63 Brushless Electric Motors

6.3.1 Introduction

In Section 6.1 we described the classical DC electric motor. The brushes of this motor are an obvious problem; there will be friction between the brushes and the commutator, and both will gradually wear away. However, a more serious problem with this type of motor was raised in Section 6.1.5. This is that the heat associated with the losses is generated in

Voltage

Time

Time

Time

Figure 6.23 Current/time graphs for the simple three-phase AC generation system shown in Figure 6.22, assuming a resistive load. One complete cycle for each phase is shown. Current flowing out from the common point is taken as positive the middle of the motor, in the rotor. If the motor could be so arranged that the heat was generated in the outer stator, that would allow the heat to be removed much more easily, and allow smaller motors. If the brushes could be disposed of as well that would be a bonus. In this section we describe three types of motor that are used as traction motors in vehicles that fulfil these requirements.

One of the interesting features of electric motor technology is that there is no clear winner. All three types of motor described here, as well as the brushed DC motor of Section 6.1, are used in current vehicle designs.

6.3.2 The brushless DC motor

The brushless DC motor (BLDC motor) is really an AC motor! The current through it alternates, as we shall see. It is called a brushless DC motor because the alternating current must be variable frequency and so derived from a DC supply, and because its speed/torque characteristics are very similar to the ordinary 'with brushes' DC motor. As a result of brushless DC being not an entirely satisfactory name, it is also, very confusingly, given different names by different manufacturers and users. The most common of these is self-synchronous AC motor, but others include variable frequency synchronous motor, permanent magnet synchronous motor, and electronically commutated motor (ECM).

The basis of operation of the BLDC motor is shown in Figure 6.24. Switches direct the direct from a DC source through a coil on the stator. The rotor consists of a permanent magnet. In Figure 6.24(a) the current flows in the direction that magnetises the stator so that the rotor is turned clockwise, as shown. In 6.24(b) the rotor passes between the poles of the stator, and the stator current is switched off. Momentum carries the rotor on, and in 6.24(c) the stator coil is re-energised, but the current and hence the magnetic field, are

Momentum keeps rotor moving

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Figure 6.24 Diagram showing the basis of operation of the brushless DC motor

reversed. So the rotor is pulled on round in a clockwise direction. The process continues, with the current in the stator coil alternating.

Obviously, the switching of the current must be synchronised with the position of the rotor. This is done using sensors. These are often Hall effect sensors that use the magnetism of the rotor to sense its position, but optical sensors are also used.

A problem with the simple single coil system of Figure 6.24 is that the torque is very unsteady. This is improved by having three (or more) coils, as in Figure 6.25. In this diagram coil B is energised to turn the motor clockwise. Once the rotor is between the poles of coil B, coil C will be energised, and so on.

The electronic circuit used to drive and control the coil currents is usually called an inverter, and it will be the same as, or very similar to, our universal inverter circuit of Figure 6.21. The main control inputs to the microprocessor will be the position sense signals.

A feature of these BLDC motors is that the torque will reduce as the speed increases. The rotating magnet will generate a back EMF in the coil which it is approaching. This back EMF will be proportional to the speed of rotation, and will reduce the current flowing in the coil. The reduced current will reduce the magnetic field strength, and hence the torque. Eventually the size of the induced back EMF will equal the supply voltage, and at this point the maximum speed has been reached. This behaviour is exactly the same as with the brushed DC motor of Section 6.1.

We should also notice that this type of motor can very simply be used as a generator of electricity, and for regenerative or dynamic braking.

Although the current through the motor coils alternates, there must be a DC supply, which is why these motors are generally classified as DC. They are very widely used in computer equipment to drive the moving parts of disc storage systems and fans. In these small motors the switching circuit is incorporated into the motor with the sensor switches. However, they are also used in higher power applications, with more sophisticated controllers (as of Figure 6.21), which can vary the coil current (and hence torque) and thus produce a very flexible drive system. Some of the most sophisticated electric vehicle drive motors are of this type, and one is shown in Figure 6.26. This is a 100 kW oil-cooled motor, weighing just 21 kg.

