Motor Insulation

Insulation systems must be used in a motor to electrically insulate the windings from the mechanical parts of the motor as well as to insulate the phase winding conductors from one another. Extra insulation is also required between any adjacent phase windings for 230 or higher voltage motors. In general, the higher the operating voltage, the better the insulation system must be.

All NEMA-designed electric motors are manufactured with cord laced end turns to assure that they are positioned so they cannot touch any mechanical parts. In addition, the phase windings are nearly always impregnated with a varnish by either dipping or trickling into the heated end turns. The purpose of this is to secure the conductors to prevent them from vibrating between one another, which would wear through the wire insulation and cause turn-to-turn shorts. This stator varnish also provides some additional insulation protection to the system. For example, this coating makes the windings resistant to ambient conditions such as moisture. Consequently, a motor insulation system is very complex, utilizing several different materials, parts, and processes to effectively insulate the windings. The parts considered in an insulation system include slot cells, phase barriers, conductor insulation, slot wedges, end turn supports, tie material, and winding impregnation material.

Since the introduction of PWM inverters for use with all types of variable speed drives, such as ac induction, pM brushless and switched reluctance brushless, a new insulation system failure mode has emerged. This is a most significant problem in installations where the inverter is located 50 ft (15 m) or more away from the motor. The long motor lead cables used in these applications cause very high voltage spikes not present at the inverter end of the lead cables. These voltage spikes across the first turns of each phase winding result in the degradation of the wire insulation due to the corona insulation failure. Special magnet wire insulation and motor manufacturing methods are available to overcome this problem. Therefore, if an inverter is to be used to drive a pump motor with long connection cables, an "inverter duty" motor should be specified.

There are four basic classes of insulating materials currently recognized by the motor industry. Each differs according to its physical properties and can withstand a certain maximum operating temperature (frequently termed total temperature or hot-spot temperature) and provide a practical and useful insulation life. The insulation classes and their maximum operating temperatures are

Those factors that contribute to the maximum operating temperature of a motor insulation system are the ambient temperature, the temperature rise in the motor winding caused by motor losses, and any overload allowance designed into the motor (service factor).

The current standard for motors within the range of NEMA ratings (frames 140T to 449T) requires nameplate marking for the maximum allowance ambient temperature, the power rating, the associated line current needed to develop this power, the class of insulation used, and the service factor provided. Motors larger and smaller than NEMA frame sizes have nameplate marking for maximum allowable ambient temperature, temperature rise in degrees Celsius either by thermometer or resistance measurement, power rating, line current needed to develop this rating, and any service factor provided.

In addition to ambient temperature, there are several additional environmental conditions that must be considered when applying an electric motor. In applications where chemical fume or moisture levels are abnormal and can cause decomposition of an insulation system, standard insulation will be inadequate. These applications require a motor with a premium insulation system that will incorporate highly resistant components and may include special impregnation techniques. Chemical fumes and moisture can also be destructive to the mechanical parts of a motor, and special protective treatment should be provided to these parts. NEMA frame-size motors have a special motor for chemical industry applications that has standard features to resist these environmental factors. For example, these motors usually contain shafts made from stainless steel.

Applications with excessive vibration can destroy a winding and damage the mechanical parts of a motor. In such cases, it is advisable to provide (1) extra treatment for the winding to ensure that it is rigid and will not vibrate and chafe the insulating materials and (2) a mechanical construction that will have the strength to withstand the above.

If abrasive dust is present, the motor insulation should be protected with a resilient surface coating to withstand the impact of the abrasive particles.

Because all insulation systems employ components that can in some degree support fungus growth in tropical locations, motors applied in such areas should incorporate fungus-proofing treatment on the insulation.

Class B Class F Class H

Obviously, applications exhibiting a combination of any or all of the environmental conditions discussed should have special protection for each condition. Applications in unusual environmental conditions may require special protection and should be discussed with the motor manufacturer or distributor.

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