In conventional AC locomotives, 25 kV AC 50 Hz single phase supply received from OHE system is reduced to low voltage AC single phase supply (approximate 1000 volts) using step down traction transformer. The traction transformer has power rating of approximately 5000 KVA and depends on the type of locomotive. Traction transformer has different tapping in secondary winding and the same are connected to on-load tap-changers. Thus, the output voltage of the on-load tap-changer can be increased as per the requirement by selecting tapping towards more number of secondary winding turns and vice-verse. The output of on-load tap-changer is fed to thyristor based controlled rectifier block, which convert AC into controllable DC. Further, DC supply is fed to DC series motors through smoothing reactor, DC switchgear & starting resistance. For traction purpose, high starting torque is required during starting phase to start the train and further the value of developed torque is reduced gradually with increase in the speed. Reduction in torque at higher speed provides operation of the motor at rated power in the complete range of speed (Power=Torque * Speed). As per the torque-speed characteristic of DC series motor, it develops high starting torque at low speed and less torque at higher speed i.e. developed torque is inversely proportional the speed for DC series motor, which is suitable for traction purpose. Further, series-parallel control of traction motors is used in locomotive to achieve the required torque-speed characteristic for traction purpose. All DC traction motors of locomotive are connected in series through switchgear at the time of starting and supplied with low voltage DC. At higher speed, all DC traction motors are operated in parallel connection and supplied with rated DC voltage. Traction motors are connected to the wheel through reduction gears system, which further reduce the speed & increase the torque at wheel-axle. By changing the ratio of reduction gears, the same traction motor can be used for both freight and passenger locomotives.
Microprocessor controlled, IGBT based converter-inverter & three phase induction motor based energy efficient technology has been introduced in advanced trains. Such trains are capable to regenerate electrical energy during braking and the same is fed back to the OHE supply. Major electrical equipment associated with advanced traction system are given below:
1. Over Head Equipment (OHE) system to provide 25 kV, 50 Hz AC power supply
3. Lightning Arresters
4. OHE voltage sensing unit or Potential Transformer
5. Vacuum Circuit Breaker
6. Traction Transformer
7. Current Transformer
8. Single Phase Four Quadrant Traction Converter
9. DC link & Capacitor bank
10. Three Phase Four Quadrant Traction Converter
11. Three Phase Induction Motor
12. Auxiliary Converter to supply auxiliary load of train
13. Microprocessor based train control system
14. Wheel & Rails provide return path for OHE current
Functional descriptions of main equipment are given below:
It consists of power sub-station, switchgear & protection system and over-head line ( i.e. mast & foundation, arm assembly, tensioning pulley, insulators and conductors etc. ). Generally, two conductors are used in over head line; upper one is called messenger wire or catenary and lower one is called contact wire. To achieve good current collection at higher speed, it is necessary to keep the contact wire straight. It is achieved by supporting the contact wire from catenary wire. In physics and geometry, a catenary is the curve that an idealized hanging chain or wire assumes under its own weight when supported only at its ends. This curve superficially similar in appearance to a parabola, but it is not a parabola; it is a graph of the hyperbolic cosine. Contact wire is supported from the catenary wire at regular intervals by vertical wires known as "droppers" or "drop wires". Catenary and contact wires are also connected in parallel by separate jumpers for proper distribution of electric current in both wire. The whole system is then subjected to a mechanical auto tensioning device. The function of auto tensioning device (ATD) is to maintain a constant tension in OHE conductors i.e. contact wire and catenary's wire under varying temperature conditions, so that profile of OHE can be maintained for better current collection. On straight track, the contact wire is staggered or zigzagged slightly to the left and right of center at each successive support so that the pantograph wears evenly. Contact wire is made of hard-drawn copper with small amount of silver(Cu-Ag), cadmium, magnesium, tin etc. On a larger electrified railway, different areas of track are fed from different traction substations which are normally connected to different phases of power grid supply for proper load balancing. Generally, each traction sub-station covers a distance of about 30-40Km for feeding all the trains in that zone. Short circuit between two different phase supply of adjacent section fed from separate traction sub-station can be eliminated by providing isolation between them. For this purpose, neutral section is provided between two adjacent sections. Neutral section is defined as short insulated dead overhead equipment separating sections fed by two adjacent traction sub-stations. The transition from one section to another section is set up so that the pantograph remains in continuous contact with the contact wire.
