Part five: Electric and hybrid vehicles

In the fifth part of his look into EV and hybrid technology Peter Coombes of Tech-Club continues to look at electric motors

By Peter Coombes | Published:  02 April, 2018

We previously looked at the basic principles of brushless electric motors that use an alternating current to continuously reverse or swap the polarity of the magnetic field in the stator. However, for many high power applications including electrically propelled cars, the motors are supplied with a 3-phase alternating current rather than just a single alternating current.

The illustrations in Figs 1A, 1B and 1C show the very basic concept of a 3-phase permanent magnet motor. The rotor has a permanent magnet with a single North and single South Pole; but the stator contains three electro-magnets formed from three pairs of windings (EM-1, EM-2 and EM-3), with each pair of windings being supplied by a separate alternating current. When the alternating current passes through the windings, the magnetic polarity of the electro-magnets will alternate between North and South. Note that for convenience, the connections for only one pair of windings are shown in each illustration. Importantly, the three alternating currents are phased (by 120 degrees on a genuine 3-phase motor), so that EM-2 is behind EM-1, and EM-3 is behind EM-2.

In Fig 1A, the circuit and alternating current waveform are shown for electro magnet EM-1. The alternating current will energise the electro-magnet so that the North pole is at the top and the South pole is at the bottom. The rotor is positioned so that the North and South poles are adjacent to the North and South poles of the electro-magnet; and because like poles repel each other, the rotor will turn. The rotor North pole could in fact be attracted to the South pole
of EM-2 thus helping to turn the rotor clockwise.

In Fig 1B, as the rotor turns clockwise, the alternating current applied to EM-2 creates another stator North Pole opposite the rotor North pole, therefore the rotor will continue to turn clockwise. Although the alternating current for EM-1 will have moved part way through its cycle, it is still creating a North pole at the top of the stator.

In Fig 1C, the rotor has continued to turn, but now the alternating current applied to EM-3 is creating a North Pole that again repels the rotor North pole, and the rotor will continue to turn. Note that at this stage the North pole for EM-1 has remained at the top, but when the rotor has turned through another few degrees, the alternating current at EM-1 will have moved further through its cycle (by 120 degrees from the start position), which will cause the current flow through EM-1 to reverse (connection-A will now become Negative and connection B will become Positive); therefore the magnetic poles at EM-1 will swap positions. Also note that the group of three stator North poles and the three stator South poles effectively rotate around the stator, which effectively forces the rotor to keep turning; and this process will continue as long as the three Alternating Currents are applied to the three electro-magnets.

The turning of the rotor is therefore ‘synchronised’ with the rotation of the magnetic fields (the stator North and South Poles) that rotate around the stator; and this type pf permanent magnet motor is therefore referred to as a synchronous AC motor.

Induction motors
There is then a second type of AC brushless motor known as an ‘induction motor’ that is used for many industrial and domestic applications as well as for electric vehicles (such as Tesla cars). Induction motors motor still use pairs of windings to create electro-magnets on the stator, which then create magnetic fields that rotate around the stator and motor body, but there are no permanent magnets on the rotor. The magnetic fields on the rotor are however created using basic principles of electro-magnetism known as ‘induction,’ which is where an electric current can be induced into a piece of wire (or an applicable piece of metal) by passing a magnetic field across the wire. But then, when an electric current is created in the wire, it then produces a magnetic field around the wire.

Fig 2 shows a very simple induction motor with a rotor constructed using a series of metal rods joined together by end-plates thus forming a rotating drum (often referred to as a squirrel cage). Although induction motors have a number of pairs of windings (in the same way as a permanent magnet AC motor), for simplicity, the stator is shown as having just one pair of electrical windings to create an electro-magnet. As with permanent magnet motors, the alternating current will cause the North and South poles of the electro-magnet to continuously alternate or swap.

Fig 3A then shows an end view of the Induction motor with 3-pairs of windings that are supplied with the 3-phases of alternating current in the same way as the permanent magnet AC motor. Therefore, when the phased Alternating Currents cause the North and South poles of the electro-magnets to repeatedly swap positions, the effect is that the North poles and the South poles rotate around the stator and the motor body (as shown in the sequence Fig 3A and 3B). In Fig 3A, because there are now magnetic fields rotating around the motor assembly, these rotating magnetic fields will induce an electric current into the metal rods in the rotor; and this induced electric current will in turn create magnetic fields around the rods. In effect there will be a series of North poles (F, A and B) and South poles (C, D and E) arranged around the rotor rods. In theory, whilst the stator magnetic fields are rotating around the motor body, the rotor will be forced to turn because of the repelling action of the rotor’s North and South poles against the stator’s rotating North and South poles.