These BLDC motors need a strong permanent magnet for the rotor. The advantage of this is that currents do not need to be induced in the rotor (as with, for example, the induction motor), making them somewhat more efficient and giving a slightly greater specific power. However, the permanent magnet rotor does add significantly to the cost of these motors.

6.3.3 Switched reluctance motors

Although only recently coming into widespread use, the switched reluctance (SR) motor is, in principle, quite simple. The basic operation is shown in Figure 6.27. In Figure 6.27(a) the iron stator and rotor are magnetised by a current through the coil on the stator. Because the rotor is out of line with the magnetic field a torque will be produced to minimise the

Figure 6.26 100kW, oil cooled BLDC motor for automotive application. This unit weighs just 21 kg (photograph reproduced by kind permission of Zytek Ltd.)

air gap and make the magnetic field symmetrical. We could lapse into rather medieval science and say that the magnetic field is 'reluctant' to cross the air gap, and seeks to minimise it. Medieval or not, this is why this type of motor is called a reluctance motor.

At the point shown in Figure 6.27(b) the rotor is aligned with the stator, and the current is switched off. Its momentum then carries the rotor on round over one-quarter of a turn, to the position of 6.27(c). Here the magnetic field is re-applied, in the same direction as before. Again, the field exerts a torque to reduce the air gap and make the field symmetrical, which pulls the rotor on round. When the rotor lines up with the stator again, the current would be switched off.

In the switched reluctance motor, the rotor is simply a piece of magnetically soft iron. Also, the current in the coil does not need to alternate. Essentially then, this is a very simple and potentially low cost motor. The speed can be controlled by altering the length of time that the current is on for in each power pulse. Also, since the rotor is not a permanent magnet, there is no back EMF. generated in the way it is with the BLDC motor, which means that higher speeds are possible. In the fuel cell context, this makes the SR motor particularly suitable for radial compressors and blowers.

The main difficulty with SR motor is that the timing of the turning on and off of the stator currents must be much more carefully controlled. For example, if the rotor is 90° out of line, as in Figure 6.24(a), and the coil is magnetised, no torque will be produced, as the field would be symmetrical. So, the torque is much more variable, and as a result early SR motors had a reputation for being noisy.

The torque can be made much smoother by adding more coils to the stator. The rotor is again laminated iron, but has 'salient poles', i.e. protruding lumps. The number of salient poles will often be two less than the number of coils. Figure 6.28 shows the principle. In 6.28(a) coil A is magnetised, exerting a clockwise force on the rotor. When the salient poles are coming into line with coil A, the current in A is switched off. Two other

Momentum keeps rotor moving

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Figure 6.27 Diagram showing the principle of operation of the switched reluctance motor

salient poles are now nearly in line with coil C, which is energised, keeping the rotor smoothly turning. Correct turning on and off of the currents in each coil clearly needs good information about the position of the rotor. This is usually provided by sensors, but modern control systems can do without these. The position of the rotor is inferred from the voltage and current patterns in the coils. This clearly requires some very rapid and complex analysis of the voltage and current waveforms, and is achieved using a special type of microprocessor called a digital signal processor.5

5 Although they were originally conceived as devices for processing audio and picture signals, the control of motors is now a major application of digital signal processors. BLDC motors can also operate without rotor position sensors in a similar way.

Figure 6.28 Diagram showing the operation of an SR motor with a four salient pole rotor

An example of a rotor and stator from an SR motor is shown in Figure 6.29. In this example the rotor has eight salient poles.

The stator of an SR motor is similar to that in both the induction and BLDC motor. The control electronics are also similar: a microprocessor and some electronic switches, along the lines of Figure 6.21. However, the rotor is significantly simpler, and so cheaper and more rugged. Also, when using a core of high magnetic permeability the torque that can be produced within a given volume exceeds that produced in induction motors (magnetic action on current) and BLDC motors (magnetic action on permanent magnets) (Kenjo 1991, p. 161). Combining this with the possibilities of higher speed means that a higher power density is possible. The greater control precision needed for the currents in the coils makes these motors somewhat harder to apply on a 'few-of' basis, with the result that they are most widely used in cost-sensitive mass-produced goods such as washing machines and food processors. However, we can be sure that their use will become much more widespread.

