Many electric motors in use today are marked as “inverter duty.” However, this does not necessarily mean that the motor has been certified for use with a VFD in a hazardous location by a third-party like the Federal Motor Carrier Safety Administration (CSA) or Underwriters Laboratory (UL). Putting a noncertified motor into a VFD application can cause a temperature rise that is outside the limits of the T-code rating for the location.
As a result, motors must include labeling that indicates they can be used with VFDs. They must also be operated within the speed limits specified on the labeling. Some motors may be approved for use in hazardous locations and have VFD ratings on the nameplate but still not be certified to do both (variable speed and hazardous location). Operating these motors with a VFD may void the hazardous location certification. Bottom line: be certain the motor shows certification for the location and the VFD ratings (CT or VT).
Explosion-Proof and Ignition-Proof Motors
Motors designed for use in Class I, Division 1 environments must be labeled as “explosion proof.” A misconception regarding “explosion-proof” motors is that they are designed to prevent an explosion. This is not accurate—these motors are built to simply contain an explosion internally without rupturing. This is achieved through a number of different design measures, one of which includes using high-strength materials for both the motor hardware and its enclosure. Additionally, in the event of ignition and pressure buildup inside the casing, explosion-proof motors must be able to sufficiently cool hot gases as they escape through long, tight passageways known as flame paths. This reduces the temperature of gas to a value that is less than the AIT of the atmosphere so that ignition cannot occur.
Another common misconception with hazardous duty motors is that Class I, explosion-proof motors will always satisfy Class II requirements. However, the design principles behind the two types of motors focus on the prevention of entirely different scenarios.
A Class II motor’s design, for instance, assumes that eventually, as the motor takes in air from the surrounding atmosphere, its internal temperature will equalize with the operating environment. As a result, an internal fault can cause an explosion, which (ideally) will be contained. The design of a Class I motor, on the other hand, focuses on keeping the motor’s skin temperature below the AIT of the surrounding atmosphere. This type of motor is often referred to as “dust ignition-proof.” For both Class I, Division 2 and Class II, Division 2 locations, the use of dust ignition-proof motors is permissible, but it is not mandatory.
Overall, it is important for operators and facility managers to understand that all explosion- and dust ignition-proof motors are not created equal. For instance, two motors may include the same explosion-proof rating, but it does not mean they are identical (or even similar).
As a result, operators should approach hazardous duty motor selection comprehensively by examining the unique design and construction of the motor. This should include consideration of elements such as ingress protection (is the motor IP65 vs. IP56), preventative flame paths, ruggedness of components, etc. Since a hazardous duty motor is a critical safety investment, installing a product that simply meets the minimum requirements for “explosion-proof” is not a sound strategy.
Simplifying Hazardous Duty Motor Selection
Even with a grasp on the principles of motor design and hazardous location classifications, it is easy to see how an error can be made when selecting a motor for use in a combustible environment. Because such an error can have wide-ranging impacts—from lost production to fines to worker fatalities—facilities will often go to great lengths to ensure that their motors are qualified for specific environments.
Some facilities will go so far as to segregate and label motor inventory. Others color code motors so that installers have visual verification of location approvals. Both of these methods are cost prohibitive, particularly in larger facilities, since they often result in the need for a large amount of excess inventory. Stocking multiple motors with the same power and speed rating but different location approvals ultimately increases the likelihood of human error and introduces added complexity to motor selection.