Generator not Putting Out Correct Voltage.

You’re trying to run some equipment or appliances off your generator, but it’s not putting out the correct voltage.

Without the correct voltage, you could damage your equipment or appliances.

Troubleshooting your generator is key to getting it up and running correctly. This article on “Generator not putting out correct voltage” will walk you through some common causes related to incorrect generator voltage and their solutions.

What causes low voltage on a generator?

In this article, we will discuss cases where the generator can start without any trouble and runs smoothly but refuses to supply the correct output voltage. We will introduce you to various problems you are likely to encounter in this regard and the common reasons causing them.

As a piece of general advice, carry out your troubleshooting for electrical issues for any fault in the following sequence of components. This suggestion is based on the observed frequency of failures among these components.

  1. GFCI, the main breaker, and the receptacles,
  2. Stator and Rotor tests,
  3. Components that form part of excitation circuits like brushes, AVR, rectifiers, and others.
  4. Mechanical Issues.

A. Zero Voltage on a Single Receptacle at No Load

In this scenario, you get no voltage on anyone out of the various power outlets your generator has, while other outlets perform normally. The possible reasons are

Faulty ground fault interrupter circuit (GFCI) or the receptacle

These components often fail with the GFCI resulting in a fair share of problems. However, it is a device designed to protect humans from an electric shock. If it is tripping continuously, it is most likely doing its job by responding to leakage in a generator or the load, whereby the fault current is getting diverted to the ground by the ground connection of the frame.

GFCI can malfunction due to excessive vibrations caused by mismatched loads, loose rotor shaft bearings, or worn-out/hardened rubber mounts. All modern GFCI goes dead on internal failure or indicates their health status when you press their “TEST Buttons.

Check your power receptacle by disconnecting all the wires and installing a jumper wire between the hot and neutral terminals. Test for the continuity between the gold and silver-colored screws on the unit’s side. Replace the receptacle if the test fails.

Bad connection

Check the receptacle’s lead wire connections and rectify them.

Break-in wiring to the receptacle

Check the wiring up to the failed receptacle and rectify it.

B. Zero voltage on all receptacles

In the second scenario, you may not be getting any power supply on all the power receptacles of your generator sets. There are eight main possible reasons for no voltage output at any of the receptacles.

Faulty main circuit breaker

Most modern circuit breakers carry colored indications to signal the breaker’s status. Otherwise, you may need to measure the resistance to check the health of the CB. During these resistance tests, the generator must not be in running condition and ensure the component under test is disconnected from its circuit.

Close the circuit breaker and use your multimeter to test the resistance between the input and output terminals of the breaker. The breaker is good if a multimeter measures a small finite resistance value. Else, it needs to be replaced.

If you find the circuit breaker to be healthy and still tripping, this indicates a likely overload or short to ground in your power cables or the connected loads. We have dealt with this scenario in detail in our article on “How To Fix An Overloaded Generator?

Loss of residual magnetism

The conventional brushed and brushless alternator starts by using residual magnetism, which is the amount of magnetization retained by the winding cores after removing the external magnetic field. To start, the generator should develop 5 to 8 V at no load.

It is recommended to start and run the generator for some length of time every month. Otherwise, it loses its residual magnetism and may not start at all. You can restore the residual magnetism by various methods of field flashing. These methods may be unsafe for the operator or the equipment and best left to the professionals unless you have seen, trained, or carried it out before.

Shorted Stator winding

The stator on most generators has two main windings and an excitation winding as a minimum. The stator may have additional windings to produce power and sensing voltage for the AVR and a 12V supply for battery charging and other accessories.

The higher the kilowatt rating, the more the number of windings and complex output lead arrangement. The odor of burnt insulation, blackened varnish, or carbon deposits provides the first indication of the shorted or burnt stator winding.

Carry out the ohmmeter tests on each winding, compare them with the information from the manufacturer, then test for winding to ground resistance, including the exciter winding on the brushless generators. In case of any doubt, you may carry out the mega ohmmeter tests after disconnecting all electrical components and shorting the rotor diodes with jumpers.

Worn Out Rotor Brushes

Check for cracks, burning, or crumbling signs. It is recommended to replace the brushes once they have worn out to half their original length. Damaged brushes can melt their plastic holders. During their replacement, polish the slip rings with a pad and remove all abrasives.

