Intermittent Encoder Fault on a Gearless Traction Elevator
A 12-stop gearless traction elevator in a 2015 office tower has a 350 fpm, 3,500 lb capacity, 480 V regenerative drive, and a closed-loop incremental encoder. About two or three times a week the car stops between floors 8 and 9 with a position error fault. The car resets after the main disconnect is opened and closed. The fault always occurs at the same hoistway location, usually in the afternoon. The motor leads megger good. The encoder cable is shielded, but the shield is landed on ground at both the drive end and the motor end. A recent roof repair disturbed conduit near the hoistway. Maintenance has already replaced the drive once with no change.
1. What does the repeatable fault location tell you, and what are the most likely causes?
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A fault that repeats at the same hoistway location points to a position-dependent cause rather than a random electronic failure. The drive has already been replaced without change, so the problem is more likely in the encoder, its coupling, its cable, or a local noise source. Likely causes include a slipping or cracked encoder coupling, a damaged encoder cable that flexes at that point in the travel, a shield grounding problem, a loose termination, or electrical noise coupled from the disturbed conduit. Landing the shield at both ends can create a ground loop, especially if there is a potential difference between the drive and motor frames. Afternoon occurrence may correlate with building load, temperature, or a piece of equipment operating nearby, but the fixed position is the stronger clue. The mechanic should treat the fault as a system-level signal-integrity problem, not assume the encoder itself is bad.
2. Outline a diagnostic plan that avoids unnecessary parts replacement.
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Review the fault log and record the exact position, direction, speed, and load at each event. Ride the car with a diagnostic tool and watch encoder counts or position error through the fault region. Inspect the encoder coupling for slip, runout, and cracks, and check the encoder mounting. Flex and wiggle the encoder cable at the suspected location while monitoring counts; a change indicates a broken conductor or shield problem. Verify shield termination against the drive manufacturer's instruction and correct any double-grounding or missing ground. Check the conduit disturbance for a rubbed cable, sharp bend, or separation from motor leads. Confirm drive and motor grounding, inspect brake coil wiring for noise, and discharge the DC bus before working on power terminals. Use a scope or drive trace to compare command and feedback if available. This sequence isolates mechanical slip, cable damage, grounding, and noise before replacing the encoder or drive again.
3. What corrective action and verification would you require before returning to service?
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Correct the identified cause: replace a worn coupling, repair or replace the damaged encoder cable, and terminate the shield according to the manufacturer's grounding scheme, typically at one end unless the drive instructions require otherwise. Reroute the cable away from noisy conductors and restore proper conduit support. If the encoder itself is suspect, verify its output with a scope before replacement. After repair, run the car through the former fault zone at normal and inspection speeds, with full load and no load, and monitor position error. Operate for a defined observation period, such as several days of normal traffic, and review the fault log before closing the work order. Document the fault history, measurements, corrected shield termination, parts replaced, and monitoring results. Do not simply clear the fault and return the car to service; an intermittent position fault can cause a hard stop or misleveling, so verification must include repeated trips through the same location.