A pantograph is a device mounted on the roof of an electric train to collect electrical power (current) through contact with an OHE system. The most common type of pantographs have a 'Z' style shape. The pantograph is held up by compressed air pressure or spring pressure and also applies pressure on the contact wire. Current is collected from overhead lines by pantographs and feed to traction transformer through vacuum circuit breaker. For smooth operation of electrical equipment, pantograph must maintain good electrical contact with overhead line under all running conditions. At higher speed, it is more difficult to sustain a good electrical contact with contact wire. Good electrical contact between pantograph and overhead line at higher speed can be assured by increasing upward pressure applied by pantograph on the overhead line. On the other hand, the wear & tear of pantograph and overhead line also increase with rise in upward pantograph pressure. Current collecting portion of pantograph used on 25 KV OHE system, wherein amplitude of pantograph current is around 300A, is usually made of graphite / metallized carbon. Graphite is a good dry-lubricant in presence of moisture content. It is a soft material as compared to OHE contract wire material which reduces the wear & tear of OHE contact wire. With low voltage OHE system used for metro services i.e. 750V DC / 1500V DC / 3000V DC, wherein amplitude of pantograph current may be more than 1000A, soft copper strip is used for current collection because copper has higher current collection density as compared to graphite. The wheel and steel rails provide the return path to the electrical current.
A lightning arrester or surge diverter is a device used on electrical power systems to protect the insulation and equipment of the system from the damaging effects of lightning. One terminal of lightning arrester is connected to high-voltage terminal and other with the ground terminal. Lightning arrester is used on power systems above 1000V to protect other parallel connected equipment from lightning and switching surges. It does not absorb the lightning; however, it diverts the lightning to ground terminal. All lightning arresters consist of metal oxide varistor (MOV) disk. Basically the MOV disk is a semiconductor that is sensitive to the voltage. At normal voltage, the MOV disk works as an insulator, but at higher voltage, it becomes a conductor. The MOV disk is a very fast acting semiconductor switch. Generally, gap less zinc oxide surge arresters are mainly used for surge protection. The micro structures of zinc oxide (ZnO) block highly doped with antimony represents a collection of billions of microscopic junctions of ZnO grains in series and parallel arrangement and separated by inter granular junctions. Each boundary wall between two ZnO grains works as a voltage sensitive switch that turn on at a voltage of 3V. If there are 1000 series junctions from top to bottom of a zinc oxide (ZnO) disk, it will turn on at voltage of about 3000 volts. In a lightning arrestor, many solid zinc oxide discs are arranged one by one to form a cylindrical stack. The number of zinc oxide discs used per lightning arrester depends upon the voltage rating of the system. However, the current carrying capacity of the surge arrester block depends on the cross section area of block. Lightning arresters are connected at the output of pantograph.
Potential transformer (PT) is a voltage step-down instrument transformer of low power rating which reduces the voltage of a high voltage circuit to a lower level for the purpose of measurement. It is used to measure the voltage of the system and connected in parallel to the line. They are designed to have an accurate voltage ratio, minimum phase error and negligible loading on the system to enable accurate secondary connected metering. Generally, the secondary voltage of the potential transformer is 110 V. Potential transformer is connected at the output of pantograph and gives feedback to the control circuit.
A vacuum circuit breaker (VCB) is used at the output of pantograph to isolate the main traction transformer from the OHE supply in case of internal fault or for maintenance work. In vacuum circuit breaker, vacuum is used as the arc quenching medium which offers high insulating strength and superior arc quenching properties. With 25KV AC OHE system, VCB are usually installed on the roof near the pantograph. Due to less maintenance requirement and capability of repeated operation, VCB is suitable for traction purpose. For industrial purpose, Vacuum circuit breakers are suitable for mainly medium voltage & power application i.e. upto 66KV and 100MW.
