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Calculate a Portable Generator Wattages (with a Voltmeter)

Calculate a Portable Generator Wattages (with a Voltmeter)

It’s not once or twice I have taken my generator out of the shed, only to be surprised it’s producing no power. Typically, the engine runs well, but no power is coming out of it. It’s also necessary to test the outputs once to ensure the generator produces the voltage required. So, how do you check for generator output?

If your appliances aren’t running, you want to test the outlets with a voltmeter. If there isn’t anything coming out of them, you will want to check generator output by troubleshooting down to its source. You will have to test the AVR and the alternator for faultiness and troubleshoot whichever part that’s faulty.

It might not happen to you, but it’s not impossible. It’d be best to know how to check your generator’s output safely and the best time to do it. So, where does it all start?

What’s the Best & Safest Way to Check Generator Output?

Before hooking up anything to your generator, it’s always imperative to know its output to protect yourself and your appliances. And since you’re working with voltage and current, something that can injure, incapacitate, or even kill you, it’s would be best to know how to do it safely. So, safety first.

What Safety Measures to Take When Checking Generator Output?

  • Read and understand your generator operator’s manual before starting the testing to reduce the risk of injuries.
  • AVOID making any adjustments to your generator when the engine is running and the spark plug connected; stop the generator and disconnect the spark plug wire.
  • Beware of burns from hot parts; you want to ensure the engine is completely cooled down before touching the possible hot surfaces.
  • AVOID working on moving parts to reduce the risk of injuries.
  • Keep gasoline and other fuels away from the hot generator to reduce and avoid the risk of fire or explosion. Keep flames, sparks, and cigarettes away from any fuel-related parts.
  • Always work on dry ground and when your hands are dry. Wear protective electrical gloves if not experienced with electricity testing.
  • AVOID touching the voltage regulator or any open wires when the generator is running as you can get electrocuted big time.
  • Lastly, if you’re not confident of working around electricity, it’d be best to get a professional to help you out.

How to Test Generator Output from Outlets

Step 1: Start your generator

Put the choke at the ON/START, flip the power switch to the ON position or turn ON the ignition switch to start the engine. If your unit doesn’t have an ignition key, then pull the recoil cord to start it. If the generator isn’t starting, double-check the fuel valve to make sure it’s open.

Step 2: Turn your voltmeter on and set it to the ‘AC voltage’ position.

You want to move the dial from the OFF position to the section marked with AC voltage. If the text isn’t available, check for V~ or A~. Some voltmeter uses ‘ACA’ or ‘ACV.’

Pro tip: If having trouble setting your voltmeter to test for AC voltage, consult its manufacturer’s instruction manual on how to use it.

Warning: You must test your generator’s output when the voltmeter is set to AC voltage, or you’ll risk blowing the voltmeter’s fuse.

Step 3: Ground the Voltmeter

With the voltmeter on AC voltage position, attach its black lead to your generator’s frame. If your voltmeter comes with the cables out, take the black lead cable and insert it black into the black socket.

If there is an alligator clip, attach it to the generator’s frame. If not, you will need an alligator clip or some electrical tape to help hold the black-end needle to the generator frame.

Connecting the voltmeter to the frame grounds the wiring, thus protects electrical wires and components from sudden power surges.

Step 4: Plug in the red lead into any of the AC output outlet plugs

Plug it to any of the AC outlets, which you plug the appliances too. You want to make sure the red cable connects to the red socket on the voltmeter. The tip should be in either of two holes on the AC outlet.

Your generator might have multiple and different output outlets with different voltages. An example is a unit that comes with 120-volt outlets and a 220-volt outlet. Typically, these outlets will have a label near them. The good news is, you can test them both the same way.

We use 120-volt outlets for running standard electrical devices and 220-volt outlets for high-power appliances such as welders and washing machines.

Step 5: Read the voltmeter to see the generator output in volts

Check the voltmeter display for the voltage output while holding the red lead on the outlet. Keep it steady to avoid surges in the reading. What you get here is how many volts the generator puts out.

So, if the outlet is labeled 120 volts, the voltmeter will display a number close to 120 volts unless there’s a problem with the generator output.

Step 6: Test all the generator outlets

If the first outlet you inserted the red to is working right, it doesn’t guarantee that all the others are working correctly. It’d be best to test them too. So, take out the lead and connect it to the next outlet until you check all the outlets’ output.

