MA325 Automatic Voltage Regulator (AVR) - bdsar.com

1 MA325 Automatic Voltage Regulator (AVR)Specification, installation and descriptionThe MA325 is a phase controlled thyristor type AVRand forms part of the excitation system for abrushless generator. The design employs SurfaceMount Technology (SMT) to provide a host of featureswith minimum component power is derived from a permanent magnetgenerator (PMG), to reduce Radio FrequencyInterference (RFI) and isolate the AVR control circuitsfrom the effects of thyristor controlled AVR senses the Voltage in the main generatorwinding and controls the power fed to the exciterstator and hence the main rotor to maintain thegenerator output Voltage within the specified limits,compensating for load, speed, temperature andpower factor of the generator.

2 Three phase RMSsensing is employed for superior Voltage start circuitry is included to provide a smoothcontrolled build up of generator output frequency measuring circuit continually monitors theshaft speed of the generator and providesunderspeed protection of the excitation system byreducing the generator output Voltage proportionallywith speed below a presettable threshold. A furtherenhancement of this feature is an adjustable volts perHertz slope to improve engine recovery time on turbocharged circuitry is included which cuts off theexcitation power in event of over limiting may be included to allow control overthe amount of short circuit current flowing during shortcircuits of the generator excitation is limited to a safe period byinternal shutdown of the AVR output device.

3 Thiscondition remains latched until the generator complete protection, a circuit breaker option isavailable providing circuit isolation in event of a shortcircuit power is made for the connection of a remotevoltage trimmer allowing the user fine control of thegenerators specificationSensing inputVoltage170-250 Vac maxFrequency50-60 Hz nominalPhase1 or 3 Wire2 or 3 Power input (PMG)Voltage170-220 Vac maxCurrent5A/phaseFrequency100-120 Hz nominalPhase3 Wire3 OutputVoltagemax 120V dcCurrentcontinuous 8A for 10 secsResistance15 ohms minimumRegulation( 4% engine governing) +/- RMST hermal per degree centigradechange in AVR ambientSoft start ramp time3 secondsTypical system responseField current to 90%80msGenerator volts to 97%500msExternal Voltage adjustment+/-6% with 1k ohm 1 watt trimmerUnder frequency protectionSet point97% HzSlope100-300% down to 30 HzDwell (recovery) to 2 secondsUnit power dissipation25 watts maxAccessory input+/-1 volt input = +/- 5% generator voltsQuadrature droop Amps for 5% droop @ pfCurrent to 1A (10 ohm burden)Over excitation protectionSet point65 V dcTime delay10 seconds (fixed)

4 Over-volts protectionSet point300 voltsTime delay1 secondCircuit breaker trip coil Voltage 10 to 60 V dcCircuit breaker trip coil resistance 50 to 100 ohmsEnvironmentalVibration20-100Hz50 humidity 0-60c95% RHOperating temperature-40 to +60cStorage temperature-55 to +80cAccessories are available for this AVRD esign detailThe main functions of the AVR are as follows:-Potential divider takes a proportion of the generator outputvoltage and attenuates it. This input chain of resistors includesthe hand trimmer adjustment. Isolating transformers areincluded to allow connection to windings of different polarity droop circuit converts the current input from a CTinto a Voltage which is phase mixed with the sensing result is a net increase in the output from the sensingnetwork as the power factor lags, causing the reduction inexcitation needed for reactive load sharing of converter converts the AC signals from the potentialdivider into a DC signal representing the mean squared value ofthe wave circuit provides an interface between the AVR andaccessories.

5 Allowing the accessory output Voltage to besummed with the sensing Voltage for control supply components consist of dropper resistors, zenerdiodes and smoothing to provide the required voltages for Voltage reference is a highly stable temperaturecompensated zener diode used for dc Voltage start circuit overrides the precision Voltage referenceduring run up to provide a linear rising compares the sensing Voltage to the referencevoltage and amplifies the difference (error) to provide acontrolling signal for the power circuit provides adjustable negative ac feedback toensure good steady state and transient performance of thecontrol control driver controls the conduction period of theoutput device.

