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What Are Common Faults of AC Alternator in Diesel Generator

Dec. 16, 2020

AC alternator is an important part in diesel generator, if it has faults, whole diesel generator can not work. Some customers asked question about this last time, so Starlight Power will answer details about the common faults of AC alternator, hope this article is helpful to everyone.


A.There is abnormal voltage between alternator neutral line and ground.

1.Under normal condition, due to the influence of high-order harmonics or manufacturing process, the air gap under each magnetic pole is uneven and the magnetic potential is unequal. If the voltage is between one and several volts, there will be no danger and it is not necessary to deal with it.

2.The alternator winding has short circuit or poor insulation to the ground, which leads to the deterioration of electrical equipment and alternator performance and easy to heat up. Therefore, timely maintenance should be carried out to avoid the expansion of the accident.

3.There is no voltage between neutral line and ground under no-load, but voltage appears when there is load, which is caused by three-phase imbalance. Therefore, three-phase load should be adjusted to make it basically balanced.

B.Alternator overheating.

1.The alternator does not operate according to the specified technical conditions. Over high stator voltage, iron loss increasing, too large load current, startor winding cooper loss increasing, too low frequency, those factors will slows down the cooling fan speed, affect the alternator heat dissipation. And too low power makes the rotor excitation current increase, resulting in rotor heating.

2.The three-phase load current of the alternator is unbalanced, and the overload one phase winding will overheat; if the difference of the three-phase current exceeds 10% of the rated current, it is a serious unbalanced phase current. The unbalanced three-phase current will produce negative sequence magnetic field, thus increasing the loss and causing the heating of the pole winding and ferrule. The three-phase load should be adjusted to keep the current balance as much as possible.

Yuchai generator

3.The air duct is blocked by dust and the ventilation is poor, which makes heat radiation difficult. We should clean dust and oil dirt to make the air duct unobstructed.

4.Air intake temperature is too high or water intake temperature is too high, cooler is blocked. We should cool down Air intake temperature and water intake temperature, and remove the blockage in the cooler. Before troubleshooting, we should limit the load of alternator to reduce the temperature of alternator.

5.Too much or too little grease for bearing. Grease should be added according to the regulations, usually 1/2 ~ 1/3 of the bearing chamber (the upper limit for the lower speed and the lower limit for the higher speed), and it should not exceed 70% of the bearing chamber.

6.Bearing wear. If not serious wear, will make bearing overheated locally. If serious wear, it may cause friction between stator and rotor, resulting in overheating of stator and rotor. We should check whether there is noise in the bearing. If friction between stator and rotor is found, stop the machine immediately for maintenance or replace the bearing.

7.The insulation damage of stator core will cause the short circuit between pieces, which will increase the eddy current loss of the iron core and generate heat. In serious cases, the stator winding will be damaged. Stop the machine immediately for maintenance.

8.The parallel conductor of stator winding is broken, which makes the current of other wires increase and generate heat. Stop the machine immediately for maintenance.

 Yuchai generator set

C.Alternator overcurrent.

1.Overload. We should reduce load.

2.In case of phase to phase short circuit or grounding fault of transmission line, the line should be overhauled, and it can return to normal after troubleshooting.


D.Generator terminal voltage too high.

1.If the grid voltage of the generators paralleled with the power grid is too high, the voltage of the generators in parallel should be reduced.

2.If the fault of excitation device causes over excitation, the excitation device should be repaired in time.

E.Insufficient power.

Due to insufficient compound excitation compensation of voltage source of excitation device, the excitation current required by armature reaction can not be provided, so that the generator terminal voltage is lower than the grid voltage and the rated reactive power cannot be delivered. The following measures should be taken:

1.A three-phase voltage regulator is connected between the generator and the excitation reactor to increase the terminal voltage of the generator and gradually increase the magnetic potential of the excitation device.

2.By changing the phase between the voltage and magnetic flux potential of the excitation device and the voltage at the generator terminal, the resultant total magnetic flux potential can be increased, and a resistance of several thousand Ω and 10W can be paralleled at both ends of each phase winding of the reactor.

3.Reducing the resistance of rheostat increases the excitation current of generator.

F.Alternator loses the residual magnetism and cannot generate electricity when starting.

1.The reason why the residual magnetism is often lost after shutdown is that the material used for the magnetic pole of the exciter is close to the soft steel, and the residual magnetism is less. When there is no current in the excitation winding after shutdown, the magnetic field will disappear. Batteries should be provided and magnetized before power generation.

2.When the magnetic pole of the generator loses magnetism, the DC current larger than the rated current (for a short time) should be introduced into the winding for magnetization, that is, sufficient remanence can be recovered.

G.The temperature of excitation reactor of automatic excitation device is too high.

1.If the reactor coil is short circuited locally, the reactor should be repaired.

2.The air gap of reactor magnetic circuit is too large, so the air gap of magnetic circuit should be adjusted.


H.After generator is started, the voltage cannot rise.

1.The excitation circuit is disconnected, so that the voltage can not rise. Check whether the excitation circuit is broken and whether the contact is good.

2.If there is no instruction on the exciter voltmeter that the residual magnetism disappears, the exciter should be magnetized.

3.The polarity of the magnetic field coil of the exciter is reversed, and its positive and negative connecting lines should be exchanged.

4.When doing some tests in the maintenance of the generator, the magnetic field coil is wrongly connected with reverse direct current, which leads to the disappearance or reversal of the residual magnetism, so the magnetization should be conducted again.


Starlight Power is not only provide technical support, but also sell and produce diesel generators, we have focused on high quality product for more than 44 years. If you need to buy diesel generator recently, please call us +86 13481024441( same as WeChat number). 

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