How an explosive atmosphere is divided into zones

he ATEX directive 99/92/EC distinguishes between two types of explosive atmospheres: gas and dust  Areas subjected to these two kinds of explosive atmospheres are each divided into three zones  The zone’s characteristics are identical for gas and dust, but their numbering is different  Zones 0, 1, 2 refer to gas and zones 20, 21, 22 refer to dust

Zone 0 / 20: Constant danger

Permanent presence of explosive gasses or com- bustible dust  Minimum category 1 equipment 

Zone 1 / 21: Potential danger
Occasional presence of explosive gasses or com- bustible dust during normal duty  Minimum cat- egory 2 equipment

Zone 2 / 22: Minor danger

Presence of explosive gasses or combustible dust not likely to occur or only for a shorter period of time  Minimum category 3 equipment
Grundfos manufactures pumps, with motors in both category 2 and category 3  The illustration on your right shows the division of an area into zones with different levels of danger of explo- sion  For each of the three zones it is only a cer- tain category of equipment – in this case motors – that can be used due to danger of explosion
The owner of the equipment is responsible for defining whether an area is to be considered haz- ardous within the regulations stated in the ATEX directive  However, if the user has any doubts about the definition of hazardous areas, he has to contact the proper authorities for advice
In Denmark the proper authority is the local Emergency Management Agency
The link between zones and equipment categories, is a minimum requirement  If the national rules are more strict, they are the ones to follow

What is explosive atmosphere?

According to the new directives, dust is now considered an explosive atmosphere
An explosive atmosphere is an atmosphere that develops explosively because an uncontrolable combustion  Explosive atmosphere consists of air and some sort of combustible material such as gas, vapours, mists or dust in which the explosion spreads after ignition  Typical exam- ples of productions where combustible dust is of major concern, is the handling of cereals, animal feed, paper, wood, chemicals, plastics and coal
Examples of sources of ignition that can cause the atmosphere to explode:

• Electrical sparks
• Flames
• Hot surfaces/ spots
• Static electricity
• Electromagnetic radiation
• Chemical reaction
• Mechanical forces
• Mechanical friction
• Compression ignition
• Acoustic energy
• Ionising radiation

An explosion is an uncontrolled combustion wave that produces a rapid increase in temperature and pressure  For an explosion to take place, three elements have to be present at the same time: fuel, (such as explosive gas) an oxidiser, (such as the oxygen in the air) and a source of ignition, (such as electrical sparks)  The combination of these three elements is generally referred to as the Fire Triangle 

To generate a potentially explosive atmosphere, the mixture of fuel and oxidiser has to have a cer- tain concentration  This concentration depends on the ambient pressure and the content of oxygen in the air, and is referred to as the explosion limits  Outside these limits, the mixture of fuel and oxi- diser will not ignite, but has the potential to do so if the proportions change  For an explosive atmosphere to form, a certain concentration of combustible material must be present 

                            

What is ATEX?

ATEX (ATmosphère EXplosible) refers to two new EU directives about danger of explosion within different areas  The first ATEX directive (94/9/ EC) deals with requirements put on equipment for use in areas with danger of explosion  The manufacturer has to fulfil the requirements and mark his products with categories  The sec- ond ATEX directive (99/92/EC) deals with the minimum safety and health requirements that the owner of the equipment has to fulfil, when working in areas with danger of explosion 

IEC 60034-7 Mounting arrange- ments and types of construction (IM code)

Basically, three types of standard motors exist: Foot-mounted motor, flange-mounted motor with tapped-hole flange and flange-mounted motor with free-hole flange  The motor types differ from one another in the way they are mounted in differ- ent applications 

Foot-mounted motor:

This kind of motor is mounted in the application by a foot with holes  The foot can either be integrated, (normal for cast iron motors) or it can be retrofit- ted, (normal for motors with stator housing made of aluminium) 

Motor with tapped-hole flange:

This type of motor is mounted in the application by means of bolts, which are screwed into the drive-end flange  In the drive-end flange there are threaded holes with standardised thread size and placed in a standardised pitch circle diameter 

Motor with free-hole flange:

This type of motor is mounted in the application by means of bolts through free-holes in the drive- end flange  The diameter of these free-holes is stand- ardised and the holes are placed in a standardised pitch circle diameter 

Combination of flange and foot:

The above-mentioned motor types can be com- bined in different ways:
• Horizontally or vertically
• With the shaft end pointed in different
directions
• With the foot turned in different directions The combinations are described in mounting des- ignations and are defined with codes according to IEC 60034-7 

IEC 60034-6 Methods of cooling of electric motors (IC code)

The three most frequently used motor cooling methods have the following designations - IC codes according to the IEC 60034-6 standard IC 411, IC 410, and IC 418 are applied 

IC410: The motor is cooled by free convection

IC411: The motor is cooled by a fan mounted on the motor shaft 

IC 418: The motor is cooled by an air flow typi- cally coming from an external fan 

Care of Windings and Insulation

Except for expensive, high horsepower motors, routine inspections generally do not involve opening the motor to inspect the windings. Therefore, long motor life requires selection of the proper enclosure to protect the windings from excessive dirt, abrasives, moisture, oil and chemicals.
When the need is indicated by severe operating conditions or a history of winding failures, routine testing can identify deteriorating insulation. Such motors can be removed from service and repaired before unexpected failures stop production. 

