Allis-Chalmers TLH-21 LTC Maintenance Guide

The Allis-Chalmers TLH-21, later built by Siemens-Allis, is a reactance-type load tap changer found on large power transformers throughout the utility system. It has two assemblies that wear and get serviced on the same visit. A dial switch on each phase does the tap changing and takes the arcing, using two moving contacts tied to a preventive autotransformer to bridge adjacent taps. A separate collector ring, center hub, and set of contact fingers carry the main load current out of the rotating assembly. The dial switch wears from arcing; the ring and fingers wear mechanically. Both drive the maintenance priorities on this changer.

How the TLH-21 works

The dial switch selects the tap and breaks the current. Its two moving contacts, tied to opposite ends of the preventive autotransformer, bridge two adjacent taps for an instant while the autotransformer limits the circulating current, and a quick-break mechanism snaps them apart so the arc clears fast in oil. The collector ring, hub, and contact fingers are the current-collection path: the fingers ride the ring continuously to carry load current out of the rotating assembly, so they wear mechanically rather than from arcing. The reversing switch handles the winding polarity change each time the mechanism passes through neutral.

Because the contact fingers ride against the collector ring continuously during operation, the wear pattern is different from a reciprocating contact design. The ring develops grooves where the fingers travel. The fingers themselves wear on their contact faces. Both are normal and manageable, but they require inspection on a known schedule, wear that goes unchecked eventually results in a finger losing contact with the ring, which causes a failed tap change under load.

Maintenance intervals

The Siemens-Allis instruction book calls for a first routine check at 6 months or 50,000 operations, whichever comes first, with annual checks thereafter and oil checks every six months. On an active feeder transformer or a unit serving an industrial load with frequent tap changes, that operation count can arrive much earlier than the calendar interval. Check the operations counter at every annual oil test and plan accordingly.

LTCs on lightly loaded units or those serving stable loads that rarely require voltage adjustment may go the full five years with minimal wear. The oil condition in those cases is often the limiting factor, aged oil that has oxidized without significant arcing activity still needs to be changed on schedule.

Collector ring inspection

With the LTC compartment drained and the collector assembly accessible, inspect the ring surface for groove depth. The contact fingers wear a groove into the ring along their travel path. Measure groove depth at the center of travel, where it will be deepest. The instruction book specifies the maximum allowable groove depth before ring replacement is required. A ring that has been worn unevenly, with deeper grooves at certain tap positions that are used more frequently, may need replacement before the maximum depth is reached everywhere.

Check the ring surface for pitting, scoring, or signs of burning. The ring is a current-collection surface, not an arcing surface, so it should show smooth mechanical wear, not arc damage. Any burning or pitting on the ring is a warning sign. It usually means a finger has lost spring pressure and is chattering or arcing where it should be riding solidly, or the collector is passing current it should not because of a problem upstream at the dial switch. Track down the cause rather than just dressing the surface. A weak contact spring that lets a finger hover, a sluggish drive, or oil in poor condition are the usual culprits.

Inspect the gaps between ring segments for carbon bridging. Carbon buildup across a gap reduces the insulation between adjacent tap positions, which can cause flashover at voltages the LTC normally handles without issue. Clean carbon bridges from gaps before reassembly.

Contact finger inspection

Remove the collector hub and inspect each contact finger individually. Check the contact face for wear depth, fingers have a wear indicator or a minimum dimension specified in the instruction book. A finger worn below minimum must be replaced. Since the fingers operate as a set, replace the full set when any individual finger is at minimum to avoid uneven contact pressure on the ring from a mix of new and worn fingers.

Check contact spring pressure on each finger. The spring holds the finger against the ring surface under the operating conditions inside the LTC compartment. A spring that has lost tension allows the finger to chatter against the ring at operating speed, which accelerates both ring and finger wear and creates additional arcing between operations. Verify spring tension against the specification and replace any spring that does not meet it.

Inspect the hub body for cracks, deformation, or evidence of contact with the ring support structure. A hub that has been running eccentric, possibly from a worn drive shaft bearing, may show uneven wear across the finger set or physical damage where the hub has been contacting something it should not.

Reversing switch contacts

The TLH-21 reversing switch operates each time the tap changer passes through the neutral position. On a transformer that routinely runs in the middle taps, the reversing switch may see very few operations and appear in good condition. On a unit that frequently runs at the edge of its range, the reversing contacts take proportionally more operations and wear accordingly.

Inspect the reversing moving and stationary contacts for wear, pitting, and adequate contact surface. Verify the reversing switch mechanism moves freely and seats solidly at both positions. A reversing switch that is hesitant or that shows play in the mechanism is a risk, a partial operation of the reversing switch under load can be a serious event.

Drive mechanism and timing

The TLH-21 drive turns the rotating assembly and snaps the dial switch through each tap change. The speed of that quick-break operation determines how long the dial switch arcing contacts spend bridging adjacent taps, and faster is better for contact life. Time the operation and compare to the manufacturer specification. A sluggish transition, caused by worn gears, inadequate lubrication, or a weak motor, extends the arc duration at the dial switch and accelerates wear.

Check drive shaft alignment and bearing condition. An eccentric shaft causes the hub to wobble as it rotates, producing uneven contact pressure across the fingers and uneven wear on the ring. Bearings that are worn or running dry will also show up in a timing test as inconsistency in the rotation speed through the transition.

Lubricate all specified points with the manufacturer-approved lubricant. The gear surfaces, shaft bearings, and cam followers all have specific lubrication requirements. Do not substitute, the specified grease viscosity and additive package are chosen for the operating conditions inside the LTC compartment.

Oil condition

Sample and test the LTC oil before draining at each inspection. The dielectric breakdown voltage, moisture content, and carbon loading tell you what the unit has been doing since the last oil change. Black or heavily darkened oil with a low BDV needs to be replaced, do not filter it back in. Filter-processed oil that still has elevated carbon content and low dielectric strength is not restored to service condition, it is just cleaner-looking degraded oil.

DGA sampling on TLH-21 oil is worth doing at each major inspection. Elevated acetylene in the LTC compartment is a flag for abnormal arcing, arcing that is beyond what normal tap change operation should produce. This can precede a contact failure by enough lead time to schedule a repair rather than respond to an outage.

When refilling, use only oil that meets the specification for LTC service. The LTC compartment oil must be dry, moisture in the fill oil negates the dielectric benefit of the oil change and immediately begins degrading the fresh oil.

OEM wear limits from the TLH-21 instruction book

The Siemens-Allis instruction book for the TLH-21 publishes hard numbers. Replace main moving contacts before the remaining flat contact surface narrows to 1/8 inch. Replace main stationary blades before two thirds of the copper-tungsten edge is eroded. On alignment, the moving contact centerline must engage at least 3/16 inch beyond the copper-tungsten division of the stationary contacts, and the reversing finger centerline at least 1/4 inch beyond the stationary contact bevel. The book calls for a first routine check at 6 months or 50,000 operations, and rates typical arcing contact life at 100,000 to over 1,000,000 tap changes, varying with rating, loading, and operation frequency.

Two OEM diagnostics worth keeping: arcing marks on the reversing switch contacts mean the drive is out of adjustment, because a correctly timed reversing switch never breaks current. And for units that sit on one tap, the book prescribes exercising the mechanism several positions each side of neutral monthly to wipe off the oil film that otherwise builds resistance at the contact faces. We keep the surviving OEM documentation and factory part references for these units on file. Send a nameplate photo through our contact form and we will confirm the model and quote the correct kit.