Motors used in hazardous locations or severe-duty applications must meet a variety of stringent and sometimes unique requirements. An awareness and understanding of these requirements are critical to maintaining a productive and safe operating environment.
This article provides information to assist end-users in the application and maintenance of the appropriate motor for hazardous locations or severe-duty applications. It also provides checklists for new installations and preventive maintenance of hazardous-location and severe-duty motors.
Understanding classes and groups
Hazardous locations are environments in which explosive or ignitable vapors or dusts are present or might be present. Motors for such environments must be designed in a way that prevents external motor temperatures from igniting the vapor or dust and that contains any internal motor failures within the motor enclosure under all operating conditions.
To select the proper motor for a hazardous location, end-users must understand both the Underwriters Laboratories (UL) listed mark and the National Electrical Code (NEC) class, group and division designations and operating temperature codes.
Two primary classifications for hazardous location motors are available ," Class I and Class II. Each class has several subcategories called "groups."
A Class I motor is designed to withstand and confine the effects of an internal motor explosion, as well as to meet surface temperature requirements under all normal operating and fault conditions. Group letters A through D belong to Class I and designate different atmospheres in which the motor operates. Group A covers motors that operate in the most combustible atmospheres (acetylene), followed by Groups B, C and D. Table 1 describes the environments covered by each class.
A Class II motor covers dust in amounts sufficient to create explosive atmospheres, as well as dusts that are electrically conductive. A Class II motor is designed to prevent dust from entering the motor, while keeping the motor's surface temperature under the ignition temperature for the particular atmosphere during operation or fault condition. Group G (grain dust) has the lowest external surface temperature (165 Degrees C); Group E (metal dust) and Group F (carbon dust) have external surface temperatures of 200 Degrees C. The lowest ignition temperature of any material within the group determines the surface temperature limit for each group.
Not all Class I units are suitable for a Class II operation, and not all Class II units are suitable for a Class I operation. Very few manufacturers produce hazardous-location motors suitable for Class I, Groups A and B atmospheres.
Hazardous locations are further broken down into Division 1 and Division 2. A Division 1 location is one in which ignitable substances are likely to be present continuously or intermittently during normal operations. In a Division 2 location, ignitable materials are handled or stored in a way that could allow them to escape in the event of a spill or equipment failure.
Division designations do not appear on the UL label for Class I and Class II hazard location motors. All Class I and Class II motors are designed to meet Division 1 requirements and, therefore, are suitable for both Division 1 and Division 2 locations.
When a hazardous location motor is equipped with thermostats, the maximum temperature limit will not be exceeded both during operation and after the motor has been taken off-line.
All hazardous location motors manufactured after February 1975 must carry a temperature code that identifies the maximum motor surface temperature that could develop under all operating conditions ," everything from overload to motor burnout.
The photo shows a typical severe-duty motor application in a petrochemical plant.
(Photograph by EASA.)
Repairing hazardous-location motors
UL permits the repair of hazardous-location motors, but sets specific requirements. UL also inspects the repair facility to ensure compliance. Always make sure the repair facility has the proper UL approvals to repair these motors.
To retain UL certification, a repaired motor also must meet a number of requirements. First, it must have been listed by UL when it was originally manufactured. Second, it can be rebuilt and listed ONLY for the class and group for which it was originally designed.
Internal temperature protection (thermostat) is required for every rebuilt hazardous location motor. No modifications are permitted that could change the motor's temperature rise or ability to contain an internal explosion. UL does allow a voltage change as long as the original winding data are available.
For motors used with variable-frequency drives, it is especially important to obtain from the manufacturer the operating speed range for which the motor will still meet the temperature requirements of the listing. A motor that was not originally listed for inverter duty cannot be used on variable-frequency power unless the manufacturer has tested the design and determined that motor skin temperature will not exceed the required maximum throughout the frequency range for the application. For non-sinewave power, it is assumed that the motor will run 20 Degrees C hotter than it would on sinewave power.
Maintaining hazardous-location motors
Maintenance of hazardous location motors is essentially the same as for totally enclosed fan-cooled (TEFC) motors. Normally, maintenance includes lubrication at prescribed intervals; inspection for damage or evidence of corrosion, dirt and other debris accumulation on the frame and fan cover; and a check of vibration levels. If the motor is equipped with a thermostat, the thermostat circuit integrity also should be checked periodically.