6 This is achieved by pedestal and ramp controlfollowed by a level detector and driver control devices vary the amount of exciter field currentin response to the error signals produced by the circuit provides a short pulse at the beginning of eachcycle of the PMG wave form and is used to synchronise theUnder Frequency Roll Off (UFRO) and power control circuits tothe generator cycle converter converts the outputs from currenttransformers into a signal representing the average value of thecurrent wave circuit measures the period of each electrical cycle andreduces the reference Voltage linearly with speed below apresettable light emitting diode (LED) gives indication of relief or load acceptance circuit provides adjustment forincreasing the Voltage roll off(steepness of V/Hz slope), to aid engine recovery afterapplication of block load.

7 A "Dwell" feature is included whichdelays the recovery of generator volts after application of load,providing further detector continuously monitors the excitation voltageand provides signals to shutdown the output device if overloadslast for more than ten seconds. An overload condition producesa latched fault requiring the generator to be stopped for monitor continuously monitors the Voltage at theGenerator terminals and provides signals to shut down theoutput device and trip an optional circuit circuit breaker isolates power from the exciter and AVR ifsustained over- Voltage occurs. A one second timer is includedin the circuit to prevent operation during transient over-voltages.

8 Which are normal after load generator must be stopped to reset an over Voltage OF AVR CONTROLSUFRO TableStability Table0 50 Hz 6 Pole0 Under 100 kW2 60 Hz 6 Pole1 100-500 kW4 50 Hz 4 Pole2 500-1000 kW5 60 HZ 4 Pole3 1000-1500 kW4 1500-2000 kW5 2000-2500 kW6 Over 2500 kW7 Special8 Special9 SpecialControlsFunctionDirectionVoltsTo adjust generator output voltageClockwise increases output voltageStabilityTo prevent Voltage huntingClockwise increases the damping effectStability selectionTo optimise transient performanceSee tableUFROTo set under frequency knee pointClockwise reduces knee point frequencyFrequency selectionTo set UFRO control rangeSee tableDroopTo set Voltage droop to 5% at 0 PFClockwise increases the droopTrimTo match accessory output to AVR inputClockwise increases AVR sensitivityDipTo adjust frequency related Voltage dipClockwise increases the dipOver voltsTo set over Voltage protection cut off levelClockwise increases the overvoltage levelRMSSet and sealed at the factoryI limitTo set maximum short circuit currentClockwise increases the currentExc adjust over excitation trip levelClockwise increases the trip levelDwellTo set Voltage recovery time after block loadingClockwise increases the recovery timeADJUSTMENTS OF AVR CONTROLSV oltage adjustmentThe generator output Voltage is set at

9 The factory, butcan be altered by careful adjustment of the volts controlon the AVR board, or by the external hand trimmer !DO NOT INCREASE THE Voltage ABOVE THERATED GENERATOR Voltage . IF IN DOUBT,REFER TO THE RATING PLATE MOUNTED ON THEGENERATOR a replacement AVR has been fitted or re-setting of theVOLTS adjustment is required, proceed as follows:-Before running generator, turn volts control fully anti-clockwiseTurn remote volts trimmer (if fitted) to midway positionTurn stability control to midway positionConnect a suitable voltmeter across line to neutral of thegeneratorStart generator set, and run at a no-load condition atnominal frequency 50-53 Hz or 60-63 HzIf the red light emitting diode is illuminated, refer to theunder frequency roll off adjustment (UFRO)

10 Carefully turn the VOLTS control clockwise until ratedvoltage is reachedIf instability is present at rated Voltage , slowly turn thestability control clockwise until Voltage is steady, then re-adjust Voltage if necessaryVoltage adjustment is now completeStability adjustmentThe AVR includes a STABILITY or damping circuit toprovide good steady state and transient performance ofthe switch is provided to change the response of thestability circuit to suit different frame size machines andapplications, (see stability table on previous page).The correct setting of the stability control can be foundby running the generator at no load and slowly turningthe stability control anti-clockwise until the generatorvoltage starts to become optimum or critically damped position is slightlyclockwise from this point, ( where the machine voltsare stable but close to the unstable region ).