How Does Brushless Alternator Works? (With Diagram)

Operation Principle of Brushless Alternator
              In all alternators, voltage may be generated by rotating a coil wire in the magnetic field or by rotating a magnetic field within a stationary coil wire .It doesn't matter whether the coil is moving or the magnetic field is moving. Either configuration works equally well and both are used separately or in combination depending on mechanical, electrical and other objectives.

              In the case of brushless alternator both combinations are used
together in one machine.
             Before going to the explanation part you should know about stator,rotor,field windings and armature. The stationary part of an alternator is called the stator and the rotating part is called the rotor. The coils of wire used to produce a magnetic field are called the field winding and the coils that the power are called the armature winding. Here you can see both Armature and Field winding used as rotor as well as startor.


Working Of Brushless Alternator

          Brushless alternators have two parts; one is Excitation alternator part and the other is  main alternator part 

Here we have provided a schematic diagram of Brushless Alternator

Excitation alternator
       Here Armature is rotor and  Exciter Field winding is stator. Exciter field produce a magnetic field by the help of AVR or Residual magnetism. When it starts rotating a voltage is generated in Exciter armature which gives current to the main field to produce magnetic field in Main alternator.

Main Alternator
.      Here main field is rotor and armature is stator. Voltage Produced in the exciter armature pass through the Diode Mounting plate(Rectifier) and goes to the Main Field which produce a magnetic field. When this magnetic field cuts the Main Armature a potential difference is procured. we can take supply out directly, no brushes required . Here the voltage produced can be regulated by exciter field current

AVR(Automatic Voltage Regulator)
       Avr controls the Output voltage by controlling exciter field current. AVR , sensing the out put voltage from main armature , compares it  with the set value and then changes the exciter field current. AVR having diodes which converts A/c into D/c for exciter field 
 3-Phase Brushless Alternator Circuit Diagram




31 Comments


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  1. how does the exciter field i.e the exciter stator gets charged initially ?

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    Replies
    1. residual magnetism ie,when the power is switched off to an electro magnet, it will retains some magnetic power

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    2. Sometimes there is one permanent magnetic and two electromagnets which start the process

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    3. Exciter field consists of 1 permanent magnetic and two electromagnets... https://www.flight-mechanic.com/wp-content/uploads/2020/06/12-324.gif

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    4. So Ive been reading and watching a lot of videos online converting old brushed alternators into a brushless DC motor using a Bldc controller to energize the three phase coils and a low voltage power source as an excite through the old brushes. My question is... Can this same set up or a simlar set up be used to turn the brushless alternator into a Brrushless motor... what would be the advantages and disavantages of this approach.

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  2. Replies
    1. automatic voltage controller

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    2. Residual magnetism or it has a battery supply to...increase the residual magnetism ...so that req volatage can be produced

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    3. After alternators are manufactured, they have no magnetism at all. The exciter stator(exciter field) is temporarily (for few seconds) energized by external 12V battery with the rotor running. Exciter field and main field become electromagnets. The exciter field and main field retain the magnetism till the next operation and there will be no need for external battery use. Little Emf will be induced on main armature and voltage will be tapped on its terminals to the AVR. The AC current flowing to the AVR is rectified and regulated (increased or decreased) to the level that matches required output voltage level then fed to exciter field. This is a closed loop control system. It gives feed back which is inturn used as ref.

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    4. Thank you very much @Hlomza, @bijoy.. This is what i was looking for almost a day in all the sites, the explanation gave me the clarity now.

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  3. Why diode plate is fitted in between exciter armature and main field.exciter armature is already excited and connected on same shaft?

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    Replies
    1. Yes you are correct exciter armature get excited and produce A/C, Diodes convert this A/C to DC for the Main field

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    2. When rotor start rotating magnetic field cut by the rotor induce emf on rotor conductor is it correct ?

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  4. can someone help me ? i get an assignment about 'explain the principle of operation of a brushless type DC altenator' so is this the answer im searching for ?

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    Replies
    1. Operations are same, but at the out put thr will probably have 6 diodes to convert the output from the main alternator to DC.

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  5. 1.Initially Residual magnetism will be in exciter field or main field?

    2.Does Exciter stator or field is made up of permanent magnet?

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    Replies
    1. 1. both exciter field and main field having residual magnetism
      2. No

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  6. how to connect a GAVR-25A to a single phase brushless generator?

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  7. Greatly explained, no doubts at all

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  8. Hi, does brushless alternator is the same as we see in automobile alternators, or Is it that you are referring to you is Permanent magnet Alternator?

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    Replies
    1. Automobile alternators typically have brushes to power the field, and produce 3-phase AC power, so they are basically brushed type 3-phase alternators. This output is then fed into a diode bridge rectifier (just like the type mounted on the rotor of the brushless alternator) to produce DC voltage. The voltage regulator monitors the DC output voltage and varies the field current (power to brushes), which is supplied from the vehicle battery, to keep the voltage constant at about 14 volts, which is a good voltage to charge 12-volt lead acid batteries. Some newer cars apparently vary the voltage to avoid over-charging the batteries, but the principle is the same.

      Permanent magnet generators are not used because the field power cannot be changed: the only way to regulate the power of a PMG is to vary the speed which can't be done the way an alternator is typically attached to a vehicle engine.

      Vehicle alternators are not very efficient, but this is not a problem since the vehicle engine is much more powerful than the maximum alternator power.

      In some newer alternators, a brushless design is used, similar to the diagram above, and the field exciter is powered from a voltage regulator, but the voltage regulator (AVR) is powered from the vehicle battery so the issue of needing residual magnetism is avoided. Like the brushed alternator, the main field power is put into a rectifier bridge, and the resulting DC output voltage is what is monitored by the AVR, to result in a constant ~14 volts DC at any engine speed.

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  9. Also is the torque requirements same as that for a normal alternator and an Permanent magnet Alternator?

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    Replies
    1. A normal alternator torque depends on both the attached load and the field current, the field current being regulated by the internal voltage regulator. With no power applied to the field, there is very little resistance to turn a normal alternator. A permanent magnet alternator's torque is controlled by the load only, but still has more torque required than a normal alternator with no load applied. (I based this on a simple real world test - I have both a normal alternator and permanent magnet generator sitting here, and tried to spin both)

      In terms of running with an actual load, in theory, a PMG and a normal alternator will require the same amount of input power (speed x torque) to produce the same output power. However, since the normal alternator requires power to energize the field, assuming the alternator itself is ultimately powering the field, more input power is required to drive the alternator, vs a PMG with the same load applied.

      But, since the alternator's field can be varied, the alternator will require less torque as its speed increases, since the voltage regulator will reduce the field current as the speed increases to maintain the same output voltage.

      If you attach a static (resistive) load to the alternator, the power applied to the load will remain constant as the speed of the alternator increases, as the torque required to turn the alternator decreases, maintaining a constant power to the load. This means that if the field is powered at 0 rpm, the alternator will be almost impossible to turn. In a vehicle, the alternator is only powered after the engine is started, which is easy to do by switching off the field current while the key is in the "start" position.

      For the PMG with a static load connected, the torque required to turn the rotor will remain constant from 0 rpm up to any speed, however the power applied to the load will increase as the speed increases. This is why a PMG is not used as a vehicle alternator, since a PMG output power can only be controlled by the rotor speed.

      So even though a normal alternator is inefficient compared to a PMG, the ability to vary the output voltage by regulating the field is very useful.

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  10. Inside the rotor wheee the claw is, are there suppose to be windings inside the claw as well?

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  11. How we get output of the alternator practically, explain please

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  12. The brushless generator that uses capacitor how does it works?

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