During rush hour, a passenger elevator stopped in one of the large shopping centers in Tashkent. Visitors were stuck inside, the flow of customers was paralyzed, and the administration was losing revenue every minute. This case is a clear example of how therepair of an elevator motorMitsubishi YPM-15K with a power of 15 kW is organized under conditions of a tight deadline. The full cycle - from emergency call to commissioning - took us exactly 36 hours.
Initial data: what happened
The object is a four-story shopping center in the Yunusabad district of Tashkent. High-speed passenger elevator with a lifting capacity of 1000 kg, machine room on the roof. Motor-drive - Mitsubishi YPM-15K, asynchronous motor with squirrel-cage rotor, power 15 kW, rotation speed 1450 rpm, built-in 110 V DC brake electromagnet.
Chronology of the accident according to the operating engineer:
- 14:20 - the elevator stopped between the 2nd and 3rd floors, the frequency converter protection was triggered overheating;
- 14:35 - the operation service manually lowered the cabin, evacuated passengers through the shaft;
- 15:00 - restart: the engine hums, the rotor does not rotate, the body heats up to 90 °C in 30 seconds;
- 15:40 - the administration calls our team urgent repair electric motors.
Hour 0–4: departure and initial diagnostics
The team arrived at the site 50 minutes after the call. The first thing we do if we suspect an elevator motor failure is to remove the voltage, check the brake manually and measure the resistance of the stator windings. The measurements showed a characteristic picture:
| Phase | Resistance, Ohm | Certificate standard | Deviation |
|---|---|---|---|
| U–V | 0.82 | 1.15 | −28.7% |
| V–W | 1.14 | −0.9% | |
| U–W | 0.83 | 1.15 | −27.8% |
| Insulation (megaohmmeter 500 V) | 0.4 MOhm | ≥ 5 MOhm | critical |
The picture is clear - an interturn short circuit in one of the sections of phase U and a simultaneous loss of insulation between the winding and the housing. When 110 V was applied, the brake electromagnet retracted, but with a noticeable rattling sound - the armature stroke was incomplete. To a first approximation, the bearings had no play, but we made the decision to replace them after disassembly.
Hours 4–8: dismantling and delivery to the workshop
Mitsubishi YPM series elevator motors are installed on a rigid frame along with a traction pulley and gearbox. Dismantling was complicated by the fact that the machine room is on the roof: we had to work with a hoist and a freight elevator from the adjacent building. A motor weighing 142 kg was delivered to our workshop in Tashkent in 1.5 hours.
In the workshop, the engine underwent repeated inspection and was included in the repair card. At the same time, we started purchasing materials: PETV-2 enamel wire with a diameter of 1.18 mm, KO-916 insulating varnish, groove cardboard, interlayer film, SKF 6309-2Z and 6206-2Z bearings.
Hour 8–20: stator rewinding
This was the key operation of the entire case. After disassembling and pressing out the old winding, it turned out that not only the section in the U phase was damaged - some of the turns in the W phase had thermal darkening of the varnish. The solution is a complete rewinding of the electric motor stator while maintaining the original connection diagram.
Winding parameters restored from the nameplate and core measurements:
- number of poles - 4;
- number of stator slots - 36;
- step along the slots — 1–9;
- number of turns in a section - 28;
- phase connection - star;
- insulation class - F (155 °C).
After laying and settling the winding, we carried out vacuum impregnation with varnish and drying in an oven at 130 °C for 4 hours. Control measurements after impregnation: resistance between phases - 1.14–1.16 Ohms, insulation resistance - 850 MOhms. This fully meets the requirements for an elevator drive.
Hour 20–28: rotor, bearings and brake electromagnet
The Mitsubishi YPM-15K rotor is cast aluminum, squirrel-cage. Visual inspection and testing on the machine showed the absence of broken rods, the imbalance was 9 g mm with an allowable 12 g mm. Balancing was not required, but we still ran the rotor on our balancing stand - this is standard for elevator machines. Read more about the technology in the material about balancing the electric motor rotor.
The bearings were replaced preventively, according to regulations: after 7 years of operation in the “lift-TC” mode (about 280 cycles per day), the rolling life has already been exhausted by more than 80%. SKF 6309-2Z (front, pulley side) and 6206-2Z (rear) installed. This is a typical job - we do it every day: replacing bearings in an electric motor.
The brake electromagnet is the most “elevating” unit of this machine. During disassembly, we discovered burnt coils and wear of the copper armature washer by 0.7 mm. The coil was rewound, the washer was replaced, the gap was set to the specified 0.3 mm. Test: 110 VDC response time: 38 ms, release time: 65 ms. Mitsubishi's norm is 30–45 ms and 55–80 ms, respectively.
Hour 28–34: assembly, testing, return delivery
After assembly, the motor went through a standard test cycle on our stand: idling for 30 minutes, load mode 75% of the nominal, vibration measurement at three points. Final indicators:
| Parameter | Metering | Normal |
|---|---|---|
| No-load current | 9.8 A | ≤ 11 A |
| Vibration of bearing shields | 1.4 mm/s | ≤ 2.3 mm/s |
| Winding temperature under load | 62 °C | ≤ 110 °C (class F) |
| Insulation resistance | 820 MOhm | ≥ 5 MΩ |
Hours 34–36: installation and start-up
Return delivery and lifting into the machine room took about an hour. Installation on a standard frame, alignment on the pulley (misalignment no more than 0.05 mm), connection of power and brake circuits, testing of operation with an empty cabin, then with a reference load of 800 kg. The elevator passed 12 control cycles “1st–4th floor” without any comments. 36 hours after the first call, the shopping center was operating normally again.
What we learned from this
This case is a typical example of how speed and quality are combined when working with an elevator drive. It is worth drawing three practical conclusions from it for the technical services of shopping centers and business centers in Tashkent:
- Regular monitoring of insulation resistance (once every six months) would allow detection of varnish degradation before an accident.
- Overheating of the frequency converter is almost always a symptom of a problem in the motor, and not in the VFD itself. Do not reset the error - call diagnostics of electric motors.
- Preventive inspection of an elevator motor is cheaper every 3-4 years than downtime of an object for a day.
Need help with an elevator drive, frequency drive or asynchronous motor? Contact Elektromotors engineers - we travel to any point in Tashkent, diagnose on the spot and make a decision within an hour. Contacts and application form →