The Allis-Chalmers TLS is a load tap changer used in power transformers manufactured through the mid-twentieth century. It is a transfer switch design with a separate collector ring and collector hub assembly for each phase. While the TLS shares lineage with the TLH-21 and TLG, it uses a different contact geometry and a distinct moving arm configuration — parts between these models do not interchange. Replacement contact components are manufactured to specification for transformers still active in the field.
The TLS uses a two-part collector system: a collector ring assembly that distributes current through the tap winding, and a collector hub assembly that mounts the moving contact arm. The collector hub geometry differs between Phase A/B and Phase C — these are not the same part and must be ordered to match the specific phase position in the transformer. Installing the wrong hub creates a contact alignment problem that will not be visible until the transformer is back in service.
The moving contact arm mounts to the hub and contacts the transfer switch stationary contact during each tap change operation. Two arm variants exist: a hub-finger assembly and a ring-finger assembly. These are not interchangeable — the correct variant must be identified before ordering a replacement or refurbished assembly. Inspect the existing arm to determine which type is installed before disassembly.
The main stationary contact is the primary wear component on the TLS. Each tap change operation carries the arcing duty at the stationary contact surface, and material loss accumulates with each operation. Inspect the contact face for erosion depth, pitting, and any evidence of contact surface cracking. A contact that has worn past the manufacturer’s minimum thickness must come out before the transformer goes back into service — a stationary contact at minimum thickness has no safety margin for unexpected operations.
The main moving contact — the finger contact mounted to the collector hub — makes and breaks contact with the stationary on each operation. Inspect finger contact pressure, surface condition, and the integrity of the contact tip. A moving contact that has lost its spring pressure will not make firm engagement with the stationary, producing elevated contact resistance and accelerated arcing at both surfaces.
The neutral stationary contact carries current in the neutral tap position between operations. It does not see arcing duty but develops contact resistance from oxidation and surface deterioration over time. Measure contact resistance at the neutral position and compare to the in-service baseline. An elevated reading at neutral with a good reading across the other tap positions points to the neutral contact specifically.
The reversing switch changes the polarity of the tap winding to allow voltage regulation in both the raise and lower directions from a single set of taps. It operates infrequently compared to the transfer switch but requires inspection at each maintenance interval. Inspect the reversing moving finger contact for wear and tip condition. Check the left-hand and right-hand stationary contacts for pitting and surface erosion at the contact engagement zone. Verify that the reversing switch drive mechanism moves the contact arm fully to both positions without binding.
TLS maintenance intervals are operation-count based. Most manufacturers specify contact inspection at 50,000 to 75,000 tap change operations, with full contact replacement at or before end-of-life wear limits. UnderNERC PRC-005, calendar-based programs must not exceed six years for LTC-equipped transformers unless the utility has adopted a performance-based maintenance program with documented trending. Transformers in voltage regulation service with high tap change rates reach operation-count limits well before the calendar limit — check the operations counter at each scheduled outage and do not rely on calendar alone to schedule contact inspection.
LTC oil should be sampled at each maintenance interval and tested for dielectric breakdown voltage, moisture, and dissolved gas. The LTC oil compartment is separate from the main tank on most transformer designs — the compartments must not communicate. A DGA sample from the LTC compartment that shows elevated ethylene or acetylene indicates arcing in the oil above what the contact wear level would predict, and warrants investigation before reassembly.
Southern Switch manufactures TLS contact components and performs LTC maintenance in the field. Contact us with your transformer nameplate and operation count and we’ll identify the correct parts and service interval.