STAMFORD AS480 Automatic Voltage Regulator (AVR)

A compact SMT regulator with an excitation boost interface

The STAMFORD AS480 is a half-wave phase-controlled automatic voltage regulator for brushless generators. It uses surface-mount technology, custom mouldings and heatsinks to make a compact assembly, and adds a feature the smaller AVRs lack: a connection for the optional Excitation Boost System (EBS), used where short-circuit current has to be maintained on a self-excited machine.

The AVR is linked to the main stator windings and the exciter field to form a closed loop with load regulation of +/-1.0 percent, assuming 4 percent engine governing. Sensing and power inputs share one 100 to 264 V ac single-phase supply. Positive voltage build-up from residual levels is ensured by efficient semiconductors in the power circuit.

Technical specifications

How the AS480 is set up

Stability links select the response of the damping circuit to suit the generator size, with separate link positions for sets above and below 100 kW. A slow response setting is offered for small sets, where it helps reduce lamp flicker. The red LED shows under-speed running and doubles as the over-excitation indicator. The over-excitation trip level is set at the works at 65 V dc and cannot be altered; the condition is allowed for approximately eight seconds so that normal transients and short-term overloads do not cause nuisance trips, and the generator must be stopped to reset it.

110 V sensing and the Excitation Boost System

Operation with 110 V ac sensing is possible by replacing the hand trimmer link with a fixed resistor, but the hand trimmer then cannot be used and generator overload capability is reduced. The EBS is a small externally mounted rotary power supply, short-term rated, that provides excitation in heavy overloads or short circuits; its own electronics protect the generator against sustained overload. Where the AVR is mounted outside the generator the manual says to derate by 20 percent.

Choosing among STAMFORD AVRs

The AS480 is a self-excited, single-phase sensed regulator. Use the table to compare it with its siblings before ordering.

How the STAMFORD AS480 works

The regulator senses the generator output voltage and compares it with a set point. It then adjusts the DC current it feeds to the exciter field. If the voltage falls, it raises the field current. If the voltage rises, it lowers it. In this way the generator voltage stays steady as the load changes. The optional boost system lets the machine keep short-circuit current.

Where the AS480 is used

• Brushless generator sets
• Self-excited machines
• Sets needing short-circuit current
• Small sets that need slow response
• Stand-by and prime power plants

Selection checklist for the AS480

• Supply of 100 to 264 V ac
• Output of 5 A continuous
• Exciter field resistance above 15 ohms
• 50 or 60 Hz setting
• Droop input for paralleling
• Excitation Boost System option
• External voltage trimmer
• Ambient temperature

Installation, commissioning and maintenance notes for the AS480

Mount the regulator on a firm surface with the heatsink free for air. Wire the sensing and power inputs and the exciter field. Select the frequency and the stability link for the size of the set. Run the set and adjust the voltage with the trimmer. Check the LED for under-speed or over-excitation.

Requesting a quotation for the STAMFORD AS480

Please send the generator type, the supply and the quantity. Seven Star LLC serves industrial customers in Oman and the GCC and replies with a quotation based on the information provided.

AS480 input

The sensing and power input is 100 to 264 V ac, single phase, 50 to 60 Hz.

AS480 output

The output is 82 V dc at 200 V ac input and 45 V dc at 110 V ac input.

AS480 output current

The output current is 5 A continuous and 7.5 A for 10 seconds.

AS480 regulation

The regulation is plus or minus 1.0 percent with 4 percent engine governing.

AS480 response

The AVR responds in 20 ms, and the machine voltage reaches 97 percent in 300 ms.

AS480 under-frequency protection

The set point is 94 to 98 percent, with a jumper for 50 or 60 Hz.

AS480 over-excitation protection

The set point is 67 V dc, plus or minus 3 percent, with a delay of 10 to 15 seconds.

AS480 temperature range

It works from minus 40 to plus 70 degrees C, with output derating above 60 degrees C.

Regulator records for the AS480

Record the link positions, the trimmer setting and the date of each test.

Choosing the right AS480 setting

Use the slow response setting on small sets, and the faster setting on larger machines.

Documents and quotation

Seven Star LLC supplies STAMFORD AVRs on request from Muscat. The specification, installation and adjustments datasheet is linked here. Send the alternator model and serial number from the nameplate and the existing AVR code, because the correct replacement depends on the excitation system. See Diesel Generator Spare Parts for more generator components, or the Cummins C150D5 150 kVA diesel generator set for a complete set. Confirm the version and any revision against the original unit before ordering.