Brushless Excitation System is a technology for providing the field current to the Synchronous Generator without using Slip Ring and Carbon Brushes. As this method of Excitation i.e. Brushless Excitation System do not use Carbon Brushes, therefore the losses because of contact resistance of carbon brushes is eliminated.
Also, in case of initial start up of Generator we don’t need to supply external power source to Brushless Exciter as is the case in Static Excitation System. Because in Static Excitation System the power for filed excitation is taken from the output of the Generator terminal through CT and PT. But in Brushless Excitation System we don’t need any start up power supply for field excitation (Why? Ask yourself after going through the whole post. I am sure you will be able to Answer.)
Brushless Excitation System consists of two major parts: Pilot Exciter and Main Exciter.
Pilot Exciter is Permanent Magnet, also called PMG (Permanent Magnet Generator) mounted on the rotor shaft. Armature winding of PMG is a stationary part which is mounted on Stator. Thus when rotor rotates, the filed flux created by Permanent magnet will also rotate and as Armature is stationary, there will be flux linkage in the Armature winding of PMG and an EMF will be induced across the terminals of Armature of Pilot Exciter. This Armature thus produces three phase AC power by using the mechanical energy of Rotor. This AC power from Pilot Exciter is then rectified by Thyristor Bridge. This DC is then feed to the field winding of Main Exciter which is wounded on Stator of main Exciter. Note that Permanent Magnet of Pilot Exciter is mounted on Rotor, Armature of PMG is stationary as it is wounded on Stator and Field winding of Main Exciter is stationary as it is also wounded to Stator of Main Exciter. Normally PMG has 16 poles, so the AC produced by PMG will be at a frequency of 400 Hz (f = PN/120, N =3000 rpm). Why?
Because of high frequency the ripple content in the rectified DC will be low. That is why PMG produces AC power at this higher frequency. Armature of PMG is shown in figure below.
Schematic diagram of Brushless Excitation System is shown in figure below.
Main Exciter of Brushless Excitation System consists of Stator and Rotor part. Rotor of Main Exciter is coupled with the Rotor of Turbo Generator on which armature winding is wound. On Stator, filed winding is wound. Normally there are six poles on the field, so it will produce AC power at a frequency of 150 Hz. This AC power is then converted to DC by Diode Bridge. Diode Bridge is mounted on two wheels which in turn is mounted on the Rotor shaft. This wheel is called Diode Wheel. Both diode wheels are connected to complete a bridge rectifier arm. Figure of whole Brushless Excitation System is shown below.
For each arm of Diode, two fuses are connected as shown in the Schematic diagram of the Brushless Excitation System.
The above picture shows the whole Brushless Excitation System. Note that Brushless Exciter is manufactured and supplied separately. Then Turbo Generator and Brushless Exciter rotor is coupled at the Site. Form the figure, wire is wound on the PMG, actually it is wound for making the material permanent magnet by field flashing. Field flashing is done by applying 20,000-24,000 AT for fraction of second. Two poles, North and South pole is created on the PMG filed by field flashing. Moving a bit left from PMG, you can see a Fan and this is provided for cooling purpose. Brushless Exciter is normally air cooled. Thus this Fan forces the air to pass through the Air Guide (see in figure) and main Exciter Armature for cooling. Also from the figure, you can see small windows on Armature of Main Exciter, it is provided for cooling.
Now the question arises how to control the field current of Turbo Generator?
See the schematic diagram of Brushless Excitation System. Thyristor Power Regulator controls the DC output of Thyristor Bridge Rectifier. Thus by Thyristor Power Regulator / Automatic Voltage Regulator we control the field current to the Main exciter due to which we can control the AC output of Main Exciter and hence the DC output of Diode Bridge Rectifier which is feed to the Field winding of Turbo Generator Filed winding. By this way we control the field current input to the field wing of Turbo Generator.
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