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Why Can Generator Set Start Up, But No Voltage?

Dec. 27, 2019

After generator set start up, there is no voltage, the most possible reason is the generator has no magnetic field.

Why is there no magnetic filed in generator?

Fault phenomenon:

After standby generator set idle for a long period of time, when restart, there is no AC voltage output. Measure the main output winding, and the voltage between phases is "0" or lower than "5vac".

The reason:

Generator has not been used for a long time, resulting in losing remanence contained in iron core. The excitation coil can't establish the proper magnetic field. At this time, the generator set runs normally but does not generate voltage. This kind of fault often occurs in new generator set, generator set stored in humid environment or generator set not used for a long time.

Generator set

How to deal with no magnetic field problem of generator?

1.Check the main output circuit of the generator to avoid the excitation cannot be established due to the short circuit at the output end.

2.if there is excitation and magnetization button, press the excitation and magnetization button.

3.If there is no excitation button, use the battery to magnetize it. The specific method is:

a.Use 12V DC battery, connect negative pole to "XX" terminal of AVR board, the positive pole is connected to the "X" terminal of AVR through a diode.

b.Restart the generator set and record the main stator output voltage, the voltage is close to the rated voltage, or the voltage between P2-P3 of AVR terminal is 170-250V.

c.Shut down the unit, then disconnect the battery power supply of "X" and "XX" terminals, and restart the generator set. At this time, the generator set should be able to operate normally. If there is still no voltage, it can be considered that there is a problem with the generator set or AVR.


Here is an other method of resolution:

First, look at the gloss level of the two slip rings. If there is no lustre, rub it with emery cloth until it becomes shiny. If still can not solve, use a very thin fuse and connect the three-phase four wires on the generator randomly. Then start the engine, the fuse will blow automatically, and the voltage will come up.


If you don't know the diode, open one of the iron rings on the back cover of the generator, and you can see two slip rings, then see the carbon sand. At this time, take a battery of any size, start the generator and hold the positive and negative terminals on the battery by hand, press and hold the two carbon sands corresponding to the slip ring. Do not remove until the generator voltage increases.


If it still doesn't work, exchange the positive and negative poles of the battery. If the voltage disappears immediately after removal, it indicates the diode is broken. Replace the diode.


Why does the voltage not rise after generator set start up?

If the voltage does not rise after the diesel generator set start up, the unit will not work normally.

1.The excitation circuit is disconnected, resulting in preventing the voltage from rising. Check whether the excitation circuit has already connected or not and the contact is good or not.

2.The residual magnetism disappears. If there is no indication on the exciter voltmeter, it means that the residual magnetism disappears and the exciter should be magnetized.

3.The polarity of the field coil of the exciter is reversed. Its positive and negative connecting wires shall be exchanged.

4.When some tests are done in the maintenance of generator, the magnetic field coil is connected with reverse direct current by mistake, resulting in the disappearance or reverse of residual magnetism, so it should be magnetized again.

Starlight Power is a manufacturer of 20kw-3000kw diesel generator set in China, founded in 1974. If you are interested, please contact us by sales@dieselgeneratortech.com or call us +86 134 8102 4441. 

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