Brushless motor design for a medium car

3.4.1 INTRODUCTION

Here the task is to optimize the 45/70 kW driveline for the family car of the future². This involves improvements in fundamental principles but much more in materials and manufacturing technology.

The introduction of hybrid vehicles places ever greater demands on motor performance.

It is the long-term aim of the US PNGV programme to reduce the cost of ‘core’ electric motor and drive elements to 4 dollars per kW from around 10 dollars charged in 1996 for introductory products supplied in volume. The price may be reduced to 6.5 dollars using new manufacturing methods to be reviewed below. Further savings may come from very high volume production.

This will require significant investment which will not occur until there is confidence in the market place and technical maturity in a solution. In terms of design, we may increase speed from 12 000 to 20 000 rpm. For reasons to be explored, a further increase becomes counterproductive unless there is a breakthrough in materials. In the inverter area Polaron believe the best cost strategy is to use a double converter with 300 V battery, 600 V DC link and 260 V motor. This assumes power levels of 70 kW.

The motor can be induction type or brushless DC. Induction is satisfactory in flat landscape/

long highway conditions. For steeper terrain, and shorter highways as exists in Europe brushless DC is more suitable – especially for high performance vehicles and drivelines for acceleration/

braking assistance in hybrid vehicles. Excellent progress has been made in the silicon field. The introduction of high reliability wire bonded packaging in association with thin NPT chip technology for IGBTs is reducing prices and improving performance. Currently a 100 A 3 phase bridge costs around $100 in volume. The arrival of complete 3 phase bridge drivers in a single chip at low cost is a further improvement in this area. Individual driver chips provide better device protection and drive capability at this time.

Great progress has been made in batteries in recent years. However, the time has come for a change in emphasis. Previously the pure battery electric was seen as the desired solution. Even if the remaining technical issues can be addressed, we are still impeded by weight and cost of such a solution. Consequently Polaron believe they should focus on hybrid solutions and this needs batteries optimized for peak power not energy capacity. It requires batteries with geometries optimized for peak power – ultra-low internal resistance and perhaps high capacitance at the same time. It will certainly require new packaging. A capacity of 2 kWh at 2 minute rate would be adequate for the average family car. It will also require a low cost short-circuit device to bypass high resistance cells in long series strings.

There is now little doubt that brushless DC machines offer the best overall performance when used in vector control mode, with high voltage windings, Fig. 3.6. The reason is that the brushless DC motor offers the lowest winding current for the overall envelope of operation. An electric vehicle has to provide a non-linear torque/speed curve with constant power operation from base speed to maximum speed. In a brushless DC motor, the motor voltage may be held constant over this range using vector control. In an induction motor, the motor voltage must rise over the constant power speed range. If V and I are the voltage and current at maximum speed and power the values at base speed are V × (Base Speed/Max Speed)^1/2, I × (Max Speed/Base Speed)^1/2. If maximum speed / base speed = 3.5 times, the current at base speed is 1.87I. Consequently the induction motor inverter requires 1.87 times the current capacity of the brushless DC motor inverter.

The most significant improvement recently for brushless DC machines has been the development of the Daido magnet tube in Magnaquench material. This product offers the benefits of high energy magnet and containment tube. This leads to a third benefit which is not immediately obvious but very significant. Surface magnet motors usually employ a containment sleeve which adds several millimetres of air gap to the magnetic circuit. Since magnet tube does not require a sleeve if used within its speed capability, a thinner magnet tube is possible whilst maintaining

Power (3.5:1 CPSR) (kW) 45 70 70 70 150

Speed max 12 000 10 000 13 500 20 000 20 000 

Stator OD (mm) 218 200 220 200 225 

Rotor OD (mm) 141 113 141 113 145  

Active length (mm) 80.5 190 97 110 160 

Overall length (mm) 141 260 157 170 230 

Stator voltage (V) 150 360 460 360 460 

Max Efficiency 96% 96% 98% 96.5% 98.6% 

Winding L (mH) 0.1 1.78 1.37 0.85 0.28 

Winding R (mW) 9.6 66 116 38 13.4 

Poles 16 8 8 8 8 

Stator/rotor mass (kg) 19 40 21 24 44

*NOTE: 35 kW continuous, 70 kW short time rated.

Fig. 3.6 Current designs of vector controlled brushless DC machines.

200 kW

100 kW POWER

20 K 50 K 100 K

200 kW/20K rpm

80kW/55K rpm

25kW/80K rpm

SPEED RPM

the same air gap flux density. The benefit is reduced magnet weight for a given motor design.

For example, 140 mm diameter Daido grade 3F material with a 5 mm wall will operate unsupported to 13 500 rpm.

