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PT.22: TIIE BENEFITS OF MODERN 3-PHASE ELECTRIC LOCOS THE EVOLlITION OF ELECTRIC RAILWAYS Up until very recently, the series DC motor has been king for electric traction. It has very high starting torque and will run over a wide speed range. But ultimately the series DC motor will be replaced by the more efficient 3-phase induction motor. By BRYAN MAHER Three-phase induction motors in electric locomotives are not new. They were used as far back as the 1890s and were traction supplied from 3-phase overhead trolley wires. But the only way to control their speed was by pole switching. This was clumsy, made for very jerky acceleration and would only let the loco operate efficiently at a few fixed speeds. So while a few countries persisted for some time with 3-phase traction, notably Italy (see Pt.8, SILICON CHIP, June 1988), all electric and diesel electric locos have used series DC motors which are relatively easy to control in speed and torque. Some locos have used low frequency AC to feed the series traction motors (notably the 25Hz US system) but whether the traction motors have used AC or DC, they all have the drawback of using brushes and either commutators or sliprings. Series motors for traction are also are large and very heavy. Their brushes and commuators require considerable maintenance and the ingress of moisture, dirt and brake dust to the motors causes lots of problems. Heavy traction motors also place a limitation on maximum operating speeds. Part of the motor weight hangs on the wheelset axle and this unsprung weight degrades the bogie riding quality over track undulations. Another big disadvantage of series DC traction motors is low efficiency at low speeds - they draw very large currents while the actual power being developed is quite low. This is a particular problem in diesel electric locomotives because the diesel engine has to run at high speeds to generate the high currents required at starting. This means that the alternator must be over dimensioned to deliver those very high starting currents. 3-phase squirrel cage motors Invented in 1888 by Nikola Tesla, squirrel cage AC induction motors have the highest power to weight ratio of any electric motor. Moreover, the rotor has a very simple construction, consisting of a simple laminated silicon steel core with slots carrying bare copper bars, all short-circuited together at both ends (very similar to a squirrel cage, hence the name) and with no insulation. Since the rotors are very robust INDUCTION MOTORS ARE very simple in construction as these photos of a stator and rotor show. Since there are no brushes, no commutator or sliprings, and virtually no insulation on the rotor, the motor is utterly reliable and the only maintenance required is infrequent bearing replacement. 102 SILICON CHIP and · have no brushes and no commutators they suffer very little damage from vibration and are virtually maintenance-free. Rotor faults are rare in 3-phase AC squirrel cage motors because of their simplicity of construction, whereas the incidence of breakdown in the armatures of DC motors is much higher because of their complex electrical structure. Rotating magnetic field The 3-phase currents supplied to the stator coils of an induction motor set up a rotating magnetic field in the air gap between stator and rotor (the stator is so-named because it is stationary). This rotating magnetic field spins at the so-called "synchronous speed" which is proportional to the supply frequency, and inversely proportional to the number of stator poles. For example, for a 2-pole motor on 50Hz supply the "synchronous speed" is 3000 RPM while in a 2-pole motor on 25Hz supply the magnetic field rotates at 1500 RPM. An induction motor always spins a little slower than the rotating magnetic field. Typically, a 2-pole motor on 25Hz supply at full load runs at 1450 RPM, only 3.3 % slower than synchronous speed. At no load such a motor can run as fast as 1495 RPM. Motor torque/speed The torque developed by a squirrel cage motor depends on the difference between actual rotor speed and synchronous speed. Fig.1 shows the example of a 2-pole motor on 25Hz supply. Maximum torque and best efficiency occurs at a speed about 3% less than synchronous speed. In effect then, induction motors run at a virtually constant speed. For stationary motors, as in factories and workshops, this is ideal for driving drills, grinders and other machinery. But for traction motors in locomotives the requirement is controlled variable speed with high starting torque at low speeds. Clearly, induction motors on a fixed frequency supply present big problems for traction use. 3-phase locomotives THIS MODERN DIESEL 3-PHASE shunting locomotive is made by Brown Boveri. Shunting locos require very high starting tractive effort and the 3-phase inverter drive system is ideal for this, giving fuel savings of more than 30%. a more or less constant speed on a fixed frequency AC supply, the obvious need for locomotive use is a variable frequency supply which would let them run at any desired speed. Using a 2-pole motor as an example, a very high running speed could be attained with the AC supp- ly set at 50Hz; half that speed at 25Hz, one quarter with a 12.5Hz AC supply and so on. For starting, where the highest torque is required, we could use an AC frequency as low as 1Hz or 2Hz. Maximum torque would then be exerted at zero rotor speed - ideal for starting a train. +TORQUE WORKING POINT ORIVING MOTORING ORIVE SUP ZERO TORQUE L-- - - ----1..!.45-00---J/ - 1 = 5 5 = 0 - - - - -- SP-EE - O-(R~PM-) 1500 RPM=/ SYNCHRONOUS SPEED BRAKE I sup-ni-~ WORKING POINT BRAKING - TORQUE FIG.1: THE TORQUE VERSUS SPEED characteristic of a typical 3-phase induction motor, this one being a 2-pole version operating from a 25Hz supply. Note that the motor operates efficiently