Traction transformer is used to step down the available 25 kV AC OHE voltage in the range of 1000 volts. The output of transformer secondary winding is connected to the input of single phase four quadrant traction converter through a contactor. During the operation of 4QC converter, the output voltage supply of traction transformer secondary winding is short circuited through IGBTs for a short time ( "ON" and "OFF" frequency of IGBT is kept in order of few KHz ) and re-open circuit to develop high voltage pulse equivalent to ‘Ldi/dt’ at the input of 4QC converter. Thus, the DC link voltage at the output of single phase 4QC converter can be boost up based on the operational requirement. To facilitate above activity, the traction transformer accompanied by 4QC converter are designed to achieve percentage impedance of around 40%. However, the traction transformer of conventional AC locomotive is designed for low value of percentage impedance (less than 10%) to achieve better voltage regulation from no load to full load. The percentage impedance of a transformer is the volt drop on full load due to the winding resistance and leakage reactance expressed as a percentage of the rated voltage. The higher percentage impedance of transformer can be achieved by increasing the air gap between transformer winding and core. The low voltage winding of traction transformer is kept near to the iron core to reduce the overall insulation cost. In regenerative braking mode, traction transformer works as step up transformer and feed power from transformer to OHE system. The transformer is equipped with various protections like over pressure, over temperature, oil flow detection, oil level detection etc. The transformer is filled with mineral oil as cooling medium. External air blowers are installed on the transformer radiators for forced air cooling of the fins filled with transformer oil.
A current transformer (CT) is used in electrical power system for stepping down alternating current of the system for metering and protection purpose in a convenient way. The primary winding of CT may be of either a single flat turn, or just a conductor or bus bar placed through a central hole and secondary winding consists of thousand turns of thin insulated wire. Thus, CT should not be kept in open circuit condition during the operation; otherwise, high voltage will be induced in secondary winding of CT, which may damage the insulation of CT. Most current transformers have a standard secondary rating of 1A or 5A. Generally, CTs are rated as 100/5A means that the primary current is 20 times greater than the secondary current, so when 100A is flowing in the primary conductor it will result in 5A flowing in the secondary winding. For an ideal CT, the angle between the primary and reversed secondary current vector should be zero. But for an actual CT, there is always a difference in phase between primary & secondary winding due to the fact that primary current has to supply the component of the exiting current. The angle between the primary and reversed secondary current phases is termed as phase angle error in current transformer. Similarly, primary current does not exactly equal to the secondary winding current multiplied by turns ratio because primary current has to contribute the core excitation current. This error in current transformer is called ratio error. Error in Current Transformer can be reduced using a core of high permeability and low hysteresis loss magnetic materials. CTa are used to measure current of transformer primary winding and induction motors.
The output of traction transformer is supplied to the single phase four quadrant traction converter through a contactor. Each leg of 4QC converter consists of silicon based Insulated Gate Bipolar Transistor (IGBT) with anti-parallel arrangement of power diode. A four-quadrant stage is the dedicated part of a power converter used to manage load voltage and current in the four-quadrant area. Basically, the design of 4QC is based on two controlled switching device's bridges mounted in anti parallel arrangement. During the traction or motoring mode, the 4QC works as boost converter (i.e. step-up) and convert the single phase AC supply available at the transformer secondary into controlled DC voltage for the DC link. The term 4QC signifies that the phase angle between voltage and current is freely adjustable while motoring as well as braking. During train operation, single phase 4QC operates in first quadrant (during traction or motoring mode) and second quadrant (during generating or braking mode) only. The polarity of the DC link remains fixed during the operation of traction motor in all four quadrants.
