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

  • 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.  

  • Part Seven: Electric and hybrid vehicles  

    Over the past few months, we have looked at battery and electric motor technologies of electric and hybrid vehicles,
    as well as looking at the advantages and disadvantages of batter power compared to fossil fuel power.  
        
    Irrespective of whether a vehicle is powered solely by batteries and an electric motor or whether the vehicle is a hybrid that has the addition of a petrol engine for propulsion and
    re-charging the batteries, the vehicle will require a sophisticated electronic system to manage and modify the electrical energy. In effect, the vehicles have an electrical management system that is often referred to as the ‘power electronics’.

    Controlling electric motor speed and power
    The obvious task of the power electronics system is to control the speed and power of the electric motor so that the vehicle can be driven at the required speed and achieve the required acceleration. As mentioned in a previous article, with Alternating Current (AC) motors the motor speed is regulated by altering the frequency of the 3-phases of alternating current. For light load cruise driving, the current flow provided by the battery pack to the electric motor might only be in the region of a 70 or 80 amps or less, but when the vehicle is being driven under high load conditions, the current requirement will be much higher. Therefore the power electronics can allow higher current flows to be delivered to the electric motor, with some reports quoting as high as 1,800 amps for brief periods on some Tesla vehicles during hard acceleration. However, the power electronics system will monitor currents and temperatures of the electronics, the batteries and the electric motor to ensure that overheating and damage do not occur. As an additional function, the power electronics systems will also control the cooling system (often a liquid cooling system) for the electronics, the batteries and the motor to help maintain acceptable temperatures.
        
    Because most modern electric motors fitted to electric and hybrid vehicles are alternating current motors, the power electronics system must convert the direct current supplied by the battery into alternating current. The power electronics system therefore contains a DC to AC inverter.

    Battery charging from a home charger or remote charging point
    For pure electric vehicles the batteries are re-charged from home based chargers or remote charging points (and this is also true for many later generations of hybrid vehicles). The battery charging must be carefully controlled to prevent overheating and damage, therefore the power electronics system contains a charging control system to regulate the charging rate (voltage and current). Most charging devices provide alternating current, therefore an AC to DC converter forms part of the power electronics system to enable the batteries to receive direct current.
        
    Note that for rapid charging (especially with lithium based batteries), the power electronics system can regulate the charging rate so that the batteries re-charge up to about 80% capacity relatively quickly (perhaps within 20 to 30 minutes with fast chargers), but to prevent overheating and damage, the charging rate is then significantly reduced for the remaining 20%
    of charge.

    Battery charging from an engine driven generator
    Most mass produced hybrid vehicles use an internal combustion engine that can propel the vehicle, but the engine also drives a generator that can re-charge the main high voltage batteries. While the engine is running, the power electronics system again controls the charging rate; and again, the output from the generator passes through the AC to DC converter. Note that the power electronics system will be linked to or integrated with the engine management system, which will allow the power electronics to cause the engine to start and generate electricity if the batteries are low on stored electrical energy.
        
    Because the electric motors fitted to electric and hybrid vehicles can usually function also as generators, when the vehicle is decelerating or braking (or coasting), the electric motor can therefore be used to help re-charge the batteries. The electrical output from the motor/generator will vary with speed; therefore the power electronics system must control the charging rate to the batteries. As with home/remote charging and charging with an engine driven generator, because the motor/generator produces an AC current, the generator output must pass through the AC to DC converter.

    12-Volt battery charging
    A 12-Volt electrical system is still used for electric vehicles, but because there is no engine driven alternator, the 12-volt battery is charged using power from the high voltage system. The power electronics system contains a DC to DC converter that converts the high voltage of the main battery pack down to the required voltage for the 12-volt battery. The charging rate for the 12-volt battery is also controlled by the power electronics system.

    Additional functions of the power electronics system
    As mentioned previously, modern electric vehicles (and hybrid vehicles) will be fitted with cooling systems to maintain the temperatures of the batteries, the electronics and the electric motor. Pure electric vehicles are more likely to be fitted with liquid cooling systems due to the higher currents required for the electric motor that is the only source of propulsion, whereas with hybrid vehicles that also use an internal combustion engine to propel the vehicle generally have less powerful electric motors and therefore often make use of air cooling. However, whichever system is used for cooling, the cooling system can be controlled by the power electronics system to regulate the amount of cooling being applied; note that with liquid cooling systems, the control can also apply to the electric cooling pumps that force the coolant to flow around the cooling system.
        
    Another cooling or heating related function of the power electronics system is to ensure that the battery temperature is at the optimum temperature for charging (and for discharging when the battery is providing electrical power). Batteries charge much more efficiently and faster if they are at the optimum temperature of typically between 10 and 30ºC (or slightly higher for some lithium batteries); but the charging rate should be lowered for lower temperatures; and for many consumer type lithium based batteries, charging is not possible below 0ºC.
        
    Because vehicles are equipped with a cooling/heating systems (for driver/passenger comfort as well as for controlling vehicle system temperatures), the power electronics system can switch on an electrical heater (that would form part of the cooling/heating system) when the batteries are being charged. Therefore, if the vehicle is being charged from a domestic based charger or remote charging station and the ambient temperature is low or below freezing, the battery cooling/heating system can raise the battery temperature to ensure charging take place at the fastest possible rate.


  • Behr Hella Service extends compressor range for EVs and hybrids 

    Behr Hella Service, the exclusive UK distributor of SANDEN electric compressors, has unveiled an aftermarket solution with OE pedigree to combat the challenges faced by technicians when tasked with air conditioning maintenance on hybrid and electric vehicles. The Premium Line range encompasses 4,500 products made by OE manufacturers and these second-generation compressors naturally fall into the programme. They provide the solution for applications such as the Mercedes Benz B-Class and S400 Hybrid, as well as the Volvo V60 HYBRID.
    www.behrhellaservice.com

  • A shock to the system and how to avoid it  

    Hybrid and electric vehicles (H/EVs) are an ever-day reality, and are becoming more popular with drivers and carmakers.
    “Electrified powertrains are emerging as manufacturers’ preferred means of meeting stringent future emissions legislation,“ says Jonathan Levett, technical trainer for Delphi Technologies Aftermarket.

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