3-PHASE ASYNCHRONOUS DRIVE IN TRACTION

3-phase ASYNCHRONOUS DRIVE in traction

                                        By

                                                      S.RAMAKRISHNA   

RAM_SIRAPURAPU@YAHOO.COM

CONTENTS:

                    1. ABSTRACT 

                                                                                         

                   2. INTRODUCTION

                                                  

                   3.POWER CIRCUIT

                                                    

                   4. ADVANTAGE OF 3-PHASE LOCOS OVER 

                            CONVENTIONAL LOCOS 

                                                        

                   5. INDUCTION MOTOR ELECTRICAL

                             REGENERATIVE BRAKING

                             

                    6.CONCLUSION                                                                

                    

ABSTRACT:

This paper deals with the advanced technology of replacing the D.C. Series Motors by the 3-phase Induction Motors in Traction and the Electric Regenerative Braking of the Induction Motor.

Series Motors are widely using in traction because of their fulfillment in the desirable characteristics of the traction motors. But their voltage and power ratings are limited by the fact that the sparking will occur when the voltage per commutator segment exceeds certain limit. The regenerative braking is difficult and also frequent maintenance should be required for the commutator and brushes, that is way these series motors can be replaced with induction motors.

Experiments showed that at low frequencies, induction motor develops high starting torque, drawing low starting current. And they have robust and cheap construction. Due to the advent of power electronic devices, now it is possible to change constant supply frequency to a variable frequency.

The pantograph draws single-phase A.C. supply from OHE and the NSR converts it to D.C. supply. The D.C. supply is then converted to 3-phase A.C. supply by means of ASR. Thus by varying the firing angle of SCRs in the NSR & ASR, Variable Voltage Variable Frequency supply can be fed to 3-phase induction motor.

During the train-braking period, the Kinetic Energy of hauling masses can be converted into electrical energy by the method of pole changing. Thus the supply is fed back to the OHE, the D.C. link supplies the required reactive power.

INTRODUCTION:

In the last one decade the electric traction technology has undergone sea change with 3-phase ac traction motors, gate turn off thyristor (GTO) & variable voltage variable frequency traction system.  This technology offers many advantages in addition to being high energy efficient.  The advent of 3-phase passenger electric locomotives on the Indian Railways is a quantum technological jump.

            The main feature of 3-phase electric locomotives is the use of 3-phase asynchronous motor drive, which is powered by microprocessor controlled variable voltage variable frequency ac power supply from the GTO based power inverter. The 3-phase asynchronous motors are far more reliable, compact & require very little maintenance attention besides this main feature. There are several other advanced features in the 3-phase locos namely regenerative braking and unity power factor, which result in reduced energy bills, higher adhesion, fine step less control on tractive effort, facility of pre-set speed, high tractive effort, reduced harmonics, less unsprung mass resulting in less rail wear & tear and less distortions to the track geometry.  It is crew friendly & maintenance friendly.

POWER CIRCUIT:

         The conventional locomotive was equipped with dc traction motors, while these locomotives run on 3- phase squirrel cage induction motors. Let us understand the power circuit, fig.1.  The pantograph draws power from OHE at 25kv.  The same is stepped down (to 2x2180V) using a fixed ratio transformer, NSR (line side inverter) converts 1-phase AC to DC, ASR (drive side Inverter) converts DC to 3-phase AC, which provides the supply to the induction motors.  In between the two is DC link shown by a capacitor.  DC link reduces the current ripples and provides a fixed voltage source to ASR. 

        This locomotive is capable of working in motoring & regenerating modes. In regenerating mode, power is transferred from locomotive to OHE and this results in brake application on the locomotive.  In regenerative mode, the motor acts as a generator and the power so produced is fed back to OHE.  The 3-phase AC from the motors in converted to DC by ASR.  NSR converts DC to 1-phase, which is stepped up by transformer & fed to OHE. The ASR ensures that the power is drawn at unity power factor by the locomotive.

           

             

                                         There are two power converters in               NSR, ASR & DC link along with its control circuit from a unit referred to as power converter. each locomotive.  One power converter feeds all the induction motors of one bogie.  The NSR & ASR modules are made up of GTO and  other associated devices. These modules are forced oil cooled.

Drive: Before the advent of power electronics, DC series motor was used for traction as it was easy to control and satisfied the requirement of providing high torque during starting.  However now it is becoming possible to control the induction motor characteristics using the above referred power converter in such a way that they provide high starting torque.

            The output from the power converter is variable voltage & variable frequency (popularly known as VVVF).  As the speed increases, the frequency supplied to the induction motor also increases. The induction motor has two regions (1) constant torque zone (ii) constant horsepower zone as shown in fig (2).

            The ratio between the voltage & frequency is kept constant resulting in high starting torque.  This is constant torque zone.

            After reaching the rated value the voltage is maintained constant and only the frequency of supply is increased.  This is the constant power zone.

Auxiliary circuit: The motors used for auxiliary circuits are 3-phase, squirrel cage induction motor and 3-phase power required is supplied by auxiliary converter (BURs).  The auxiliary converter converts 1-phase A.C to 3-phase A. C.

            The auxiliary converter performs similar functions as that of static converters used in some locos.  There are 3 auxiliary converters each of 100KVA.  In case one fails the other one takes over its load automatically, (without driver’s intervention).  The interesting feature of auxiliary converters is that it provides soft start for the compressors.  The system does not maintain the unity power factor and is not capable of regeneration.

            The 1-phase ac is first converted to dc.  There after, dc is converted to 3-phase ac and supplied to the induction motors.  The output is square wave.

  Control Circuit:  The control system is not relay based, instead uses the distributed micro -computer control.  (The system can be thought of as conceptually similar to the SCADA being used in TRD).  The control system used is ADTranz proprietary MICAS-52 (micro -computer-Automation system).

