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Three phase generators Under Voltage On Starting and how to resolve this...


The solution to generator under voltage depends on the actual

symptoms experienced. Carefully looking at the generators output can

help you effectively diagnose the fault and hopefully correct it quickly.

Electricity is dangerous, so ensure you are suitably qualified to work on

this equipment before attempting any procedure below.


This article assumes your generator is 3 phase and your control panel

will show you all three phase readings. Welland generators come with a

digital controller as standard to monitor most functions. You should also

ensure you have a multimeter ready to measure the actual output

voltages.


There are two broad types of under-voltage faults we see. There are

genuine alternator faults, where the voltage is low at the alternator and

there are panel / wiring issues that cause low voltage alarms.

Reading / Verifying the Panel Voltage


Follow these steps:

1. Ensure the generator is off.

2. Make sure the generators breaker is also tripped (i.e. off)

3. Run the generator, monitor the voltages on all three phases.

4. Turn the generator off.


If:

Generator Voltage is zero 0V on one phase or more phases

Normally, even when the alternator has failed a small voltage would be

present. This indicates that the controller cannot read the voltage on the

phase where it is 0.


• Check the control module plugs are fitted tightly.

• Check the fuses / breakers between the alternator and the control

panel on the phase(s) where the voltage is 0, install a new phase or put

the breaker to the on position.

• Test for open circuits in the voltage sensing circuits.


Check the actual reading using a multimeter, at the generator terminals.

If that shows normal, you could have blown the voltage sensing in the

control module. Check the control module on another unit to confirm.

If the multimeter shows 0V on one phase also, further investigation of

the alternator may be required.


Generator voltage is low on all three phases

Alternator Low voltage on all three phases, 400V, 3 Phase Alternator,

Off Load, How to fix it?


Before starting this section, please ensure you read the warnings and

suggestions in the general alternator voltage fault finding section and

your alternator manual.


This section applies to machines OFF LOAD. If you have a different

problem go back to the alternator voltage fault page.

Alternator Low Voltage Between 10-20% of rated output

On a 400V L-L Machine, this means you will have a voltage of between

40-80V. In my experience this is very often near the middle of this range

at 60V. This indicates lack of excitation, which could be:


1. The Automatic Voltage Regulator (AVR) fuse (if fitted) has blown.

2. The Automatic Voltage Regulator (AVR) has failed.

3. A break in the excitation circuit between the AVR and the exciter

stator.

4. Failure of the diodes.


Low Voltage Between 0-10V

In alternators that don't have a excitation system using a PMG, the

alternator relies on its residual magnetism to initially build the voltage. If

this residual has been lost, the alternator won't produce any voltage at

all.


It is usually possible to regenerate this magnetism, you should consult

someone qualified and your alternator manufacturer on the correct

procedure for your model.

Generator Voltage Low on One Phase, Other Phases Normal or High

If the voltage is low on one phase (not 0, if it is zero see above!) then

continue to the alternator voltage low on on phase, normal / high on

other phases.


Alternator Low voltage on one phase, 400V, 3 Phase Alternator, Off

Load, how to fix it?


Before starting this section, ensure that the warnings and suggestions in

the general alternator voltage fault finding section and your alternator

manual have been followed.


This section applies to machines OFF LOAD. If you have a different

problem go back to the alternator voltage fault page.

One phase Low Voltage,


This usually indicates that the phase has a low resistance to ground - it

probably needs a rewind or replacement. In a machine with an AVR that

uses single phase sensing, this will happen if one of the good phases

are being used for sensing.


Low Voltage on One Phase, Other Phases Normal

A low voltage on one phase with the other two normal, this indicates a

main stator fault. In this instance the AVR will be sensing from one of the

phases that are at normal voltage. The phase that has a problem, will

have low insulation resistance to or shorted to ground. You should

perform an insulation test, according to the manufacturer to ensure the

unit meets its required level. If it fails, you will need to rewind the

alternator, or fit a new machine.


Low Voltage on One Phase, Other Phases High

This again indicates a main stator fault. In this instance the AVR will be

sensing from the phase reading low, boosting the excitation system to

increase the voltage, leading to an increase on the phases that are OK

and little or no change on the phase that has a low resistance or shorted

to earth.


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