Cummins STAMFORD MX321 AVR Automatic Voltage Regulator generator system

The Cummins STAMFORD MX321 AVR is a high-performance analog three-phase induction automatic voltage regulator designed for STAMFORD alternators (e.g., S6 and HC6 series) and compatible with Cummins generator systems. As a key component under the Cummins brand, it ensures stable voltage output by continuously monitoring and adjusting the excitation current of the alternator.

Key Features

Precision Voltage Regulation

Monitors the main winding voltage using three-phase RMS detection.

Maintains output voltage within ±0.5% RMS tolerance by adjusting the excitation circuit, compensating for load changes, speed variations, temperature fluctuations, and power factor shifts.

PMG-Powered Isolation

Derives its power from a three-phase Permanent Magnet Generator (PMG), isolating the AVR control circuit from nonlinear load disturbances and radio-frequency interference (RFI) at the alternator terminals.

Enhances stability during motor starting and sustains short-circuit current.

Protective Functions

Under-Frequency (UFRO) Protection: Reduces output voltage when the alternator speed drops below a critical threshold to safeguard the excitation system.

Over-Voltage Protection: Shuts down the power device and triggers an optional excitation circuit breaker if stator voltage exceeds the preset limit for a specified duration.

Current Limiting: Controls sustained short-circuit current via a current transformer (CT) connected to the AVR’s S1/S2 terminals, with a built-in 10-second overcurrent limit (also applicable to motor starting current).

Adjustability

Features adjustable parameters such as UFRO threshold, over-voltage trip point, stator current limit, ramp time, dip, dwell, and droop.

Includes a remote voltage trimmer interface for fine-tuning output voltage.

Dynamic Stability

Incorporates voltage regulation and damping circuits to ensure excellent steady-state and transient response characteristics under varying operating conditions.

Technical Specifications

Parameter Details

Input Voltage - Induction: 190–264V AC (max), 2 or 3-phase

- PMG Power: 170–220V AC (max), 3-phase, 3-wire

Input Current 3A/phase (PMG), 100–120Hz (nominal frequency)

Output Voltage Max 120V DC

Output Current - Continuous: 3.7A

- Intermittent: 6A (for 10 seconds)

Load Resistance Min 15 ohms

Thermal Drift 0.02% per °C ambient temperature change

Soft Start Ramp Time 0.4–4 seconds

System Response Time - AVR response: ~10ms

- Excitation current to 90%: ~80ms

- Voltage to 97%: ~300ms

Applications

Standard configuration for STAMFORD S6 and HC6 series alternators.

Widely used in Cummins generator systems across industrial, commercial, and power applications, including:

Factories and manufacturing plants

Data centers and healthcare facilities

Marine and offshore power systems

Emergency backup power solutions

Compatibility Note

As a product under the Cummins portfolio, the STAMFORD MX321 AVR is fully integrated with Cummins engines and alternators, ensuring seamless performance and reliability. It is often referred to as "Cummins MX321" or "STAMFORD MX321" interchangeably, reflecting the brand’s dual identity within the Cummins group.

For technical support or replacement, always refer to Cummins or authorized STAMFORD distributors to ensure compatibility with your specific generator model.