Single-Phase Motor Starter Panel with Thermal Overload Protection

Single-phase induction motors drive pumps, compressors, fans and machine tools across homes, farms and small industry — and every one of them should sit behind a proper starter panel. Direct switching with an ordinary switch gives no overload protection: a stalled or overloaded motor draws locked-rotor current until its windings burn, and after a supply interruption a plain switch leaves the motor restarting the moment power returns, which is a genuine safety hazard. The standard answer is the DOL starter: a contactor switched through start/stop pushbuttons, a thermal overload relay that trips on sustained overcurrent, and a control circuit wired so the contactor cannot re-energize by itself after a power failure. This project builds that exact panel on a bench board — real switchgear, real wiring practice, and a test procedure that proves each protection function works.

1. The 230 V AC supply enters through the MCB, which provides short-circuit protection and isolation for the whole panel.
2. Power flows through the contactor's main contacts to the motor terminals; the contactor coil is driven by the control circuit.
3. Pressing the green start button energizes the contactor coil through the normally-closed stop button, overload relay contact and (latched) e-stop; an auxiliary contact seals the coil in.
4. The motor runs direct-on-line; the thermal overload relay's heater elements carry the motor current and its bimetallic strips model the motor's heating.
5. On sustained overcurrent the overload relay's NC contact opens, dropping the contactor and lighting the trip lamp — the contactor stays out until the relay is manually reset.
6. If the supply fails, the contactor coil de-energizes and the seal-in breaks; when supply returns the motor cannot restart until start is pressed (no-volt release).
7. Pressing stop or the e-stop at any time drops the contactor immediately.

Tech stack:

• Single-phase power contactor with auxiliary contacts
• Thermal overload relay (adjustable setting range)
• MCB for short-circuit protection and isolation
• Start/stop pushbutton station + latching emergency stop
• Pilot indicator lamps (supply, run, trip)
• DIN-rail/panel enclosure with terminal blocks
• Single-phase induction motor demo load

• [DOL starting with contactor] The motor is switched through a proper power contactor, not a toggle switch — demonstrating the standard motor-control building block.
• [Thermal overload relay] A bimetallic overload relay with adjustable current setting trips the contactor on sustained overcurrent, protecting the motor against overload and single-phasing effects; its trip class and setting procedure are documented.
• [No-volt release behavior] The start/stop control circuit is wired so the contactor drops out on supply failure and cannot restart until start is pressed again — the required no-volt protection, demonstrated live.
• [MCB short-circuit protection] An MCB upstream of the contactor provides short-circuit and isolation protection with the correct breaking capacity for the demo supply.
• [Start/stop pushbutton station] Green start, red stop and a latching emergency-stop mushroom give the standard operator interface.
• [Status indication] Pilot lamps show supply healthy, motor running and overload tripped states for a readable demo.
• [Fault simulation for demo] The test procedure includes a safe simulated-overload method so the examiner can watch the overload relay trip and the panel lock out.

• Working motor starter panel (contactor, overload relay, MCB, pushbuttons, e-stop, lamps, enclosure, demo motor)
• Complete wiring diagrams (power circuit and control circuit) with wire sizing notes
• Component list with ratings
• Protection test procedure (buyer-run: verify overload trip, no-volt release and e-stop function)
• Project report PDF (motor starting theory, DOL vs reduced-voltage starting, protection, methodology)
• PPT presentation for final review
• Viva Q&A preparation document (DOL starting current, thermal overload principle, no-volt vs no-load release, trip class)
• Setup and demonstration guide

• This is an academic demonstration panel at fractional-HP demo scale; it must not be installed on real plant or building wiring without a licensed electrician's review.
• Overload relay settings are design-selected for the demo motor and verified by the buyer's own test procedure — the panel ships with the setting procedure, not a pre-certified trip curve.
• DOL starting draws high inrush current by principle; reduced-voltage starting (autotransformer/soft starter) is noted as an extension, not included.
• The thermal overload models heating approximately; it does not replace motor winding-temperature sensing.
• Repeated rapid start/stop cycling can nuisance-trip the thermal element — this behavior is documented as a teaching point.