The one critical part of the whole induction process is that the stator’s rotating magnetic fields must be moving or cutting across the rods in the rotor to be able to induce an electric current in the rods; so when the rotor is stationary, the rotating magnetic fields will induce an electric current in the rods and this will create the rotors magnetic fields. However, the rotor will then start to turn, and in theory the rotor will then turn at the same speed as the rotation of the stators magnetic fields around the motor body. If the rotor turns at the same speed as the magnetic fields, then the magnetic fields are not actually cutting across the metals rods, which means there will be no induced electric current or magnetic field in the rotor rods; so the rotor will now slow down and try to stop. As the rotor slows down, the next rotating magnetic field will cut across the rotors rods and once again this will induce an electric current and create a magnetic field at the rotor rods, so the rotor continues to turn.     

In reality, the rotation of the rotor does not normally reach the same speed of rotation as the stator magnetic fields, so there is always an amount of slip between the speed of rotation of the stator magnetic fields and the rotor speed. Therefore, the rotor speed is not synchronised to the rotation of the stators magnetic fields, which is why these types of induction motors are often referred to as ‘asynchronous AC Motors.’

Related Articles

  • Hybrid tech in motor sport  

    If you have even a passing interest in motorsport, you are probably aware of, and have an opinion regarding the use of hybrid technology in Formula 1 racing. Back in 2009, the use of electric motors was allowed, which enabled the additional electric power to supplement the power produced by the conventional engine.
    In effect, a Formula 1 car could operate in much the same way as most mass produced hybrid vehicles because electric motor could also function as a generator to charge batteries. During power-off driving (braking and deceleration) the kinetic energy of the moving car and rotating engine drove the generator, which charged the batteries; but the energy transferred from the moving car to drive the generator also helped to slow the vehicle. The stored or recovered electrical energy could then be fed from the battery back to the motor when additional power was required. This system was known as a ‘Kinetic Energy Recovery System’ or KERS; and for the purists who like the sound of a hard working petrol engine, this KERS hybrid technology was OK because the 2.4 litre V8 engine still did most of the work and sounded great.
    After a bit of a bumpy ride, for 2014 the hybrid F1 hybrid regulations evolved into a more complex set of rules that specified more complex technologies. The energy recovery systems were allowed to deliver a maximum of 12KW (160hp) of power in addition to the power delivered by a 1.6 litre V6 turbo-charged engine; but for 2014 onwards, there were two types of energy recovery systems that had to be used. Both types of energy recovery systems still use a ‘Motor/Generator Unit,’ which unsurprisingly is known as an MGU; but one system is then known as MGU-K (Motor/Generator Unit Kinetic), and the second system is known as MGU-H (Motor/Generator Unit Heat).

    The MGU-K system is much the same as the original KERS system used from 2009. The Motor/Generator Unit is usually connected by gears to the engine crankshaft, therefore when the unit functions as a motor and draws electrical energy form the battery, the motor feeds mechanical energy back to the crankshaft to provide additional power and torque (such as for acceleration). During power-off driving, the engine is still connected to the driving wheels; therefore the Kinetic energy of the moving car again rotates the engine and the electric motor, which now functions as a generator to re-charge the battery.
    The illustration (Fig 1) shows a basic layout for the MGU-K kinetic energy recovery system, but note that for convenience, the motor generator is shown connected directly to the front of the crankshaft but it can be located on one side of the engine beneath the exhaust manifold. The illustration also shows a battery management/electric power controller that regulates the power delivery of the motor and controls the re-charging process when the motor functions as a generator.
    A lithium-Ion battery pack is usually used to store the electrical energy, although super-capacitors have apparently been experimented with that can accept a re-charge and then discharge electrical energy more quickly than a battery.
    However, with the second energy recovery system, the Motor/Generator is driven by the rotation of the engine’s turbocharger , which is driven by the flow of hot exhaust gas (which contains Heat Energy). Therefore the two systems are referred to as MGU-K (for kinetic) and MGU-H (for heat).

