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A standard overhaul procedure is insufficient. If an Ex motor is repaired incorrectly, it may function electrically but fail catastrophically as a safety device, potentially igniting the surrounding atmosphere. Therefore, all overhauls must strictly adhere to international standards such as IEC 60079-19 (Explosive atmospheres – Equipment repair, overhaul, and reclamation).

This article outlines the non-negotiable technical requirements when servicing these specialized machines.

1. Preservation of Flameproof Joints (Flame Paths)

For Ex d (Flameproof) motors, the enclosure is designed to contain an internal explosion without propagating it to the external environment. This relies entirely on the Flame Paths (or Flameproof Joints). These are the mating surfaces between the end shields and the stator frame, or the shaft and the inner bearing cover.

Strict Technical Requirements:

• Dimensional Tolerance: The gap (interstice) between these surfaces is calculated to cool down hot gases escaping from an internal explosion. The Maximum Experimental Safe Gap (MESG) must strictly adhere to the manufacturer's original tolerances (often within 0.1mm or less).

• Surface Finish: The surface roughness (Ra) of these joints is critical. No scratches, nicks, or corrosion are permissible.

• Prohibition of Sealants: Never apply paint, varnish, or silicone gaskets to a flameproof joint. Doing so alters the gap dimensions and acts as a fuel source. Only non-hardening, approved anti-corrosion grease is allowed.

2. Temperature Class (T-Class) Compliance

Explosion-proof motors are rated with a specific Temperature Class (e.g., T3, T4), indicating the maximum surface temperature the motor will reach under the worst-case operating conditions.

Rewinding Constraints:

• Winding Resistance: When rewinding, the technician must replicate the original winding data exactly (turns, pitch, wire gauge). Any deviation can alter the copper losses (I^2R), leading to higher operating temperatures that exceed the certified T-Class.

• Insulation System: The insulation class (e.g., Class F or H) must be maintained or upgraded. Downgrading the insulation is strictly prohibited.

• Thermistors/RTDs: If the original design included embedded thermal protection (PTC thermistors) in the windings, they must be reinstalled and tested. They are often the primary safeguard against overheating.

3. Mechanical Parts and Enclosure Integrity

The mechanical strength of an Ex motor is far superior to a standard TEFC motor. The overhaul process must not compromise this structural integrity.

• Fasteners: All bolts used in Ex d motors are typically high-tensile steel (Grade 8.8 or 12.9). Replacing them with standard mild steel bolts (Grade 4.6) is a safety violation, as the enclosure could rupture during an internal explosion.

• Blind Holes: Bolts must not bottom out in blind holes; there must be sufficient clearance to ensure the clamping force is applied to the flange, not the bottom of the hole.

• Fan and Fan Cover: The external fan must be made of non-sparking material (conductive plastic or non-ferrous metal) to prevent static discharge or frictional sparking. The distance between the fan and the cover must meet minimum clearance standards (usually >1% of the max diameter) to prevent contact during deformation.

4. Bearings and Lubrication

While bearings are standard wear components, their selection in Ex motors impacts the Maximum Surface Temperature.

• Internal Clearance: Engineers must use the correct internal clearance (typically C3) to allow for thermal expansion without seizing. A seized bearing generates excessive heat, which is an ignition source.

• Labyrinth Seals: The grease paths and labyrinth seals often form part of the flame path assembly. These must be cleaned carefully without abrasion.

5. Marking and Certification

Upon completion of the overhaul, the motor is no longer considered "New."

• The "R" Mark: According to IEC 60079-19, a repair facility should attach an additional nameplate indicating that the motor has been repaired. This plate usually carries the symbol "R" (inverted triangle).

• Documentation: A full report detailing the measurements of flame paths, winding resistance, and replaced parts must be archived. This traceability is essential for insurance and safety audits.

Conclusion

Overhauling an explosion-proof motor is not merely a repair job; it is a recertification of safety. It requires specialized measuring tools, deep knowledge of the IEC 60079 series, and strict adherence to the original manufacturer's specifications. Any compromise—whether in the gap of a flame path or the grade of a bolt—can turn a safety device into a hazard.