Whenever a motor is opened for repair, service the windings as follows:

1. Accumulated dirt prevents proper cooling and may absorb moisture and other contaminants that damage the insulation. Vacuum the dirt from the windings and internal air passages. Do not use high pressure air because this can damage windings by driving the dirt into the insulation.

2. Abrasive dust drawn through the motor can abrade coil noses, removing insulation. If such abrasion is found, the winding should be revarnished or replaced.

3. Moisture reduces the dielectric strength of insulation which results in shorts. If the inside of the motor is damp, dry the motor per information in "Cleaning and Drying Windings".

4. Wipe any oil and grease from inside the motor. Use care with solvents that can attack the insulation.

5. If the insulation appears brittle, overheated or cracked, the motor should be revarnished or, with severe conditions, rewound.

6. Loose coils and leads can move with changing magnetic fields or vibration, causing the insulation to wear, crack or fray. Revarnishing and retying leads may correct minor problems. If the loose coil situation is severe, the motor must be rewound.

7. Check the lead-to-coil connections for signs of overheating or corrosion. These connections are often exposed on large motors but taped on small motors.Repair as needed.

8. Check wound rotor windings as described for stator windings. Because rotor windings must withstand centrifugal forces, tightness is even more important.In addition, check for loose pole pieces or other loose parts that create unbalance problems.

9. The cast rotor rods and end rings of squirrel cage motors rarely need attention. However, open or broken rods create electrical unbalance that increases with the number of rods broken. An open end ring causes severe vibration and noise.

Testing Windings :

Routine field testing of windings can identify deteriorating insulation permitting scheduled repair or replacement of the motor before its failure disrupts operations. Such testing is good practice especially for applications with severe operating conditions or a history of winding failures and for expensive, high horsepower motors and locations where failures can cause health and safety problems or high economic loss.

The easiest field test that prevents the most failures is the ground-insulation, or &127megger," test. It applies DC voltage, usually 500 or 1000 volts, to the motor and measures the resistance of the insulation.

NEMA standards require a minimum resistance to ground at 40 degrees C ambient of 1 megohm per kv of rating plus 1 megohm. Medium size motors in good condition will generally have megohmmeter readings in excess of 50 megohms. Low readings may indicate a seriously reduced insulation condition caused by contamination from moisture, oil or conductive dirt or deterioration from age or excessive heat.

One megger reading for a motor means little. A curve recording resistance, with the motor cold and hot, and date indicates the rate of deterioration. This curve provides the information needed to decide if the motor can be safely left in service until the next scheduled inspection time.

LUBRICATION

Motors made up to frame 160 are not fitted with grease fitting, while larger frames up to frame 200 this device is optional.
For frame 225 to 355 grease fitting is supplied as standard. Proper Lubrication extends bearing life.

LUBRICATION INSTRUCTIONS

- Inject about half the estimated amount of grease and run the motor at full speed for approximately a minute; switch off the motor and inject the remaining grease.
The injection of all the grease with the motor at rest could cause penetration of a portion of the lubricant through the internal seal of the bearing case and hence into the motor.
Nipples must be clean prior to introduction of grease to avoid entry of any alien bodies into the bearing.
For lubricating, use only a manual grease gun.

BEARING LUBRICATION STEPS

1. Clean the area around the grease nipples with clean cotton fabric.
2. With the motor running, add grease with a manual grease gun until the quantity of grease recommended in Tables
9 or 10 has been applied.
3. Allow the motor to run long enough to eject all excess of grease.

ABNORMAL SITUATIONS DURING OPERATION

MOTOR DOES NOT START

- Lack of voltage on motor terminals - Low feeding voltage
- Wrong connection
- Incorrect numbering of leads
- Excessive load
- Open stationary switch
- Damaged capacitor
- Auxiliary coil interrupted

LOW STARTING TORQUE

- Incorrect internal connection
- Failed rotor
- Rotor out of center
- Voltage below the rated voltage
- Frequency below the rated frequency - Frequency above the rated frequency - Capacitance below that specified
- Capacitors series connected instead of parallel

HIGH NO LOAD CURRENT

- Air gap above that specified- Voltage above that specified- Frequency below that specified- Wrong internal connection- Rotor out of center- Rotor rubbing on the stator- Defective bearing- Endbells fitted under pressure or badly fitted - Steel magnetic lamination without treatment - Run capacitor out of that specified- Stationary/centrifugal switch do not open

BEARING HEATING

-BEARING HEATING
- Excessive amount of grease
- Excessive axial thrust or radial force of the belt - Bent shaft
- Loose endbells or out of center
- Lack of grease
- Foreign bodies in the grease

MOTOR OVERHEATING

- Obstructed ventilation
- Smaller size fan
- Voltage or frequency out of that specified - Rotor rubbing on the shaft
- Failed rotor
- Stator with insufficient impregnation
- Overload
- Defective bearing
- Consecutive starts
- Air gap below that specified
- Improper run capacitor
- Wrong connections

HIGH NOISE LEVEL

- Unbalancing
- Bent shaft
- Incorrect alignment
- Rotor out of center
- Wrong connections
- Foreign bodies in the air gap
- Foreign bodies between fan and fan cover - Worn bearings
- Improper slots combination
- Inadequate aerodynamic

EXCESSIVE VIBRATION

- Rotor out of center
- Unbalance power supply voltage
- Failed rotor
- Wrong connections
- Unbalanced rotor
- Bearing housing with excessive clearance
- Rotor rubbing on the stator
- Bent shaft
- Stator laminations loose
- Use of fractional groups on run capacitor single-phase winding