🔍 Key Concepts

• Difference between thermal overload heaters and magnetic (instantaneous) protection in motor starters • How a two‑wire control circuit (like a pressure switch or maintained contact) behaves after a power loss – this relates to low voltage protection vs low voltage release • Reading the schematic symbols for the overload contacts (OL) and the contactor coil M in both the power and control diagrams

💭 Think About

• Look at the OL elements in the motor leads T1, T2, and T3: what type of device are these usually, and how do they react to sustained overcurrent versus short‑circuit current? • Trace the control circuit from the line, through the selector switch (Man/Stop/Auto), pressure switch, and OL contact to the M coil. If the supply voltage is lost and then restored with the selector still in Auto and the pressure switch calling for the pump, will the motor restart automatically or stay off? • Ask yourself whether this control scheme uses a momentary "start" pushbutton and sealing contact, or only maintained contacts. How does that choice affect whether the circuit provides low voltage protection or low voltage release?

✅ Before You Answer

• Verify that the OL devices in the power circuit are the only overcurrent elements shown between the contactor and motor, and identify whether they are thermal overload heaters or magnetic trips. • Confirm whether there is a sealing (holding) contact for the M coil that would drop out on low voltage and require manual re‑energizing, which indicates low voltage protection. • Decide, based on the restart behavior after a power failure, whether the circuit’s control provides low voltage protection (no automatic restart) or low voltage release (automatic restart when conditions are met).