64 Motor Cooling Efficiency Size and Mass

6.4.1 Improving motor efficiency

It is clear that the motor chosen for any application should be as efficient as possible. How can we predict what the efficiency of a motor might be? It might be supposed that the type of motor chosen would be a major factor, but in fact it is not. Other factors are much more influential than whether the motor is BLDC, SR or induction.

An electric motor is, in energy terms, fairly simple. Electrical power is the input, and mechanical work is the desired output, with some of the energy being converted into heat. The input and output powers are straightforward to measure, the product of voltage and current for the input, and torque and angular speed at the output. However, the efficiency of an electric motor is not so simple to measure and describe as might be supposed. The problem is that it can change markedly with different conditions, and there is no single internationally agreed method of stating the efficiency of a motor6 (Auinger 1999). Nevertheless it is possible to state some general points about the efficiency of electric motors, the advantages and disadvantages of the different types, and the effect of motor size. In Section 6.1.5 we also generated a general formula (equation (6.14)) for the efficiency of an electric motor that holds quite well for all motor types.

The first general point is that motors become more efficient as their size increases. Table 6.2 shows the efficiency of a range of three-phase, four-pole induction motors. The efficiencies given are the minimum to be attained before the motor can be classified Class 1 efficiency under European Union regulations. The figures clearly show the effect of size. While these figures are for induction motors, exactly the same effect can be seen with other motor types, including BLDC and SR.

The second factor that has more control over efficiency than motor type is the speed of a motor. Higher speed motors are more efficient than lower speed ones. The reason for this is that one of the most important losses in a motor is proportional to torque, rather than power, and a lower speed motor will have a higher torque, for the same power, and hence higher losses.

A third important factor is the cooling method. Motors that are liquid cooled run at lower temperatures, which reduces the resistance of the windings, and hence improves efficiency, though this will only affect things by about 1%.

Another important consideration is that the efficiency of an electric motor might well be very different from any figure given in the specification, if it operates well away from optimum speeds and torque. In some cases an efficiency map, like that of Figures 6.7 and 6.33

Table 6.2 The minimum efficiency of four-pole three-phase induction motors to be classified as Class 1 efficiency under EU regulations. Efficiency measured according to IEC 36.2

Power, kW

Minimum efficiency, %

6 The nearest to such a standard is IEC 34-2.

Output torque/N.m 82 84

Output torque/N.m 82 84

86 84 82

Motor speed/rpm

Figure 6.33 The efficiency map for a 30kW BLDC motor. This is taken from manufacturer's data, but note that in fact at zero speed the efficiency must be 0%

86 84 82

7500

Motor speed/rpm

Figure 6.33 The efficiency map for a 30kW BLDC motor. This is taken from manufacturer's data, but note that in fact at zero speed the efficiency must be 0%

may be provided. That given in Figure 6.33 is based on a real BLDC motor. The maximum efficiency is 94%, but this efficiency is only obtained for a fairly narrow range of conditions. It is quite possible for the motor to operate at well below 90% efficiency.

As a general guide, we can say that the maximum efficiency of a good quality motor will be quite close to the figures given in Table 6.2 for all motor types, even if they are not induction motors. The efficiency of the BLDC and SR motors is likely to be one or two percent higher than for an induction motor, since there is less loss in the rotor. The SR motor manufacturers also claim that their efficiency is maintained over a wider range of speed and torque conditions.

6.4.2 Motor mass

A motor should generally be as small and light as possible, while delivering the required power. As with the case of motor efficiency, the type of motor chosen is much less important than other factors (such as cooling method and speed) when it comes to the specific power and power density of an electric motor. The one exception to this is the brushed DC motor. We explained in Section 6.1.6 that the brushed DC motor is bound to be rather larger that other types, because such a high proportion of the losses are generated in the rotor, in the middle of the motor.

Figure 6.34 is a chart showing typical specific powers for different types of motor at different powers. Taking the example of the BLDC motor, it can be seen that the cooling method used is a very important factor. The difference between the air cooled and liquid cooled BLDC motor is most marked. The reason for this is that the motor has to be large enough to dispose of the heat losses. If the motor is liquid cooled, then the same heat losses can be removed from a smaller motor.

We would then expect that efficiency should be an important factor. A more efficient motor could be smaller, since less heat disposal would be needed. This is indeed the case, and as a result all the factors that produce higher efficiency, and which were discussed in the previous section, also lead to greater specific power. The most important of these are as follows.

Higher power leads to higher efficiency, and hence higher specific power. This can be very clearly seen in Figure 6.34. (However, note that the logarithmic scale tends to make this effect appear less marked.)

Higher speed leads to higher power density. The size of the motor is most strongly influenced by the motor torque than power. The consequence is that a higher speed, lower torque motor will be smaller. So if a low speed rotation is needed, a high speed motor with a gearbox will be lighter and smaller than a low speed motor. A good example is an electric vehicle, where it would be possible to use a motor directly coupled to the axle. However, this is not often done, and a higher speed motor is connected by (typically) a 10:1 gearbox. Table 6.3 shows this, by giving the mass of a sample of induction motors of the same power but different speeds.

The more efficient motor types, SR and BLDC, have higher power density that the induction motor.

The curves of Figure 6.34 give a good idea of the likely power density that can be expected from a motor, and can be used to estimate the mass. The lines are necessarily

Specific power, kW/kg 10

Specific power, kW/kg 10

0.10

0.10

0.05

0.02

0.01

0.10

100 200 Motor power, kW

Figure 6.34 Chart to show the specific power of different types of electric motor at different powers. The power here is the continuous power. Peak specific powers will be about 50% higher. Note the logarithmic scales (this chart was made using data from several motor manufacturers.)

Table 6.3 The mass of some 37 kW induction motors, from the same manufacturer, for different speeds. The speed is for a 50 Hz AC supply

Speed (rpm)

3000 1500 1000 750

270 310 415 570

broad, as the mass of a motor will depend on many factors other than those we have already discussed. The material the frame is made from is of course very important, as is the frame structure.

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