The Allis-Chalmers TLG is an in-tank load tap changer found on power transformers throughout the legacy North American fleet. Like all Allis-Chalmers LTC designs, it was built to last — and it does, when it receives proper maintenance. The contacts and oil are the primary wear consumables. The drive mechanism requires lubrication and periodic timing verification. None of this is complicated, but all of it matters.
Allis-Chalmers produced several LTC designs across their transformer product line. The TLG and TLH-21 are distinct units with different contact architectures. The TLH-21 uses a collector ring and contact finger system; the TLG uses a more conventional switching contact arrangement. They share some design philosophy but do not share parts. Verify which unit is installed before ordering replacement contacts — the nameplate and instruction book confirm this.
Allis-Chalmers specified contact inspection based on operation count, with calendar-based maximums for units where counter data is unavailable. The instruction book for the specific TLG variant governs — TLG units were produced across multiple voltage classes and transformer ratings, and the specifications are not uniform across all variants.
Where operation counters have not been read or the count is unreliable, default to a five-year inspection cycle for active units. A TLG that has been in uninterrupted service on a regulated feeder transformer for five years without an internal inspection is overdue regardless of what any counter shows.
With the LTC compartment drained, inspect arcing contacts for erosion depth against the minimum material specification in the instruction book. The arcing contacts absorb the interruption duty on every tap change — on an active unit, they are the most likely component to reach end of life before the calendar interval does.
Beyond thickness, inspect the contact surface for abnormal wear patterns. Pitting that is centered and uniform across the contact face is consistent with normal arcing. Pitting that is concentrated at one edge, deeply cratered, or shows evidence of material splash onto adjacent surfaces suggests an arcing anomaly. Investigate the transition resistor and drive timing before replacing contacts and returning the unit to service — otherwise the same condition will recur.
Main contacts see less arcing but accumulate oxidation and surface resistance over years of service. Inspect and clean the main contact surfaces and verify adequate contact spring pressure. After reassembly, measure contact resistance on each phase and compare to baseline. A reading significantly above the previous measurement points to a seating or spring problem that needs to be resolved before the unit is returned to service.
Inspect the reversing switch contacts at every maintenance visit. Reversing switch operations are infrequent, which makes their wear easy to overlook and their failures hard to catch until a reversing operation is actually commanded. A worn or pitted reversing contact that is never tested because the tap changer stays in one direction for years is a hidden failure mode.
The TLG drive mechanism stores spring energy for the tap change sequence. Inspect the spring for set, the latch for wear at the engagement face, all pivot points for lubrication and pin wear, and the drive motor for correct operation and charging time.
Old grease does more damage than no grease. Dried lubricant becomes an abrasive varnish on pivot surfaces that accelerates pin wear and adds friction that slows the mechanism. Remove all old lubricant completely before applying fresh grease per the instruction book specification. Do not apply grease to latch engagement surfaces in excess — a grease-fouled latch can reset incorrectly after an operation.
Time the tap change after reassembly and lubrication. Verify the result is within the specification for the unit. Slow timing after fresh lubrication indicates spring set; slow timing that appeared gradually over successive maintenance cycles confirms it. A spring that has taken a permanent set should be replaced rather than adjusted around.
The TLG compartment oil is separate from the main transformer oil. Test dielectric strength, moisture content, and acid number from a sample taken before draining at each inspection. Carbon-loaded or moisture-contaminated oil should be fully replaced rather than filtered — LTC oil in poor condition recontaminates quickly when the carbon deposits left on the compartment interior are not also removed.
After draining, clean the compartment interior and the contact assembly before filling with fresh LTC oil. Refill to the level specified on the nameplate or in the instruction book and verify the oil conservator or expansion system is functional and correctly set.
DGA sampling of the LTC compartment oil is worth including in any comprehensive diagnostic program. A rising acetylene level in the LTC oil, out of proportion to the known tap change frequency, is an early indicator of abnormal arcing that warrants inspection regardless of where the unit falls in its scheduled maintenance calendar.
We manufacture Allis-Chalmers TLG contact kits and support LTC maintenance across the Southeast. Send us your unit details and we’ll respond within one business day.