During generating or braking mode, 4QC takes current (energy) from DC link and converts it into the single phase AC supply, so the same can be fed back to the OHE system through main transformer. During the operation of 4QC as boosting converter, the two IGBTs S2 & S3 are switched ‘ON’ simultaneously (IGBTs S1 & S4 are kept switched ‘OFF’) for a short duration in positive half wave of AC supply. When IGBTs are switched ‘ON’, a short circuit current flow through the secondary winding of transformer and follow the paths – A1,J1,D1,S3,J2,A2 and A1,J1,S2,D4,J2,A2 respectively in positive half wave of AC supply. As soon as the current has reached the desired value of IGBT, it is switched ‘OFF’. This induces a high voltage pulse (equivalent to the Ldi/dt, where L is the self-inductance of the transformer) and the same applied to the DC link capacitor through the diodes. Capacitors oppose the rise in applied voltage and consequently current flow from transformer to DC link which charge the DC link at high voltage. Similarly, IGBTs S1 & S4 are switched ‘ON’ simultaneously (IGBTs S2 & S3 are kept switched ‘OFF’) for a short duration in negative half wave of AC supply. When IGBTs are switched ‘ON’, a short circuit current flow through the secondary winding of transformer and follow the paths – A2,J2,S4,D2,J1,A1 and A2,J2,D3,S1,J1,A1 respectively in negative half wave of AC supply. The switching frequency of the IGBTs is kept in order of few KHz. As a consequence, current continue to flow in DC link circuit, through freewheeling diodes and the output voltage of DC link is maintained more than the maximum value of input AC supply. The power factor at traction transformer input can also be improved by adopting suitable switching pattern of IGBTs.
Before the development of IGBT, Gate Turn Off (GTO) thyristor was used as high-power semiconductor switching device. Like thyristor, a GTO is a current controlled, minority carrier, bipolar device which consist of four p-n-p-n layers & three junctions. A GTO can be turned on by a gate signal, and can also be turned off by a gate signal of negative polarity. GTO thyristors suffer from long switch off time and this restricts the maximum switching frequency of GTO to approximate 1 kHz only. Additional snubber circuits are also required to limit the di/dt, dv/dt parameters during turn-on and turn-off. MOSFET is also a switching device and suitable for low power Switched Mode Power Supply(SMPS), works at low voltage and high switching frequency circuit (>200kHz). MOSFET has low switching loss but its on state resistance per unit area is high which results into higher conduction loss. On the other hand, bipolar junction transistor (BJT) has higher switching losses but lower conduction losses. An insulated-gate bipolar transistor (IGBT) combines the advantage of MOSFET and BJT. IGBT combines an isolated-gate FET for the control input and a bipolar power transistor as a switch in a single device. Gate control circuit of IGBT is much simpler and requires low control power. No snubber circuit is required for IGBT operation. IGBT is a suitable switching device in the frequency range of 1KHz to 10 KHz. Due to above advantages, IGBT has replaced GTO from the traction system. The IGBT is used in medium to high-power applications like switched-mode power supplies, traction motor control and induction heating etc. A single chip of IGBT has voltage rating of approximate 1.1 KV. A module of IGBT may have more than one IGBT chip connected in series-parallel connections. IGBT modules are available in the voltage range of 1.8KV, 3.3KV, 4.5KV & 6.5 KV with current rating upto 1200A. The switching loss of IGBT contributes major portion of the total losses and its value increases with increase in switching frequency, however conduction loss of IGBT depends on the load current.
During the generating or braking mode, power flow in reverse direction i.e. from DC link to OHE system. Now, single phase 4QC converter works as pulse width modulation (PWM) inverter and converter DC link voltage into alternating supply voltage of 50Hz (equal to OHE supply frequency). The same alternating voltage is step-up by the main transformer and feed back to the OHE system.
Adequate capacity of capacitor bank is placed at the DC link to reduce the ripple content of the DC voltage. A dc voltage sensor is also installed at DC link to measure its voltage for control and safety purpose. A bleeder resistance is also connected in parallel to discharge the DC link capacitor. The DC output of single phase 4QC converter stores energy in the capacitor bank. This provides the stiff supply voltage on DC link and allow the use of voltage source inverter (VSI) at the output of DC link. The vast majority of drives are VSI type with PWM voltage output. In metro train, the DC link voltage is selected in the range of 1500V DC.
