V ELECTRIC TRACTION AND DRIVE CONTROL
The electric traction is provided by the combination of electric motor and drive control algorithm. The permanent magnet based motors, reluctance based motors and induction motors have emerged as front-runners for the choice of motors for EVs and HEVs. At very high power scale applications like locomotive traction, induction motors are by and large the popular choice. They are rugged machines with very little or no maintenance. The low power applications seem to prefer the permanent magnet based motors. They are compact and efficient. Recently much research is happening in switched reluctance and synchronous reluctance motors. The permanent magnet motors are very compact, but suffer from the drawback of rare-earth material availability. Rare-earth material is needed to make permanent magnets and these are available in very few countries. Therefore, much of the research work on electric motors specifically for EVs use either the induction machine approach or the synchronous reluctance approach. Future electric vehicles will eventually use either one of these type of motors.
Drive control is generally based on the high dynamic performance vector control approach. The field oriented control based on d-q axes theory has evolved to some degree of maturity and is now the most preferred type of motor control. This approach is applicable to induction machines, permanent magnet machines and reluctance machines too. In the drive control domain, one of the main challenges that need to be resolved is self-commissioning. The motor parameters may vary with time, loading and environmental conditions. Apart from this, the motors may be swapped for repairs and maintenance. Therefore, the drive controller should be able to self-commission for any motor under all such circumstances.
VI INTER-DISCIPLINARY SYSTEM
EVs and HEVs are complex systems comprising of interdisciplinary domains. The challenges for EVs and HEVs span multiple disciplines. Core engineering disciplines like energy, electrical and mechanical are central to EV and HEV design. Power electronics with special emphasis on machine control and drives are essential to handle e-engine or e-propulsion development. Drive control algorithms for vector control of induction machines and permanent magnet machines are very mature now. However, there are robustness challenges in this specific context like machine parameter changes due to machine replacement and maintenance. This need to be addressed satisfactorily. Issues related to energy storage is a vast topic that needs a major research outlook. This is one aspect that needs to be satisfactorily addressed before any meaningful ICE to EV transition happens. At the national grid level, the energy chain budget for the EV from source to wheels need to be looked and challenges regarding the nature of source to be used from the point of view of pollution and fossil fuel reserves should be explored and addressed. There are several challenges on the IoT front too. Issues related to IoT enable steering for fully automatic and/or assisted steering have many applications in future. Assisted steering sees lot of potential to avoid accidents due to driver fatigue/sleep while driving. Auto-steering has lot of potential in synchronised fleet management and also traffic management of vehicles at busy junctions. Vehicle fleet management algorithms can reduce the time spent in waiting at traffic junctions. IoT can also play a role in inter-vehicle communication for collision avoidance. Several such relevant applications can be thought of where IoT can play a nice role of integrating power electronics, mechanical devices and sensors.
Both the EVs and the ICE vehicles have pros and cons. World transportation will eventually and inevitably move more and more towards electric drive. However, one school of thought feels that eventually a realistic transport eco-system would stabilise with both electric propulsion and petrol/diesel propulsion.
In India, more than 75% of electric power comes from fossil fuel based thermal power stations. This same converted fossil fuel energy is the one that will eventually charge the batteries and drive the electric propulsion based EVs. The energy chain efficiency considers the efficiency of coal extraction and transportation, thermal boiler efficiency, turbine efficiency, transmission and distribution efficiency, battery charging efficiency and the electric propulsion efficiency upto the wheel load demand. In the case of ICE vehicles, the petrol/diesel fossil fuel is delivered to the vehicles as fuel. Here, the energy chain efficiency considers the efficiency of petrol/diesel extraction, fuel transportation and distribution and internal combustion engine efficiency upto the wheels. Considering the energy chain efficiencies from the source to wheels, both EVs and ICE show no significant advantage over the other. Moving to EV mode of transport in the large scale would make sense only if the primary energy source that is used to ultimately charge the batteries and run the EVs is truely a renewable energy source not dependent on fossil based reserves. In this regard, one may ponder that under the current electric power generation situation in the country the one signinant advantage to shift over to the EV mode of transport is that pollution sources are centralised and easily controllable. Whereas, in the case of ICE scenario, each vehicle is a pollution source and consequently more difficult to regulate. In order to make a deep impactful shift from ICE to EVs, the entire country’s power generation must move towards renewable generation and only such green power should be used to power the EVs. This is a very serious challenge.
Another challenge that needs to be effectively addressed is “what happens to the existing ICE vehicles on the roads?”. There are millions of 2, 3 and 4 wheelers on the roads currently using ICE. One cannot just abandon or throw them away. Physical vehicle archival process is not easy if not impossible. One needs to address this by providing “retrofit” solutions that will seamlessly transfer existing ICE vehicles to HEVs and then on to EVs. This probably may be the only way to address such a mammoth challenge.
Batteries and battery charging is an important challenge that needs to be addressed if the EV eco system is to take off. The batteries have energy densities hovering around 250 Wh/kg and power densities around 200 W/kg. Both are much less compared to fossil fuel storage. Higher energy density would imply greater range and higher power density would mean greater acceleration capability. Ultra capacitors have very high power densities but low energy densities. One can use an eutectic combination of different battery types and/or batteries with ultra capacitors to improve the performance parameters of the storage system.
Fast charging of batteries may not be a feasible or realistic solution as the power generation is not commensurate with the transport sector demand. Further, most power generation in the country is based on fossil fuel and therefore should not be used for EV battery charging. Slow charging through photovoltaic arrays at parking lots is one possible solution. Charging batteries from grid at lean load demands should be attempted. If the power generation and load demand does not match, there will be power grid stability issues. An interesting challenge is to use the EV batteries connected to the grid through the charger units as VAR compensators for stabilising the grid. Battery charging should co-exist with battery swapping at the re-charge stations. In order to avoid long queues at the recharge stations, battery swapping will become an inevitable technology for EV eco system. Estimating the state of health of the battery pack is very important, but a difficult challenge to address. This is important as energy pricing and tariffs for battery swapping will be determined based on this estimate. Number of charge-discharge cycles of the battery is a history parameter that needs to be transmitted across swaps. Technology for this storage and retrieval of battery history data needs to be addressed in a robust manner.
In conclusion, it may be said that walking is the best mode of transport. For any energy output delivered from a system, the amount of energy required at the input will always be greater, as a part of the input energy is lost as irrecoverable heat energy. This is a universal law occurring in all systems, simple and complex. Moving over to an EV eco system from an ICE eco system will not solve the problems of pollution and deteriorating environmental conditions, without effort from our side to change the way we live. We have been over-consuming energy and natural resources for quite some time now. We should live with an awareness of energy conservation. The most efficient means of transport is that which is already provided by nature i.e. muscle power of humans and animals. One may start with adopting simple changes to life style like move into residences near ones work place. This will solve most transport issues and go a long way towards better quality of life.