Operating temperature has played a role in electrical machines throughout their history. Already the first electrical engineers noticed that failures are often indicated by smoke, and smoke is an indication of high temperature. On the other hand, one of my former colleagues said once that every machine winding failure is a cooling failure, because it would have been prevented if only more cooling could have been arranged.

There has always been a need for higher operating temperatures. More and more power has to be produced from the same machine frame. The output power defines the price the customer pays for a machine, but the physical size of the machine defines how much it costs to build one. An obvious way to increase the power from the same frame is to allow the machine to run at higher temperature, if only allowed by the technology, materials, and cooling system. Higher winding temperature allows higher stator current, which means more power. The machines are hardly ever dimensioned for their true physical boundaries, so they usually have margin for increased output power. For instance, traditional industrial induction motors have far higher breakdown torque margins than required by the IEC standard.

Temperature classes have historical background. In their famous paper in 1913 Steinmetz and Lamme [2] divided the insulating materials into three classes ”A”, ”B”, and ”C” based on their resistance to temperature. Class ”A” materials could be used at 90 °C and class ”B” 125 °C. Class ”C” was considered for ”fireproof” high temperature materials, but no definite temperature value was given. Today, class A refers to 105 °C and class B to 130 °C, but class C is not used. Interesting example is related to the old mica asphalt insulation system introduced in 1930s and widely used in high-voltage generators. In classification by Steinmetz and Lamme it falls into class ”A” (90 °C). Asphalt is a thermoplastic material and behaves like a fluid with high viscosity rather than a solid. But interestingly, this classification anchored many of the high-voltage generators into the same temperature class for decades to come, even though their insulation technology took major steps forward. Still in 1970s generators were designed for 90 °C maximum temperatures, even though their insulation system was valid for class F (155 °C).

The present-day low-voltage motors operate commonly below 130 °C (class B) temperature limit, and contain either a class F (155 °C) or class H (180 °C) insulation system. Materials used in the insulation systems are based on traditional technology: Nomex paper, Mylar film, or Kapton film used as groundwall insulation, polyester-derivatives as wire insulation, and polyester or epoxy resin for impregnation.

Progress in insulation technology is generally slow, even though new polymers are being constantly developed. When a new material is first released, it needs plenty of testing and evaluation until the machine workshops are willing to accept it. For example, a test series for motor insulation system according to IEEE 1776 (form-wound) or IEEE 117 (random-wound) easily takes more than a year to accomplish. In addition, certain material certificates (UL or NORSOK for example) require sometimes even more strenuous test program. Obviously, the certificates improve the quality of the material: if it tolerates extensive test series, it must be a good material. But there is another side to that story. The certificates extend the lifecycle of old and outdated materials. Even though new materials are introduced and they are better in every technical way, many company prefers to use the old materials, because they still have valid certificates, and because – provided by the out-of-date technology and reduced customer base – they are cheap. Furthermore, replacing the old insulating materials may bring surprising challenges, and costs. Replacing the impregnating resin usually means investing into a parallel, new, and costly VPI process equipment. Or alternatively, the new resin requires a VPI process, whereas the old resin works reasonably well with inexpensive dip-and-bake process.

For the reasons mentioned above, many new motors and generators are built today with materials developed in the last millennium. To be accepted in production, new insulating materials and insulation systems must provide clear performance boost, or savings in mass production scale. One form of a performance boost is provided by increased operating temperature.

Increasing the temperature class from the above-mentioned and typical class B to the succeeding classes F or H, or even further provides obviously increased output power, but also other advantages. First, increased motor power means that a certain load can be driven with a motor of smaller frame size. Second, higher temperature requires less cooling. The machines cooled with external blowers can use less blower power. Similarly, machines cooled with a shaft-installed fans can use smaller fans. Less cooling power means higher machine efficiency. Third, higher temperature class provides more safety margin. The machine can be allowed to run hotter, for longer duration, or with higher power during unexpected fault conditions. Also, all kinds of failures on site which are somehow affected by temperature can be expected to reduce. Finally, allowing higher temperature may rescue the machine from fatal design flaws. Wrongly dimensioned coils, magnetic circuit, or cooling system all tend to increase the winding temperatures. In the worst case, this leads to over-heating, and the machine will not be able to deliver its rated power. With added temperature margin, the rated point may be reached without redesigning the entire machine.

Increasing the temperature also brings risks, which need to be assessed. If the winding temperature is increased, most likely the temperatures in other parts of the machine go up as well. Few potential issues are listed here. In permanent magnet machines higher temperature increases the risk for magnet demagnetisation, and the magnet grade may have to be changed. If the bearing temperatures increase, changes may be required for the bearing design, type, or lubrication fluid. If the temperatures increase in the rotor, and the application is critical in this respect, the rotor-dynamics may be compromised. In large diameter machines the thermal expansion of rotor parts may cause mechanical issues, and require reinforcing structures. High winding temperature may also lead to hot frame surfaces, which might require preventive measures against thermal injuries, or compromise the ATEX rating in machines running in explosive environments.

The latest progress in insulating material technology enables to go beyond class H, which has been widely considered as maximum. Many suppliers (e.g. 3M, Elantas, and Vonroll) have commercially available insulating materials for 200 °C insulation system, for which we have no letter. Using the newest material data, we can sketch a low-voltage insulation system for 200 °C roughly as follows:

  • Polyesterimide + polyamideimide wire insulation
  • Mainwall insulation made of Nomex and Kapton
  • VPI with epoxy or polyesterimide resin

As we can see, the proposed 200 °C insulation system does not contain any exotic materials. Instead, it is essentially similar to the traditional class F or H insulation system, and more importantly the winding can be manufactured in the same way as before. Only minor specifications are changed. Hence, also the cost effect of skipping these temperature classes is moderate.

The above proposal is still based on the manufacturing technology and methods dating back 100 years or so. Bringing the electrical machines to the next level may require revising this logic. One potential way could be to use high-performance plastics such as PTFE (Teflon) or PEEK, which can both be used at 250 °C. PTFE is already a common material in low-voltage signal cables. PEEK has been traditionally used in various types of injection moulding components (e.g. bolts or cable ties) in demanding oil and gas applications, but lately it has been introduced as a sheet for electrical machine groundwall insulation. Also, the traditional inorganic materials like mica and glass both tolerate very high temperatures. Obviously, plenty of manufacturing headaches are involved here, and they are not speculated any further. But for example a motor stator with glass-insulated magnet wires injection-moulded in PEEK plastic could be operated at 250 °C, and could also tolerate very harsh operating environments. That should be enough of motivation to gradually open up the Pandora’s box for the next skip in temperature classes.


Further reading:

1 Shugg,W. T., Handbook of Electrical and Electronic Insulating Materials, 2nd ed., IEEE Press,
New York, NY, USA, 1995.

2 Steinmetz, C. P. and Lamme, B. G., ”Temperature and Electrical Insulation”, Transactions on
AIEE, Vol. 32, No. 1, 1913, pp 79 – 89.

3 Stone, G., Culbert, I., Boulter, E. A., Dhirani, H., Electrical Insulation for Rotating Machines
– Design, Evaluation, Aging, Testing, and Repair, 2nd ed., Joh Wiley & Sons, Ltd, 2014.