The high efficiency of an IE4 super premium motor does more than lower the energy bill; it also directly affects the thermal balance inside the motor. As efficiency rises, the losses converted to heat inside the motor decrease and the winding runs cooler. At this point two concepts that determine motor life come into play: insulation (thermal) class and temperature rise (ΔT). The insulation class describes the maximum temperature the winding can withstand, while temperature rise describes how much the motor heats up under load. The difference between the two forms the motor''s "thermal margin", that is, its safety margin. In this article we cover the F and H insulation classes, the B and F temperature-rise limits, the thermal reserve arising from combining the two, the advantage of an IE4 motor at high ambient temperature, and the 10°C rule that governs winding life.

Insulation Class: The Maximum Temperature the Winding Can Withstand

The winding of an electric motor is protected by insulation materials surrounding the copper conductor. This insulation retains its electrical and mechanical integrity up to a certain temperature; beyond that it degrades over time. The insulation class defines this withstand temperature. The two most common classes in industrial motors are F and H:

  • Class F insulation: The maximum permitted winding temperature is 155°C. Today the vast majority of standard industrial motors are produced with Class F insulation.
  • Class H insulation: The maximum permitted winding temperature is 180°C. It withstands higher temperatures and is preferred for hot environments and demanding applications.

As in the HEM Motor range, standard production is usually done with Class F insulation; Class H insulation is offered as an option in special applications requiring high temperature. The critical point here is that the insulation class alone is not a performance indicator; what really matters is how much the winding actually heats up, that is, the temperature rise.

Temperature Rise (ΔT): How Much Does the Motor Heat Up Under Load?

Temperature rise expresses how far the winding temperature climbs above the ambient temperature while the motor runs at rated load, and it is given in Kelvin (K). The reference ambient temperature is taken as 40°C by standard. Temperature-rise classes are also shown with letters:

  • Class B temperature rise: Corresponds to about 80 K rise. So in a 40°C ambient the winding reaches about 120°C.
  • Class F temperature rise: Corresponds to about 105 K rise. In a 40°C ambient the winding reaches about 145°C.

The subtle distinction to note is this: the insulation class (F/H) describes the material''s withstand, while the temperature-rise class (B/F) describes how much the motor actually heats up. Even though both are shown with the same letter, they are different things. Our article on temperature-rise class and rise (ΔT 80K), where we covered temperature rise and the thermal class in detail for asynchronous motors, reinforces this distinction.

F Insulation + B Temperature Rise: What Is Thermal Reserve?

The combination that motor manufacturers frequently apply and that is very valuable in terms of quality is Class F insulation with Class B temperature rise. Let us see the logic of this combination with numbers:

  • F insulation says the winding can withstand up to 155°C.
  • B temperature rise says the winding heats up to only about 120°C in a 40°C ambient.
  • The difference of about 25-30°C is an unused thermal margin (thermal reserve).

This thermal reserve is worth its weight in gold for the motor''s life and reliability. Because in real life the motor does not always run under ideal conditions: ambient temperature can exceed expectations, voltage unbalance can occur, load can fluctuate, or the cooling fins can become dirty over time. The F insulation + B rise combination preserves life by allowing the winding to operate without reaching its maximum withstand temperature even in these adverse situations.

Diagram showing the relationship between insulation class and temperature rise (thermal reserve) of an IE4 motor winding

How Does IE4''s Low Loss Contribute to Thermal Margin?

Here IE4 super premium motors have a hidden advantage. An IE4 motor produces fewer losses than a lower-efficiency motor of the same power; that is, less heat is generated inside it while doing the same work. Less internal heat means a lower temperature rise. When combined with F insulation, this means a wider thermal reserve.

In practice this means: a high-efficiency IE4 motor runs cooler in the same environment than a lower-efficiency equivalent, and its winding lasts longer. The efficiency gain should be seen not only as energy savings but also as a gain in life and reliability. You can find where the losses decrease in an IE4 motor in our efficiency losses (iron, copper, friction) article, and the effect of cooling and fan design on efficiency in our cooling and fan design article.

