Stamford SX421 Specification, Installation And Operation Manual

SX421 AUTOMATIC VOLTAGE

REGULATOR (AVR)

SPECIFICATION, INSTALLATION AND ADJUSTMENTS

GENERAL DESCRIPTION

The SX421 is a three phase sensed Automatic Voltage
Regulator (AVR) and forms part of the excitation system for a
brushless generator.

In addition to regulating the generator voltage, the AVR
circuitry includes a number of protective features. Excitation
power is derived directly from the generator terminals.

Positive voltage build-up from residual levels is ensured by
the use of efficient semiconductors in the power circuitry of
the AVR.

The AVR is linked with the main stator and exciter field
windings to provide closed loop control of the output voltage
with load regulation in the order of +/-0.5% RMS.

The AVR senses the output voltage from the main stator
windings and in response to this controls the power fed to the
exciter field and hence the main field to maintain output
voltage within the specified limits, compensating for load,
speed, temperature, and power factor of the generator.

Overvoltages caused by open circuit sensing terminals or
short circuit power device are avoided by overvoltage
detection circuitry which provides circuit breaker trip signals
for circuit isolation. (see note 1).

A frequency measuring circuit continually monitors the
generator output and provides underspeed protection of the
excitation system by reducing the generator output voltage
proportionally with speed below a presettable threshold. A
further enhancement of this feature is an adjustable V/Hz
slope, to improve frequency recovery time on turbo-charged
engines.

Provision is made for the connection of a remote voltage
trimmer, allowing the user fine control of the generator output.

Accessories are available for this AVR. Please refer to factory
for further details.

TECHNICAL SPECIFICATION

SENSING INPUT

Voltage 170-250 V ac max
Frequency 50-60 Hz nominal
Phase 3
Wire 3

OUTPUT

Voltage max 90 V dc at 207 V ac input
Current Continuous 4 A dc

Transient 6 A for 10 seconds

Field Resistance 15 Ω minimum

REGULATION

(See Note 2) +/- 0.5% RMS

THERMAL DRIFT

(after 10 min)

0.5% for 40°C change in AVR ambient

TYPICAL SYSTEM RESPONSE

Field current to 90% 80ms
Machine Volts to 97% 300ms

EXTERNAL VOLTAGE ADJUSTMENT

+/- 6% with 1 K Ω trimmer

UNDER FREQUENCY PROTECTION

Set Point (See Note 3) 95% Hz
Slope 100-300% down to 30 Hz

UNIT POWER DISSIPATION

20 watts maximum

BUILD UP VOLTAGE

3.5 V ac @ AVR terminals

ACCESSORY INPUT

+/- 1V = +/- 5% change in output
volts @ 415 V

QUADRATURE DROOP

Maximum sensitivity (10 Ω Burden)

0.22A for 5% droop @ 0p.f.

OVER VOLTAGE PROTECTION

Set Point 300 V dc
Time Delay (fixed) 1 second
Circuit Breaker Trip Coil Voltage 12-30 V dc
C/B Trip Coil Resistance 25-40 Ω

ENVIRONMENTAL

Vibration 20-100 Hz 50mm/sec

100 Hz-2 kHz 3.3g
Relative Humidity 0-60°C 95%
Operating Temperature -40°C to + 70°C
Storage Temperature -55°C + 80°C

NOTES

1. A miniature circuit breaker must be fitted to use the Overvoltage

Protection feature.

2. With 4% engine governing.

3. Factory set, semi-sealed, jumper selectable.

DESING DETAILS

The main functions of the AVR are:

Sensing Resistors take a proportion of the generator output
voltage and attenuate it. This input chain of resistors includes
the range potentiometer and hand trimmer which adjusts the
generator voltage.

Quadrature Droop Circuit converts the current input into a
voltage which is phase mixed with the sensing voltage. The
result is a net increase in the output from the sensing network
as the power factor lags, causing the reduction in excitation
needed for reactive load sharing of paralleled generators.

A trimmer allows control over the amount of droop signal.

RMS Converter is a square law precision rectifier circuit that
converts the ac signals from the sensing networks into a
composite dc signal representing the mean squared value of
the waveform.

Offset Control provides an interface between the AVR and
accessories.

Power Supply components consist of zener diodes, dropper
resistors and smoothing to provide the required voltages for
the integrated circuits.

Precision Voltage Reference is a highly stable temperature
compensated zener diode for comparison purposes.

Main Comparator/Amplifier compares the sensing voltage to
the reference voltage and amplifies the difference (error) to
provide a controlling signal for the power device.

Stability Circuit provides adjustable negative ac feedback to
ensure good steady state and transient performance of the
control system.

Power Control Driver controls the conduction period of the
output device. This is achieved by pedestal and ramp control
followed by a level detector and driver stage.

Power Control Devices and Rectifier vary the amount of
exciter field current in response to the error signals produced
by the main comparator.

Synchronising Circuit provides a short pulse at the zero
crossing of each cycle and is used to synchronise the Under
Frequency Roll Off (UFRO) and power control circuits to the
generator cycle period.

UFRO circuit measures the period of each electrical cycle and
reduces the reference voltage linearly with speed below a
presettable threshold. A light emitting diode (LED) gives
indication of underspeed.

Engine Relief (load acceptance) circuit causes greater
voltage roll off (makes the volts/Hz slope steeper) to aid
engine speed recovery after application of a "block" load.

Overvoltage Monitor continuously monitors the voltage at
the generator terminals and provides signals to trip a circuit
breaker to isolate power from the exciter and AVR if sustained
overvoltages occur. A one second timer is included in the
circuit to prevent operation during transient overvoltages
which are normal after load removal.

A miniature circuit breaker must be fitted to use the
Overvoltage Protection feature.