During the traction or driving mode, three phase 4QC converter works as voltage source inverter and convert DC link supply into three phase AC supply voltage of variable magnitude as well as variable frequency. The output of the 4QC inverter is fed to the three phase induction motors. The maximum possible amplitude of the phase-to-phase output voltage depends upon the magnitude of the DC link voltage and the modulation index. When the reference modulating signal is a sinusoid of amplitude Am, and the amplitude of the triangular carrier wave is Ac, the ratio of ‘Am/Ac’ is known as the modulation index. The modulation index controls the amplitude of the output voltage and harmonic content in the output voltage. However, the modulation ratio is the ratio between the frequency of the triangular carrier wave to the frequency of the reference modulating wave ‘fc/fm’. Modulation ratio is related to the dominant harmonic frequency in output voltage. The RMS value of the output voltage can be controlled by varying the modulation index and frequency of the output voltage can be increased by increasing the frequency of the reference modulating signal wave.
In sinusoidal pulse width modulation (SPWM), generation of the desired output voltage is achieved by comparing the desired reference waveform (modulating signal) with a high-frequency triangular wave (carrier signal). Depending on whether the reference sinusoidal signal voltage is larger or smaller than the triangular carrier waveform, comparator circuit provides either the positive or negative voltage to the IGBTs gate control circuit. When sinusoidal reference wave has the magnitude higher than that of the triangular carrier wave, the comparator output is high which turn ‘ON’ the related IGBTs. If the magnitude of sinusoidal reference wave is lower than that of triangular carrier waveform, the comparator output is low which switch ‘OFF’ the related IGBTs. In SPWM, each pulse has different width. With a sufficiently high carrier frequency (fc/fm=21), the frequency of harmonic content in output voltage is shifted to the higher frequency range and thus do not propagate significantly in the load due to presence of the inductive elements. It may be noted that the lower order harmonics can be shifted toward higher order harmonics to reduce the overall voltage distortion by proper selection of switching control (ratio of fc/fm); however, increase in the ratio of ‘fc/fm’ also increases the switching losses in the IGBTs. Such a situation is acceptable in most cases as the harmonic voltages of higher frequencies can be satisfactorily filtered using lower sizes of filter chokes and capacitors.
When triangular carrier wave has its peak coincident with zero of the reference sinusoidal, the number of pulse per reference sinusoidal voltage cycle (N) will be equal to
N= (Triangular carrier wave freq. fc) / (Sinusoidal reference wave freq. fm)
When zero of the triangular carrier wave and reference sinusoidal wave are coincident, the number of pulse per reference sinusoidal voltage cycle (N) will be equal to
N= ((Triangular carrier wave freq. fc) / (Sinusoidal reference wave freq. fm)) -1
For modulation index less than one, harmonics of order (N±2) have largest amplitude in the output voltage and accordingly the filter may be designed. In three phase system, it is appropriate to adopt fc/fm=3k to suppress the even harmonics, here k= Integer number. Generally, the value of modulation index ‘m’ remains less than or equal to 1. However, for m>1, there are periods of the triangle wave in which there is no intersection of the reference modulating wave and the triangular carrier wave. However, a certain amount of such ‘over-modulation’ is often allowed in the interest of obtaining a high magnitude of AC output voltage even though the harmonic content in the output voltage of inverter increases. Generally, three reference sinusoidal waves are used for three phase SPWM inverter. Each sinusoidal waves have phase difference of 120° respectively. The frequency of these sinusoidal waves is chosen based on the required inverter output frequency (50/60 Hz). The carrier triangular wave is usually a high frequency wave of several KHz.
