*3.3. Changes in the Performance of SCIMs between 1945 and 2020*

SCIMs and most of the electromechanical equipment developed in the 20th century underwent a series of improvements and refinements, from conception through the technological advances in construction processes, mainly in the improvement in the quality of the materials used.

Test results based on data from 1945 and 2020 were used to analyse the change in the performance of 359 SCIMs, with speeds corresponding to two, four, six, or eight poles, at a motor rated output power of 3.7, 37, 150 kW, in order to aid in answering the questions (I and II) that motivated this research.

Figure 14 shows the trends in performance of two-pole SCIMs over time, tested from 1945 to 2020.

Figure 14 shows test results from 68 SCIMs organized into three output power categories and arranged over time. In the years in which results were obtained from more than one SCIM of the same speed and mechanical power, the average performance was calculated for the construction of the figure. In addition, in the years in which there were no SCIMs tested at the output power used in the analysis, the linear regression method was used between the adjacent years in which data were available, in order to construct the figure. The same considerations were applied to Figure 15 (four-pole SCIMs), Figure 16 (six-pole SCIMs), and Figure 17 (eight-pole SCIMs).

**Figure 16.** The average performance of 6-pole SCIMs between 1945 and 2020.

**Figure 17.** The average performance of 8-pole SCIMs between 1945 and 2020.

Table 3 presents the cumulative performance gain between 1945 and 2020 for the three analysed power values.


Generally, high-power SCIMs are always associated with high performances. They are often subjected to more rigorous quality control routines by the manufacturers and users, who are concerned about the losses in this equipment because they are primarily the predominant industrial electrical loads. This fact results in SCIMs of higher power such as 150 kW having smaller performance gains over that time interval. Medium-power (37 kW) and low-power (3.7 kW) SCIMs are associated with high performance gains, with accumulated values of 13% and 11.1%, respectively, based on the analysed period. In other words, the reduction in losses in two-pole SCIMs between 1945 and 2020 was 53.1% for 3.7 kW, 70.3% for 37 kW, and 65.1% for 150 kW. The trends shown in Figure 14 and Table 3 for two-pole SCIMs are similar to those in Figure 15 and Table 4 for four-pole SCIMs, in Figure 16 and Table 5 for six-pole SCIMs, and in Figure 17 and Table 6 for eight-pole SCIMs.

**Table 4.** The average performance of 4-pole SCIMs between 1945 and 2020.



**Table 5.** The average performance of 6-pole SCIMs between 1945 and 2020.

**Table 6.** The average performance of 8-pole SCIMs between 1945 and 2020.


According to Table 4, the loss reduction for four-pole SCIMs was 54.8% for 3.7 kW power, 69.1% for 37 kW, and 72.9% for 150 kW between 1945 and 2020.

According to Table 5, the loss reduction for six-pole SCIMs was 54.8% for 3.7 kW power, 69.1% for 37 kW, and 68.1% for 150 kW between 1945 and 2020.

According to Table 6, the loss reduction for eight-pole SCIMs was 45.1% for 3.7 kW power, 65.1% for 37 kW, and 67.2% for 150 kW between 1945 and 2020.

The three curves (3.7 kW, 37 kW, and 150 kW) showed similar trends in the four figures presented (Figures 14–17), making it possible to separate three periods:


Several elements influenced these trends for each of the three periods described above. At first, between 1945 and the mid-1960s, an intensive process of technological innovation was identified, highlighting the following elements that directly influenced the performance gains of SCIMs:


In the second period, between the 1960s and 1980s, SCIMs showed significant drops in performance, making it possible to identify the influence of the following elements. In this period, insulation from varnish was developed. The varnish made it possible to withstand high temperatures without compromising the insulation. For this reason, SCIM designs emerged that admitted more significant losses in the stator winding wires due to increased temperature in the coils. Temperatures up to 180 ◦C, already standardized in the 1970s (Table 7), were observable in some SCIMs.


**Table 7.** Thermal class of insulation of electrical conductors. Source: [86].

Cotton and silk operated only as electrical insulators. In contrast, the varnish used, in addition to being an electrical insulator, is a thermal conductor. This factor made it possible to accommodate the winding wires in even more miniature housings without damaging the insulation and to improve cooling with an increased transfer of heat produced mainly in the stator winding wires to the external surface, via the design of the fins on the housing.

When varnish is used to insulate the winding wire, it conducts the temperature rise resulting from the losses in the stator winding wires to the housing (Figure 18). In the process, the fins are designed to increase the contact area with air, thus improving the heat dissipation process and changing the geometry of the SCIM housing.

