Designing for reliability in pool and spa applications

June 2018

Understanding the operating environment

Designing an electric motor involves a deep knowledge of the motor's application beyond horsepower, mounting and other nameplate specifications. This is just as true for pool and spa applications as it is for non-leisure pump applications. Understanding the environment in which the motor will operate is a key step in designing a more reliable, longer-lasting pool and spa motor.

According to the Association of Pool & Spa Professionals U.S. Swimming Pool and Hot Tub Market 2015 Report, there are 8.535 million installed in-ground and above-ground pools. The highest density of pool owners live in California, Florida, Arizona and Texas, representing approximately 45% of the market. These states pose unique engineering environmental challenges that include elevated temperature, increased rainfall and coastal salt exposure.

To survive in these environments, pool motors are designed with moisture resistance in mind. Bearing selection is one step toward improving moisture protection. Typically, sealed bearings are chosen to help prevent moisture intrusion to moving surfaces and grease/lubrication. Bearings are life tested to several criteria including performance, endurance, lubrication breakdown and corrosion resistance. Sealing materials, such as lip seals, O-rings and water slingers on motor shafts, are incorporated to help prevent water from entering critical motor components. Motor designs are tested extensively in rain chambers before going in the field. In the rain chamber, the motor is placed on a large rack surrounded by water sprayers that operate during on and off cycles while the motor is in operation. The sprayers deliver a rainwater simulation equivalent to five inches of daily rainfall directly onto the motor; testing the design reliability to help prevent water intrusion failures.

Ambient temperature also drives design considerations. With summer temperatures frequently reaching over 37.8℃ (100℉) in many areas of the country, it is important to test motors for their ability to withstand temperature extremes. Most pool motors are designed to operate at temperatures of 50℃ (122℉), and most spa motors are designed to operate up to approximately 60℃ (140℉). The motors are attached to a dynamometer, or loaded water pump, and then placed in an insulated chamber. The air is heated to either 50℃ or 60℃, and the motor must operate at full load without failure to carry load.

Material selection and engineering to help reduce failures 

Material selection and engineering also help to reduce corrosion failures. Pool motors are exposed to several corrosive agents: salt water proximity, chlorine, pool cleaning chemicals and extensive rainfall. Motor frames are traditionally made from steel, and protection from the elements can help prevent rust. Powder coating bare metals is generally considered an economical way to help prevent corrosion in pool and spa motors, as evidenced by its use in everything from picnic tables to industrial machinery. Powder-coated parts are tested for their resistance to corrosion by being placed in a heated chamber and sprayed with an atomized solution of salt water or other corrosive fluid.

Since motor shaft extensions can be vulnerable to corrosion as well, stainless steel is usually the preferred material. Although several varieties and grades of stainless steel exist, 300 series stainless steel is often a popular choice.

Some pool motors are constructed with aluminum frames. These are typically a totally enclosed fan cooled frame design to help prevent moisture intrusion and incorporate a rear-mounted fan and external cooling fins. Since there is a lower likelihood of water entering the motor, corrosion failures are usually minimized. Open drip proof motors can be constructed to help prevent moisture intrusion and buildup inside the motor by incorporating internal fans and air channels to maximize air movement. Engineers develop simulations of air movement called Computational Fluid Dynamics (CFD) models. These models allow engineering teams to optimize air speed, temperature and direction by varying the fan profile and air channel size and routing. Such design elements help prevent condensation and water entrapment from accumulating inside the motor.

Quality control to help ensure reliability and energy efficiency 

Outside of traditional mechanical failures, electrical component failures are extensively studied to help improve overall motor reliability. Motor starting switches and capacitors are among common contributors to pool motor failures. To prevent this issue, switch components are routinely tested to 500,000 to 1 million cycles before being approved for use. Some motors have switchless designs to eliminate the possibility of switch failures entirely. Extensive quality control efforts, including regular testing of switches and capacitors, help prevent electrical component failures that could reduce the product lifespan.

Moisture prevention, reliable operation in extreme temperatures, corrosion resistance and dependable electrical components are just a few of the considerations engineers keep in mind when designing pool and spa motors to help ensure consistent performance and longer life. By considering the various ways that pool and spa motors interact with their environments, design and application engineers, like those at Nidec Motor Corporation, help ensure high levels of reliability and energy efficiency. This contributes to pool and spa equipment running more smoothly each summer.

Sources:
APSP.Org