In a Sinusoidal PWM Inverter, the output AC voltage between two phases may be calculated based on the following formula (considering modulation index=1) :
VLINE VOLTAGE = (√3 × VDC) (2 × √2) = 0.6124 × VDC
During the generating or braking mode, three phase induction motors work as asynchronous induction generator and convert kinetic energy of train into electrical energy. The generated electrical energy is supplied to three phase 4QC converter which converts it into DC voltage and supply to the DC link. Further, single phase 4QC converter works as pulse width modulation (PWM) inverter and converts DC link voltage into AC supply voltage of 50Hz (equal to OHE supply frequency). The same alternating voltage is step-up by the main transformer and feed back to the OHE system.
Motor coach of metro train has four traction motor per coach i.e. one traction motor on each axle. Similarly, locomotives have one traction motor on each axle. In a metro train, the number of motor coaches may be 33 to 66% of the total number of coaches. With Variable Voltage Variable Frequency (VVVF) Control, squirrel cage three phase induction motors of about 250KW are used in metro train. Generally, each three phase PWM converter feeds two three phase induction motors connected in parallel. In case of fault in any traction motor, the concern PWM converter and connected two traction motor get switched off automatically, while remaining propulsion system work normally. The three phase PWM converter have harmonics content in the output voltage, which lead to overheating of traction motor. During the train operation, traction induction motors are operated with its short time rating, a wide range of frequency & speed, which make its design much complicated as compared to general purpose induction motor. During the train operation, traction motors are operated in following four quadrants:
1. Quadrant I - Traction or motoring mode (i.e. positive torque) in forward direction (i.e. positive speed),
2. Quadrant II - Generating or braking mode (i.e. negative torque) in forward direction (i.e. positive speed),
3. Quadrant III - Traction or motoring mode (i.e. negative torque) in reverse direction (i.e. negative speed),
4. Quadrant IV - Generating or braking mode (i.e. positive torque) in reverse direction (i.e. negative speed),
The output power of traction motor at any speed can be calculated using following formula :
Output Power of T.M. (kW)= Developed Torque of T.M. at given speed (kN-m) * Angular Velocity (radians per second)
Outpur Power of T.M. (kW)=(Developed Torque of T.M. at given speed (kN-m) * 2 * π * Rotor Speed (rounds per second))
Outpur Power of T.M. (kW)=(Developed Torque of T.M. at given speed (kN-m) * Rotor Speed (r.p.m.))/9.55
The auxiliary converter is used to supply auxiliary load of the train. It is connected either from the DC link or additional auxiliary winding provided in main transformer for auxiliary load. It consists of IGBT based inverter, isolation transformer, sine wave filters and switchgear devices. Sine wave filters are low pass frequency filters, which convert the rectangular pulse width modulated (PWM) output of auxiliary converter into a smooth sine wave voltage. The output of auxiliary converter are 3-Phase 415V supply, 1- Phase supply and 110V or 24V DC supply; which are used to feed the load of cooling blowers, battery charger, control electronics and light & fan load etc.
Before understanding of VVVF control, facts related to functioning of induction motor are described below :
1. Induction motor develops maximum running torque while the rotor resistance is equal to the rotor reactance i.e. R2=S*X2, where S=slip. The value of rotor reactance X2, depends upon the rotor supply frequency and rotor supply frequency depends upon the slip of the motor i.e. fr = S × fs.
2. The maximum torque developed by induction motor is independent of the rotor resistance while the slip at which it occurs is directly proportional to it.
3. Torque developed by a squirrel cage induction motor is the same whenever slip-speed is the same
4. The torque developed in induction motor is proportional to the product of flux per stator pole, rotor current and the power factor of the rotor.
5. During the normal operation, both the stator and rotor flux move with the synchronous speed and relative motion between them is always remain zero.
