ALTERNATORS

ALTERNATORS:

There are two rating of alternators- 18kW & 25kW. 18kW is used in under slung coaches & 25kW is used in RMPU coaches. Alternators those are used here are of special type that is brushless statodyne type of alternator in which neither commutator nor brushes are used. Two windings – field winding & stator winding are done on the stator. Field windings are concentrated in the two bigger slots. And stator windings are distributed in the small slots so each field coin spans half the total number of stator slots. Stator coils is connected in parallel star and field coils in series. Rotor consists of stacked stamping resembles a cogged wheel having 12 sets of teeth and slot uniformly distributed on the rotor surface.

Principal of operation:

Core of the stator which is completely embraced by the field coil will retain a residual magnetism, if excited by a battery once. This residual flex finds it path through the rotor. When the rotor is rotated teeth and slots offer a periodic variation in the reluctance of flux path. Due to this variation in reluctance, the flux linked with the phase coil will bar7 periodically and include an alternating voltage in the stator coil. The frequency of the include voltage depends on the speed of the rotor. The magnitude depends on the speed of the rotor and level of residual magnetism. The residual flux is strengthened by a positive feedback from the induced voltage by the regulator to attain the desired output voltage. In case the output voltage exceeds 150 volt (for 25kW) due to failure of any regulating components the over voltage protection unit will cut off the field supply.

Description:

Rail generating equipment’s for AC coaches consists of:

        i.            Brushless Alternator Type-  BA 18130 for 18KW    

               BA 23130 for Optimized (22.75KW)

               BA 25130 for 25 KW

      ii.            Universal Regulator type-    Mar  181330 for 18KW

                                             Mar 25130 for KW

                                             (22.75 KW and 25KW)

The DC output of the equipment’s supply to coach loads consisting of:

a.      Incandescent lamps.

b.      Invertors to supply loads to florescent lamps and Roof mounted AC package unit,

c.       Air circulating fans,

d.      A bank of lead acid/alkaline batteries of suitable voltage and ampere hours,

e.       Motors for compressors, condenser and evaporators for under slung AC coaches,

f.       Other electrical equipment’s in AC coaches,

Alternator data:

Voltage                                                                                    : 130 Volts Dc

Current                                                                                     : 138 Amp DC for 18 KW

                                                                                                 : 175 Amp DC for 22.75 KW

                                                                                                 : 193 Amp DC for 25KW

Cut in Speed                                                                            : 400 RPM

Min. Speed for full O/P(MFO)                                                : 750-800 RPM

Max. Speed                                                                              : 25000 RPM

Max. Test Speed                                                                      : 3050 RPM

RECTIFIER/REGULATORUNIT:

The R/R panel has following functions:-

a.      Rectifying 3- phase AC output of the alternator to DC using full wave diode rectifier Bridge.

b.      Regulating the voltage generated by the generator at set value.

c.       Limiting the output voltage to the set value.

Power Rectifier:

               This consist of six member of silicon diode connected in three phase full wave bridge. The three phase AC output of the alternator is rectified by these diode to obtain DC output at terminal +DC and –DC of the regulator/rectifier unit. Each diode is protected against transient surge voltage by capacitor C1. The whole bridge is protected against high frequency surges by capacitor C3 and the DC output is filtered by capacitor C and C2.

Voltage regulator:    

               The voltage include in the phase winding of the alternator depends on the speed of the rotor, excitation current and the load current. The speed of the rotor varies with the speed and load current varies depending on the load requirement in the coach. In the absence of a regulator, the alternator terminal voltage cannot be kept constant when the speed and load are varying.

 When the voltage regulator is connected, it will monitor the output voltage and pass a command to the control circuit to reduce the excitation current as soon as the output voltage exceeds the set value. The regulator has two input and an output, one input is the voltage input taken from the alternator output terminals and the other is current feedback taken from the CTs. The output of the regulator is the excitation fed to the field winding.

Voltage detector (DT):

                The voltage detector serves the function of providing necessary ‘error signal’ for voltage regulation. It consist of a network of zener diode, potential divider and rheostat. When the output voltage of the alternator exceeds the set value, the zener D8 breaks down and this will send a current through winding 10-11 of magnetic amplifier. This causes an increase in the impedance of load winding hereby decreasing field current maintaining alternator at set value.