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Causes Of No Or Low Generator Voltage

Jun 9, 2023 | Technical Literature | 0 comments

The fault phenomenon of generator low voltage alarm can be checked by confirming that there is no deviation in the display instrument, confirming that the speed/frequency of the generator set is normal, and confirming that the load change is not large when the alarm occurs, etc. to do related investigations. The reason is generally that the magnetic pole junction of the generator changes, the output voltage is unstable, or the voltage is low due to the damage of the winding, or it may be the damage of the winding connection wire, resulting in insufficient current, which affects the output voltage of the generator.


After the generator set is running, when the generator speed reaches the rated speed, the AC exciter stator excitation circuit switch is closed, and when the voltage regulating potentiometer is adjusted to the upward direction to the maximum value, the generator has no output voltage or the output voltage is very low.

Cause of Issue

  1. Generator iron core magnetism loss or large or weak newly installed generator sets are affected by long-distance transportation or the generator is placed for too long, and the generator iron core disappears or weakens, resulting in the residual voltage of the generator disappearing or lower than the normal voltage. Value, that is, multiplying the magnetic line voltage is less than 10v, and the residual magnetic phase voltage is less than 6V. Since the stator and rotor of the synchronous generator and the stator and rotor cores of the AC exciter are usually punched/indented with 1~1.5mm thick silicon steel sheets, the residual magnetism is easily disappeared or weakened by vibration after excitation.
  2. Excitation circuit wiring measures When overhauling the generator, the polarity of the excitation winding is accidentally reversed during work. After the power is turned on, the magnetic field generated by the excitation winding current is opposite to the direction of the residual collision and cancels out, causing the multiplier to disappear. In addition, during maintenance, if you measure the DC voltage of the excitation winding or test the automatic voltage regulator AVR to pass DC current to the excitation winding, you do not pay attention to its polarity, which will also cause the residual magnetism of the iron core to disappear.
  3. The excitation circuit circuit is broken. The electrical contact in the excitation circuit of the generator is poor or the terminals of each electrical component are loose, and the lead wire is broken, causing the circuit to be interrupted. The excitation winding of the generator has no strong current excitation.
  4. Interruption of the circuit on the DC side of the rotary rectifier Due to the interruption of the circuit on the DC side of the rotary rectifier, after the AC exciter is rectified by the rotary rectifier, the excitation current provided to the excitation winding cannot be sent to the excitation winding, resulting in the failure of the AC synchronous generator to generate electricity.
  5. The AC exciter is faulty and has no output voltage. The AC exciter is faulty and cannot send out voltage, so that the excitation winding of the AC synchronous generator has no excitation current.
  6. The generator excitation winding is disconnected or grounded, resulting in no excitation current or very small excitation current of the generator.
  7. Poor contact of the carbon brush or serious wear of the carbon brush. For the poor contact of the carbon brush, the contact between the carbon brush and the rectifier ring should be properly adjusted to make the contact between the two completely reliable. For the serious wear of the carbon brush, only Replace with new accessories of the same specification.
  8. There is a break in the circuit. The disconnection point should be carefully searched, and for the desoldered solder joint, solder should be added to make it firm.


  1. When the iron core of the generator disappears, magnetization treatment should be carried out. The magnetization method is as follows. For self-excited generators, an external battery is usually used, and the negative pole line of the generator is connected to the lead-out point of the excitation winding. It is enough to connect to itself, but it is necessary to recognize the polarity of the DC power supply and the excitation winding, that is, the positive pole of the DC power supply is connected to the positive pole of the excitation winding, and the negative pole of the DC power supply is connected to the negative pole of the excitation winding. If there is a magnetizing circuit on the control panel of the diesel generator set, the button switch should be turned to the “magnetizing” position to magnetize the AC exciter. For generators with third harmonic excitation, when the no-load excitation voltage cannot be established, a DC power supply can also be used for magnetization.
  2. Excitation circuit wiring error, correct it after searching.
  3. Use the ohm gear of the multimeter to find the disconnection of the excitation circuit, and connect it; at the fault point with poor contact, polish the surface oxide layer with fine emery cloth, and tighten the loose wiring bolts and nuts.
  4. If the grounding and disconnection of the excitation winding, use a 500v “megohmmeter (shake meter) to check the ground insulation of the winding, find out the grounding point, use a multimeter to find out the disconnection, and repair it.
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