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Author Topic: How To Test Your Generator - How to Polarize Your Generator  (Read 43070 times)
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« on: August 05, 2007, 06:52:08 AM »

Is my generator charging?

If you have a voltmeter:
  • Measure the voltage across the battery. Should be around 12.6 volts with the engine off. As the engine speeds up, the voltage should increase to somewhere around 14 volts. If it does, the generator is working and charging the battery.
  • Check brushes are not worn excessively. If you can, apply a little pressure to the brushes while the engine is running and see if the gen light goes out or the voltmeter reading increases.

If you do not have a voltmeter:
  • Get the car started, even by jumping if required.
  • Remove the + cable at the battery
  • The car should remain running. If it dies your generator is not charging.
  • It the car remains running, begin increasing the electrical load. Turn on lights, radio, everything that will increase your electrical load.
  • If at any time your car struggles to stay running, you are not producing enough power for your cars demand.
  • Check brushes are not worn excessively. If you can, apply a little pressure to the brushes while the engine is running and see if the gen light goes out or the voltmeter reading increases.

Replace brushes if required.

Generator Testing

  • Disconnect the generator from the regulator  (probably easiest to simply take the wires off the generator).
  • Connect a jumper from DF on the generator to the generator frame. Now run the engine and measure the voltage from ground to  D+  on the generator.
    NOTE: Some generators have different types of terminals; verify which is DF and which is D+.
  • As you increase the engine speed, the voltage should jump up to  +35 volts or so  (@3000 RPM).

If it passes this test, the generator is good. Don't run this test longer than necessary as it will overheat the generator.
If it fails this test, the generator may be good but need to be polarized. Check out "How To Polarize Your Generator".

NOTE: These tests are for 12V generators. The same tests will work on 6V generators but the voltages will be roughly one half the 12V values.
Second, a little bit about polarizing
Why do generators need to be polarized? Generators need some magnetism to get started. This "residual" magnetism remains in the field pole pieces even after the engine has stopped. The next time the generator starts up, the residual magnetism creates a small voltage in the Armature windings. Not enough to charge the battery, but enough to allow the field windings to draw current. As the field current increases, the pole pieces create even more magnetism. That makes even more voltage in the armature, and the cycle continues until the generator is capable of producing maximum output.

Anytime a generator is disconnected from the system, there is a possibility the residual magnetism may have decreased to the point where it can no longer get the generator started producing voltage. In the case of a new generator or one which has been mis-treated, the residual may even be of the wrong direction (north and south poles reversed).

Polarization is a simple process used to restore the field pole residual magnetism and ensure the magnetic direction is correct.

Now, How to Polarize Your Generator

  • Remove the fan belt.
  • Connect a jumper wire from DF to ground.
  • Connect a wire from the battery + terminal to D+ on the generator.
  • The generator shaft should start to spin.
  • Don't run this way for more than a few seconds to avoid overheating.
  • If the generator did not spin, the generator is likely defective.
  • The generator will now be properly polarized.
  • If it did, put the belt back on and re-test for generator voltage with DF grounded.   If the output voltage is still low, the generator is defective.

Do regulators need to be polarized?
No, regulators are not polarity-sensitive. The instructions supplied with some voltage regulators, actually polarize the generator, not the regulator.
« Last Edit: September 18, 2009, 05:58:43 AM by Don » Logged
AirCooled VW and Dune Buggy Discussion Forum From
« on: August 05, 2007, 06:52:08 AM »

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