Glossary of Used Diesel Generator Terms

Terminology and Concepts For Electric Generators

The following list of terms and word-pictures is offered to help our clients understand the relevant terms and concepts when purchasing, maintaining and selling electric generators. While we believe that many of the following terms need only be defined for our purposes today, comparing electrical terminology to water flowing in a pipe, while not perfect, presents a good analogy of electricity. This glossary is intended for Everyman and not overly technical.

Glossary Of Used Diesel Generator Terms (2025 Update)

This technical glossary aims to clarify common terminology used in the purchasing, maintenance, and operation of used diesel generators. Drawing parallels between electrical principles and flowing water, these simplified explanations provide a practical foundation for understanding generator systems.

AC vs. DC

• Direct Current (DC): Flows in one constant direction, such as from a battery (positive to negative).
• Alternating Current (AC): Changes polarity (positive to negative) at a fixed frequency (e.g., 60 Hz in the U.S.). This cyclical change is measured in hertz (Hz)

Alternator

A device that converts mechanical energy (e.g., from a diesel engine) into electrical energy (AC).

Amperage (AMPS)

The intensity of an electric current, representing how many electrons flow through a conductor per second.

Apparent Power, Real Power, and Power Factor

• Apparent Power (kVA): The total power supplied, irrespective of load characteristics.
• Real Power (kW): The portion of power that performs actual work (e.g., heating, turning a motor).
• Power Factor (PF): The ratio of real power to apparent power, reflecting load efficiency.

Battery Charger & Rectifier

• Battery Charger: Converts AC power to DC to recharge batteries.
• Battery Charge Rectifier: Converts AC to DC within a generator’s stator to maintain battery charge.

Brush

A conducting element (often graphite/copper) enabling continuous electrical contact between a moving part (rotor/commutator) and a stationary component.

Calculations

• Horsepower to Watts: HP=W745\text{HP} = \frac{W}{745}.
• Power in Resistive Loads: P=IVP = IV.
• Power in Inductive Loads: P=IV×PFP = IV \times \text{PF}.

Capacitor Run/Start Electric Motors

• Capacitor Start Motors: Use a capacitor to boost starting torque.
• Capacitor Run Motors: Maintain a capacitor circuit during operation for higher torque requirements (e.g., air compressors).

Circuit Protection & Breakers

Circuit breakers protect wiring from overloads or short circuits and can serve as a service disconnect. 

Conductor

Any wire or cable facilitating electric current flow. 

Contactor & Relay

• Contactor: An electrically operated switch for higher current loads.
• Relay: Similar to a contactor but typically handles lower currents.

Commutator

Found in DC or universal motors, it switches winding polarity with rotating brushes.

Core & Cradle

• Core: Laminations in a generator forming the magnetic circuit.
• Cradle: The metal frame encasing and protecting the generator/engine assembly.

Current

The flow rate of electric charge, measured in amperes (A). Analogous to water flow in a pipe.

Cycle & Frequency (Hz)

• Cycle: One full AC waveform (zero → positive max → zero → negative max → zero).
• Frequency (Hz): Number of cycles per second (e.g., 60 Hz).

Deep Cycle Battery Charger

Designed for batteries repeatedly discharged and recharged with minimal capacity loss.

Diode

A semiconductor device allowing current to flow in one direction only, used for rectification.

Direct Current (DC)

Electric current with a unidirectional flow, often from batteries or DC generators. 

Dynamo (Generator)

A machine converting mechanical energy to electrical energy via electromagnetic induction.

Efficiency

The ratio of power output to power input, factoring in losses such as friction or winding resistance.

Electricity in Commercial Applications

• Single-Phase 120 V/240 V: Common residential and light commercial power in the U.S.
• Three-Phase (e.g., 120/208, 277/480): Used for heavier industrial/commercial loads.

Electromagnets

Coils of wire that become magnetic when current flows. Their polarity and strength vary with current changes. 

Electro-Motive Force (EMF)

The force driving current flow in a conductor, measured in volts. 

Engine Low Oil Shutdown & Mechanical Governor

• Low Oil Shutdown: Automatically stops the engine if oil pressure drops below safe levels.
• Mechanical Governor: Regulates engine RPM under varying loads.

Four-Cycle (Four-Stroke) Engine

A common diesel/gasoline engine design with distinct intake, compression, power, and exhaust strokes. 

Full Pressure Lubrication Engine

Uses an oil pump to lubricate internal components under pressure, increasing reliability.

Full Power Outlet

An outlet designed to deliver the generator’s maximum power capacity. 

Generator

A broad term for devices converting mechanical to electrical energy (AC or DC).

Ground

An intentional connection between electrical circuits and the earth for safety and stability.

Idle Control

A system adjusting engine speed based on load to save fuel and reduce wear.

Ignition Coil

Transforms low-voltage DC into high voltage for spark plugs in gasoline engines.

Induction

When a conductor moves near a magnetic field, a current is induced. Fundamental for generator operation.

Inductors

Coils (motors, transformers) that store energy in a magnetic field, influencing AC power flow and phase.

Magnetic Poles

Magnets have north/south poles, with like poles repelling and opposite poles attracting.

Magneto

A permanent magnet-based generator for ignition systems in some engines.

Neutral vs. Hot

In modern wiring, Neutral is anchored to Ground at the breaker panel, while Hot carries supply voltage.

Ohm & Ohm’s Law

• Ohm (Ω): A unit of electrical resistance.
• Ohm’s Law (V=IR): Fundamental relationship of voltage, current, and resistance.

Parts of the Alternator

Generators have rotating (rotor) and stationary (stator) magnetic components for producing electricity.

Phase

The angular timing difference between AC waveforms. Single-phase (two wires) vs. three-phase (three wires) systems.

Power & Power Factor

• Real Power (W): Actual work done.
• Apparent Power (VA): Product of current and voltage regardless of phase shift.
• Power Factor (PF): The ratio of real power to apparent power, indicating efficiency.

Power Transfer System

Equipment (e.g., Automatic Transfer Switches) enabling safe generator integration with facility wiring.

Pressure vs. Voltage

Voltage in electrical circuits is analogous to water pressure in pipes, driving current flow.

Rated Speed & Rated Voltage

Generator sets have specific RPM and voltage ratings ensuring stable output under designed load conditions.

Rotor & Stator

• Rotor: The rotating electromagnetic assembly in generators or motors.
• Stator: The stationary magnetic field assembly.

Single Phase vs. Three Phase

• Single Phase: Typically 120/240 V systems for residential/light commercial.
• Three Phase: Common in industrial settings for efficient power distribution (e.g., 120/208 V, 277/480 V).

US Power & Environment (USP&E)

A global leader in providing power solutions, known for its dedicated customer service and high-effort approach.

Vibration Mount

Rubber or elastomeric components reducing vibrational transfer between engine/generator and its frame.

Volt & Voltage

• Volt (V): The unit of electromotive force.
• Voltage: The potential difference driving current flow.

Voltage Regulator

A device automatically controlling generator output voltage by adjusting rotor excitation. 

Watt

The SI unit of real power. One kilowatt (kW) equals 1,000 watts.

Winding
Coils of wire in the generator forming electromagnetic fields. Stator windings are stationary; rotor windings rotate.