Explosion Proof Motors

Explosion proof motors are a class of electric motors that are constructed to both contain an explosion if it occurs within the motor as well as prevent the release of explosive gases or vapors to the surrounding environment. ¹ They are used in hazardous locations and are commonly applied in the chemical, coal mining, textile and petrochemical industries. ² To be classified as explosion proof, the motors must be manufactured to the requirements of Underwriters Laboratories (UL) and the National Electrical Code (NEC) in “class, group and temperature code restrictions.” ³

The Basics: Explosion Proof Enclosure Design

Motor enclosure design is central to the design an explosion proof motor. The essential requirement of explosion proof enclosures is that they must “must prevent the ignition of explosive gases or vapors that may surround it.” ⁴ They are bulkier and heavier than non-explosion proof motor enclosures. They are designed around the ability of the motor to resist corrosion and limit temperature rises or overheating. They are manufactured to be flametight, which requires their joints and flanges be constructed with narrow tolerances so hot gases from an internal explosion will cool to the point that when they are released they are too cool to ignite an explosion. ⁵

Overview of Motor Classifications

Explosion proof motors are essentially a motor enclosure classification based on envirnomental protection and motor cooling methodology. ⁶ In general, motor enclosures are classified by the National Electrical Manufacturers Association (NEMA) and the International Electrotechnical Commission (European standard) in two general groups ⁷: open enclosures and totally enclosed enclosures. Open enclosures have openings, which allow air to ventilate over and around the motor’s windings. There are four types of open enclosures. They are:

• Drip-proof
• Splash-proof
• Guarded
• Weather protected

Open enclosures are not designed for hazardous locations. Totally enclosed motors are bettered suited for these locations. Totally enclosed enclosures prevent air to flow from inside the motor to the outside environment but they are “not intended to be gas tight ⁸. There are four types ⁹ of totally enclosed motors. They are:

• Totally Enclosed, Non Ventilated (TENV)
• Totally Enclosed, Fan Cooled (TEFC)
• Totally Enclosed, Air Over (TEAO)
• Explosion Proof

Explosion Proof Motor Classifications by Hazardous Locations

Explosion proof motors are rated for use in hazardous locations as defined by Underwriters Laboratories (UL) ¹⁰ and the National Electrical Code (NEC) ¹¹ (or OSHA ¹²) as follows:

• CLASS I: Gases, vapors, and liquids
• Group A: Acetylene
• Group B: Hydrogen, etc.
• Group C: Ether, etc.
• Group D: Hydrocarbons, fuels, solvents, etc.
• Division 1: Normally explosive and hazardous
• Division 2: Not normally present in an explosive concentration (but may accidentally exist)
• CLASS II: Dusts
• Group E: Metal dusts (conductive and explosive)
• Group F: Carbon dusts (some are conductive and all are explosive)
• Group G: Flour, starch, grain, combustible plastic or chemical dust (explosive)
• Division 1: Ignitable quantities of dust normally are or may be in suspension, or conductive dust may be present
• Division 2: Dust not normally suspended in an ignitable concentration (but may accidentally exist). Dust layers are present.
• CLASS III: Fibers
• Textiles, wood-working, etc. (easily ignitable, but not likely to be explosive)
• Division 1: Handled or used in manufacturing
• Division 2: Stored or handled in storage (exclusive of manufacturing)

Explosion proof motors are rated for environment by class, group and division and must be identified as such (e.g., Class I, Division 1, Class I Division 2, etc.) The differences in the classes depend on temperature limitations and the protection methods for the material (groups) specific to an application. ¹³

Explosion Proof Construction

Explosion proof motors are constructed with parts, materials and methods that give it a more robust construction suited for hazardous locations. These include:

• Class F insulation
• Non-sparking brass shaft slingers
• Oversized conduit/connection boxes with threaded conduit holes
• Non-sparking, corrosion resistant, cooling fans
• Cast iron end plates and fan covers
• Stainless steel breathers and drains
• Oversized, double-shielded, anti-friction, sealed ball bearings with high temperature grease
• Low loss steel laminations for higher efficiency
• Precision dynamic balancing
• High temperature polyester varnish impregnated armatures
• Dynamically balanced to reduce vibrations

Class F winding insulation provides greater protection against high temperatures than other classes of insulation. ¹⁴ Class H polyester varnish to impregnate motor armatures also provide high temperature protection. ¹⁵ The end plates are made with cast iron for both enhanced strength to handle the high pressures generated by explosions as well as corrosion resistance. All parts are machined with close tolerances to impede the expulsion of explosive gases from the motor until after they have cooled off sufficiently so they will not ignite the surrounding atmosphere. ¹⁶ Non-sparking brass shaft slingers are used on the motor shaft to provide a tighter shaft seal. Oversized conduit/connection boxes add room to manipulate the cabling and wires; they are made of cast iron for greater strength and corrosion resistance, as previously cited.