Manufacturer of custom designed motors Ohio Electric Motors reports that special considerations must be taken when using mechanically commutated electric motors direct current (DC) motors in hazardous locations to ensure that it not only solves the motion control needs of the application, but also that it is operated safely and does not become an ignition source for a hazardous event.
“The National Electrical Code (NEC) defines hazardous locations as areas where fire or explosion hazards may exist owing to flammable gases or vapours, flammable liquids, combustible dust, or ignitable fibres or flyings. Fire or explosive hazards require not only the presence of a flammable substance but also an oxidizer and a source of ignition,” says the company.
Since electrical equipment is a common ignition source, the NEC classifies and sets the standards for the safe use of this equipment in hazardous locations.
The company claims that DC motors are accepted for use in hazardous locations and are a special type of motor, which is rated as explosion proof as it is housed in a motor enclosure that can withstand an explosion of a specified gas, vapour or dust within it, and prevent an internal explosion from igniting any gas, vapour or dust surrounding it.
In addition, the motors are Underwriters Laboratory and/or CSA Canadian Standards Association approved and marked to show the class, group, and operating temperature for which they are approved.
The most common hazardous locations are in the agricultural, aviation, mining, industrial, chemical processing and petroleum industries. But the causes of fires or explosions in these industries vary.
The NEC classifies hazardous locations in three ways; class 1 is flammable gases or vapors in the air, such as natural gas or gasoline vapor such as petroleum refineries and gasoline storage, class 2 is combustible dust or finely pulverized material, including grain elevators or flour and feed mills and class 3 is easily ignitable fibres or flyings that are collected around machinery or on lighting fixtures such as textile mills or cotton gins.
“Each class is associated with a type of condition called a division. Division 1 hazards are under normal conditions, while division 2 hazards occur in abnormal or fault conditions,” says the company.
Coal dust explosions are also well documented and can occur in thermal dryers, cyclones, bag houses, pulverised-fuel systems, grinding mills, and other process or conveyance equipment owing to small particle coal dusts suspended in the air that are ignited.
Grain elevators, rice mills, dust pellet mills, flour mills and feed mills commonly have combustible dusts in layers or suspended in the air. When oxygen and ignition sources are present, flames can ignite surrounding the ignition source and, if not contained, will spread out directionally with a shock wave causing a chain reaction, resulting in an explosion.
“Gasoline pumping stations that are used for aviation, that have gas vapors in the presence of air, can be ignited from static electricity, which depends the linear velocity of the fuel through the piping systems and the type of filter and water separator used, as well as the quantity of fuel being pumped,” says the company.
Fire and explosion hazards are found in dye works owing to the flammable solvents and dyestuffs used in the processes, soap manufacturing can lead to explosive atmospheres owing to the alcohol in scent perfumes, while food manufacturers can experience explosions from dusts, flour or powdered gelatin.
Eliminating a Source of Ignition
While flammable gases, combustible dusts, vapours, or fibres in the presence of air provide the fuel and oxidizer for a fire or an explosion, an ignition source is needed to ignite the reaction to create an explosive atmosphere.
There are many types of ignition sources, including sparks or arcs from electrical equipment or wiring, open flames, chemical reactions or biological processes, oxygen levels or temperatures, lightning, ionizing radiation, compression and shock waves, and static electricity.
Regarding electrical ignition sources, explosion-proof DC motors and associated electrical devices, such as wiring, conduits and connectors, are used to deprive a potential hazardous event of its ignition source.
Explosion-proof DC motors can be in any wiring configuration, whether brushed or brushless, permanent magnet or electromagnetic field. “What makes these motors safe for use in hazardous locations is an explosion-proof motor enclosure that can withstand, prevent or isolate an explosion, so it cannot spread into the surrounding atmosphere,” notes the company.
To meet explosion-proof requirements, explosion-proof motors are designed with specifications. The motors must have limited temperature so any flames that happen to escape the motor are cooled to a level that cannot ignite the external environment. This is accomplished by creating extra-long flame paths and special clearances.
The company says that an explosion-proof motor should also have definite length paths, air gaps, widths and tight clearances between motor parts - rotating and stationary - so rubbing and arcing is avoided.
Further, the motors should have the ability to withstand an internal explosion without suffering damage. “Pressure testing of motor housing, end bells, terminal boxes, and covers prior to use, as well as eliminating the use of light metals on external surfaces to avoid friction-based arcing, is important for these motors,” says the company.
Explosion-proof motor enclosures are designed to meet standards set by different organisations throughout the world.