Governors and Switches

Many single phase motors used in hard-to-start applications, such as conveyor belts, oil burner pumps, carbonated beverage pumps, belted fans and blowers, and commercial garage door openers, use a set of parts called a rotating governor and stationary switch assembly. Motors in these applications must have a starting circuit that produces high starting torque while at the same time limiting a high starting current. The rotating governor and stationary switch assembly enable the starting circuit to energize for a brief period of time (typically a fraction of a second) to quickly get the motor up to running speed, limiting the starting circuit to a short "burst."

Rotating governors and stationary switch assemblies have been around for decades. Manufacturers continue to make improvements to these components, developing easier-to-assemble designs with better functionality. Some of this evolution has come as the result of testing the devices under conditions peculiar to certain applications. Depending upon the end use of the product, manufacturers evaluate component performance under very low temperatures or rapid fluctuations in temperatures. In other instances, they may create a test where the switch reverses after each operating cycle. Typically, the engineers will run the switch to failure under these extreme conditions while monitoring every step in the process using high-speed photography combined with other recording instruments. Their objective is to capture the exact moment and (hopefully) the cause of failure.

From there, the research team will analyze the problem going through a cycle of failure analysis, design improvement, and process improvement. New components or design changes are tested once again, often by restarting the failure analysis process. The results are numerous improvements not always perceptible to the casual observer (or even the trained service technician). Companies take these improvements seriously, however, and often they are kept as trade secrets of the manufacturers.

Here are some examples of how this exhaustive process yields improvements in switch design. One discovery made about cold-temperature operation is that most switch designs are not affected by just the cold. Repeated exposure to humidity while the part is cold can build up damaging layers of frost and ice on components. One such application would be an overhead garage door opener, where the repeated opening and closing subjects the opener (and the switch) to a wide range of temperature variations. Manufacturers have created designs that clear offending ice from the switch without causing malfunction.

Another factor, discovered through the use of high-speed photography, is that the switch needs to make and break contact "cleanly." If the contacts are not broken cleanly, the result may be "bounce" that causes the switch contacts to cycle on and off several times before breaking a starting circuit. This bouncing could cause the start circuit to arc unnecessarily and fail prematurely in the application.

You wouldn’t think that so much work would be needed to create a relatively simple device. But consistency and reliability—especially in the face of unusual operating conditions—call for a lot of engineering know-how. It’s that kind of effort that allows switches—and motors—to be used in so many different applications.

by Neil Simon, aka the Motor Doctor - regional sales manager for A. O. Smith Electrical Products Company.