My Generator is Producing No Voltage with No Load.

How can I test it?             How can I fix it?

Why does your generator not produce any voltage?

The most common causes of a generator producing no voltage is the loss of residual magnetism in the alternator and or a failed AVR or another excitation component.

If the voltage is around 50-70V on each phase, then it is likely the AVR needs replacement.

If the voltage is 0-5V per phase, it is probably a loss of residual magnetism.

If your generator does not have a PMG system, (Generally the PMG is attached to the rear of the alternator rotor and protrudes in its own protective case) then it is almost certainly a self-excited alternator. These rely on the residual magnetism of the alternator, left from it being run previously, to build up its initial voltage.

How to Test and Fix a Generator that has Low voltage on one phase, 400V, 3 Phase Alternator

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.

How to Fix High or Low 3 Phase or Single Phase Alternator Voltage Problem

You can fix and diagnose a problem with an alternator with careful diagnosis, using a multi-meter and / or an insulation tester, like a Megger.

To perform these tests, you should ensure you are electrically competent to do so, ideally having passed a current electrical testing qualification IE: Electrician. If you are not an electrician, I would suggest you consult your Trades person. We have instructions on how to diagnose and fix different common problems. These instructions are generic and should always be undertaken in accordance with your alternator’s operation manual.

Important: Don’t reply on the voltage readings from your control panel to diagnose an alternator failure. You should only read the voltages with no load from a multimeter, at the generator terminals.

This section applies to machines with no load, OFF LOAD.

  • You must adhere to strict safety protocols here as there is a chance of electrocution, Only competent people should do these measurements.
  • Is your Generator 3 Phase (400v to 415v) P to P or Single Phase (220v to 240v) P to N?
  • If single phase, does it have an AVR (Voltage regulator) or Capacitors for voltage regulation?
    • Typical AVR       
    •  Typical Capacitor 
  • If Single phase with capacitor
    • Visually inspect the capacitor. If leaks, cracks, bulges, or other signs of deterioration are evident, replace the capacitor.
    • Remove the capacitor from the generator noting wired connections (be careful not to touch the bare connectors as the capacitor could be charged)
    • To safely discharge a capacitor: After power is removed, connect a 20,000 Ω, 5-watt resistor across the capacitor terminals for five seconds. Use your multimeter to confirm the capacitor is fully discharged.
    • Using your multi meter turn the dial to the capacitance measurement mode, the symbol often shares a spot on the dial with another function, a function button may need to be depressed to use the correct function. “ -I(- “ , Function Symbol.
    • If the capacitor states its size in (u F) you are generally allowed + or – 5% so if your capacitor is 40 u F you can say its ok at 38 u F to 42 u F on your meter, if its outside these readings replace it.
  • If Singe Phase using your multi meter check your voltage P to N = ____
  • If 3 Phase check the following.
    • Is your 3 phase alternator AVR regulated or is it a compound alternator?
    • Using your multi meter check your voltage phase to phase.
      • L1 to L2 = ____  L2 to L3 = ____  L3 to L1 = _____
    • Using your multi meter check your voltage phase to neutral.
      • L1 to N = ____ L2 to N = ____ L3 to N = ____

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 alternator.

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.

How to Fix an Alternator Low voltage on all three phases, 400V, 3 Phase Alternator

Alternator Low Voltage Between 10-20% of rated output

On a 415V 3 phase generator measuring phase to phase , this means you will have a voltage of between 41-82V. In my experience this is very often near the middle of this range at 60V. This indicates lack of excitation, which could be:

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

The Automatic Voltage Regulator (AVR) has failed.

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

Failure of the diodes.

Low Voltage Between 0-10V

In alternators that do not have an 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 will not produce any voltage at all.

It is usually possible to regenerate this magnetism (Flashing the alternator), you should consult someone qualified and your alternator manufacturer on the correct procedure for your make and model.

How to Fix Alternator High Voltage on all phases, 415V, 3 Phase Alternator

High alternator voltage on all three phases, Off Load

In generators that have separate circuits to power the AVR (fitted with auxiliary windings or PMG systems) the AVR can be powered, but it could lose the sensing circuit, either because of a break in the sensing cable, or an AVR fault. This loss of sensing will make it appear to the AVR that the voltage is low and it will therefore boost the excitation system to try to increase the voltage, causing the voltage on all three phases to rise.

To fix this, check the resistance of the sensing circuit according to the alternator manual (or at least check it is not an open circuit) and swap the AVR to see if it fixes the problem.

I need to stress again only perform the above items if you are qualified. If you’d like professional assistance, Powerlite offers generator repair services.

Some useful links

Share this post: