Automatic Transfer Switch Trainer for Mains–Generator Changeover

Hospitals, data centers and apartment buildings depend on automatic transfer switches to move critical loads from mains to a standby generator when the grid fails — but students study ATS only as a block diagram, never seeing the sensing, timing and interlocking that make it safe. A wrong changeover welds contacts, backfeeds the generator into the grid, or drops the load twice. This trainer makes the sequence physical: a mains-sensing relay watches the supply, a timer module implements the start delay and transfer delay, and two interlocked contactors perform the actual changeover to a simulated generator source, with indicator lamps and a transfer-time measurement point at every stage. Every timing in the sequence is configurable and measurable, so the viva can be answered with the panel in front of the student.

1. With mains healthy, the sensing relay holds the mains contactor closed and the load runs on mains; the GENERATOR contactor is locked out by the interlock.
2. When mains voltage drops below the sensing threshold, the start delay timer begins — this rides through momentary dips without transferring.
3. After the start delay, the simulated generator source is "started" (energized) and the transfer delay runs to let it stabilize.
4. The mains contactor drops out, the interlock releases, and the generator contactor pulls in — the load is now on the generator supply, with the changeover interval measurable at the test points.
5. When mains returns and stays healthy through the retransfer delay, the sequence reverses: load back to mains, generator source shut down.
6. Throughout, the electrical + mechanical interlock guarantees the two sources can never be paralleled, and the emergency stop drops both contactors.

Tech stack:

• 2 × power contactors with mechanical interlock accessory
• Voltage-sensing / phase-monitoring relay
• ON-delay timer modules (start, transfer, retransfer delays)
• MCBs for mains input, generator input and load output
• Panel indicator lamps, emergency stop pushbutton
• Steel enclosure trainer panel with labeled terminals
• Stopwatch / oscilloscope for the buyer-run transfer-time test

• [Mains-failure sensing] Voltage-sensing relay monitors the mains input; dropout below the threshold starts the transfer sequence.
• [Configurable transfer timing] Start delay, transfer delay and retransfer delay set on timer modules — every timing in the sequence is adjustable and measurable.
• [Interlocked contactors] Two contactors with both electrical and mechanical interlocking, so mains and generator can never parallel — the core safety principle of any ATS.
• [Simulated generator source] Second input source standing in for the generator, with its own breaker, so the full changeover can be demonstrated on the bench.
• [Status indication] Panel lamps for MAINS ON, GENERATOR ON, TRANSFER IN PROGRESS and LOAD ON GENERATOR at every stage.
• [Transfer-time measurement] Test points for measuring the changeover interval with a stopwatch or scope — the buyer-run test procedure documents exactly how.
• [Safety-first construction] Input MCBs, clear terminal shrouding, emergency stop and a documented safe test sequence in the build guide.

• Assembled ATS trainer panel (contactors, timers, sensing relay, MCBs, lamps, e-stop)
• Complete wiring diagram and bill of materials with exact part numbers
• Buyer-run test procedure: transfer-time measurement, interlock verification, timing adjustment
• Timing configuration guide (start/transfer/retransfer delay setting)
• Project report PDF (background, ATS theory, system design, wiring, test procedure, safety)
• PPT presentation for final review
• Viva Q&A preparation document (changeover sequencing, interlocking, backfeed prevention, timer selection)

• The "generator" is a simulated second AC source on the bench — the trainer demonstrates the ATS logic and switching, not generator control or auto-start of a real genset.
• Transfer time is a design target (< 1 s), not a measured claim — the buyer measures it on their own panel with the documented procedure.
• Single-phase 230 V demonstration scale; three-phase ATS extension is documented as future scope.
• This is a training panel: it demonstrates changeover correctly, but it is not a certified switchgear product for building installation.
• Mains-voltage wiring demands respect: the build guide's safety sequence must be followed exactly.