Although the peak efficiency of the SR motor may be slightly below that of the BLDC motor, SR motors maintain their efficiency over a wider range of speed and torque than any other motor type.

Figure 6.29 The rotor and stator from an SR motor (photograph reproduced by kind permission of SR Drives Ltd.)

6.3.4 The induction motor

The induction motor is very widely used in industrial machines of all types. Its technology is very mature. Induction motors require an AC supply, which might make them seem unsuitable for a DC source such as batteries or fuel cells. However, as we have seen, AC can easily be generated using an inverter, and in fact the inverter needed to produce the AC for an induction motor is no more complicated or expensive than the circuits needed to drive the brushless DC or switched reluctance motors we have just described. So, these widely available and very reliable motors are well suited to use in electric vehicles.

The principle of operation of the three-phase induction motor is shown in Figures 6.30 and 6.31. Three coils are wound right around the outer part of the motor, known as the stator, as shown in the top of Figure 6.30. The rotor usually consists of copper or aluminium rods, all electrically linked (short circuited) at the end, forming a kind of cage, as also shown in Figure 6.30. Although shown hollow, the interior of this cage rotor will usually be filled with laminated iron.

Three coils A, B, and C are wound on the stator.

The arrows show the direction of the magnetic field when there is a positive current in the coil.

Three coils A, B, and C are wound on the stator.

The arrows show the direction of the magnetic field when there is a positive current in the coil.

Figure 6.31 Diagrams to show how a rotating magnetic field is produced within an induction motor

The three windings are arranged so that a positive current produces a magnetic field in the direction shown in Figure 6.31. If these three coils are fed with a three-phase alternating current, as in Figure 6.23, the resultant magnetic field rotates anti-clockwise, as shown at the bottom of Figure 6.31.

This rotating field passes through the conductors on the rotor, generating an electric current.

A force is produced on these conductors carrying an electric current, which turns the rotor. It tends to 'chase' the rotating magnetic field. If the rotor were to go at the same speed as the magnetic field, there would be no relative velocity between the rotating field and the conductors, and so no induced current, and no torque. The result is that the torque speed graph for an induction motor has the characteristic shape shown in Figure 6.32. The torque rises as the angular speed slips behind that of the magnetic field, up to an optimum slip, after which the torque declines somewhat.

Torque

Torque

Speed of rotation of magnetic field

Figure 6.32 Typical torque/speed curve for an induction motor of

Angular speed

Speed of rotation of magnetic field of

Angular speed

Figure 6.32 Typical torque/speed curve for an induction motor

The winding arrangement of Figure 6.30 and 6.31 is knows as two-pole. It is possible to wind the coils so that the magnetic field has four, six, eight or any even number of poles. The speed of rotation of the magnetic field is the supply frequency divided by the number of pole pairs. So, a four-pole motor will turn at half the speed of a two-pole motor, given the same frequency AC supply, a six-pole motor one-third of the speed, and so on. This gives a rather inflexible way of controlling speed. A much better way is to control the frequency of the three-phase supply. Using a circuit such as that of Figure 6.21, this is easily done. The frequency does not precisely control the speed, as there is a slip, depending on the torque. However, if the angular speed is measured, and incorporated into a feedback loop, the frequency can be adjusted to attain the desired speed.

The maximum torque depends on the strength of the magnetic field in the gap between the rotor and the coils on the stator. This depends on the current in the coils. A problem is that as the frequency increases the current reduces, if the voltage is constant, because of the inductance of the coils having an impedance that is proportional to the frequency. The result is that, if the inverter is fed from a fixed voltage, the maximum torque is inversely proportional to the speed. This is liable to be the case with a fuel cell or battery system.

Induction motors are very widely used. A very high volume of production makes for a very reasonably priced product. Much research has gone into developing the best possible materials. Induction motors are as reliable and well developed as any technology. However, the fact that a current has to be induced in the rotor adds to the losses, with the result that induction motors tend to be a little (1 or 2%) less efficient than the other brushless types, all other things being equal.

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