The Advantage of IE4 at High Ambient Temperature

Standard temperature-rise values are given for a 40°C ambient. But in many industrial facilities the ambient temperature is above this: foundries, areas around furnaces, boiler rooms, outdoor field cabinets under the summer sun, and so on. As ambient temperature exceeds 40°C, the total temperature the winding reaches also rises, approaching the insulation class limit.

This is where the low temperature rise of an IE4 motor becomes decisive. Thanks to the lower ΔT, even when ambient temperature rises, the winding reaches the withstand limit of the insulation class later. This both reduces the need for power derating and preserves the life of an IE4 motor in hot environments. We covered the subject of power derating at high ambient temperature in our derating at high ambient temperature and derating at high altitude articles. You can find insulation class selection in hot and dusty environments in our insulation class in hot/dusty environments article.

Winding Life and the 10°C Rule

The life of insulation materials is directly related to the temperature at which they operate, and this relationship follows an exponential (logarithmic) curve. A practical approach commonly used in engineering is the 10°C rule: each roughly 10°C permanent increase in winding temperature roughly halves the expected life of the insulation. The reverse is also true; running the winding 10°C cooler can roughly double its life.

This rule clearly explains why thermal reserve is so important. Considered through the 10°C rule, the 25-30°C margin provided by the F insulation + B rise combination makes a striking contribution to winding life. So a cooler-running IE4 motor does not just consume less energy; it also serves trouble-free for much longer. Using protection elements such as PT100 and PTC thermistors to monitor winding temperature in the field secures this life; we addressed the subject in our thermal relay, relay and fuse selection article.

IE3, IE4 and the Thermal Consequences of Efficiency Class

As efficiency class rises (from IE3 to IE4, from IE4 to IE5), internal losses decrease and thermal behavior improves. For this reason, efficiency class selection relates not only to energy cost but also to the thermal life of the motor. We covered in detail when which efficiency class makes sense in our IE4 vs staying with IE3 article, and mechanical compatibility in the IE4 transition in our mechanical compatibility (frame, foot, shaft) article. To see the effect of insulation class on life in IE3 motors, our IE3 winding and insulation class (F/H) article also completes the topic. For the full product range you can visit the HEM Motor home page.

What to Look at Thermally When Purchasing?

When buying a motor, instead of glancing at a single efficiency value on the nameplate, you should evaluate the motor''s thermal identity as a whole. The insulation class (F or H), the temperature-rise class (B or F) and the thermal reserve left by the design determine how long the motor will last in the real world. A good supplier documents these values clearly and recommends the right combination according to the ambient temperature of your application. Especially in applications requiring high ambient temperature, frequent start-stop, or continuous torque at low speed with a VFD, thermal margin should be placed at the center of motor selection.

How Is Temperature Rise Measured and Why Is the Resistance Method Used?

A motor''s temperature rise must be measured so as to represent the average temperature of the winding. Since it is not possible to insert a thermometer directly into the inner layers of the winding in the field, the most reliable and standardized method is the resistance change method. Because the electrical resistance of the copper conductor increases proportionally with temperature, the winding resistance is measured when the motor is cold and after it has heated under load, and the difference is converted to temperature rise. This method is essential in assessing insulation life because it gives the average temperature of the whole winding rather than a single point read from the surface.

Understanding this measurement logic also explains why the nameplate values are a reliable quality indicator. The manufacturer documents temperature rise by testing the motor at rated load, rated voltage and reference ambient temperature. A good supplier can share these test results, because temperature rise is measurable proof of the motor''s true thermal quality. If you want to monitor winding temperature continuously in operation, embedded PT100 sensors or PTC thermistors take on this task and protect the motor in case of overheating.

The Effect of Voltage Unbalance, Harmonics and VFDs on Winding Temperature

Temperature rise is not determined only by load and ambient temperature; supply quality also has a direct effect. Voltage unbalance in the grid creates current unbalance between phases, which in turn causes additional heating in the winding. Even a small voltage unbalance can cause a disproportionate increase in winding temperature; therefore supply quality is a hidden factor that directly consumes thermal margin.

