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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