    The second energy recovery system (MGU-H) also makes use of a Motor/Generator Unit; but instead of being connected to the engine crankshaft, this unit is connected to the engine turbocharger (Fig. 2). As with road vehicle turbocharging, hot exhaust gas from the internal combustion engine drives a turbine that is connected to a compressor that then draws in air and forces it into the engine intake under pressure. But because the engine only produces high volumes of hot exhaust when the engine is under load and the throttle is open sufficiently to allow a higher mass of air to enter the engine, the turbocharger is only effective under higher load driving conditions.
    With the F1 engines, the turbocharger (which can rotate at speeds in the region of 100,000 RPM or more) is then also connected to the MGU-H Motor/Generator Unit, so as well as forcing air into the engine, the turbocharger also drives the MGU-H and generates additional electrical energy to charge the battery.
    The clever bit however is the use of the MGU-H to then drive the turbocharger. When the throttle of a turbocharged engine is initially opened to obtain more power (especially after decelerating when the engine might be at low RPM), the turbocharger speed will have reduced to low or almost zero RPM. It therefore takes a brief period for the turbo charger to spin up, but this is then also dependent on the engine responding to the open throttle and then creating higher volumes of exhaust gas to drive the turbocharger. Therefore there is a time lag between opening the throttle and when the turbocharger can actually increase the airflow into the engine to produce increased power and torque; and this inevitably has an effect on how quickly the vehicle accelerates. Because the MGU-H motor/generator is also connected to the turbocharger assembly, it can actually spin-up the turbocharger immediately when additional power is required (which will be before the exhaust gas is able to drive the turbocharger). In effect, the turbocharger also becomes an electrically driven supercharger.

    Controlling electrical power and electrical generation
    The operation of MGU-H Motor/Generator Unit is again controlled by the battery management/electric power controller, which therefore controls when the MGU-H functions as a turbocharger drive motor and when it functions as a generator. The control unit therefore has a complex task of regulating both the MGU-K and MGU-H motor/generator units so that the additional power provided by the electric motors is within the specified limits imposed by the regulations, and that the additional power is only available for the specified periods during a lap of the circuit.
    The electronic control system then has one other important control function, which relates to braking. During deceleration and braking, when the MGU-K system is recovering kinetic energy from the moving car to drive the generator, it creates a significant braking effect on the rear wheels. If the driver is also applying the normal brakes at the same time, there will a combined braking force from the brakes and from the MGU-K. Any increase or decrease in the braking force provided by the MGU-K could then alter the total amount of braking force applied to the rear wheels, which could either lead to brake lock up or to a lack of rear braking. The electronic control system for the MGU-K must therefore communicate and influence operation of the braking system, to ensure that the driver remains in overall control of the braking forces.
    Although the use of hybrid technology in F1 does accelerate the technology learning curve (pardon the pun), one big disadvantage is that use of the turbocharger muffles the exhaust noise, which does tend to upset the purist petrolheads.

  • IMI launches new international EV training solution   

    Launching today (Tuesday 11 September) at Automechanika Frankfurt, the IMI is showcasing its new Electric Vehicle eLearning modules designed to transform the way people undertake training within the workplace.

    With full-electric car sales in the EU set to reach 200,000 this year, the IMI has connected with Germany’s training academy, Lucas Nülle, to make continual learning convenient and interactive for individuals of all abilities.

    Steve Nash, Chief Executive at the IMI, said: “Making sure that an employer and its employees are ready for the increased number of ultra-low emission vehicles is paramount to future-proofing a business. Being able to service and maintain these vehicles safely should be the key focus, especially when the industry is experiencing the biggest growth in automotive technology that we’ve ever seen.

    “Advances in new technology are creating hundreds of thousands of new jobs across the world, and individuals working in the industry should be adopting this new training to make themselves leaders in their area of expertise. It’s an exciting time for the motor industry and the IMI is committed to making sure we’re ready to embrace the changes that are set to transform the sector.”

  • IMI: UK garages unprepared for EV surge 

    The Institute of the Motor Industry (IMI), has voiced its concern for the safety of technicians after electric vehicle sales reach a record high.

  • technologies of electric and hybrid vehicles  

    In the previous two issues, we looked at the way batteries store energy. We could in fact compare a battery to a conventional fuel tank because the battery and the tank both store energy; but one big difference between a fuel tank and a battery is the process of storing the energy. Petrol and diesel fuel are pumped into the tank in liquid/chemical form and then stored in the same form. Meanwhile, a battery is charged using electrical energy that then has to be converted (within the battery) into a chemical form so that the energy can be stored.

    One of the big problems for many potential owners of pure electric vehicles is the relatively slow process of
    re-charging the batteries compared to the short time that it takes to re-fill a petrol or diesel fuel tank. If the battery is getting low on energy, the driver then has to find somewhere to re-charge the batteries, and this leads to what is now being termed ‘range anxiety’ for drivers.

    Whilst some vehicle owners might only travel short distances and then have the facility to re-charge batteries at home, not all drivers have convenient driveways and charging facilities. Therefore, batteries will have to be re-charged at remote charging points such as at fuel stations or motorway services; and this is especially true on longer journeys. The obvious solution is a hybrid vehicle where a petrol or diesel engine drives a generator to charge the batteries and power the electric motor, and for most hybrids the engine can also directly propel the vehicle. However, much of the driving will then still rely on using the internal combustion engine that uses fossil fuels and produces unwanted emissions. The pure electric vehicle therefore remains one long term solution for significantly reducing the use of fossil fuels and unwanted emission, but this then requires achieving more acceptable battery re-charging times.

