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
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    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
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    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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