Blown Capacitor

Alternators have two types of stators – Brushless and Brushed. All brushless and a few brushed types use one or two electrolytic capacitors for voltage regulation purposes. These capacitors are of 450V and 18 to 100 MF rating. They are installed near the end cover or behind the Control Panel.

Always discharge the capacitors before testing them. You may use a screwdriver with an insulated handle or moderate load for discharging. The capacitor may become nonconductive or develop a short between their plates. The bulging top near the terminals is a tell-tale sign of a faulty capacitor.

To test it, shut down the engine, disconnect the wiring to the capacitor, and connect an analog ohmmeter to its terminals after setting the major scale at R x 100. This will charge the capacitor, and the needle will move to the right, indicating the increase in resistance till the capacitor is fully charged.

Bad Automatic Voltage Regulator (AVR)

All AVRs carry separate adjustment potentiometers for voltage and frequency. Industrial quality AVRs may have a few additional adjustments. Using these, the generator should be set to run at 3750 rpm (+- 50 rpm) and produce 125 to 130 V at a frequency of 61 to 62 Hz.

Problems in any automatic voltage regulator may result in either no output, low output, high output, and unstable output voltages or a drop in voltage under load. We shall be discussing these failures in individual scenarios. As far as no output voltage is concerned, it can be due to disconnected or damaged wires to AVR, failure of rectifier bridge, and defective voltage regulator.

You can check and replace or rectify wires or rectifiers if found faulty. If you have a brushed Genset with minimal solid-state components in the AVR, you can go in for a rotor excitation test described at the article’s end. AVR may have to be replaced if necessary.

Low engine rpm triggers the mechanism for shutting down output.

Low engine rpm can be a result of

  1. Fuel restriction caused by clogged fuel filters, fuel lines, air leaks, or malfunction of engine stop solenoid, or
  2. Bad quality, stale (long storage in the fuel tank), water mixed fuel, or
  3. Insufficient air for combustion, or
  4. Switching the generator without turning off the loads might disturb the factory settings.

A loose mechanical connection between the generator rotor and the engine crankshaft

This can prevent it from producing power. Check and tighten as required.

C. Half the Normal Voltages

In this scenario, you will get 60 V at 120 V receptacles and 120 V at 240 V receptacles, even if the engine runs at rated speed. The common reasons for this fault are:

Faulty bridge rectifier or rectifier connections

This scenario is the most likely fault, whereby one or more rotor diodes have failed. Diodes have very low resistance in the forward direction and may blow up like a fuse in the presence of excessive rotor excitation currents.

With an ohmmeter, test the diodes in forward and reverse directions. The diode is functional if you get a very low resistance in the forward direction and a very high resistance reading in the reverse direction. However, sometimes the diodes pass the tests but do not function under load. As these are very inexpensive components, you may change them in case of doubt. Proper soldering iron feed application is necessary to get a low resistance point.

A high resistance connection

A high resistance connection in the output circuit causes a voltage drop.


The problems in the AVR that can cause low voltages on the receptacles are

  1. Insufficient generator rpm – check the engine.
  2. Disconnected or partially connected wires to the AVR – check and rectify the wiring issues.
  3. Incorrect voltage setting – adjust the reference setting value for the AVR.

D. Excessive Voltage on All Receptacles

The common reasons for the voltage output to go high on all the receptacles include:

Very high engine speed

High engine speed will make the magnetically charged rotor rotate fast, inducing higher EMF in the stator and overvoltage in the electrical panel. Regulate the engine speed by adjusting the speed governor’s throttle adjustment screw with a screwdriver.

Faulty sensor circuit wiring

A faulty sensing circuit will provide the voltage regulator with an erroneous sensing voltage, making its PID controller generate an incorrect value of the control signal and, thereby, the rotor excitation current.

The capacitor size is too large.

A higher capacitor value than the required rating will result in higher generator output voltage. Find out the specified rating advised by the manufacturer. Replace your existing capacitor with the recommended capacitor.

Misadjusted or a bad regulator

Check the AVR voltage pot setting. Adjust if incorrect settings are found. Else, replace the regulator.

So far, we have discussed faults that are apparent to you without imposing any load on your generator. Now, we shall focus on voltage-related issues that manifest themselves once the generator is loaded.

