargers run cooler and last longer than oil-cooled units.

Wet-Stacking

Carbon and unburned fuel accumulation in the exhaust system from prolonged low-load operation (<30% load). Causes reduced efficiency, smoke, and exhaust fires. Prevented by monthly load bank testing.