Moloney Electric was a Canadian transformer manufacturer whose transformers remain in service throughout North American utility fleets. The MA, MB, MA1, MB1, and MAMB2 load tap changers installed in Moloney transformers are in-tank, oil-immersed units with selector-switch architecture. Like all LTCs of this era, OEM support is no longer available, which makes contact availability and field expertise the two limiting factors in keeping these units in service.
The MA and MB designations indicate different transformer designs and current ratings within the Moloney product line. The MA1 and MB1 are variants with different contact dimensions from the base MA and MB — a critical distinction when ordering replacement parts, since a contact machined for the MA will not be dimensionally correct for the MA1. The MAMB2 is a further variant with its own selector blade and reversing blade geometry. Always confirm the specific model from the transformer nameplate and the LTC instruction book before ordering replacement contacts or blades.
The nameplate on the LTC compartment — separate from the main transformer nameplate — will typically identify the LTC type and serial number. If the LTC nameplate is missing or illegible, the instruction book filed with the transformer records is the next reference. Contact dimensions can be measured from the removed parts and matched to the correct replacement specification, but this takes field time that is better spent on inspection rather than identification.
Moloney specified contact inspection intervals in terms of operations — typically 50,000 to 100,000 tap changes depending on the model and the load current being interrupted. Calendar intervals of five to seven years apply where operation counters are unavailable or have not been consistently read. Units in active voltage regulation service accumulate operations faster than units on infrequently adjusted transformers; adjust the inspection schedule accordingly.
Sample the LTC compartment oil annually for dielectric strength, moisture, and acid number. DGA of the LTC oil at the same interval gives early warning of abnormal arcing conditions between inspections. A sharp increase in acetylene between annual samples on a unit that was clean at the previous test warrants a contact inspection before the next scheduled calendar interval — do not wait.
For bulk electric system assets,NERC PRC-005Maintenance interval requirements apply. Document each maintenance activity with dates, findings, contact measurements, and oil test results to satisfy the standard's record-keeping requirements.
The stationary contacts are the fixed current-path elements that the selector mechanism bridges and then lands on at each tap position. They receive arc erosion at their contact face during every tap change and carry full load current while the transformer is on that tap. Inspect each stationary contact for erosion depth, surface condition, and secure mounting. Measure remaining contact material against the minimum thickness in the instruction book.
Contact material worn below minimum must be replaced. Do not attempt to re-surface or build up worn contacts by any means — restore to OEM dimensions by installing machined replacement contacts. Southern Switch machines MA, MB, MA1, and MB1 stationary contacts to OEM specification. Replacement contacts are available as individual pieces or as a complete set covering all tap positions on the unit.
When stationary contacts show uneven erosion — more wear on one side than the other — investigate the selector mechanism alignment before replacing contacts. A selector arm that is not centering on the stationary contact distributes arc energy unevenly across the contact face, and the same pattern will reappear on the new contacts within a fraction of the normal service life if the alignment issue is not corrected.
The selector switch blade is the moving arm that sweeps across the stationary contacts as the LTC steps through its tap range. On the Moloney MA/MB series, the selector blade carries the load current between positions and makes the electrical bridge during transition. Inspect the blade for erosion at its contact tips, cracks or deformation in the blade body, and secure attachment to the drive shaft. A blade that is eroded at its contact tip will not make full contact with the stationary contact surface, producing elevated contact resistance and potentially elevated arc energy during subsequent tap changes.
The MAMB2 selector blade has a distinct geometry from the MA and MB blades. Inspect it using the same criteria — erosion at the contact tip, blade body integrity, and secure mounting — but do not attempt to substitute an MA or MB blade in an MAMB2 application. The contact geometry is different and the substitution will result in incorrect contact engagement that accelerates wear on both the blade and the stationary contacts.
Southern Switch machines replacement selector blades for the MAMB2 to match the original geometry. When the selector blade is replaced, verify that the new blade engages each stationary contact correctly through the full tap range before closing the compartment — any binding or misalignment will be visible as a mechanism load spike on the drive motor and will show up in contact resistance measurements on the affected tap positions.
The reversing switch changes the polarity of the LTC tap range, effectively doubling the number of voltage adjustment steps available from the same winding design. It operates far less frequently than the selector switch — typically only when the transformer is being adjusted to a new voltage band or is reversing through neutral — but it must be in correct condition when it does operate.
The MAMB2 reversing blade (large) is a specific component that should be inspected at every maintenance visit regardless of how infrequently the reversing switch has operated. Low operation frequency does not mean the blade is in good condition — it means the blade has been sitting in its position for an extended period, during which corrosion, oil varnish buildup, or contact oxidation can develop without the self-cleaning action of regular operation.
Operate the reversing switch through its full range as part of the maintenance procedure and verify that it seats correctly at each position, that contact resistance is within specification, and that the drive mechanism completes the operation within the specified time. A reversing switch that has not been operated in several years may exhibit sticking or binding from dried lubricant in the mechanism — clean and re-lubricate before returning the unit to service.
The drive mechanism positions the selector blade at each tap and initiates the reversing switch operation. Inspect all pivot points, linkage connections, and the gear train between the drive motor and the operating mechanism. Clean dried lubricant from all surfaces and apply fresh lubricant of the type specified in the instruction book. Old varnished grease adds friction that the drive motor must overcome, which slows tap change timing and increases motor wear.
Time the tap change from motor start to completion at each tap step and verify it is within the specification. A timing test that reveals one or more tap positions that are consistently slower than others indicates a mechanical interference at that position — a burr on a gear tooth, a worn bushing that allows side load on the drive shaft, or a contact that is not disengaging cleanly before the selector begins to move.
Verify limit switch operation at both ends of the tap range. The limit switches prevent the selector from being driven past the end tap positions, and a failed limit switch can allow the mechanism to over-travel and damage the selector blade or stationary contact assembly. Test each limit switch by driving to the end tap and verifying that the motor de-energizes correctly. A limit switch that is corroded, misaligned, or has a worn actuating cam should be replaced during this outage.
Drain the LTC compartment and sample the oil before internal inspection. The Moloney MA/MB compartment is sealed from the main transformer tank — treat the two oils as separate systems with separate service schedules. Carbon from arc interruption accumulates in the LTC oil over time and, if the oil is not changed on schedule, eventually coats the contact surfaces and contributes to additional arcing on subsequent operations.
Clean the compartment interior thoroughly before refilling. Remove visible carbon deposits from the walls and bottom of the tank. Inspect and replace compartment seals that show compression set or extrusion. Refill with fresh oil of the type specified for the LTC compartment — not main tank transformer oil.
We machine MA, MB, MA1, MB1, and MAMB2 contacts and blades in-house and support Moloney LTC maintenance across the Southeast. Tell us your model and we’ll respond with options and lead time within one business day.