**Figure 18.** Stator temperature measurement points (A, B, C, D and E). Adapted from [87].

The temperature reduction in SCIMs between points A and E, expressed in Figure 18, can be described as follows:

**A**—the hottest point of the SCIM, inside the slot that generates the heat from the Joule losses of the stator winding wires;

**AB**—the temperature reduction resulting from heat transfer from the hottest point to the outer wires of the coil. As air is not a good conductor of heat, there must be no "voids" inside the groove. Therefore, the windings must be compacted and impregnated with varnish, filling the voids as much as possible;

**B**—the temperature reduction caused by the insulator inserted between the winding wires and the metal plates. It is common to use special paper or synthetic insulating foil to line the groove;

**BC**—the temperature reduction by thermal conduction in the SCIM core plates;

**C**—the temperature reduction in the contact between the core and the housing. Precision machining of the housing to reduce surface irregularities is essential in heat conduction;

**CD**—the temperature reduction by thermal conduction through the shell thickness;

**DE**—the temperature reduction due to the increase in the SCIM surface exposure caused by the fins.

The reduction in copper mass meant that SCIM manufacturers were able to reduce the final cost of the equipment, since copper is the highest cost input in the construction of SCIMs. This trend was verified in the test reports of the analysed period. An increase in Joule losses (I2R) in the stator winding wires was mainly observed in relation to previous decades. When the section of the copper conductors reduces, the total mass of the SCIM also reduces. The reduction in copper increased the Joule losses and consequently increased the operating temperature of the SCIMs. The heat generated internally could be more easily dissipated in the housing with varnish.

In the third period, between the 1980s and 2020, improvements in the average performance of SCIMs were evidenced mainly by the following observations.

Minimum performance level policies were applied in the world's largest economies between the 1990s and 2020. The policies that indicate the minimum energy performance of equipment are entitled "minimum energy performance standards (MEPS)," which specify minimum levels of energy performance for commercial purposes. The main objective of MEPS is to guide the performance of the equipment for the consumer and establish a minimum legal requirement for commercialization.

Government bodies usually institute MEPS policies. In the case of SCIMs, MEPS are divided into performance classes, allowing different levels that increase the requirement of a specific minimum performance value according to technological advances and market acceptance. Performance classes for SCIMs internationally are harmonized with the IE code in IEC 60034-30-1 [88], which is widely accepted as the global standard, making performance classes comparable across the various regional energy policy documents for SCIMs. The standard defines efficiency classes from IE1 to IE4 (Figure 19), where IE1 is the lowest, and IE4 is the highest. Similarly, in the United States, performance classes IE1 to IE4 are called Standard, High efficiency, Premium efficiency and Super-Premium efficiency, according to NEMA [89]. The new IE5 class has not been defined in detail; however, it is foreseeable in a future edition of the standard. For IE5 SCIMs, the goal is to reduce losses by about 20% compared to the IE4 class [88,90].

**Figure 19.** Efficiency levels in the IEC 60034-30-1 (2014) classification standard curves for 50 Hz, 4-pole SCIMs. Source: [88,90].

The SCIMs tested in 2020 were already IE3. Therefore, in the next few years, it should be possible to make another short jump in the performance gain of SCIMs.

The implementation of MEPS for SCIMs took place in the USA and Canada in 1997 and was later gradually applied in other countries, with modifications implemented by each energy agency of the various countries, but maintaining the harmonization as shown in Figure 20.

**Figure 20.** Timeline of global minimum performance standards for SCIMs. Source: [91–97].

To comply with the new legislation, which imposes higher performance indices, the central intervention of the manufacturers, verified in the test reports of the analysed period, was the reduction of Joule losses in the stator, because stator windings started to be built with more copper mass compared to previous decades. This movement also meant that the

mass of SCIMs, which until then had decreased with time, began to increase, returning to the levels verified in the 1960s.

During this period, other secondary elements were observed that also influenced the improvement of the performance of SCIMs:

1. Advances in the design of SCIMs through the use of modelling software, enabling structural improvements in the coupling and a reduction in vibrations and noise;

2. Three-dimensional computational modelling of electromagnetic fields, enabling project optimization;

3. Advances in the processes of the casting of steel-silicon sheets;

4. Use of more efficient cooling systems (ventilation).

The three periods described led to profound changes in the mass/power ratio of SCIMs. The analysis presented in Figure 4 demonstrates the falling mass/power ratio and points to the lower levels in the following years needing to be updated. For this reason, Figure 21 was created to answer question III, which was one of the questions motivating this research.

