Voltage Regulator Stamford MX321

Stamford MX321 Voltage Regulator: Analog AVR Powered by PMG

The MX321 is a three-phase analog AVR powered by a permanent magnet generator (PMG), which isolates it from nonlinear loads and allows for sustained short-circuit current. It includes stability control, adjustable UFRO protection, current limiting, overvoltage and overexcitation protection, and an input for a parallel droop transformer. It is offered as a standard option on Stamford S6 and HC6 alternators.

Technical characteristics

• Product reference: PAFMX321E00
• Manufacturer: Stamford (Newage/AvK, Cummins Generator Technologies)
• Type: Analog AVR, three-phase detection, powered by PMG
• Compatible alternators (original equipment): Stamford S6, HC6 (optional on the rest of the lineup)
• Sensing: 190–264 V AC max., 2- or 3-phase, 50–60 Hz
• PMG Power Supply: 170–220 V AC max., three-phase, 3-wire, 3 A/phase, 100–120 Hz nominal
• Output: 120 V DC max., 3.7 A continuous (linear derating to 2.7 A at 70 °C), 6 A intermittent (10 s), minimum resistance 15 Ω
• Regulation: ±0.5% RMS (with 4% motor control)
• Thermal drift: 0.02% per °C
• Soft-start ramp: 0.4 to 4 seconds
• Typical response: AVR 10 ms, field current at 90% in 80 ms, machine voltage at 97% in 300 ms
• External voltage adjustment: ±10% with a 5 kΩ, 1 W external potentiometer
• Low-frequency protection (UFRO): setpoint 95%Hz, slope 100–300% down to 30 Hz
• Surge protection: 300 V DC, fixed delay of 1 second
• Overexcitation protection: 75 V DC, fixed delay of 8 to 15 seconds
• Analog input: ±5 V DC max., adjustable sensitivity (1 V = 5% alternator voltage), input resistance 1 kΩ
• Quadrature droop input: max. sensitivity 0.22 A for 5% droop at 0 PF, max. input 0.33 A
• Current limit input: sensitivity range 0.5 A to 1 A
• Power dissipation: 18 W maximum
• Adjustable controls: Volts, Stab, Ufro, Dip, Droop, Trim, Over V, Exc, Dwell, Ramp, I Limit
• Operating temperature: -40°C to +70°C
• Storage temperature: -55°C to +80°C
• Relative humidity: up to 95% at 0–70°C, non-condensing
• Permissible vibration: 20–100 Hz, 50 mm/s; 100 Hz–2 kHz, 3.3 g
• Weight: 0.5 kg
• Country of manufacture: United Kingdom

Is it compatible with my Stamford alternator?
The MX321 is the original AVR for the Stamford S6 and HC6 alternators, and it is also available as an option for the rest of the lineup. If your alternator is a different model, the most reliable way to check is to look at the part number of the AVR already installed or the alternator's nameplate. If you have any questions, contact us at with the exact model of your alternator or generator set.