1 Alternator

The Permanent Magnet Generator (PMG) provides power for the excitation of the exciter field via the Automatic Voltage Regulator (AVR) which is the controlling device governing the level of excitation provided to the exciter field. The AVR responds to a voltage-sensing signal derived, via an isolating transformer, from the main stator winding. By controlling the low power of the exciter field, control of the high power requirement of the main field is achieved through the rectified output of the exciter armature.

                                                   Synchronous Generator

MX321 Automatic Voltage Regulator (Avr)

Specification, Installation And Adjustments

Thyristor type and forms part of the excitation system for a brushless generator. In addition to regulating the generator voltage, the AVR circuitry includes protective features to ensure safe reliable control of the generator. Excitation power is derived from a permanent magnet generator (PMG) to guarantee low Radio Frequency Interference (RFI) and immunity from thyristor type loads. The AVR is linked with the main stator windings and controls the power fed to the exciter stator and hence the main rotor to maintain the machine output voltage within the specified limits, compensating for load, speed, temperature and power factor of the generator. Soft start circuitry is included to provide a smooth controlled build up of generator output voltage.

                                             Generator excitation control system.

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 an adjustable pre-set threshold. A further enhancement of this feature is an adjustable volts/Hz slope to improve frequency recovery time on turbo charged engines.

Uncontrolled over excitation is limited to a safe period by internal shutdown of the AVR output device.

This condition remains latched until the generator has been stopped. For complete protection, a circuit breaker option is available providing circuit isolation in event of a short circuit power device. In addition it detects engine speed and provides adjustable voltage fall off with speed, below a pre-selected speed (Hz) setting, preventing over-excitation at low engine speeds and softening the effect of load switching to relieve the burden on the engine. It also provides over-excitation protection that acts following a time delay, to de-excite the generator in the event of excessive exciter field voltage.

Provision is made for the connection of a remote voltage trimmer allowing the user fine control of the generator's output. Accessories are available for this AVR. Please refer to factory for further details.

                                                    Automatic Voltage Regulator

Adjustment of AVR Controls

AVR control functions:

Control              Function

Volts                  To adjust generator output voltage

Stability 1                   To prevent voltage hunting

Stability 2         To prevent voltage hunting

UFRO                 To set under-frequency roll off knee point

Droop                To set generator droop to 5% at full load 0 PF

V/trim               To match AVR input to accessory output

EXC                   To set the over-excitation cut off level

DIP                     To set the initial-frequency related voltage dip

Over/V              To set the over-voltage protection cut off level

I/limit                To set the maximum short circuit current

 

1 Voltage adjustment

The generator output voltage is set at the factory, but can be altered by careful adjustment of the volts control on the AVR board, or by the external hand trimmer if fitted. Terminals 1 & 2 on the auxiliary terminal block in the generator terminal box will be fitted with a shorting link if no hand trimmer is required. 
Do not increase the voltage above the rated generator voltage. If in doubt, refer to the rating plate mounted on the generator case.

If a replacement AVR has been fitted or re-setting of the VOLTS adjustment is required, proceed as follows:-

1) Before running generator, turn VOLTS control fully anticlockwise.

2) Turn remote volts trimmer (if fitted) to midway position.

3) Turn STABILITY control to midway position.

4) Connect a suitable voltmeter (0-300V ac) across line to neutral of the generator.

5) Start generator set, and run on no load at nominal frequency e.g. 50-53Hz or 60-63Hz.

6) If the red Light Emitting Diode (LED) is illuminated, refer to the Under Frequency Roll Off (UFRO) adjustment.

7) Carefully turn VOLTS control clockwise until rated voltage= is reached.

8) If instability is present at rated voltage, refer to stability adjustment, and then re-adjust voltage if necessary.

9) Voltage adjustment is now completed.

2 Stability adjustment:

The AVR includes a stability or damping circuit to provide good steady state and transient performance of the generator. The correct setting can be found by running the generator at no load and slowly turning the stability control anti-clockwise until the generator voltage starts to become unstable.

The optimum or critically damped position is slightly clockwise from this point (i.e. where the machine volts are stable but close to the unstable region).

3 Optimum response selection:

The "jumper" selector lead should be correctly linked (A, B, C at the bottom of the board) for the frame size of the generator

4 Under Frequency Roll Off (UFRO) Adjustment:

The AVR incorporates an under speed protection circuit which gives a volts/Hz characteristic when the generator speed falls below a presettable threshold known as the "knee" point.

The correct linking must first be set with "jumper" lead for 4 or 6 pole, 50 or 60Hz operation (see diagram).

The red Light Emitting Diode (LED) gives indication that the UFRO circuit is operating.

The UFRO adjustment is preset and sealed at the works and only requires the selection of 50/60Hz using the jumper link.

Adjustment of the UFRO control will be necessary if the unit is used on a 6 pole machine to replace an earlier AVR type.

For optimum setting, the LED should illuminate as the frequency falls just below nominal, i.e. 47Hz on a 50Hz system or 57Hz on a 60Hz system.

If the red LED is illuminated and no output voltage is present, refer to Over Excitation adjustment.

5 DIP adjustment:

The 'DIP' adjustment allows some control over the generator voltage dip upon the application of load.

This feature is mostly used when the generator is coupled to turbo-charged engines with limited block load acceptance, and operates only when the speed is below the UFRO knee point (LED illuminated).

With the 'DIP' control fully anti-clockwise the generator voltage characteristics will follow the normal V/Hz line as the frequency falls below nominal.

With a more clockwise setting, the slope of the V/Hz line will be increased until the rate of engine falling speed recovers.

Turning the 'DIP' control clockwise provides greater voltage dip allowing easier engine recovery.

6 DROOP adjustment:

Generators intended for parallel operation are fitted with a quadrature droop C.T. which provides a power factor dependent signal for the AVR. The C.T. is connected to S1, S2 on the AVR.

The DROOP adjustment is normally preset in the works to give 5% voltage droop at full load zero power factor (cos Ø).

Clockwise increases the amount of C.T. signal injected into the AVR and increases the droop with lagging power factor.

If a three phase sensing unit is fitted, droop adjustment is made on this separate unit.

With the control fully anti-clockwise there is no droop.

7 TRIM adjustment (V/TRIM):

An auxiliary input is provided to connect to the VPF controller, (A1,A2). It is designed to accept dc signals up to +/- 5 volts.

The dc signal on this input adds to or subtracts from the AVR sensing circuit, depending on polarity.

The V/Trim control allows the user to adjust the sensitivity of the VPF controller. With V/Trim fully anti-clockwise the VPF controller has no effect. Clockwise it has maximum effect. Normal setting is fully clockwise.

8 Over Excitation (EXC TRIP) adjustment:

The adjustment is set and sealed in the works and should not be altered.

An over excitation condition is indicated on the common red LED which also indicates underspeed running.

The generator must be stopped to reset an over excitation trip.

                                     Muhammad Asif Munir
                                     m_asiforient314@yahoo.com