over a very narrow rev range. If the speed is to be varied, so must the input frequency. Since induction motors do run at AUGUST 1989 103 THIS POWERFUL GERMAN LOCO uses 3-phase inverter drive and is rated at 5600kW (7500hp). Using a Bo-Bo axle arrangement, it weighs only 84 tonnes and yet generates a starting tractive effort of 340kN. As well, with a low frequency AC supply, the motor efficiency remains high even at very low rotor speeds. Though these facts have been well known for many years, the problem was how to achieve a variable frequency AC power supply with a capacity of several megawatts or more. The French railways made a creditable attempt with their class CC1400 electric locomotives of which 20 were built between 1955 and 1959. Though these showed the way for 3-phase traction, they were unsuccessful because the only way to achieve a variable frequency AC supply in 1955 was by a variable speed motor-alternator set carried in the locomotive. The advent of high power silicon controlled rectifiers (SCRs) opened the way to the design of DC-AC inverters which could produce 3-phase outputs with an approximate sinewave shape. These could work at any frequency depending only on the rate at which the SCRs are triggered. There was just one catch with the early high power thyristors. For locomotive use inverters of two to 10 megawatts rating are re104 SILICON CHIP quired and until about 1980 the highest available power thyristors were all too slow in response for inverter service. Nowadays, in their locomotives, the ASEA-Brown Boveri Company uses GTOs (gate turn-off SCRs) rated at between 2000 and 3000 amps and 2kV to 4.5kV, with switch-on times of 5-15µs and a current rise time of 500A/µs. The use of a single large GTO for each phase is preferred over multiple semiconductor devices in parallel for reasons of cost, weight and space. Traction circuit As outlined in previous articles in this series, typical electric locomotives operate from a single phase high voltage AC overhead supply of either 11, 15, 25 or 50kV at frequencies of either 16.6, 25, 40, 50 or 60Hz, depending on the system used in various countries. In every case a transformer steps the high voltage supply down to a convenient voltage of around 500V AC. This is then rectified and fed to the traction motors. The motors themselves are controlled in speed by varying their field currents or by varying their DC supply voltage. In a locomotive with 3-phase traction motors the high voltage step-down transformer and rectifier are still required but in this case the DC supply is regulated to a constant voltage. This voltage feeds a DC-to-AC 3-phase bridge inverter which uses 6 large fast GTOs, 6 large free-wheel diodes and associated trigger components. The general circuit arrangement is shown in the diagram of Fig.2. The 3-phase AC supply provided by the inverter drives all traction motors in parallel. The frequency of this AC thereby determines the motor speed and this is directly variable by the driver's speed controller. Either 4 motors in a Bo-Bo locomotive or 6 motors in a Co-Co machine are used. Starting voltage Well, now we have a 3-phase inverter system which will let the induction motors run at any speed but there is another problem. Because the current of an induction motor also depends on inductive reactance [which is proportional to frequency), it will tend to draw a lot more current when the frequency is lowered. That's just what we don't want. HIGH VOLTAGE, SINGLE PHASE AC OVERHEAD LINE +2kVOC REGULATED BUS HARMONIC FILTERS OC-AC 3-PHASE INVERTERS 3-PHASE VARIABLE FREQUENCY MOTOR BUS 6xGTO OVDC BUS RAIL ALL TRACTION MOTORS IN PARALLEL FIG.2: THE ELECTRICAL CONFIGURATION of a 3-phase electric locomotive. The high voltage single phase supply is stepped down in the main transformer, rectified and regulated to 2kV DC and then fed to the solid state 3-phase inverter. The inverter output is continuously variable to frequencies of less than 1Hz. This allows the induction motors to generate very high starting effort, while only drawing modest currents. , - - 3~~~TB[Ri~~ii ~ POL YPHASE THYRISTOR BRIDGE RECTIFIER + 2kvDc REGULATED Bus DC-AC 3-PHASE INVERTERS 3-PHASE VARIABLE FREQUENCY BUS OVOC BUS ALL TRACTION MOTORS IN PARALLEL FIG.3: IN A DIESEL 3-PHASE electric loco, the inverter effectively decouples the alternator from the traction motors and lets the diesel engine operate at the optimum speed for minimum fuel consumption. The alternator can be smaller too, because it does not have to supply very high current at starting. So at the same time as the AC frequency is reduced to achieve low speed, the voltage at the motors must also be reduced. This is achieved by pulse-width modulation of the GTOs. Thus at low speed the whole system is operating on reduced voltage and hence greatly reduced power, while still being able to produce high tractive effort. This means that the efficiency of the locomotive is high even at the lowest speeds. This important characteristic is in stark contrast to all other types of locomotive drives using commutator motors on either DC or single phase AC. Ideally, railway systems would like a single locomotive class having both high speed and high power so that one class can handle all jobs from heavy freight to express passenger. The older ideas of separate passenger and freight loco classes is inefficient in terms of plant usage. Now, with 3-phase induction motors able to operate over a very wide range of speeds, and with very high starting tractive effort, one loco class is possible. As an example of this, consider the E120 class 5600kW Bo-Bo locomotive on the German Federal Railway. It is equally suited to pull- ing a 2700 tonne freight train on gradients up to 5 o/o and speeds up to 80km/h or pulling fast passenger trains weighing 550 tonnes on gradients up to 2.5% and speeds up to 200km/h. All this is