The engine supplies mechanical power; the alternator determines how that power becomes usable electricity. End users do not need to design windings, but they should know why the same genset rating can use different frames, why motors and UPS loads change the selection, and what evidence matters beyond the brand.
Define loads, voltage, frequency, environment and duty first; then confirm capacity, excitation, AVR, temperature rise and protection; compare brand, local support and lead time last. “International alternator brand” or kVA alone is not a purchasable specification.
1. What the alternator does—and what the brand can prove
The alternator converts engine torque into electrical power and maintains the required voltage, frequency and waveform. It affects motor-start voltage dip, waveform with nonlinear loads, fault current, paralleling, efficiency and temperature rise.
STAMFORD, Leroy-Somer / Nidec Power and Marathon provide product families, options, records and a service foundation. The brand does not replace the model. One brand may offer different frames, windings, AVRs, auxiliary-winding or PMG excitation, single or double bearings and low or medium/high voltage.
The alternator is not the complete set. Engine output, coupling, controller, breaker, cooling, grounding, distribution and load management must work together. A useful decision explains why one exact model fits the load.
2. Collect nine project inputs before comparing brands
- Set output: kW, kVA, power factor and prime or standby rating.
- Voltage and frequency: 50/60 Hz, phase, line voltage and connection.
- Critical loads: motors, pumps, compressors, lifts, UPS, VFDs, rectifiers, welders and data equipment.
- Starting sequence: what returns together and what can be delayed.
- Permitted variation: transient voltage, frequency and waveform limits.
- Duty: standby, daily prime, continuous, island parallel or utility parallel.
- Environment: temperature, altitude, humidity, salt, dust, ventilation and corrosion.
- Installation: open or enclosed, airflow, shaft line, access and termination.
- Local capability: technicians, rewinding, AVR and routine stock, response time.
A load list with only total power is not enough. Identify the largest motor, starting method, UPS/VFD share and simultaneous recovery sequence.
3. Use six steps to define the alternator platform
Step 1: align rating definitions
Alternators commonly use kVA while engines and loads use kW. Reconcile power factor, efficiency and duty; do not equate engine mechanical power, set electrical power and catalogue alternator capacity.
Step 2: validate dynamic loads
Direct motor starting needs short-duration excitation and voltage recovery. UPS, VFD and rectifier loads can add harmonics and crest current. Use manufacturer transient data and the actual sequence rather than one fixed oversizing percentage.
Step 3: select excitation and AVR
Self-excited, auxiliary-winding and PMG arrangements behave differently during load steps, nonlinear loads and faults. AVR sensing, response, paralleling and protection must coordinate with the controller.
Step 4: check temperature rise, insulation and environment
Heat, altitude, dust, humidity and salt affect cooling, insulation life and usable output. Confirm insulation, temperature rise, IP, heaters, winding treatment and derating.
Step 5: check mechanical and electrical interfaces
Confirm speed, poles, flywheel housing, coupling, bearing arrangement, rotation, terminals, breaker, grounding and neutral. One interface error can defeat the right brand.
Step 6: compare local execution
Among technically qualified models, compare real lead time, serial traceability, AVR and bearing stock, rewinding capability, service cities and warranty.
4. Comparing STAMFORD, Leroy-Somer and Marathon
This is a project-fit comparison, not an absolute ranking. Every brand contains several families and options.
| Candidate | Favourable conditions | Relative direction | Verify |
|---|---|---|---|
| STAMFORD / AvK | Global OEMs, consultant approval, broad ratings and MV/HV | STAMFORD roughly 7.5–5,000 kVA plus larger AvK range; recognition and records | Series, winding, excitation, AVR, serial, manufacture and local parts |
| Leroy-Somer / Nidec Power | Industrial, data-centre, LV/MV/HV and special applications | Official roughly 10 kW–25 MW scope, established OEM families and serial-data entry | Current brand, model, AVR, excitation, factory data and service |
| Marathon Electric | Americas, installed base and rugged industrial sets | Official roughly 5–4,000 kW entry and long motor/generator background | kW/kVA basis, regional family, PMG/AVR, repair and parts |
Another brand should remain eligible when its exact model has stronger transient, environmental, service and schedule evidence. Brand choice should reduce load-recovery, delivery and repair risk.
5. Ten quotation fields that actually matter
- Model and frame for identity, dimensions, capacity and parts.
- Rated kVA, power factor and temperature rise on one comparison basis.
- Voltage, frequency, phases and poles.
- Winding and connection for voltage, neutral and harmonic behaviour.
- Excitation aligned with transient and protection needs.
- AVR model for regulation, paralleling, sensing and spares.
- Insulation and temperature-rise class; high insulation class does not remove heat.
- Protection and environment: IP, heaters, anti-condensation, filters and treatment.
- Bearing and mechanical interface: single/double bearing, coupling and housing.
- Serial and tests: factory data, no-load/load voltage, alarms and set report.
6. How priorities change by project
- Occasional commercial standby: starting, stability, routine parts and cost.
- Large motors or refrigeration: starting kVA, excitation, voltage recovery and staged loading.
- UPS and data centres: harmonics, short-circuit ratio, winding, AVR, load steps and paralleling.
- Heat and altitude: derating and airflow before frame selection.
- Coastal or humid sites: protection, anti-condensation, winding treatment and heaters.
- Multiple sets: AVR paralleling, reactive sharing, droop/communication, CTs and controls.
- Medium/high voltage: protection, switchgear, grounding, insulation coordination and site tests.
7. Common mistakes: same rating and brand can still mean different machines
- Brand without model, serial, winding, AVR and excitation.
- Catalogue maximum treated as usable capacity at the actual voltage, frequency, rise and environment.
- Steady-state kW sizing that ignores the largest motor, UPS and simultaneous recovery.
- PMG specified as automatically superior without a fault-current or nonlinear-load need.
- Insulation class checked while temperature rise, airflow, dust and humidity are ignored.
- A global network treated as local stock and response.
- The alternator checked without engine output, coupling, controls, protection and complete-set testing.
Compare on one rating definition, load list, environment and delivery scope. Otherwise price and parameters are not directly comparable.
8. Final delivery and acceptance checklist
The specification should state brand, full model, serial traceability, rated kVA and power factor, voltage/frequency, winding, excitation, AVR, insulation and rise, protection, derating, bearing/interface, fault and transient requirements, paralleling, spares, warranty and tests.
On delivery, reconcile the alternator and set nameplates, datasheet, wiring diagram, AVR document, insulation test, no-load/load voltage and alarm records. The decision is not a name; it is a configuration that starts the loads, operates at the site and can be repaired.
Official entries: STAMFORD | AvK, Nidec Leroy-Somer and Marathon Electric. Portfolio ranges define shortlist boundaries, not the current model specification.