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.’

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