The rotor of the machine, Fig. 3.7, is assembled with the magnet tube glued to the flux return tube, with the magnets de-energized. The pole pattern is applied with a capacitor discharge magnetizer from inside the flux return tube. The end plates and motor shafts are then fitted using a central bore for precise axial alignment. Use of a solid rotor is not practical unless a rotor material which does not saturate until 3 tesla is used. Since such material costs

$50 per kg the hollow tube is the best alternative. The use of magnet tube makes complete automation of rotor construction possible achieving significant savings in labour costs, Fig. 3.7a.

Many designers are attracted by the possibility of running motors faster than the current 12 000 rpm. The objective is to reduce the peak torque requirement in an effort to reduce weight and cost of active materials. One obvious method is to compromise the constant power over the 3.5:1 speed:range requirement. Polaron’s own investigations into faster speed suggest any increase above 20 000 rpm will be counterproductive. There are many reasons for this:

(a) The maximum frequency of operation is limited to 1500 Hz using Transil 315 in 0.08 mm thickness (3.15 W/kg at 50 Hz). Most designers are concerned with no load line losses and are endeavouring to optimize this.

(b) Consequent on (a), as the speed rises above 20 000 rpm the pole count has to be reduced from 8 to 6 to 4 poles. This results in thicker magnets and longer flux return paths.

(c) Optimum machine geometry is rotor OD = stator length. The Polaron 70 kW machine has rotor OD = 140 mm and rotor length of 95 mm which is close to optimal. The machine has 8 poles and gives 70 kW from 4000 to 13 500 rpm.

(d) Machines that are below 100 mm rotor diameter are not easy to make as the windings cannot be inserted by automatic machinery. This is especially true of heavy current windings.

(e) Machines with low pole count have poor rotor diameter to stator diameter ratio, which increases the mass of stator iron and results in large winding overhangs increasing copper losses.

(f) Laminations for these machines should have a large number of teeth to reduce the thermal resistance from copper to water or oil jacket. The limitation is when the tooth achieves mechanical resonance in the operating frequency range of the machine. Typically it is the 6f component that causes excitation (6f = 6 times motor frequency). Silicon steel (Transil) has good thermal conductivity. High nickel steels such as radiometal exhibit poor thermal conductivity but lower

(a) (b)

Fig. 3.7 Rotor design and machine performance: (a) a 150 kW, 20 000 rpm brushless DC stator-rotor; (b) power/speed for brushless DC motor with 3.5:1 constant power speed range.

iron losses. Machines with a high peak torque requirement are better in Transil where the copper losses of peak torque can be safely dissipated.

(h) If a better core material at a sensible price were available it would be a real boon. This is one area where there is much room for improvement. Polaron are aware of powder core technology using sintered materials but the tooth tip flux density is only 0.8 tesla. Ferrites are worse at 0.5 tesla.

(i) If makers are prepared to use containment sleeves, a power–speed graph for high speed radial brushless DC machines would look like that in Fig. 3.7a (based on 3.5:1 constant power torque/speed curve). This is the maximum power achievable in consideration of dynamic stability requirements. This graph assumes two point suspension and that the first critical speed must be 20% higher than the top speed of operation (25 kW rotor from 25 000 to 80 000 rpm would be 57 mm OD × 100 mm long).

(j) One problem with high speed machines is the increased kinetic energy stored in the rotor.

This can place a severe strain on subsequent speed reducers unless torque limiting devices are provided.

(k) Acoustic noise is often severe at high speed. For a reduction try: (i) impregnation of stator;

(ii) removing sharp edges on outside of rotor; (iii) operating rotor at reduced pressure using magnetic seals or (iv) using machine with liquid cooling jacket.

(l) Speed reduction is another difficult area at high speed. Since torques are low, friction speed reducers are quieter than gears by a factor of ten.

(m) Bearings and mechanical stability are challenging problems at turbomachinery speeds.

Polaron believe the best cost/performance ratio can be achieved for 70 kW system by: (1) using a Transil 315 stack 0.08 mm thick made as a continuous helix using the punch and bend technique;

(2) using a rotor made from 5 mm magnet tube of surface mount structure mounted on 12.54 mm of 14/4 stainless steel; (3) magnetizing the rotor after assembly to flux density of 3 tesla for 2 millisecs for maximum flux density; (4) choosing a stator frequency of less than 1500 Hz, mean air gap flux density 0.6 tesla; (5) using a liquid cooled stator; (6) insulating the stator from earth with low capacitance coupling; (7) choosing stator of 215 mm OD with 48 teeth stack of 95 mm giving 70 kW from 4000 to 13 500 rpm. Alternatively a stator of 185 mm with 24 teeth and rotor of 110 mm OD × 140 mm long will give 70 kW from 6000 to 20 000 rpm; (8) winding the machine for 460 V in constant power region (460 V at 4000 rpm) with machine driven as a generator open circuit. This gives good efficiency and substantial winding inductance to minimize carrier ripple, Fig. 3.7b.