As described above that the torque developed by a traction motor is proportional to the air gap flux, rotor current & the power factor of the rotor current however the speed of induction motor depends upon the frequency of the applied voltage. The value of air gap flux inside induction motor depends upon the ratio of applied voltage and its frequency. Therefore, high starting torque can be developed by induction motors at any speed if the flux in the air gap at that speed is maintained to its desired level. Three phase four quadrant converter provides the variable voltage and variable frequency supply to the induction motor. In practical, supply voltage and frequency are varied in the same proportion in order to maintain a constant flux in the air-gap. If voltage is reduced to half then the frequency is also reduced to half to maintain a constant flux in the air-gap. At reduced supply frequency, the rotor frequency also reduces in same proportionate ( fr = S × fs) which results into low rotor reactance X2 (i.e. X2 =2×π×fr×L) and improved power factor of rotor current ( p.f.=R2/√[(R2)2+(S×X2)2] ). Under these conditions, shape of the torque/speed curve remains the same but its position along the X-axis (i.e. speed axis) shifts with frequency. Since the shape of the torque/speed curve remains the same at all frequencies, it follows that torque developed by a squirrel cage induction motor is the same whenever slip-speed is the same. The difference between synchronous speed of flux and actual rotor speed ( ‘Ns – Nr’ ) is called slip-speed. The slip is defined as S=(Ns–Nr)/Ns and %slip is defined as S=(Ns–Nr)×100/Ns. The normal full-load slip of the traction induction motor is of the order of 1 to 2%. The synchronous speed of stator flux (Ns) can be calculated by using formula:
Synchronous Speed = 120×f / P r.p.m., where f is supply frequency and P is number of stator poles.
Torque (N-m) developed by traction motor (TM) can be converted into tractive effort (TE) or force (Newton) on wheel
by using following formula:
T.E. or Force developed by train = (No. of TM × TM Torque × Gear Ratio × Gear Efficiency) / Radius of Wheel;
To full fill the requirements of train operation, high starting torque is required at low speed and less torque is required at higher speed. Therefore, tractive effort of train is kept constant upto required speed and beyond that, the developed torque is reduced inversely to the speed. The tractive effort developed by the traction unit or locomotive should have shape as given below :
To achieve the required Tractive effort vs Speed graph, traction motor control can be classified into following category:
In this control of induction motor, the ratio of applied voltage and frequency is kept constant to maintain the value of flux constant which results into maximum developed torque at the time of starting. At the time of starting, low AC voltage of low frequency is applied to the induction motors through SPWM inverter. As the traction motors start and attain the speed, the voltage and frequency applied to the induction motor are also increased in same proportionate considering the V/f ratio constant. Thus, continuous maximum torque can be developed upto required speed. In above figure, section ‘OA’ indicates the constant torque control. This type of control is also known as volts/hertz control or constant tractive effort control.
The maximum output voltage of SPWM inverter depends upon the DC link voltage. Therefore, the value of inverter output voltage can not be increased beyond a certain limit. This restriction limits the constant V/f zone at point ‘A’. Beyond this point, the applied voltage across induction motor is kept constant however the frequency of the supply voltage is increased in proportional of required speed. With the increase in supply frequency only, the value of air gap flux is reduced (i.e. V/f ratio reduce). Therefore, the developed torque of traction motor is also reduced in inversely proportional to the speed. Thus, it can be concluded that in this zone, traction motors operate at constant power (Power (kW)=Tractive Effort(kN) × Speed(m/s)). The value of traction motor power & its operation period in this zone decide the power rating of traction motor. This region is also called the field weakening because in this zone, the value of air gap flux reduces with increase in the speed of traction motor.
During generating or braking mode, the synchronous speed of rotating stator flux in induction motor is always kept lower than the actual rotating speed of the rotor by -1 to -2% slip. In generating mode, traction induction motors work as asynchronous genrator (with negative slip i.e. S<0) and active power flow from induction motor to SPWM inverter. However, the reactive power required to develop magnetic flux in stator of induction motor is supplied by SPWM inverter. Therefore, the synchronous speed of stator flux can be controlled by governing the switching pattern of IGBTs in SPWM inverter. Now, induction motors work as asynchronous induction generator and convert kinetic energy of the train into electrical energy of variable frequency. Now, three phase 4QC works as converter and converts the electrical energy of variable frequency into DC voltage and feed to the DC link. Further, single phase 4QC converter works as pulse width modulation (PWM) inverter and converter DC link voltage into alternating supply voltage of 50Hz (equal to OHE supply frequency). The same alternating voltage is step-up by the main transformer and feed back to the OHE system. VVVF control is also used in windmills asynchronous generator because windmills run at non-fixed speed.