**Figure 21.** Changes to SCIMs in the mass/power ratio between 1891 and 2020.

The research relied on SCIM test data from 1945 to 2020. However, SCIM mass data was only available in technical reports from 1997 onwards. Before this date, few reports presented a record of the mass of the SCIM under test. Between 1945 and 2020, records of seven SCIMs with power and speeds compatible with Figure 4 and with mass records were discovered. The mass/power ratio found in these seven SCIMs was compatible with the data published by AEG. Thus, Figure 4, containing results between 1891 and 1984, was updated with data obtained in this research (Figure 21).

To create Figure 21, in 2000, 12 two-pole SCIMs with power between 3.7 kW and 4.4 kW were used, and in 2020, 16 SCIMs were used in the same power range and for the same speed. After calculating the mass/power ratio for each SCIM, the arithmetic mean was calculated for each of the two years under analysis.

A significant result verified in Figure 21 was the increase in the mass of SCIMs from the 2000s onwards, reaching the level of 10.2 kg/kW for the same power and speed, returning to levels verified in the 1950s.

The increase in mass was produced mainly by using conductors of a larger section, to reduce the block where the most significant losses in SCIMs are found, that is, the losses from the Joule effect in the wires of the stator windings.

The reduction in volume of an electrical machine can also result in challenges in keeping components cool. In the case of high heating, deterioration of the properties of most materials (such as insulators, coils, and sheets of ferromagnetic material) can occur, causing a reduction in the useful life of the equipment. This is one of the reasons that justify the average increase in the carcass of SCIMs in the last two decades.

There was also an increase in the lengthening of the rotor package, and consequently of the stator windings, significantly increasing the amount of material used in the construction of the high-efficiency motor, as seen in Figure 22.

**Figure 22.** The difference in the material quantity between Standard SCIM and High-Efficiency SCIM. Source: [98].

In Figure 22, the most significant change made to increase the performance of a 5 HP (3.7 kW) electric motor from 84% to 90.2% was an increase in mass of 27 kg or approximately 33%, while maintaining the same carcass.

For performance gains superior to those shown in Figure 22, increasing the carcass to accommodate the new stator and rotor dimensions was necessary. Figure 21 shows that SCIMs went from a 100L housing in 2000 to a 112L housing twenty years later (2020).

The mass/power ratio depends on the power range and speed, so Figure 21 cannot be directly generalized to other power values without proper adjustments. However, the shape of the curve presents a similar trend for the other power ranges and speeds.

There is no forecast of a continuous increase in the mass/power ratio of SCIMs, as this has been optimized in recent years through technological innovations. Other viable technologies have been presented to reach the IE5 standard. Synchronous operation motors include permanent magnet synchronous motors (PMSMs) and synchronous reluctance motors (SynRMs). Synchronous motors employ a drive that can also control the speed, and they have introduced a series of improvements in motor drives, such as ease of automation, the possibility of pre-diagnosis, ease of application of intelligent sensors, the possibility of collection and analysis of electrical quantities, etc.

PMSMs, for the same power range (4 kW) and speed (two poles) as those shown in Figure 21 can present a mass/power ratio of approximately 4 kg/kW, with a performance above 93%, even for low power and a power factor above 0.95.

SynRMs, for the same power range (4 kW) and speed (two poles) as those shown in Figure 21 can present a mass/power ratio of approximately 7.5 kg/kW, with a performance above 92.8%, even for low power and a power factor above 0.95.

Synchronous operation electric motors do not have rotor losses, and this is one of the main reasons this equipment can raise the level of performance. Synchronous motors also have a smaller physical volume than traditional SCIMs and are touted as the immediate future of variable-speed motor drives. If the economic factor also becomes an attraction, synchronous motors may also be viable in fixed-speed systems.

For SCIMs to reach IE5, two possibilities are currently considered. One is the use of amorphous materials with high magnetic permeability to reduce core losses. Another is the use of copper to minimize losses in rotors traditionally constructed of aluminium.

The magnetic package of SCIMs can be particularly suited to amorphous laminations, as demonstrated by Hitachi with an 11 kW motor prototype that achieved IE5 efficiency [99]. The Hitachi prototype had a reduced size compared with a traditional SCIM and performance above 93% over a wide load range.

Traditional medium- and low-power SCIMs have a rotor constructed primarily of cast aluminium. However, since 2002, it has been possible to find, for some applications, SCIMs with rotors made with copper [100].

The copper squirrel-cage rotor enables a 15% to 18% reduction in total motor losses (this can represent an efficiency gain of 2 to 4%, depending on the power and number of poles) [101]. A copper rotor is made of electrical steel laminations in which the rotor bars and end rings are made of cast copper instead of cast aluminium. Copper is an excellent material for rotors because it has higher electrical conductivity than aluminium [102].

The use of the copper rotor can also support the resumption of size reduction and overall weight reduction of the motor, since the reduction in losses in the rotor allows the reduction of the total length of the rotor and consequently the stator.
