Why do variable speed drives fail and how do we test them?
Freezing cold, intense heat, vibration, corrosive chemicals… It’s impossible to know what sort of dangers drives might face. The answer is to design drives for durability and subject them to rigorous testing.

The physics behind product failure

The first step is to think about the factors that could make drives fail. We take the physics of failure (PoF) approach that divides products into two types and provides two reasons why they fail.

The two reasons for failure are overstress and wear-out, which are related to the product’s strength and durability, respectively. Overstress failure occurs when a product is subjected to stress that exceeds its strength. Wear-out is a longer term failure process: each time a product is exposed to stress it suffers some damage, and the cumulative effect builds up and eventually causes failure when it exceeds the product’s durability (see figure 1).

Why do variable speed drives fail and how do we test them?
Fig. 1: Things fail when stress exceeds their strength. Defective products fail at nominal stress.

Under the PoF approach, products are considered as either nominal or defective. Nominal products will withstand nominal stress. And as long as this stress is not exceeded they will last their entire design lifetime. But defective products will fail at less than nominal stress. You might conclude that defective products will fail quickly when put into service. Yet many do in fact continue to function without any problems. This is because the stress levels in most applications are well below the nominal design limits, so defects don’t turn into failures.

The sorts of stresses that can affect the reliability of drives are mechanical, thermal, electrical, radiation and chemical. Figure 2 shows how these stresses can lead to overstress and wear-out failures.