In this material we analyze a real overhaul of an electric motor - a case study from the Elektromotors service center in Tashkent. Our engineers received a three-phase asynchronous motor AIR-280M4 with a power of 75 kW from the cement plant in Akhangaran. The customer gave us a strict 4 working days - stopping the crushing unit could not cost more. Below is a step-by-step report on what we found, what we did, and what conclusions we drew for future maintenance of similar equipment.
Customer and problem statement
The customer is a large cement production enterprise located in the Tashkent region (we do not disclose the name due to a confidentiality agreement). The engine drove the ball mill of the second line. According to the company's regulations, scheduled maintenance was performed once every 12 months, however, during the period between services, the engine was operated in conditions of increased dust and ambient temperatures up to +42 °C.
The main requirement of the client: to restore the unit within 96 hours from the moment of acceptance to the warehouse in Tashkent. The alternative is a simple production line and losses of the order of tens of thousands of dollars per day. There was no backup engine at the enterprise.
Problem: what happened at the facility
In the morning, the line operator recorded the activation of thermal protection and a characteristic burning smell from the control cabinet. After two restarts, the engine switched off completely, and bluish smoke poured out of the ventilation grilles. On site, the company's equipment performed basic measurements and reported the following symptoms:
- stator insulation resistance is below 0.2 MOhm (the norm for 380 V is at least 1 MOhm);
- extraneous noise in the bearing assembly on the drive side;
- noticeable vibration at idle above 7.1 mm/s;
- visually - darkening of the frontal parts of the winding and melting of the varnish.
On the same day, the engine was dismantled, transported to our workshop at the entrance to Tashkent and an urgent order was issued. For more information about signs of failure and how we conduct an initial inspection, read the electric motor diagnostics page.
Diagnostics: what was found during disassembly
Complete disassembly of the AIR-280M4 took about 3 hours. At the same time, electrical measurements and microscopic inspection of the winding were carried out. Below is a summary table of key indicators based on the results of input diagnostics.
| Parameter | Normal | Actual | State |
|---|---|---|---|
| Insulation resistance, MOhm | ≥ 1.0 | 0.15 | Critical |
| Winding phase resistance, Ohm | 0.21 ± 2% | 0.18 / 0.22 / 0.29 | Interturn short circuit in phase C |
| Bearing clearance 6316, mm | 0.03–0.08 | 0.21 | Wear |
| Shaft runout, mm | ≤ 0.05 | 0.11 | Requires editing |
| Rotor condition | No cracks | 2 damaged rods | For surfacing |
Diagnostic result: classic scenario - overheating due to contamination of the ventilation ducts led to a breakdown of the interturn insulation in phase C, the load imbalance destroyed the bearing assembly, and then the rotor was damaged in a chain reaction. It was impossible to restore the winding by varnishing - a complete rewinding of the stator and a comprehensive repair of the rotor were required.
Solution: step-by-step work plan
Technologists drew up a plan with a 96-hour deadline. We paralleled operations where possible: while the stator was being cut, the rotor was being repaired in a separate area and bearings were being ordered. Below is the work schedule.
| Day | Work | Control |
|---|---|---|
| Day 1 | Dismantling the old winding, firing the core in an oven at 360 °C, cleaning the grooves, application for PETV-2 copper with a diameter of 1.4 mm | Measurement of iron resistance, checking the package on the blades |
| Day 2 | Winding coils, laying in grooves, phase-to-phase insulation, star connection | Phase continuity testing, 1000 V megger |
| Day 3 | Impregnation with ML-92 varnish, drying for 8 hours, rotor repair: surfacing of two rods, shaft grooving, replacement of bearings 6316-2RS | Insulation resistance monitoring after impregnation |
| Day 4 | Assembly, dynamic rotor balancing, load testing, body painting | Vibration measurements, heating, no-load current |
Key technical decisions we made along the way:
- Increased the wire cross-section by 5% relative to the factory one - a reserve for heating in Akhangaran dust conditions.
- We applied heat resistance class F instead of B - the permissible winding temperature increased to 155 °C.
- Replaced the bearings with sealed 6316-2RS with long-term lubrication - reducing the need for maintenance.
- We performed dynamic rotor balancing in two planes up to class G2.5.
Result: tests, warranty and fate of the engine
On the fourth day at 18:40 the engine passed final tests at the stand. The final indicators are in the table.
| Parameter | Before repair | After repair |
|---|---|---|
| Insulation resistance, MOhm | 0.15 | 520 |
| No-load current, A | 59 | 41 |
| Vibration, mm/s | 7.1 | 1.8 |
| Heating after 1 hour of operation, °C | — | 68 |
| Efficiency, % | — | 93.2 |
The engine was loaded onto the transport overnight, and by the morning of the fifth day it was already in its normal place in Akhangaran. A 12-month warranty card was issued for the unit, subject to compliance with the ventilation cleaning regulations once a quarter. After 6 months, the customer sent a report: the vibration remains within 2.1 mm/s, the service downtime is zero hours. We have arranged the full cycle of services - from acceptance to dispatch - as repair of industrial electric motors at the "urgent" tariff.
Conclusions: what does this case show to customers
From four days of work, Elektromotors engineers learned several lessons that are useful to any technical director in Uzbekistan:
- Early intervention saves money. If the operator had stopped the engine after the first trip of the protection, the repair would have been cheaper and without rewinding - the cost of work increased by about 2.5 times compared to preventive maintenance.
- Insulation class F is the standard for dusty regions. In cement, brick and cotton industries, class B lives 1.5 times less than the calculated period. The transition to F pays off already in the third season of operation.
- A backup engine is not a luxury. The cost of one backup unit is recouped by one daily line downtime. For critical components, we recommend keeping at least one copy in stock.
- Air duct cleaning routineis more important than it seems. 70% of overheating in our statistics is due to clogged ventilation. Simple blowing once a quarter reduces the accident rate significantly.
- The insulation measurement log allows you to catch a breakdown several weeks before failure. A 1000 V megohmmeter and five minutes once a week is enough.
If your engine of similar power fails - from 30 to 200 kW - we can connect around the clock, including under the urgent repair program. We have a visiting team ready to pick up the unit from the site within a radius of 200 km from Tashkent - this covers Chirchik, Yangiyul, Bekabad, Angren and most of the Fergana Valley. A separate technologist is assigned to each case, who conducts the work from acceptance to delivery and is responsible for meeting deadlines.
Comparison with typical repairs in the region
So that the client understands that 4 days for a machine of such power is a short but realistic period, we have collected average figures for similar orders completed by our service in Tashkent over the past year:
| Engine power | Average repair period | Term under “urgent” tariff |
|---|---|---|
| 30–55 kW | 5–7 days | 2–3 days |
| 55–110 kW | 7–10 days | 3–5 days |
| 110–200 kW | 10–14 days | 5–7 days |
| 200–400 kW | 14–21 days | 7–10 days |
The numbers above are the median for our workshop. The timing is affected by the availability of copper of the required cross-section, the condition of the core after firing and the availability of bearings. For orders with bearings of European brands, we maintain a warehouse program - the main standard sizes FAG, SKF and NSK are always available in Tashkent.
Do you need help with an emergency engine? Contact Elektromotors engineers via the contact page - we will assess the condition of the unit within an hour after acceptance and draw up a transparent work plan with a fixed deadline. A complete list of available works is available in the our services section.