Load Tap Changer Problems: Sticking Taps, Voltage Faults, and Contact Wear

A load tap changer does its work thousands of times a year, and it fails in ways that show up on the system before they show up on a teardown. Taps that hesitate or stick, voltage regulation that drifts, a rising trend in the LTC oil's dissolved gas, or contact resistance that climbs on one tap position all point back to wear inside the diverter and selector switches and the drive mechanism. Reading those symptoms correctly is the difference between a contact change at the right interval and an unplanned transformer outage.

What the LTC does and where it wears

The LTC changes the transformer's turns ratio under load to regulate voltage, moving the circuit between tap positions with arcing contacts that interrupt current on every change. Those arcing contacts are the primary wear component. Behind them, the diverter and selector switch hardware, the drive mechanism, and the LTC oil all age on their own schedules, and a symptom on one is often driven by wear on another.

Symptoms and what they mean

Taps that hesitate, stick, or fail to complete a change point to the drive mechanism: binding in the linkage, fatigue in the spring or cam hardware, or dried lubricant slowing the transfer through the critical bridging interval. A slow transfer also increases the arc energy the contacts absorb on every operation.

Voltage regulation that drifts or hunts can indicate a contact that is not making cleanly on a tap position, a position-sensing problem, or elevated resistance on one tap from a worn or lightly loaded contact.

A rising trend in the LTC oil's dissolved gas analysis, beyond the normal signature of tap-change arcing, is early warning of abnormal arcing inside the compartment. LTC oil degrades faster than main tank oil because it sees arc products on every operation, and it must be sampled and tested as a separate system.

Contact resistance that reads high on one tap after service, compared to baseline, points to a contact finger that has lost spring tension or a contact worn past its useful face.

Field warning signs

Sluggish or hesitating tap changes. Voltage that will not hold regulation or that hunts around a setpoint. Motor operators that hunt or cycle after an operation. DGA on the LTC oil trending up against baseline. Contact resistance above baseline on a specific tap. Any of these is a reason to inspect the LTC before the next scheduled interval.

What not to do

Do not replace the contacts and close the unit back up without finding what drove the wear. A contact kit installed into a mechanism with worn diverter hardware or degraded spring tension will not last its rated interval. You will have replaced the cheap part and left the failure mode in place. Abnormal or uneven contact wear is usually a symptom of a timing or mechanism problem upstream, and it will reappear on the next set of contacts if the cause is not addressed.

Contact replacement versus full rebuild

Contact replacement gets most LTCs back in service. The main moving, arcing, and reversing switch contacts wear on a predictable schedule, and changing them at the right interval prevents the failure modes that cause outages.

A full rebuild is the right call when the unit has accumulated significant switching operations and mechanism wear is visible beyond the contacts, when oil analysis shows contamination consistent with internal arcing rather than normal erosion, or when the drive mechanism shows binding, irregular motion, or fatigue in the spring or cam hardware. We evaluate each unit on arrival and tell you what we find before we proceed. If contact replacement is all it needs, that is what we quote.

Common questions

Why is my tap changer sticking or hesitating?

Sluggish or incomplete tap changes usually point to the drive mechanism: binding in the linkage, dried lubricant, or fatigue in the spring or cam hardware slowing the transfer. A slow transfer also raises the arc energy the contacts absorb, so it accelerates contact wear as well.

What does rising DGA in LTC oil mean?

Dissolved gas trending up beyond the normal tap-change arcing signature is early warning of abnormal arcing in the compartment. Because LTC oil sees arc products on every operation, it degrades faster than main tank oil and must be sampled and tested as its own system.

Do I need a full rebuild or just new contacts?

Most LTCs need contact replacement at the right interval. A full rebuild is warranted when mechanism wear is visible beyond the contacts, when oil analysis shows internal-arcing contamination, or when the drive shows binding or spring and cam fatigue. Installing contacts into a worn mechanism will not last its rated interval.

Which tap changers do you rebuild?

We machine replacement contacts in-house and rebuild GE, McGraw-Edison, Allis-Chalmers, Westinghouse, Moloney, and more, from contact replacement through full remanufacture with test and documentation.