The grade of insulating material is divided into five grades according to its heat resistance limit temperature.

(1) Class A insulation: Its limit temperature is 105 degrees Celsius, and the insulation material is organic fibers such as cotton, silk, wood, paper and so on, such as laminated cardboard, yellow wax cloth, yellow wax silk, etc. The commonly used electromagnetic wires are single yarn oil-coated wire, double yarn wrapped wire, paper wrapped wire, etc.

(2) Class E insulation: the limit temperature is 120 degrees Celsius, and the insulation material is a heat-resistant silicone organic paint film, such as polyester film composite highland barley paper, triacetate fiberboard, etc. The commonly used electromagnetic wires are high-strength polyester enameled wire, high-strength polyvinyl acetal enameled wire, etc.

(3) Class B insulation: the limit temperature is 130 degrees Celsius. The insulation material is based on inorganic materials such as mica, asbestos, glass fiber, etc. It is reinforced with Class A insulation material and bonded with organic paint, such as mica paper (board), alkyd glass enameled cloth polyester film, asbestos paper, etc. The commonly used electromagnetic wires are high strength polyester enameled wire, epoxy enameled wire, etc.

(4) Class F insulation: the limit temperature is 155 degrees Celsius, the insulation material is the same as Class B insulation material, but it is bonded with heat-resistant silicone organic paint, such as silicone organic glass cloth, etc. The commonly used electromagnetic wires are polyimide enameled wire, silicone organic enameled double glass wire, etc.

(5) Class H insulation: the limit temperature is 180 degrees Celsius, the insulation material is the same as Class B insulation material, but it is not reinforced by Class A insulation material; the commonly used electromagnetic wires are double glass filament wrapped wire impregnated by silicone organic acid, polyesterimide enamelled wire, etc.