Analyzing premature winding failures (part 1)

There is a shared responsibility when motors fail.

By Peter Walker, Walker Enterprises

PlantServices.com

Whose fault is it?

That is the question frequently asked when a motor recently repaired by an outside vendor is returned to operation and then fails shortly thereafter. Premature motor insulation failures cause tension between the process plant and the motor repair vendor. The reason for the tension is obvious. For the operator, an unexpected motor failure means an unplanned shutdown with a loss of tens of thousands of dollars of production. To the motor shop, the failed motor is an expensive liability, and there is an understandable reluctance to accept responsibility. The vendor is often forced to choose between "eating" the cost of a rewind or losing customer good-will and future business.

It is very important that the two parties get together and agree on the underlying cause of the failure. Unfortunately, in the case of winding failures, the underlying cause is not always obvious. In contrast to mechanical failure where the operator is usually able to see and understand the failure mechanism, the circumstances are quite different in the case of winding failures. Such failures often originate in the internal parts of the stator and are not readily detected during the early stages. Indeed, in many cases, by the time the problem becomes evident, a portion of the stator may be partially destroyed.

Deducing a possible cause of failure does require a knowledge of the factors that may be involved.

The nature of insulating materials.

Organic materials used for electrical insulation are subject to aging and deterioration from a variety of stress factors. The aging is a gradual process during which the material loses the properties which initially made it desirable as an insulation. In its final stage, the material is no longer able to perform its function and a winding fails.

Normally insulation aging proceeds slowly taking many years to reach an ultimate failure. If the various stress factors that cause accelerated aging are kept to low levels, the life of motor insulation can be very long indeed. Older machines that were designed conservatively remain in operation for more than 45 years. Achieving such a long life requires a conscientious maintenance effort.

Of the factors that affect insulation life, the most basic is temperature. Operating motors with high temperature rise in high ambient temperatures shortens the ultimate insulation life. Conversely, minimizing operating temperature enhances life expectancy. Achieving a low temperature rise becomes a design trade-off against manufacturing cost. A lower temperature rise requires more pounds of steel and copper per horsepower. For that reason, it is common to see large high-cost motors (1,000 hp plus) with a conservative temperature rise of less than 80 degr  C. Smaller motors characteristically have a higher temperature rise with reduced life expectancy.

Insulation classes
 Because of the important role that temperature plays in determining insulation life, insulation materials are classified according to their temperature capability as shown in Table I.

Table 1

Insulation Classes
Class	Max Temp degr C
O	90
A	105
B	130
F	155
H	180
C	220

The basis for the insulation classification is an accelerated life test. Specifically, the material must maintain its insulating properties after 20,000 hours  (833 days, 2.28 years) at the designated temperature. Commercial insulation is subjected to thermal aging tests as well as other tests prior to becoming available for use.

During the past 20 years, the class F insulation system has become the standard for industrial motors. Most motors do not operate at 155 degr C (311 degr F ). However, keep in mind that the <I>internal<I> hot spot temperature determines the thermal life of the system. It is likely to be substantially greater than any <I>external<I> or measured temperatures.

About 50 years ago it was shown that thermal aging is the result of chemical reaction within the material itself. The process, accelerated by higher temperature, embrittles and degrades the properties of the material.

The temperature sensitivity of organic insulation materials gives rise to the "ten degree rule" for extrapolating a life expectancy. The ten degree rule states that each ten degree increase in temperature cuts insulation life in half. Conversely, each ten degree decrease in temperature doubles insulation life. The relationship can be expressed mathematically as L = 2<+>DT/10<+> x 20,000 where L is expected life in hours, DT is the difference between the maximum temperature for the class of the material and the actual operating temperature.

Sample calculation
A class F motor is known to have a hot spot temperature of 120 degr  C.  If there are no other factors present to influence the outcome, what is the theoretical life expectancy of the insulation? For class F the maximum temperature is 155 degr  C. Then DT = 155 - 120 = 35 degr C. The term (DT/10) = 35/10 = 3.5, and L = 2 <+>3.5<+> x 20,000 = 226,000 hours, approximately 26 years.