On-Load Tap Changer Working Model for Transformers

Distribution voltage must stay within limits as load changes through the day; the classic way to hold it is changing the transformer's turns ratio — its tap position. Off-circuit tap changers move only with the transformer dead — an outage every time voltage needs correction. On-load tap changers solve this by moving taps while the transformer stays energized and loaded, and the trick that makes it possible is the diverter switch: it hands the load current from one tap to the next through transition resistors, so the winding section between taps is never shorted and current is never interrupted. This project builds that mechanism as a working model. A small single-phase transformer is wound with five accessible taps; a rotary selector with moving contacts pre-selects the next tap; a diverter chamber with transition resistors performs the transfer; a voltmeter on the secondary shows each voltage step; and a lamp load bank keeps it loaded so every tap change happens genuinely on-load. The hand-operated, exposed selector-diverter sequence lets the student perform the exact steps a real OLTC executes automatically.

1. The transformer is energized from the mains through an isolation transformer, and the lamp load bank is switched on so the secondary carries load current.
2. The tap position lamp shows the current tap, and the voltmeter reads the corresponding secondary voltage.
3. To raise the voltage, the rotary selector is turned to pre-select the next higher tap — the selector moves with no load current flowing through it.
4. The diverter lever is then operated: it bridges the two taps through the transition resistors, so load current momentarily flows through the resistor path.
5. The diverter completes its travel, handing the full load current to the new tap and taking the old tap and resistors out of circuit — the make-before-break transfer.
6. The voltmeter shows the stepped-up voltage and the tap lamp updates; the sequence is repeated to step through all five taps, then reversed to step down.
7. The selector interlock is demonstrated by attempting to move the selector mid-transfer, which the interlock blocks.

Tech stack:

• On-load tap changing theory
• Tap selector and diverter design
• Transition resistor bridging
• Small power transformer construction
• Selector drive mechanism
• AC voltmeter instrumentation
• Interlock logic
• Lamp load bank

• [Five-tap transformer model] A small single-phase transformer wound with five accessible secondary taps gives a real voltage step at each position.
• [Rotary tap selector] A hand-operated rotary selector with moving contacts pre-selects the next tap while carrying no load current, exactly as in a real OLTC.
• [Diverter switch with transition resistors] The exposed diverter chamber transfers load current between taps through transition resistors, demonstrating the bridging sequence that prevents tap-to-tap shorts.
• [On-load demonstration] A lamp load bank keeps the transformer loaded during every change, so the voltage step is observed under genuine on-load conditions.
• [Step voltmeter] A panel voltmeter on the secondary shows the voltage rise or fall at each tap change, making regulation visible.
• [Selector-diverter interlock] A mechanical interlock prevents the selector from moving while the diverter is mid-transfer, enforcing the correct operating sequence.
• [Tap position indication] Lamps on the panel show the active tap at all times, so the audience follows each change.

• Small transformer with five accessible secondary taps
• Rotary tap selector and diverter switch assembly with transition resistors
• Control panel with voltmeter, tap lamps and interlock
• Lamp load bank
• Wiring and schematic diagram of selector-diverter connections
• Project report PDF (background, OLTC theory, diverter sequence, readings)
• PPT presentation for final review
• Viva Q&A preparation document (selector vs diverter, transition resistors, tap steps)

• The model works at low voltage (24 V secondary); real OLTCs operate at high voltage with completely different insulation and clearances.
• The diverter switches small currents — arc interruption phenomena of real diverters are simplified, not replicated.
• There is no vacuum or oil diverter chamber; the transfer is visible in air by design, for teaching.
• The manual selector moves far slower than a real motor-driven diverter, which completes transfer in milliseconds.
• Five taps demonstrate the principle; real units commonly have 17 or more positions.
• This is a teaching model, not grid equipment — it is not rated for any real distribution duty.