Kohler · Brushless PM (Gaseous/Commercial)

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Kohler Brushless PM Alternator

Alternators

Kohler-designed 4-pole brushless alternator with rare-earth permanent-magnet pilot exciter for gaseous-fuel commercial generators. Class H insulation, 40-500 kW range. Standard on KG, REZG, REZX, and RZG/RZX platforms.

Overview#

Kohler's brushless permanent-magnet alternator family serves the gaseous-fuel and light commercial generator platforms — from 40 kW KG-series natural gas generators through 500 kW REZX-series commercial standby units. These alternators share the same core design philosophy as the larger KH-series industrial alternators: 4-pole brushless construction, Class H insulation, and permanent-magnet pilot excitation for sub-3% THD power quality.

The key differentiator across Kohler's entire alternator lineup — residential through industrial — is the commitment to PM excitation. While competitors like Generac use shunt-excited alternators in most of their commercial gaseous lineup, Kohler specifies PM excitation as standard even at the 40 kW level.

Platform coverage#

PlatformkW rangeFuelAlternator type
RCL (Residential Liquid-Cooled)24–60 kWNG / LPGBrushless PM
CCL (Commercial Compact)25–36 kWNG / LPGBrushless Wound-Field
KG (Industrial Gaseous)40–400 kWNG / LPGBrushless rare-earth PM
REZG / RZG100–150 kWNG / LPGBrushless PM
REZX / RZX180–500 kWNG / LPGBrushless PM Pilot Exciter

Why PM excitation matters for gaseous generators#

Gaseous engines produce less power per liter of displacement than diesel engines. This means gaseous generators are more susceptible to voltage droop during large load steps — the engine takes longer to recover speed, and the alternator must maintain excitation through a deeper and longer transient.

PM excitation provides:

  • Independent excitation source — voltage regulation is maintained even when the main stator output drops during transients
  • Full short-circuit current — critical for protective device coordination in commercial buildings
  • Better motor starting — Kohler markets this as "PowerBoost" in the residential RCL line, but the principle is the same across all PM-excited platforms

Common service issues#

In our field experience with gaseous-platform generators:

  • Spark plug fouling affects engine speed stability, which manifests as voltage and frequency fluctuation. This is an engine issue, not an alternator issue, but customers often report it as "alternator problems."
  • Cooling air restriction — gaseous generators in enclosed mechanical rooms accumulate lint, dust, and debris faster than outdoor diesel units. Restricted airflow causes alternator overheating and insulation degradation.
  • AVR/controller interaction — voltage regulation on KG and REZX platforms is managed by the Decision-Maker controller, not a standalone AVR. Controller firmware updates occasionally change regulation parameters.

OnPoint service notes#

We service Kohler gaseous alternators across the central California coast as part of commercial standby maintenance contracts. The most common service items are annual megger testing, load bank testing with voltage regulation verification, and cooling system inspection. The brushless PM design is extremely reliable — alternator-specific failures are rare compared to engine and fuel system issues on gaseous platforms.

Frequently Asked Questions

What does "rare-earth PM" mean in Kohler alternator specs?
Rare-earth PM refers to the permanent-magnet pilot exciter that uses rare-earth (neodymium) magnets. This small PM generator on the alternator shaft provides independent, high-energy excitation to the voltage regulator — independent of the main stator output. The rare-earth magnets provide stronger magnetic flux density than ferrite magnets, enabling better voltage regulation during load transients and full short-circuit current for protective device coordination.
Which generators use the Brushless PM alternator?
Kohler's brushless PM alternators are standard across the gaseous and light commercial platforms: KG-series natural gas generators (KG40 through KG400), REZG/RZG-series gaseous standby generators (100-150 kW), REZX/RZX-series gaseous standby generators (180-500 kW), and the residential RCL liquid-cooled series (24-60 kW). The same PM excitation architecture is used across all sizes, with frame scaling for the power rating.
How does power quality compare to diesel alternators?
The brushless PM gaseous alternators deliver the same sub-3% THD specification as Kohler's diesel alternators. The PM excitation system is identical in principle — the difference is frame sizing, cooling, and winding configuration to match gaseous engine characteristics (typically lower power density per displacement than diesel).
What is the difference between wound-field and PM alternators?
Kohler uses two alternator types in the residential/light commercial range. The smaller RCA air-cooled residential units (14-26 kW) and CCL commercial compact units (25-36 kW) use brushless wound-field alternators, while the larger RCL liquid-cooled residential units (24-60 kW) and all KG commercial units use brushless permanent-magnet alternators. The PM type provides better voltage regulation and short-circuit performance, which matters more at higher power levels and for commercial applications.
What maintenance does a brushless PM alternator need?
Minimal. The brushless design eliminates brush and slip ring maintenance entirely. Annual service consists of insulation resistance testing (megger), voltage regulation verification during load bank testing, and visual inspection of connections and cooling air paths. The PM exciter itself has no wear components — the rare-earth magnets maintain their flux indefinitely under normal operating temperatures.

Service Specifications

ServiceFrequencyNotes
Insulation resistance test (megger)AnnuallyTest winding-to-ground at 500V DC; minimum 100 megohms when new
Bearing inspectionEvery 10,000 hours or 5 yearsSingle-bearing design; listen for noise during operation
Voltage regulation verificationAnnually during load bank testVerify at no-load and full-load; compare against commissioning baseline
Air inlet / cooling path inspectionEvery 6 monthsGaseous generators in enclosed spaces accumulate more dust; clean inlet screens and verify cooling airflow

Key Terms

Rare-earth permanent magnet
Neodymium-iron-boron (NdFeB) magnets used in the pilot exciter. Provide 5-10x the energy density of ferrite magnets, enabling compact exciter design with strong, stable magnetic flux.
Brushless wound-field
An older alternator excitation design where the exciter field is electromagnetically energized rather than using permanent magnets. Used in smaller Kohler residential units (RCA series, CCL series). Still brushless (rotating rectifier), but the excitation source is less robust under fault conditions than PM excitation.
PowerBoost excitation
Kohler's marketing term for the PM pilot exciter system on residential and light commercial alternators. Provides enhanced motor-starting capability by maintaining full excitation during the voltage dip caused by large inrush currents (air conditioners, well pumps).

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