Similarly, when the motor is supplied by a variable frequency drive (VFD), the harmonics produced by the drive create additional loss and heating in the winding. Especially in applications requiring continuous high torque at low speed, the motor''s own fan may not cool sufficiently; in this case an external forced-cooling fan may be needed. This is exactly where the low base loss of an IE4 motor and the F insulation + B rise thermal reserve come into play, helping to keep the temperature rise caused by these additional loads within safe limits. We covered VFD- and harmonic-induced heating and protection in detail in our VFD and harmonic-induced heating article.

The Practical Meaning of Thermal Margin for the Operator

Although the concept of thermal reserve looks technical, it produces very concrete results for the operator. A motor with a wide thermal margin keeps running without reaching the winding limit even during sudden load increases, hot summer days, maintenance intervals when cooling fins are dirty, and voltage fluctuations. This is a safety buffer that prevents unexpected motor burnouts and unplanned downtime. On a production line, the burnout of a single motor often means not just the motor cost but a far greater production loss.

Therefore, in motor selection, choosing a motor that leaves margin suited to the application''s thermal conditions, rather than "buying the cheapest suitable motor", is a decision that lowers lifecycle cost. This approach quickly pays for itself especially in motors that run continuously, take part in critical processes or operate in hot environments. The low losses of an IE4 motor start this thermal margin from a wider point in the first place, giving the operator a two-way gain of both energy savings and reliability.

Choosing the Right Thermal Combination by Application

Not every application requires the same thermal margin. For a motor running at normal ambient temperature under continuous, balanced load, the standard F insulation + B rise combination is often more than sufficient. But if the ambient temperature is high, the motor frequently starts and stops, it produces continuous torque at low speed with a VFD, or it takes part in a critical process, keeping the thermal margin even wider and, if necessary, moving to H class insulation should be considered. Making this decision correctly requires evaluating the motor''s ambient temperature, duty type (such as S1, S6) and load profile together. You can find duty type selection in our duty type (S1-S6) selection article.

Frequently Asked Questions

Are F insulation and F temperature rise the same thing?

No, although often confused they are different concepts. The F insulation class says the winding material can withstand up to 155°C; this is a withstand value. The F temperature-rise class says the motor heats up by about 105 K at rated load, meaning the winding actually reaches 145°C; this is a realized value. In good design, F insulation is combined with the lower B temperature rise to create thermal reserve.

I will run in a hot environment; should I get H insulation or is low ΔT enough?

The two are evaluated together. An F insulation motor with a low temperature rise (for example B rise) often provides sufficient thermal margin at moderately high ambient temperatures. If the ambient temperature is very high or the safety margin is critical, the H insulation option may be preferred. The best decision is made together with your supplier by sharing your ambient temperature and load profile.

Does the 10°C rule really double motor life?

The 10°C rule is a practical approach describing the exponential relationship between insulation life and temperature. A roughly 10°C permanent drop in winding temperature can roughly double the expected life of the insulation; conversely a 10°C rise roughly halves it. Although the exact figure varies with the material and conditions, the trend is clear: running the motor cooler extends life. That is why thermal reserve and low-loss IE4 motors are advantageous in terms of life.

Get a Quote

To determine the IE4 motor with the insulation class and temperature-rise combination most suitable for your application''s ambient temperature and load profile, consult the HEM Motor expert team. Share your ambient temperature, operating hours and load information; we will offer you the best thermal solution. To get a quote right away, call +90 (532) 345 49 86 or reach us via our contact page.

Purchasing Checklist

  • Confirm the motor''s insulation class (F=155°C or H=180°C).
  • Learn the temperature-rise class (B≈80 K or F≈105 K).
  • Ask whether there is a thermal-reserve combination such as F insulation + B rise.
  • Determine the real temperature of the operating environment (is it above 40°C) and consider derating or H class if necessary.
  • Take into account the additional thermal margin gained thanks to the low loss of an IE4 motor.
  • Plan the winding temperature monitoring (PT100/PTC) need and terminal connection.
  • If there is high ambient temperature, frequent start-stop or continuous torque at low speed, prioritize thermal margin in selection.