Motion Control Resources
IP69K-Rated Motors Survive Ultra-Harsh Environments
by Kristin Lewotsky, Contributing Editor
Motion Control & Motor Association Posted 03/19/2012
Focusing on seals, connectors, thermal management, and redundant design delivers motors capable of tolerating repeated washdown.
Motion control provides critical high-speed positioning in demanding areas like food packaging, pharmaceutical manufacturing, medical, and automotive. All too often, though, precision positioning is only half the battle. These applications expose systems to punishing treatment like autoclaving, high-pressure spray, and frequent washdown with caustic chemicals. To tolerate these types of harsh environments, components must be designed to meet standards like those defined by IP69K, which specifies components capable of enduring high-temperature, high-pressure water jets.
Know the Code
IEC 60529 defines ingress protection using a multi-symbol code, the most common elements of which are below. The basic syntax is IPYZ where Y represents the level of protection against incursion by solids (see table 1) and Z represents the level of protection against incursion by liquids (see table 2). Note that each level is defined for a very specific set of conditions, and often for a limited amount of time.
A variety of standards around the globe define ingress protection (IP) ratings for electric motors. Perhaps the most broadly applicable global standard is IEC 60529, from the International Electrotechnical Commission. The document establishes performance levels for electrical components in a range of operating conditions (see sidebar). IP ratings allow end-users of motion control technology to specify and reliably purchase components to serve their application.
The IP rating covers the survivability of components upon exposure to incursion of solids ranging from the dust to larger objects such as wire or even fingers. The standard also addresses component survivability after exposure to moisture in a variety of forms ranging from dripping water to complete submersion. The standard includes exposure to jets of water; an IP66-rated motor, for example, must be able to tolerate 100 L/min of water at 0.1 MPa from a 12.5-mm nozzle located 3 m away. This rating covers a large volume of water but not necessarily one applied at high pressure. In 1993, the German Institute for Standardization (Deutsches Institut für Normung (DIN) extended IP ratings with DIN 40050-9, which provided a definition/test protocol that encompassed high-temperature, high-pressure water jets. Originally developed for road vehicles - to cover spray cleaning of cement mixers, for example - the extension also applies to the conditions presented by food, pharmaceutical, and medical applications, among others.
The IP69K standard calls for exposing the component under test to 14 to 16 L/min of water at 8 to 10 MPa from a nozzle no more than 15 cm away. The standard specifies placing the jets at four positions between zero and 90°, and rotating the component to ensure multiple angles of exposure to the water, which must be held at 80°C.
Of course, it's one thing for a component to survive a test of a few minutes. It's another to develop something that will perform over the long haul. "It’s a pretty brutal environment," says Matt Hanson, vice president of business development at Bison Gear and Engineering Corp. (Charles, Illinois). "For us, the testing is the easy part. The hard part is designing a component that can survive out in the field for years.”
Designing to meet IP69K
Developing IP69K-rated components presents a number of challenges, including seals, thermal management, and connection/termination of leads. Not surprisingly, redundancy plays an important part in design philosophy. "Generally, industrial gear motors and high-pressure water are not spoken of in the same sentence, so you’ve got to have something designed to survive,” says Hanson. Passivized steel housings provide good long-term protection from caustic chemicals. Merely enclosing the components does not provide sufficient protection, however. There has to be a seal where the rotating (for a rotary motor) or sliding (for a linear motor) shaft exits the housing. A multi-tiered approach to seals can help block the entrance of moisture by combining a multi-lip seal with metal flange or some other sort of mechanical barrier to protect the seal material.
This approach is not enough to protect the motor from years of washdown. Even the best seal will eventually wear and degrade. It can be replaced. The stator, for example, with its vulnerable wiring requires additional barriers to guard against condensation or any moisture that might penetrate the case. A good solution is to place the stator inside a ruggedized housing and then fill it with potting compound (see figure 1).
“You have a stainless steel shell on the outside and on the inside you’ve got a fully potted machine, so in the rare event that water should find a way to get in, it won’t adversely affect the motor,” says Hanson.
The dual approach of enclosure and encapsulation provides a good solution for linear motors, as well. Typically, the magnet track of a linear measure is formed into U-shaped channel and a carriage, or slider, moves back and forth within that track. Although the slot of the channel can be shielded for certain types of applications, it's obviously a bad fit for a washdown environment.
A more robust approach involves designing the stator and the slider as two separate, enclosed elements (see figure 2). Instead of a U-shaped magnet track, the configuration consists of a ring of magnets placed in housing. The coils are enclosed in a steel tube and potted, and then the tube is nickel plated separately. The two components can thus move as separate elements while remaining protected, with the exception of wiring.
It does impose a modest performance trade-off. "Our repeatability is plus or minus two thousandths of an inch [about 50 ?m]," says John Zenner, CEO of LinMot Inc. “We provide a motor capable of a great amount of acceleration, very high speeds, and great controllability, but we are limited as far as accuracy and force. [For the right application,] we are probably the best bet in terms of cost, and simplicity. However, if you are going to try to get into the submicron range for applications, it is not the right solution.”
Cooling and Connecting
One of the challenges in building a motor for a washdown environment is thermal management. Although fans are obvious options, they require air inlets. Such openings can not only admit liquid but can create nooks for bacteria to proliferate in the warm, moist environment. Ideally, washdown motors should not have fans or fans shrouds. A good alternative is to use thermally conductive epoxy as the potting material so that it not only encapsulates the component but it transfers heat. “A lot of people look at the potting material and think it's an insulator, but actually, it dissipates heat better than air," says Hanson. "From a motor standpoint, the unit cools better with the potting material than without."
Water cooling provides another useful option for thermal management. “If we have an application where the usage is rather heavy or continuous, then we water cool the jacket of the motor using very small tubing inside the stainless steel tube," says Zenner.
Of course, every motor needs power, but in the case of IP69K-rated motors, termination is one of the biggest challenges. One common option is to mount a stainless steel junction box on the side of the housing. The cabling is terminated into the junction box and encapsulated to protect it. The solution provides an easy way for the user/OEM to connect. On the downside, integrating the junction box and cabling requires holes in the housing, which compromises the integrity of the case. As an alternative, users can specify IP69K-rated multi-pin connectors. On the inside of the housing, the leads on the connector match up with the motor, protected and encapsulated by the same epoxy potting. On the outside, a threaded multi-pin connector links the unit to the outside world. The trade-off is cost - IP69K connectors can cost several times more than conventional versions - but the bonus is a sleek, high-reliability solution.
Building electrical components that can survive repeated washdown doesn't have to be an insurmountable task. With careful attention to redundancy, seals, thermal management, and connectors, manufacturers can provide end-users and OEMs with IP69K-rated motors capable of surviving some of the most punishing environments around.