The next development in three phase technology is use of permanent magnet synchronous motor (PMSM) as traction motor. In a 3-phase PMSM, the rotor consists of permanent magnet. During the operation, PMSM rotor always operates at synchronous speed. Hence, the relative motion between the stator flux and rotor is zero, however as the load on the motor is increased, the rotor progressively tends to fall back in phase by some angle (not in speed as induction motor) and still continues to run synchronously. As the relative motion between the stator flux and rotor is zero; no voltage induces in the rotor of PMSM, which results into no iron & copper losses in the rotor. Therefore, PMSM has higher efficiency as compared to squirrel cage induction motor (SCIM). In addition of above, PMSM has higher power rating for a given size (higher power to weight ratio), low losses, comparatively less weight & volume, low noise and higher starting torque. Due to above advantages, PMSM is mainly used in high speed trains and allows the use of higher output power motor on each axle. SCIM traction motor has efficiency of 94-95% and PMSM has efficiency of 96-97%. However, the PMSM is more costly as compared to SCIM and requires a complex 4QC control system.
Silicon carbide is comprised of silicon and carbon in equal shares coupled with covalent bonding. A single crystal SiC is extremely hard; in fact the third hardest substance on the earth. Silicon Carbide (SiC) devices have several advantages i.e. wider bandgap (2.2 to 3.3 eV), high-breakdown voltage, high-operating electric field (around 10 times higher than that of Si), higher thermal conductivity (about 3 times higher than that of Si), high-operating temperature, lower resistivity, high-switching frequency operation and low switching & conduction losses. Resistance of high-voltage devices is predominantly determined by the width of the drift region. A SiC semiconductor die is much thinner due to its high dielectric strength. Therefore, SiC can reduce the resistance per unit area of the drift layer as compared to Si for the same breakdown voltage i.e. less conduction loss. Due to above properties, SiC devices allow to operate at higher temperatures (more than 400 degree centigrade which is much higher than the maximum permissible junction temperature of silicon i.e. 150 degree centigrade), higher current density (2 to 3 times the maximum current density of Si devices) and higher blocking voltages than Si power devices. However, the cost of SiC switching devices is much higher than that of Silicon semiconductor switching devices.
Commercially available SiC power devices are Schottky diodes, JFETs, MOSFETs and BJTs. Schottky diodes are majority carrier unipolar devices, and do not have reverse recovery phenomena which is favourable characteristic for high-voltage and high-frequency applications. Schottky diodes is a metal-semiconductor junction diode with no depletion layer.
The SiC switching module used for traction purpose at the place of IGBT-Diode module consists of SiC MOSFET (SiC Metal-Oxide Semiconductor Field-Effect Transistor) with anti-parallel arrangement of SiC-SBD (SiC Schottky Barrier Diode). The SiC MOSFET, as a majority carrier switch, effectively eliminates the minority carrier current tail experienced with silicon IGBTs, resulting in much lower switching losses (turn-on & turn-off losses) and facilitates much easier paralleling of switching devices. Therefore, the elimination of the tail current is a significant advantage of the SiC MOSFET over the silicon IGBT. Low switching & conduction losses, higher thermal conductivity and higher operating temperature of SiC switching devices as compared to Si IGBT has the potential to significantly reduce the size and number of heatsinks, as well as a reduction or elimination of cooling fans. The SiC converter also has higher efficiency due to their low switching and conduction losses.
Use of silicon carbide switching devices allow the operation of three phase 4QC converter at higher switching frequency which results into overall reduction of harmonic content in the output voltage and shifting of harmonic content to the higher frequency range. Therefore, harmonic current do not propagate significantly in the load (i.e. traction motors) due to presence of the inductive elements. This results into reduction in traction motor losses (i.e. low temperature rise or higher rating of motor).