20-09-2014, 12:14 PM
3-PHASE ASYNCHRONOUS DRIVE IN TRACTION
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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.
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.
©. 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 © 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
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
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
[attachment=68236]
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.
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.
©. 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 © 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
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
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