Step 7: Unplug Everything

Now that you have confirmed the amount of voltage the generator delivers, what does it mean? Is the outlet delivering enough or fewer volts of power? If your 120-volt outlet is putting out 120 volts or closes to this, it’s all right. If the outlets are delivering less voltage, lower than 180 volts

Step 8: Remove the red lead from the outlet and shut off

Now that you know how much voltage the generator is putting out disconnect the red lead from the generator outlet and detach the black lead from the frame. Turn the multimeter dial to the off position.

Step 9: Turn the Generator Off

Now you can switch the ignition key off or flip OFF the power switch to shut off the generator engine.

How to Troubleshoot Your Generator AVR, Brush, Rotor, AVR, or Exciter

There might be a problem with the AVR, alternator, or brushes if you did not get any power on any 120-volt or 220-volt outlets. You want to test what part isn’t producing electricity.

It will make it easier for you to understand what parts need repairing or replaced. Typically, you might need to replace the brushes, AVR, or windings outside the alternator, rotor, or armature inside the alternator.

Remove the cover on the alternator: Start by placing the generator with the alternator end facing you. Remove the bolts holding the cover. Use the correct screwdriver, typically a 10-mil, 8-mil, or 7-mil, depending on your model.

Flash Testing the DC Voltage across the Brush Assembly

At the top-most position of the wiring arrangement, there are two wires connected to the brush assembly with spade connectors, red wire (+), and a white wire (-).

With the generator running and the multimeter set to DC current, touch the black lead to the (-) terminal and the red wire to the (+) terminal. You want to get 5-10 volts here. If less or more, there is a problem that will want to check out.

Remove the brush holder gently and examine the carbon brushes. You want to check for damage, excessive wear, and uneven surface.

Unscrew the bolt underneath the red wire terminal, and you will have access to the brush support plate. It’ll be easier to lift brushes up and out. Make sure the brushes are between 3/8″ and 3/16″ equal in length.

The brushes can wear out with regular or extended generator use. If yours are damaged or worn out much, you’ll need to replace them.

If they are in good condition, move to the next test but leave the brush assembly out as you need as it is.

Test the Resistance across the Rotor

With the brush assembly out and the multimeter set to the Ohms 1k scale, touch the leads to each copper slip ring. The ideal range is between 45 and 75 ohms.

Warning: Do this test with the generator off.

If the reading is below 45 ohms, the rotor is failing, and it’ll be best to replace it. But, if the reading is above 75 ohms, check the rotor wiring soldering to the slip rings. You want to make sure it’s solid and wholly connected.

If the reading is between 45 and 75 ohms, continue to the next test. You will have to reassemble the brush assembly, attach the brush support plate and secure everything with the previously removed bolt. But keep the metal cover out; you’re not done yet.

Check for DC Output across the AVR to Brush Assembly

Turn the generator on and allow it to run for the next test to work. Make sure the red and white wires on the brush assembly are snugly tight.

Set the multimeter to DC volt settings and place the red probe to the red wire and the black probe to the white wire. You should get 15-18 volts of DC power.

If you’re getting a reading below 15 volts, you will want to text the exciting field before considering changing the voltage regulator, the AVR (Automatic Voltage Regulator.)

Test the Exciter Field Winding Output

Look for the quick nylon connector with four wires from the stator, connect to the Automatic Voltage Regulator, and disconnect it.

Locate two yellow wires from the set and their terminal on the quick connector. Start the generator and keep running.

Set the multimeter to the AC voltage scale and touch the probe to these terminals. Don’t worry; polarity doesn’t apply here. You should get 4-8 volts AC.

If your generator produces more than 3500 watts and there are no yellow wires, test the blue wires – they are blue, not yellow for most of these models.

If the voltage is outside the 4-8 volt AC range, you’ll need to replace the stator. However, there is still a tiny chance the stator is okay, and the problem is with the rotor magnets; they have either become loose, cracked, or broken.

It’ll be best to remove the generator stator from the rotor and check for these three problems before you start ordering the replacement parts.

However, if you’re getting a reading between 4 and 5 volts, then you’ll have to replace the AVR.

Related Questions

How can you adjust generator voltage?

You will need to access the AVR (Automatic Voltage Regulator), unscrew it, and turn it over. You will see a small screw that you can turn counterclockwise direction to lower the voltage (recommended) or clockwise direction to increase the voltage.

About the author

Sharif Gen

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