possible without changes to gear ratios. Thus, this loco is able to do the work of previous 6-axle freight locos or high speed 4-axle passenger locos. The 3-phase induction motors are considerably lighter than series DC traction motors of the same power ratings and they can deliver higher tractive effort without the time/ temperature limits of DC motors. AUGUST 1989 105 THIS INTERESTING AMERICAN locomotive is diesel powered but can also be electrically powered via a 3-rail system. The 3-phase inverter drive system allows the engine to run at low speed while still giving very high tractive effort. DC locomotives And what about DC electric railways where the overhead supply is 1500V DC such as in New South Wales? Three-phase traction motors can still be used. For this application a circuit similar to Fig.2 is used without the transformer and bridge rectifier. Instead the DC overhead line voltage is fed via a DC chopper thyristor to provide the regulated DC voltage supply for the GTO DCAC 3-phase inverter. Three-phase motors follow as before. The reason the DC overhead supply cannot be directly connected to the inverter is that the inverter must run from a regulated supply. In DC overhead systems, the line voltage varies widely depending on train loadings and distance from the substation. The future use of 3-phase traction motors thus opens up the prospect of locos with even higher tractive effort than the 86-class locos featured in last month's episode, without the need to deliver the extremely high starting currents of thousands of amperes. Diesel electric Where 3-phase traction motors are applied to diesel electrics using 106 SILICON CHIP this inverter system, big advantages apply. The electrical system is the same as described above except that the diesel engine, alternator and thyristor rectifier bridge provide the DC supply. Fig.3 shows the details. Besides being ideal for high speed diesel electric locomotives, this system has outstandingly high efficiency at starting and very low speeds. The 3-phase inverter system effectively decouples the diesel engine and alternator from the traction motors. This brings about a number of advantages apart from those already mentioned. Since the diesel-powered alternator no longer has to provide very high starting currents for the traction motors, it does not have to be as large or as heavy, for a given total output rating. Not only that but the system can be designed so that the diesel engine operates at a speed which gives the optimum specific fuel consumption for a given tractive effort. This is a particular advantage in shunting locomotives and can lead to fuel savings of more than 30%. Power factor In electric locos with 3-phase motors, this system can also be used to correct the total locomotive power factor. This in turn means less voltage drop and power loss in the overhead line, thus reducing the size and cost of the trackside transformers required. In large railway systems, this power factor improvement reduces costs all the way back to the power station itself. Wheel slip/slide Fig.1 also points up a unique advantage of 3-phase traction motors in the very steep slope from maximum torque at full speed to zero torque at slightly higher speed. This crucial fact implies that all the 3-phase motors in a locomotive are automatically forced to run at the same speed, since they are all supplied at the same frequency (at any one setting of the driver's speed controller). Should one wheel set lose traction and attempt to run faster than the others, that traction motor can never run faster than synchronous speed. So uncontrolled wheel slip is impossible. This excellent characteristic is far in advance of older locomotives using DC traction motors. It results in greatly reduced wear on the wheels and rails. When all the advantages of 3-phase variable frequency induction motors are added up, we have an ideal locomotive. No other motor system can provide full torque at standstill without damage. However, the catalog of advantages is not quite finished. Regenerative braking In contrast to locomotives running from a high voltage AC supply and using DC series motors, locos using 3-phase motors (as in Fig.2) can apply full regenerative braking without contactors, switches or any change in connections. The inverter system needs to be modified with additional SCRs but when this is done, regeneration can feed power back into the high voltage supply wire. The moment a train tends to run under gravity or momentum above the speed called for by the driver's controller, the motor immediately becomes an asynchronous alternator. The power generated is fed back via the inverter and thyristor bridge to the transformer where it is stepped up to overhead line voltage. This regenerative action has a braking effect on the train, right down to zero speed. THREE-PHASE TRACTION MOTORS are also ideal for use in diesel-electric locomotives. This Di-4 type loco from the Norwegian State Railways uses a CoCo axle arrangement and is rated at 2450kW. As an alternative, the power produced from regenerated power can be dissipated in braking resistors or used for heating on passenger trains. Our photos show some of the many applications of locomotives using 3-phase traction motors. Both diesel electric and electric locos in all sizes from shunters to main line high power machines have fully validated the concept. Acknowledgements The author thanks Lars Persson, Paul Bennet, ASEA-Brown Boveri, ASEA and ABB Journals for data, photos and permission to publish.~ ABOVE: A COMPLETE 3-phase inverter with a rating of 1420kVA (dimensions in millimetres). With four of these connected in parallel, a loco of 4450kW (6000hp) can be powered. Right: induction motors do not slip, even if the track is deliberately oiled. This is because the motors can never run at more than synchronous speed. AUGUST 1989 107