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Working Principle of AC Generator

Dec. 25, 2019

Electric energy is one of the most important sources of energy in modern society. A generator is a mechanical equipment that converts other forms of energy into electricity.

There are many forms of generator, but the working principle is based on the law of electromagnetic induction and the law of electromagnetic force. Therefore, the general principle of its construction is to use appropriate magnetic conductive and conductive materials to form magnetic circuits and circuits for mutual electromagnetic induction to generate electromagnetic power and achieve the purpose of energy conversion.

Working principle of alternating-current generator

The mechanical energy of the prime mover is converted into electrical energy output by using the principle of electromagnetic induction in which the cutting of magnetic lines of force by a wire induces electric potential. A synchronous generator consists of a stator and a rotor.

The stator is the armature that emits power, and the rotor is the magnetic pole. The stator is composed of armature iron core, three-phase winding with uniform discharge, base and end cover. The rotor is usually a hidden pole type, which is composed of excitation winding, iron core and shaft, guard ring, center ring and so on. A direct current flows into the rotor's excitation winding to generate a nearly sinusoidal distributed magnetic field (called the rotor field), and its effective excitation flux intersects the stationary armature winding. When the rotor rotates, the rotor's magnetic field rotates with it. With each revolution, and the magnetic field lines sequentially cut each phase winding of the stator, and the three-phase AC potential is induced in the three-phase stator winding. When the generator is running with a symmetrical load, the three-phase armature current is combined to produce a rotating magnetic field with synchronous speed. The stator magnetic field interacts with the rotor magnetic field to generate braking torque.

AC generator

Alternating-current generator/alternator is divided into single-phase generator and three-phase generator. Alternator is often used in diesel generator sets.

The AC generator installation inspection steps are as follows:

1) When installing the generator, ensure that the cooling air inlet is unobstructed, and avoid exhausted hot air from entering the generator. If there are blinds on the ventilation cover, the window should face down to meet the requirements of the protection level. For mechanical coupling of single-bearing generator, special attention should be paid to the uniform air gap between the stator and rotor.

2) The coupling of AC generator and diesel generator requires the parallelism and concentricity of the coupling to be less than 0.05 mm. The requirements in actual use can be slightly lower, within about 0.1 mm, too much will affect the normal operation of the bearing, resulting in damage, the coupling should be fixed with a positioning pin. Retest the coupling before installation.

3) The U, V, W, N marks are printed on the terminal head in the generator outlet box, which does not indicate the actual phase sequence and depends on the direction of rotation. The UVW printed on the certificate indicates the actual phase sequence of clockwise rotation, and VUW indicates the actual phase sequence of counterclockwise rotation.

4) When the generator with sliding bearing is coupled, the height of the center of the generator should be adjusted slightly lower than the center of the diesel engine, so that the weight of the flywheel on the diesel engine will not be transferred to the generator bearing, otherwise the generator bearing will bear the additional weight of the flywheel, which is not conducive to the formation of the oil film of the sliding bearing, resulting in heating and even burning of the bearing.

5) If the neutral points of each generator in the power supply system are connected to each other, or when the generator neutral point is connected to the transformer and its load neutral point, the genset will have a neutral current of 3 times the frequency on the neutral line in operation. Therefore, the neutral-line current of the generator must be measured under various load conditions that may occur during operation. In order to prevent the generator from overheating, the line current shall not exceed 50% of the rated current of the generator. The neutral-line power is too large, and a neutral line reactor should be installed on the neutral line to limit it.

6) According to the schematic diagram or wiring diagram, select the appropriate power cable and use copper connectors for wiring, copper joint and busbar. After the busbar and the busbar are firmly fixed, the local gap at the joint is not greater than 0.05 mm, and the distance between the wires is not greater than 10mm, also need to install the necessary grounding wire.

Generator Maintenance

No-load test once a week, running time is 10-15 minutes, so that problems can be found in time, and ensure that the battery has enough capacity for the generator to start.

1. Before maintenance, the automatic air switch of the generator should be cut off. The main switch should be placed in the stop position, and the positive and negative power cables of the battery should be folded to confirm that the generator cannot start.

2. Check the battery electrolyte water level. When it is insufficient, add sulfuric acid solution or distilled water according to its concentration.

3. Calculate the cumulative running time of the generator from the last maintenance. The air filter should be cleaned every 50 hours, and the oil filter, diesel filter, and water filter should be replaced every 250 hours.

4. Check the unit for water and oil leakage and clean the generator body.

5. Connect the battery power cord and connector. The test fault indicator should be on.

6. Check if the ground wire is good.

7. Check the running condition of the exhaust fan in the machine room, whether the diesel oil is enough to run for 8 hours.

8. Check whether the cooling water level and engine oil level are normal.

9. After the manual test machine runs normally, turn on the automatic air switch of the generator, switch the main switch to the automatic start position, perform a simulated power failure test, and observe the automatic start of the generator.

10. Run the generator with load for one hour, and observe whether the indicators of each instrument and unit operation are normal.

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