Kohler · KH Series (Industrial)

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Kohler KH Series Alternator

Alternators

Kohler-designed 4-pole brushless alternator with permanent-magnet pilot exciter, Class H insulation, and sub-3% THD at all linear loads. Standard on KD-platform industrial diesel generators from 900 kW to 4000 kW.

Overview#

The Kohler KH Series is the proprietary alternator platform used across KD-platform industrial diesel generators, from the KD900 (900 kW) through the KD4000 (4000 kW). Kohler designs and manufactures these alternators in-house — they are not sourced from Stamford, Marathon, or other third-party alternator OEMs.

The defining characteristic of the KH Series is its sub-3% total harmonic distortion (THD) specification at all linear loads. This is the technical foundation behind Kohler's positioning as the power quality leader in industrial standby generators, and the reason Kohler is the default specification for hospitals, data centers, and facilities with sensitive electronic loads.

Model nomenclature#

KH alternator model codes follow the pattern KHxxxxxTO4D:

ModelTypical gensetStandby kW
KH04070TO4DKD900900
KH04830TO4DKD10001000
KH05520TO4DKD12501250
KH05641TO4DKD13501350
KH05740TO4DKD15001500
KH06721TO4DKD1600–KD17501600–1750
KH07770TO4DKD2000, KD28002000–2800
KH08100TO4DKD22502250
KH08430TO4DKD25002500
KH08590TO4DKD30003000
KH09390TO4DKD32503250
KH10171TO4DKD3500–KD40003500–4000

Why sub-3% THD matters#

Most industrial alternators are specified at 5% THD or "less than 5%." The difference between 5% and sub-3% is significant for facilities with:

  • IT infrastructure: Server power supplies and UPS rectifiers are sensitive to harmonic content. Lower THD reduces heating in input capacitors and extends equipment life.
  • Medical imaging: MRI and CT equipment requires clean power. Excessive THD causes image artifacts and calibration drift.
  • LED lighting systems: Modern LED drivers with poor power factor correction amplify harmonics. Starting with lower source THD reduces cumulative distortion.
  • Variable frequency drives (VFDs): VFD input rectifiers generate harmonics that reflect back to the source. Lower source THD provides more harmonic headroom before requiring active filtering.

Excitation architecture#

The KH alternator uses a three-stage excitation system:

  1. PMG stage: A permanent-magnet generator on the alternator shaft produces AC power independent of the main stator output.
  2. AVR stage: The digital AVR (integrated with the Decision-Maker controller) rectifies and regulates the PMG output to feed the exciter field.
  3. Rotating rectifier stage: The exciter armature output is rectified by a rotating diode bridge and fed to the main field winding.

This architecture ensures excitation is maintained even during a bolted short circuit on the output terminals — critical for protective device coordination in NFPA 110 Level 1 installations.

OnPoint service notes#

We service Kohler KH alternators across the central California coast as part of industrial genset maintenance contracts. The most common service items are annual megger testing, AVR diagnostics via the Decision-Maker controller, and bearing inspection on units with high runtime hours. The rotating rectifier diode assembly is the most common failure component in the field, typically presenting as unstable voltage output or failure to build voltage at start.

Frequently Asked Questions

What is the THD specification on Kohler KH alternators?
Kohler specifies less than 3% total harmonic distortion (THD) at all linear loads across the KH series. This is achieved through the proprietary winding geometry and the permanent-magnet pilot exciter system, which maintains stable excitation independent of the main stator output. The sub-3% THD specification is a key differentiator versus competitors that typically specify 5% or higher.
What excitation system do KH alternators use?
KH alternators use a permanent-magnet pilot exciter (PMG) system. A small permanent-magnet generator on the alternator shaft provides independent excitation power to the AVR, which then drives the main exciter. This architecture delivers superior voltage regulation under nonlinear loads (VFDs, UPS systems, LED lighting) compared to shunt-excited alternators, and provides full short-circuit current for downstream protective device coordination.
Which generators use KH Series alternators?
Kohler KD-platform industrial diesel generators from the KD900 through KD4000 use KH Series alternators. Each generator size has a matched alternator model — for example, the KD1000 uses the KH04830TO4D and the KD3500 uses the KH10171TO4D. The alternator model code encodes the frame size and winding configuration.
How does the KH compare to Stamford alternators?
Stamford (Cummins Generator Technologies) alternators are excellent general-purpose industrial units commonly specified at 5% THD with shunt-excited AVR. Kohler KH alternators are designed specifically for the KD platform with sub-3% THD and PMG excitation as standard. For mission-critical facilities with sensitive electronics — hospitals, data centers, broadcast studios — the tighter THD specification reduces the need for downstream harmonic filtering.
Are KH alternators field-serviceable?
Yes. The AVR, rotating rectifier assembly, PMG, and bearings are all field-replaceable. However, the alternator is specifically matched to the KD engine platform — frame adaptation, SAE flange, and coupling disc are not interchangeable with other OEM generator platforms. Service and replacement parts are available through Kohler (Rehlko) authorized distributors.

Service Specifications

ServiceFrequencyNotes
Insulation resistance test (megger)Annually or after extended shutdownTest winding-to-ground at 500V DC; minimum 100 megohms when new. Values below 5 megohms indicate moisture ingress requiring space heater operation before start.
Bearing inspectionEvery 10,000 hours or 5 yearsSingle-bearing design coupled to engine flywheel. Inspect for noise, vibration, or temperature rise during loaded operation.
AVR calibration verificationAnnuallyVerify voltage regulation at no-load and full-load via Decision-Maker controller diagnostics. Confirm THD at rated load with power analyzer.
PMG output verificationEvery 2 yearsMeasure PMG output voltage at rated speed. Degraded PMG output causes poor voltage regulation under load transients.
Rotating rectifier diode testEvery 5 years or on voltage regulation faultTest forward and reverse resistance of each diode in the rotating rectifier assembly. Failed diodes cause AC ripple in the main field.

Key Terms

PMG (Permanent Magnet Generator) excitation
A small permanent-magnet alternator on the main shaft provides independent power to the AVR. Unlike shunt excitation, PMG output is available even during short circuits, ensuring full fault current for protective device coordination.
Sub-3% THD
Total harmonic distortion below 3% at all linear loads. Kohler achieves this through proprietary winding pitch and PMG excitation. Lower THD reduces heating in sensitive electronic loads and improves power quality for mission-critical facilities.
Rotating rectifier
A diode bridge mounted on the rotor that converts AC from the exciter armature to DC for the main field winding. Eliminates brushes and slip rings, providing maintenance-free excitation transfer.

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