Thursday, November 18, 2010

Motors for Hazardous Locations

Areas prone to or contaminated with explosive gases, vapours or volatile liquids are at risk from fire or explosions. A  hazardous area is a location where there is a risk of fire or explosion due to the formation of an explosive mixture of air and gas or inflammable vapour. Normal motors may emit sparks or some of their parts accessible to such environments may reach a temperature high enough to ignite inflammable surroundings during normal running. Special motors have thus been developed for such locations and may be one of the following types:
1 Flameproof (FLP) or explosion proof type (Ex ‘d’)
2 Increased safety type (Ex. ‘e’)
3 Pressurized type (Ex. ‘p’)
4 Non-sparking type (Ex. ‘n’)
Motors for Zone 0 locations

For Zone 0 locations only intrinsically safe, low-energy apparatus is recommended. Induction motors, being large energy sources, release high energy, particularly during switching or a fault condition are not suitable for such locations.

Motors for Zone 1 locations

For Zone 1 locations, the following types of enclosures are recommended

Flame- or explosion-proof motors (Type Ex-d),

IEC 60079-1 defines the basic requirements for such motors which, besides limiting the maximum temperature of any part of the motor, accessible to the contaminated area, also maintain definite lengths of paths, air gaps, widths, and diametrical clearances between various rotating and stationary parts to avoid any rubbing and arcing. The following design considerations may also be noted. 1. These motors should be able to withstand an internal explosion of inflammable gases, chemical vapour or volatile liquids without suffering damage or allowing the internal inflammation to escape to external inflammable substances through joints or other structural openings in the enclosure. (The explosion may have been caused by the gases. vapour or volatile liquids that might have entered or originated inside the enclosure). Apart from withstanding the internal explosion, the construction must be such, that the flame escaping from the interior is cooled down to such an extent that it is rendered incapable of igniting the surrounding hazardous atmosphere. This is achieved by providing joints with extra long surfaces (flame paths) and special clearances (gaps). The flame path is the breadth or the distance across the face of the flange, and the gap is the distance between the two faces of the flange, as shown in Figure.

The requirements of minimum lengths of flame paths and maximum gaps for various gas groups are specified in IEC 60079-1. Some of the important constructional features of such enclosures are as follows:

1. All components such as stator housing, end shields, terminal box and covers etc. are pressure tested before use.
2. No light metal such as aluminium is used as an external surface to avoid frictional arcing.
3. The maximum surface temperature must remain below the temperature class specified  for a specific application.

Increased Safety Motors (type Ex-e)

These motors will also suit areas defined for Zone 1. Use of HT Ex. ‘e’ motors, however, should be avoided in this zone. Such enclosures do not produce arcs internally and also restrict the temperature rise of any part accessible to such an environment to a limiting value, during start-up or run, in accordance with the applicable class of insulation. The limiting temperature must be less than or equal to the ignition temperature of the prevalent atmosphere, otherwise the limiting temperature will become the same as the ignition temperature. The rotor temperature is also restricted to 300°C during start-up, unless surface temperature table shows a lower limiting temperature.

Design considerations

IEC 60079-7 outlines the basic requirements for increased safety, type Ex-e motors as follows:

1. The enclosure must have a high degree of protection to prevent entry of dust, water or moisture. The minimum protection specified is IP:54 (IP:55 is preferred) according to IEC 60034-5.
2. All terminals must be the anti-loosening and anti-rotating types.
3. The minimum clearance and creepage distances must be maintained for conductors as specified.
4. The temperature rise of windings must be 10°C lower than that specified for normal machines.
5. The mechanical clearance between rotating parts, e.g., fan and fan cover or the radial air gap between the stator and the rotor, should not be less than specified to prevent sparking.
6. The temperature of the windings and other parts must not exceed the limiting temperature, even if the motor, after a prolonged operating period, remains energized in a stalled condition, for a specified time of tE seconds while tE will not be less than 5 seconds.

Pressurized enclosures (type Ex-p)

These may be standard TEFC motors suitable for operating under an internal pressure of 0.05 kPa, or a pressure slightly above atmospheric. The minimum specified is 5 mm water-gauge above atmosphere. During operation this pressure is maintained with inert gas or air through an external closed circuit, preventing inflammable gases from reaching the motor's inner components. Before re-switching such a motor after a shutdown, inflammable gases which may have entered the enclosure must first be expelled. For pressurizing, two holes are normally drilled on the motor's end shields, one for entry and the other for exit of the air or gas. For large motors, a totally enclosed dual-circuit type enclosure is normally used so that the motor's interior is pressurized with air or nitrogen to about 75 mm water-gauge above that of the atmosphere surrounding the enclosure. The enclosure is sealed and the leakage rate controlled to about 250 litres/min. Some of the basic safety features and devices are noted below.

Such enclosures are recommended only when there are a number of these machines installed at the same location so that a common pressurizing and piping system may be employed to reduce costs on piping network and pressurizing equipment and its accessories, in addition to its regular maintenance.

Safety features

The equipment is usually fitted with interlocks to ensure that if the internal pressure or flow rate of air or inert gas falls below a certain minimum level, the supply to the motor is cut off.

Pressure-measuring device: This is provided for the operation of alarm or trip devices in the event the pressure within the casing falls below the permitted minimum.

Safe-starting device: This is provided to ensure that no apparatus within the enclosure is energized until the initial atmosphere within the casing has been completely displaced.
Motors for Zone 2 locations

Locations falling within this category can employ motors that are more economical than the other types discussed above. IEC 60079-14 has defined the basic requirements for such enclosures which are obviously less stringent than the others. In addition to maintaining specified creepages and clearances between the rotating and the stationary parts, the following are the two main requirements specified for such enclosures:

1. They should produce no arcing during a start-up or run.
2. Surface temperature should not exceed the ignition temperature for a particular temperature class under any conditions of operation. There is no limit to the temperature rise to the permissible limits for a particular class of insulation of windings or other parts of the machine, except the limiting surface temperature. For such an application, a normal IP 55 enclosure may also be employed.

Non-sparking motors, (type Ex-n)

A subsequent study of construction features of motors for Zone 2 locations, resulted in the development of non-sparking, type ‘n’ motors. The basic design consideration for such motors is similar to that of type ‘e’ motors but now there is no restriction in the limiting temperature, by 10°C, as in type ‘e’ motors. Frame sizes for these motors are generally the same as for general-purpose motors. Thus they tend to be smaller and less expensive than type ‘e’ motors for the same output.

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