                The control system is user friendly for the drivers as well as the maintenance staff.  In case of any fault, message comes on the display screen of the driver, with necessary instructions to staff to act upon.  The appropriate control like isolation of

                                                     Fig.2

 subsystem also takes place automatically.  All the faults are logged with time stamp along with related locomotive parameters.  At the time of fault in non-volatile memory of computer, which can be down loaded using a PC for off-line analysis in workshops or sheds.  Seven bus stations carry out the control functions of the locomotive.  Each bus station consists of input/output cards, signal-processing cards, and micro -processor cards.  The bus stations communicate with each other, the status of various electrical parameters.  The control system programming is done using high-level graphic compilers (FUPLA/ALS) and also on some occasions, characteristics have been written in C++ or assembly language.

Bogies: Locomotive has bogies with fully suspended traction motors (TMs).  Or it may have bogies with axle hung, nose-suspended TMs.  These are many common features for the two bogies, as discussed below.

            The bogie has two stages of suspension.  The bogie frame rests on the axle box supported by springs.  The loco rests on two pairs of secondary springs located on the long beams of the bogies.  There is no center pivot or side bearer.  The vertical load is transmitted through two stages of suspension hydraulic dampers have been provided at various places.

            The tractive effort is transmitted from the axle to the axle box and then from axle box to bogie frame by guide rod.  Tractive effort from bogie frame of the loco is transmitted through traction bar.

ADVANTAGES OF 3-PHASE LOCOS OVER CONVENTIONAL LOCOS:

1). Squirrel cage induction motors, which have the following advantages, have replaced the dc series motors.

      (a). Less maintenance of induction motors as compared to dc motors.  No carbon         brushes are required to be replaced. Cleaning of commutator is not required.

       (b). Due to precise control of induction motors using power converters, it is possible to obtain improved adhesion (higher tractive effort) in comparison to dc series motors.

        (c). The rated voltage of induction motor is around 2000V, in comparison to 750V in dc series motors.  Hence for the same power the amount of current to be fed is low.  This results in lesser rate of induction motors, which means that lower unsprung mass.  This reduces the unsprung mass.

    2). Regeneration:  This loco is capable of regeneration, which results in direct         saving of 10 to 15% of energy.  Due to electrical braking, the wear of wheel & brake blocks reduces.

    3). Unity power factor:  This loco operates at smaller unity power factor.  The conventional locos operated at power factor around 0.8.  unity power factor results in (a) saving penalty imposed by SEBs (b) improved voltage regulations (c) It enhances the system capacity & (d) reduces the copper losses.

    4). Harmonics: 3-phase locos produce less harmonics, in comparison to    conventional locos.                         

    5). Diagnostics: In conventional locomotives, in case of any failure on line the driver has to do the troubleshooting, which results in considerable wastage of time.  In these 3-phase locos, a driver is provided with menu-based screen, where the fault messages are displayed with necessary instructions to staff to act upon.  The proper control action results in isolation of subsystem.  The fault & loco parameters are logged which can be used for “off line” analysis in workshops or sheds.

    6). Riding comfort:  This loco has two steps of suspension.  WAP5 has fully suspended motors. These features result in better riding comfort. This makes the drivers more comfortable and also reduces failures, which result due to vibrations in the loco.

7). Three-phase locos will require less maintenance & duration between two schedules is much longer than the conventional locos.  The failures in these loco are lower than the conventional locos.  Mean time, between failure/equipment (MTBF) is higher & availability of these locos is high.  

Induction Motor Electrical Regenerative Breaking:

Motor, without disconnecting it form the supply, is made to generate (instead of made to motor) and feed back energy to supply. Magnetic drag, produced on account of generation action, offers the braking torque. This method of braking is most efficient. In many cases, the transition from motoring action to generating action is smooth and without any switching operation. As soon as overhauling load drives the motor, it works as generator.

In case of induction motor Fig. 3, we find that for speeds above synchronous, motor torque becomes negative. Machine is now working as induction generator. Depending upon the speed, motoring or generation action of the machine will be automatic. By this method of braking, overhauling load may be prevented form rising much above synchronous. If the driving torque of load exceeds the maximum braking torque which motor can develop, the motor will cross over to unstable operation. Rise in speed will then decrease braking torque. Motor in that case in heading for run away conditions.

            We can bring the speed below synchronous only where arrangement of pole changing is available. If number of stator poles is increased, its new synchronous speed will be less than the actual running speed. Machine will now work as induction generator and bring the motor below its first synchronous speed till new operating speed will be little less than synchronous speed corresponding to increased number of poles. It will then continue to work as induction motor at this reduced speed. This method of electric braking is applicable to squirrel cage motors because rotor winding of slip ring motor cannot be reconnected for different number of poles

            As soon as motor speed exceeds its synchronous speed, it starts delivering active power P to the 3 - phase line.  However, for creating its own magnetic field, it absorbs reactive power Q from the line to which it is connected.  As seen, Q flows in the opposite direction to P.

            The active power is directly proportional to the slip above the synchronous Speed.  The reactive power required by the motor can also be supplied by a group of capacitors connected across its terminals.  Hence the capacitor bank must be large enough to supply the reactive power normally drawn by the motor.

Fig. 3

CONCLUSION

            We can conclude from this paper that dc series motors can be replaced by 3-phase asynchronous drive (i.e. Squirrel cage induction motor).

The 3-phase induction motor in traction is still in developing stage. The basic idea of operation and regenerative braking of the 3-phase induction motors in electric locomotives have been mentioned. The 3-phase A. C. Technology in traction is still in developing stage and now is running at some places in India. The technology of using linear induction motor is also in the development stage by using the magnetic levitation technique.