What are Generators and How Do They Work?
Generators are machines that convert mechanical energy into electrical energy. They produce electricity by creating a relative motion between conductors and magnetic fields, inducing voltage in the conductors according to Faraday's law of electromagnetic induction. Generators range from small portable units providing a few kilowatts to massive utility-scale machines generating hundreds of megawatts.
Generators work by rotating a coil of wire (or in some designs, rotating magnetic fields around stationary coils) within a magnetic field. This rotation creates a changing magnetic flux, which induces an electromotive force (EMF) in the coil according to Faraday's law. In AC generators (alternators), this produces alternating current as the coil rotates through different positions relative to the magnetic field. In DC generators, a commutator converts the alternating current in the armature to direct current in the external circuit.
Key components of generators include:
- Stator: The stationary part containing electrical windings or permanent magnets
- Rotor/Armature: The rotating part that typically contains windings or magnetic fields
- Prime Mover: The mechanical energy source that drives the generator (engine, turbine, etc.)
- Excitation System: Provides the magnetic field needed for generation
- Voltage Regulator: Maintains stable output voltage under varying loads
- Cooling System: Removes heat generated during operation
- Control Systems: Monitors and manages generator operation
- Protective Devices: Guards against electrical faults and abnormal conditions
Uses and Applications
Generators serve essential functions across various sectors:
Power Generation
- Utility-scale power plants
- Renewable energy systems (hydro, wind)
- Distributed generation facilities
- Cogeneration and combined heat and power systems
- Peak shaving and demand response applications
Emergency and Standby Power
- Commercial building backup systems
- Healthcare facility emergency power
- Data center uninterrupted power supply
- Telecommunications backup systems
- Critical infrastructure emergency power
- Residential standby generators
Industrial and Specialized Applications
- Construction site temporary power
- Mining operations power supply
- Oil and gas field operations
- Mobile power units
- Marine and shipboard power systems
- Agricultural operations
- Remote location power generation
Important Considerations Before Buying Used Generators
Usage and Maintenance History
The operational history significantly impacts reliability and remaining service life:
- Verify total running hours
- Request maintenance records and service history
- Check load history (continuous vs. standby operation)
- Confirm adherence to recommended maintenance schedules
- Verify oil and coolant change intervals
- Check for major component replacements or overhauls
- Inspect for evidence of overheating or mechanical damage
- Review any recorded fault history
Specifications and Compatibility
Ensure the generator meets your system requirements:
- Output capacity (kW/kVA) sufficient for your load
- Voltage and frequency match your requirements (120/240V, 208V, 480V; 50Hz or 60Hz)
- Phase configuration (single-phase or three-phase)
- Prime power vs. standby ratings
- Engine type and fuel compatibility (diesel, natural gas, propane, gasoline)
- Cooling system appropriate for installation environment
- Physical dimensions compatible with installation space
- Noise level acceptable for location
- Emissions compliance with local regulations
Purpose and Application
Select equipment appropriate for your specific needs:
- Continuous vs. standby power requirements
- Expected duty cycle and load profile
- Starting requirements for motors and other inductive loads
- Indoor vs. outdoor installation
- Environmental conditions (temperature extremes, altitude, humidity)
- Required autonomy and fuel storage
- Noise restrictions
- Integration with existing electrical systems
- Automatic vs. manual starting requirements
How Much Should I Budget for Generators?
Pricing varies significantly based on size, features, and condition:
New Generators
- Small portable generators (5-15kW): $1,000-$5,000
- Residential standby generators (15-30kW): $5,000-$15,000
- Small commercial generators (30-100kW): $15,000-$40,000
- Medium commercial/industrial (100-500kW): $40,000-$150,000
- Large industrial generators (500-2000kW): $150,000-$500,000
- Major power systems (2MW+): $500,000+
- Installation costs: 50-100% of generator cost
Used/Reconditioned Generators
- Typically 40-70% of new pricing depending on age and condition
- Low-hour units (under 500 hours): 65-80% of new pricing
- Medium-hour units (500-2000 hours): 50-65% of new pricing
- High-hour units (2000+ hours): 30-50% of new pricing
- Fully rebuilt/reconditioned with warranty: 60-75% of new pricing
- Testing and certification costs: $500-$5,000 depending on size
- Transportation: $1,000-$10,000 depending on size and distance
- Installation and commissioning: $5,000-$100,000+ depending on size and complexity
Cost Factors to Consider
- Age: Newer generators typically command higher prices
- Running Hours: Lower hours significantly increase value
- Manufacturer: Premium brands maintain higher resale value
- Fuel Type: Diesel often retains value better than gasoline
- Enclosure Type: Weather-protected and sound-attenuated units cost more
- Control Systems: Advanced controls add value
- Emissions Tier: Newer emissions standards compliance adds value
- Availability of parts and service: Critical for older or less common models
- Accessories: Transfer switches, load banks, fuel systems add value
When purchasing used generators, thorough testing under load is essential to verify performance and reliability. A comprehensive inspection should include engine compression testing, insulation resistance measurements, oil analysis, and load bank testing. The electrical output should be verified for voltage stability, frequency regulation, and harmonic content. Generator age is less important than maintenance history and operating hours, as well-maintained older units can often outperform neglected newer ones. For critical applications, consider hiring a qualified technician to inspect any used generator before purchase, as this investment can prevent costly problems later.