E. Fluctuating Voltage

The main reasons for obtaining rapid fluctuations in voltage on the application of load are:

Misadjusted AVR voltage potentiometer

If you have an incorrect setting on the voltage potentiometer, the control module of AVR will generate incorrect control signals resulting in a fluctuating voltage. Check the reference voltage and correct it.

Low stator or field winding resistance to the ground

Test the stator and rotor winding resistance with respect to the ground. A low resistance value in either of the two could be the reason for the fluctuation. In most cases, replacement is the likely solution.

Worn rotor shaft bearing

If you observe noise and terminal voltage instability, it could indicate excessive wear on the rotor shaft bearing, leading to changes in the air gap between the stator and rotor windings. The rotor can come in rubbing contact with the stator insulation, and eventually, both the rotor and stator can get damaged.

Faulty AVR

Replace the AVR.

F. Voltage falls Sharply as Loads are Imposed.

In this scenario, you obtain a low voltage reading beyond the acceptable range once the loads are imposed on the generator. Let us see the causes.

Open Stator Windings

Test all the individual stator windings for the resistance values using a multimeter between one end of the wire coil and the other end of the wire. A high or infinite value will indicate an open circuit. Getting a replacement stator is difficult and time-consuming, and often very costly.

Defective Diodes (may pass the resistance tests)

We had discussed the diodes in scenario C, Item “Faulty bridge rectifier or rectifier connections.”

Misadjusted AVR voltage potentiometer

Same as Scenario E, Item 1.

Unresponsive Engine Governor

As more load is imposed on the generator, it slows down till the engine governor opens the throttle valve to increase the fuel supply for combustion and increase the engine rpm back to its normal value. Sudden voltage drops are imminent if the governor cannot sense or respond to the rpm changes.

Poorly Performing Engine

Other mechanical factors, such as fuel restriction, clogged lines, filters, air supply, inadequate compression, etc., can result in a situation where the generator can’t match the demands of the load, though within limits, and produces a low voltage supply.

Loads may have a poor power factor.

If you connect low power factor loads to the alternator, they will draw more current, causing higher voltage drops in the stator conductors resulting in low voltages at the terminals. You need to check all the loads and isolate the one with a very low power factor.

Excessive overload

If the power requirement of all the devices connected to the generator is beyond its full load capacity, keeping in mind its starting and running watts ratings, it is overloaded beyond its limit to handle. You need to relook at your loads and reduce some of them, particularly the motor loads, to fix the problem.

Faulty AVR

Replace the AVR.

G. Persistently low voltage under load.

If you have ensured that your loads are within limits and do not have low power factors, then look out for the following reasons if you are experiencing persistently low voltage under load conditions.

Engine speed droop is excessive.

Check engine and governor. The droop setting of any generator defines the drop in frequency with an increase in load.

Faulty AVR power or sensing circuit

If either of power or sensing signal is incorrect, the AVR will try to produce the control signals to match those incorrect signals and may continuously run the generator at low voltage levels. Check and rectify the circuit or the set reference, whichever is faulty.

AVR Power & Sensing Circuit fault resulting in Generator Not Putting Out Correct Voltage.

Faulty rotor diodes

The faulty rotor may not be providing the correct excitation current to the rotor to produce electricity with the correct terminal voltage. Same action as scenario C, Item 1.

The faulty field winding

If you have noticed, all the elements being discussed in the last three reasons belong to the excitation circuit for the generator. Any reason that may result in low excitation current to the rotor and consequently the magnetic flux will cause persistent low voltage. Any damage in the rotor field winding will cause the same.

Excessive impedance and voltage drop in the power cord

If the power cord connected to the receptacle with a plug has a high impedance or longer length, it may have a high voltage drop, creating a low voltage at the load end. Check the size of the cable and calculate the voltage drop. Replace with an adequately rated cable.

Faulty AVR

Replace the AVR.

H. Noise – Worn Rotor Shaft Bearing

We have already discussed this in scenario E, Item 3.

  1. You need to check for contact damages on the rotor and stator. If you find them, the problem in the bearing is confirmed and needs to be replaced.


Low voltage on a generator can be caused by many things. This article looked at the most common reasons for low voltage on a generator. We also provided some tips to help you troubleshoot the issue and get your generator up and running again. Please let me know in the comments section below if you have any questions or need assistance. I will do my best to help you out. Thanks for reading!

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