STAMFORD S4 Low Voltage Alternator 250-450 kVA

The workhorse alternator for 250 to 450 kVA diesel sets

The STAMFORD S4 is the S-Range frame for generator sets in the 250 to 450 kVA class at 50 Hz (about 288 to 560 kVA at 60 Hz), the size range of most containerised standby sets used for hotels, hospitals, malls and construction sites. It is a four-pole, wire-wound, brushless alternator with a single bearing, 380 to 480 V output and class H insulation.

The S4 introduces what STAMFORD calls CoreCooling technology: a redesigned core and air path that improves thermal performance and raises power density, so the same frame delivers more kVA. The brochure also points to a new 450 kVA core length, an adaptor foot option that lets a set base switch between S4 and S5, and a flexible foot option for changing between alternator brands. The brochure also publishes S4 Special ratings at 50 and 60 Hz in the same three duty classes, notes that its general-arrangement drawing is indicative of the S4-G core, and refers requests for other voltages to STAMFORD applications engineering.

Technical specifications

S4 Dedicated ratings, 400 V, 50 Hz, class H 125/40 (winding 311)

Standby 150/40 ratings at 400 V are 280, 330, 385, 445 and 480 kVA for the same five cores. 60 Hz ratings at 480 V run from 325 to 560 kVA.

Reading the model code and picking the core

The S4 name follows a fixed pattern: S for the brand, 4 for the family, L for low voltage, 1 for the revision, D for dedicated (S for special, M for marine), a letter C to G for core length, 4 for poles and a final digit for the bearing. Core length is what sets the kVA, so match the letter to the engine output at your frequency, and check the reconnectable voltage options before the set is built.

Regulators, options and accessories

The brochure lists the MX321, MX341 and AS440 AVRs as available. The Controlled Short Circuit option is marked for the MX321 only, and a Dual AVR option is shown for the MX321 and MX341. Factory build options include a quadrature droop kit, bearing RTD, air inlet filter, remote voltage trimmer, radio-frequency interference suppressor kit, excitation loss module and diode failure detector.

How the STAMFORD S4 works

The STAMFORD S4 is a brushless alternator. The diesel engine turns the rotor through the single bearing and the SAE adaptor. A small exciter and the automatic voltage regulator feed direct current to the rotor field. The rotating field induces three-phase voltage in the stator windings, which are wire wound. The AVR (MX321, MX341 or AS440) senses the output on two phases and adjusts the field to hold voltage steady as the load changes. A PMG can be added to keep the excitation up during a motor start.

Where the STAMFORD S4 is used

• Containerised standby generator sets
• Hotels and hospitals
• Shopping malls
• Construction sites
• Prime power sets of 250 to 450 kVA

Selection checklist for the STAMFORD S4

• Rating of 250 to 450 kVA at 50 Hz
• Core size C, D, E, F or G
• Output voltage of 380 to 480 V
• Duty class: continuous, prime or standby
• Self-excited or PMG excitation
• AVR type
• Winding RTDs, thermistors or heater options
• Adaptor foot or flexible foot

Installation, commissioning and maintenance notes for the STAMFORD S4

Align the alternator with the engine through the SAE 1 adaptor, and support the free end on the foot. Check the air path, because CoreCooling needs clear intake and exhaust. Connect the 12 terminals for the winding in use and set the AVR voltage and stability. Test with load steps before handover. Keep the air filters clean if fitted, and check the bearing and winding insulation on a schedule.

Requesting a quotation for the STAMFORD S4

Please send the kVA, the voltage, the frequency, the duty class, the excitation and the options. Seven Star LLC serves industrial customers in Oman and the GCC and replies with a quotation based on the information provided.

STAMFORD S4 ratings at 400 V and 50 Hz

At class H 125/40, the S4L1D-C4 is 260 kVA, the D4 is 310, the E4 is 360, the F4 is 415 and the G4 is 450 kVA.

STAMFORD S4 standby ratings

The standby 150/40 ratings at 400 V are 280, 330, 385, 445 and 480 kVA for the same five cores.

STAMFORD S4 construction

It has four poles, class H insulation and protection to IP23. Air filters can be added.

STAMFORD S4 model code

S is the brand, 4 the family, L low voltage and 1 the revision. D means dedicated, and a letter from C to G picks the core.

Alternator records for the STAMFORD S4

Keep the serial number, the AVR settings and the insulation test results.

Choosing the right STAMFORD S4 core

Choose the core from the generator set rating, the duty class and the ambient temperature.

Documents and quotation

Seven Star LLC supplies STAMFORD S4 alternators for generator builders and plant owners in Oman and the GCC. Send the kVA and duty, voltage, frequency, engine model with SAE housing, preferred AVR and whether PMG excitation is required. More parts are listed under Diesel Generator Spare Parts; for a finished set, see the Cummins C275D5 275 kVA diesel generator set. Confirm ratings and drawings against the final data issued with your quotation, as the brochure notes ratings are subject to change.