One of the first things an electrical engineer will learn is that the number-one enemy of designing and manufacturing any electrical/electronic product is heat. It's the one characteristic that can cause your idea of a hand-sized unit to turn into a tabletop unit. When discussing insulation with a manufacturer, the first question that seems to come up is temperature resistance. It is a very important characteristic but there are a few others that need to be addressed as well. In addition to temperature rating, voltage along with mechanical strength play a very important role in the selection of insulation.

Temperature Ratings

Some of the most common insulation and their locations within the motor.

In the electrical apparatus industry, we have a few different temperature ratings, which are categorized into classes (see table). These classes are standard, set forth by the National Electrical Manufacturers Association (NEMA). At the lowest end is Class A insulation. This entry-level insulation class has a rating of 105 °C, average winding temperature rise of 55 °C, hot spot temperature rise of 65 °C, and a maximum winding temperature of 105 °C, which goes back to the insulation rating of 105 °C. Temperature classes in ascending order include Class B at 130 °C, Class F at 155 °C, Class H at 180 °C, Class N at 200 °C, and Class R at 220 °C. Each of these classification ratings has different insulation materials that work best for meeting the desired insulation class.


Once you select your temperature class, you can move on to the next most important characteristic: voltage. Here, you start to really separate insulation options and begin to prioritize the options available for your particular application. Most everyone understands that 125 Volts is the residential standard for voltage for your home, and most know 220 Volts is the voltage of many household dryers. These are just two of the many different voltages used in the electrical apparatus industry. When you begin to really research how many different voltages there are, you will be very surprised. Some of the primary voltages used in electrical apparatus machines are 12v, 24v, 125v, 208v, 220v, 460v, 575v, 950v, 2300v, 4160v, 7.5kv, and 13.8kv. This is why when you start selecting your voltage requirements, the insulation choices really begin to pare down.

The varnish insulation shown on this motor will protect the wire as well as act as a glue to protect from abrasion while keeping the wire and insulation from moving or flexing.

Mechanical Strength

Now that you have these two primary characteristics selected — temperature and voltage — you can move into mechanical strength. This is primarily important in the flexible insulation and tape products; for example, flexible insulation is used in electric motors to insulate the metal stator core from the magnet wire. If the current in the magnet wire were to exceed the strength of the flexible insulation, or if the insulation allowed the magnet wire to touch the metal core, the motor would ground and fail.

Classes of temperature ratings.

The mechanical strength is very critical to the insertion of the product as well as resisting abrasion. All of the products will also have different techniques that need to be utilized when cutting or fabricating them to fit into the electrical apparatus you are building or repairing. This is good to know as some products will increase your cost due to inflated fabrication cost.

Magnet wire being hand-insulated from the metal on this armature. If magnet wire touches the metal, the motor will fail.

Another product used in insulating an electrical apparatus unit is resin or varnish — this is not the varnish that protects the finish of the furniture in your home. This varnish does, however, protect in a similar way. These varnishes and resins are specifically designed to meet a corresponding insulation class and help to protect, bond, and insulate the magnet wire, flexible insulation, and tape insulation system. Selecting this primary insulation component is quite important because it's tied to the other insulating materials that are considered compatible in the insulation system.

Additionally, the electrical apparatus will typically carry the class rating of insulation in that system. As logic would dictate, you would expect that if all class H products are used in an electrical apparatus, the unit should carry a class rating of H; however, this is not always the case. In some instances, there have been cases where the unique and intimate insulation products used in the total system could decrease and sometimes increase the class rating due to the way they may behave together within a seal tube test.

Each apparatus should be tested in a full UL and or IEEE system to truly confirm that the apparatus meets a specific classification; various UL systems are available from manufacturers of varnishes and flexible insulation. Within that UL rating, one argument among the electric motor industry is whether the lead wire for the connections counts in the system. Currently, the lead wire is not considered part of the insulation within the motor. With it not being included in the insulating of the motor, the lead wire can be rated at a lower temperature classification than the insulation within the motor and will not reduce the overall temperature rating. As mentioned earlier, this is and continues to be a point of contention for many manufacturers and rebuilders.

Other Considerations

Decisions around temperature class, voltage, and mechanical strength are the main characteristics that will get you well on your way to selecting the best insulation for your build, but there are other decisions to make that are more specific to insulation needs for your application. These include, but are not limited to, dielectrics, ambient air temperature, moisture absorption (humidity), physical weight, physical size availability, corona resistance, and many more.

Once you have reached this level, get in touch with a qualified insulation distributor expert and provide them additional details of your application. These experts are very good at recognizing and selecting the best products to meet your application needs.

This article was written by Jimmy Walker of EIS, Electrical Specialties Group of Motion Industries, Atlanta, GA. For more information, visit here .