The Allis-Chalmers TLH-21 is a collector-ring type load tap changer found on large power transformers throughout the utility system. Its design differs fundamentally from resistor-transition LTCs like the McGraw-Edison 550: instead of discrete arcing contacts bridging adjacent taps, the TLH-21 uses a rotating collector hub riding against a stationary ring structure to make contact with each tap segment in sequence. That difference drives a different set of maintenance priorities.
The collector hub carries multiple contact fingers that press against the collector ring segments. As the drive mechanism rotates the hub, the fingers move from one ring segment to the next. The arcing occurs at the trailing edge of one segment and the leading edge of the next as the fingers bridge the gap during transition. The main contacts carry load current continuously between tap changes; the reversing switch handles polarity changes at the extremes of the tap range.
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.
Allis-Chalmers specified inspection at 50,000 operations or 5 years, consistent with most LTC manufacturers of the era. 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.
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 burning from arc activity. Light pitting between ring segments is normal — that is where the arc strikes during transition. Heavy burning concentrated in one area, or burning that extends onto the main contact face of a segment rather than staying at the edge, indicates abnormal arcing that warrants investigation. Possible causes include a sluggish drive mechanism that slows the transition, a weak contact spring that allows the finger to hover rather than seat firmly, or oil in poor condition that is not quenching the arc effectively.
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.
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.
The TLH-21 reversing switch operates at the voltage extremes of the tap range. 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.
The TLH-21 drive mechanism rotates the collector hub. The speed of rotation through the transition determines how long the arcing contacts spend bridging adjacent tap segments — faster is better for contact life. Time the transition and compare to the manufacturer specification. A sluggish transition, caused by worn gears, inadequate lubrication, or a weak motor, extends the arc duration and accelerates ring and finger 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.
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.
We manufacture TLH-21 collector rings, hubs, contact fingers, and reversing contacts, and perform LTC field maintenance across the Southeast. Send us your unit details for a quote.