Hazardous horsepower

Explosion-proof motors need special attention

by Cyndi Nyberg

Motors used in hazardous locations must meet a variety of special requirements. If these motors aren’t properly specified and/or repaired, serious safety consequences can occur.

Hazardous locations are environments where explosive or ignitable vapors or dusts are present, or may become present. Explosion-proof motors are designed to prevent the external temperatures of the motor from igniting the vapor or dust and to contain any internal faults within the motor enclosure under all operating conditions. NFPA 70 of the National Electrical Code (NEC) covers requirements for electrical installations in hazardous locations.

If the code contains no specific classification rule for a particular location, it’s up to the facility owner, the owner’s insurance company, or the local electrical inspector or fire department to determine whether an explosion-proof motor should be used in that application. The NEC doesn’t specify that an Underwriters Laboratory (UL) listed motor be used, but only that the motor is “approved” to be in that location. In practice, though, the inspector will look for a listed motor if the environment warrants it.

Motor characteristics
Before looking at requirements for repairing and rebuilding explosion-proof motors, it helps to know that UL classifies these motors by class, group and division.

Class I motors are designed for use where explosive gases or vapors are present. They must be able to withstand and confine the effects of an internal motor explosion. They also must meet surface temperature requirements under all normal operating and fault conditions.

Class II motors are designed to operate where there is enough dust to create explosive atmospheres, and where dusts are electrically conductive. These motors must exclude dust and maintain a surface temperature below the dust’s minimum ignition temperature.

A motor can be dual-rated for both Class I and II environments. Not all Class I units are suitable for a Class II operation, however, and not all Class II units are suitable for a Class I operation.

In a Class I environment, special seals confine an explosion inside the motor so that flames won’t escape. In a Class II environment, the seals prevent dust from mixing with and hardening the bearing lubrication. This keeps bearings from overheating and increasing the motor’s surface temperature.

Assigned code letters
Within Class I and Class II, group designations are assigned to different combustible substances by code letters. The letters A through D belong to Class I; and letters E, F and G belong to Class II. The Class I group designation letters categorize gases and vapors by degree of combustibility. Within Class I, Group A has the most combustible substance (acetylene), followed by B, C and D. Group A motor designs have the longest flame paths and tightest fits.

All explosion-proof motors made after February 1975 must carry a temperature code indicating the maximum motor surface temperature that will develop under operating conditions, including overload up to motor burnout.

Class II, Group G (grain dust) has the lowest external surface temperature of 165 degrees C (329 F); and E (metal dust) and F (carbon dust) have external surface temperatures of 200 C (392 F). The lowest ignition temperature of any material within the group determines the group’s surface temperature limit. Few motor manufacturers produce hazardous location motors suitable for a Class I, Group A and B atmosphere.

The chart on this page summarizes materials within each group for Class I and Class II locations.

Hazardous locations are further broken down into Division 1 and Division 2. The distinctions are defined in detail in NEC Article 500. In a Division I location, ignitable substances are likely to be present continuously or intermittently during normal operation. In a Division 2 location, ignitable materials are handled or stored so that flammable material is present only under abnormal conditions.

Class I and II motors for hazardous locations have no division designation on the UL label. All of these motors are designed to meet Division 1 requirements and are, therefore, suitable for use in both Division 1 and Division 2 locations. Standard totally enclosed fan-cooled (TEFC) and sometimes even open drip-proof (ODP) motors are used in Division 2 locations.

Maintaining the listing
The UL listing mark on a new motor means it complies with and has been built to UL requirements. A hazardous location motor listed by UL when originally manufactured can be repaired and recertified if it’s restored to its original specifications. A motor can be rebuilt for any class and group it was originally designed for, but it can’t be rebuilt and listed in a class or group for which it was not originally designed. UL will inspect a service center to ensure that it complies with these requirements, but the motor’s owner assumes responsibility for maintaining its explosion-proof listing.

Service responsibility
All electrical work must be done by a UL-approved service center for the motor to maintain its listing. Mechanical or electrical work can be outsourced, but the original service center must ensure that none of the work alters the motor’s ability to contain an explosion.

Winding over-temperature protection (thermostats) is required for every rebuilt explosion-proof motor. No modifications are permitted that could change the motor’s surface temperature or its ability to contain an internal explosion. However, UL does allow a service center to change a motor’s voltage as long as the original winding data is available. Potting of the motor leads is required for explosion-proof motors, but will also be beneficial in other repairs. In a very dirty or wet environment, potted leads help to seal out some of the contaminants.

Variable-frequency drives
For explosion-proof motors used with variable-frequency drives, it’s especially important to have (from the manufacturer) the speed range where the motor will still meet the listing’s temperature requirements. A motor not originally listed for inverter duty can’t be used on variable-frequency power unless the motor maker tested it and concluded that the motor surface temperature won’t exceed the required maximum throughout frequency range for the application. For non-sinewave power, it’s assumed that the motor will run 20 C (36 F) hotter than it will on sinewave power.

Maintaining hazardous location motors is essentially the same as for all TEFC motors. Don’t assume that since an explosion-proof motor has such stringent requirements, it requires less maintenance. In fact, a regular maintenance schedule should include lubrication at proper intervals, inspection for damage or evidence of contamination and other debris accumulation on the frame and fan cover, checking thermostats, and checking vibration levels.

All of this will ensure that safety won’t be compromised in the hazardous environment.

Cyndi Nyberg is a technical support specialist at the Electrical Apparatus Service Association, a trade association for firms that sell and service electrical, electronic and mechanical apparatus. To learn more, visit www.easa.com.

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