    Charging process and fast charging
    Compared with just a few years ago, charging times have reduced considerably, but there are still some situations where fully re-charging a completely discharged electric vehicle battery pack can in take as long as 20 hours.  It is still not uncommon for re-charging using home based chargers or some remote chargers to take up to 10 hours or more.

    Although there are a few problems that slow down charging times, one critical problem is the heat that is created during charging, which is a problem more associated with the lithium type batteries used in nearly all modern pure electric vehicles (as well as in laptops, mobile phones and some modern aircraft). If too much electricity (too much current) is fed into the batteries too quickly during charging, it can cause the battery cells to overheat and even start fires. Although cooling systems (often liquid cooling systems) are used to help prevent overheating, it is essential to carefully control the charging current (or charging rate) using sophisticated charging control systems that form part of the vehicle’s ‘power electronics systems.’

    Importantly, the overheating problem does in fact become more critical as battery gets closer to being fully charged, so it is in fact possible to provide a relatively high current-fast charge in the earlier stages of charging; but this fast charging must then be slowed down quite considerably when the battery charge reaches around 70% or 80% of full charge. You will therefore see charging times quoted by vehicle manufacturers that typically indicate the time to charge to 80% rather than the time to fully charge. In fact, with careful charging control, many modern battery packs can achieve an 80% charge within 30 minutes or less; but to charge the remaining 20% can then take another 30 minutes or even longer.   

    Battery modules
    Many EV battery packs are constructed using a number of individual batteries that are referred to as battery modules because they actually contain their own individual electronic control systems. Each battery module can then typically contain in the region of four to 12 individual cells.  One example is the first generation Nissan Leaf battery pack that contained 48 battery modules that each contained four cells, thus totalling 192 cells; although at the other extreme, the Tesla Model S used a different arrangement where more the 7,000 individual small cells (roughly the size of AA batteries) where used to form a complete battery pack.

    The charging control systems can use what is effectively a master controller to provide overall charging control. In many cases  the electronics contained in each battery module then provides additional localised control. The localised control systems can make use of temperature sensors that monitor the temperature of the cells contained in each battery module. This then allows the localised controller to restrict the charging rate to the individual cells to prevent overheating. Additionally, the localised controller can also regulate the charging so that the voltages of all the cells in a battery module are the same or balanced.

    One other problem that affect battery charging times is the fact that a battery supplies and has to be charged with direct current (DC) whereas most charging stations (such as home based chargers and many of the remote charging stations) provide an alternating current (AC). Therefore the vehicle’s power electronics system contains a AC to DC converter that handles all of the charging current. However, passing high currents through the AC to DC converter also creates a lot of heat, and therefore liquid cooling systems are again used to reduce temperatures of the power electronics. Even with efficient cooling systems, rapid charging using very high charging currents would require more costly AC to DC converters; therefore, the on-board AC to DC converter can in fact be the limiting factor in how quickly a battery pack can be re-charged. Some models of electric vehicle are actually offered with options of charging control systems: a standard charging control system which provides relatively slow charging or an alternative higher cost system that can handle higher currents and provide more rapid charging.

    Home & Away
    One factor to consider with home based chargers is that a low cost charger could connect directly to the household 13-amp circuit, which would provide relatively slow charging of maybe 10 hours for a battery pack. However, higher power chargers are also available that connect to the 30-amp household circuits (in the same way as some cookers and some other appliances); and assuming that the vehicle’s AC to DC converter will allow higher currents, then the charging time could be reduced to maybe 4 hours operate (but note that all the quoted times will vary with different chargers and different vehicles).

    Finally, there are high powered chargers (often referred to as super-chargers) that are usually located at motorway services or other locations. These super-chargers all provide much higher charging currents to provide fast-charging (as long as the vehicle electronics and battery pack accept the high currents); but in a lot of cases, these super-chargers contain their own AC to DC converter, which allows direct current to be supplied to the vehicle charging port. In effect, the vehicle’s on-board AC to DC charger is by-passed during charging thus eliminating the overheating problem and the high current DC is then fed directly to the battery via the charging control system.

    In reality, the potential for re-charging a battery pack to 80% of its full charge in 30 minutes or less usually relies on using one of the super-chargers, but battery technology and charging systems are improving constantly, so we
    will without doubt see improving charges times for
    newer vehicles.  

  • technologies of electric and hybrid vehicles  

    In the first article in this series, published in the November issue, we looked at some of the issues relating to the batteries used in electric and hybrid vehicles. As brief summary: Modern lithium based batteries typically store four times more energy than a traditional lead-acid battery of the same weight.  

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