The **JR28-R-4MDP3** is a thermal overload relay designed primarily for motor protection. It is often used in conjunction with AC contactors to protect electrical motors from phase failures and long-term overloads. --- ### ## Technical Specifications The following table outlines the core electronic and mechanical parameters of the JR28 series (specifically the R-4MDP3 variant): | Parameter | Specification | | :--- | :--- | | **Current Setting Range** | Typically 2.5A - 4.0A (Verify via dial) | | **Rated Voltage (Ue)** | Up to 690V AC | | **Trip Class** | 10A (Trips within 2-10s at 7.2x current) | | **Auxiliary Contacts** | 1 NO (Normally Open) + 1 NC (Normally Closed) | | **Reset Type** | Manual or Automatic (Switchable) | | **Phase Failure Protection** | Yes (Differential mechanism) | --- ### ### Key Electronic Components & Functions 1. **Bimetallic Strips:** * The heart of the device. It consists of two different metals bonded together. As current flows through, the strips heat up; because the metals expand at different rates, they bend. 2. **Trip Mechanism:** * When the bimetallic strips bend beyond a specific threshold (due to overcurrent), they trigger a mechanical linkage that opens the **NC (95-96)** contacts, cutting power to the contactor coil. 3. **Adjustment Dial:** * Allows the user to calibrate the tripping current to match the Motor’s Full Load Amps (FLA). 4. **Temperature Compensation:** * Includes a compensating bimetal strip to ensure the relay trips accurately regardless of the ambient temperature in the control cabinet. 5. **Test/Stop Button:** * Allows manual testing of the auxiliary contacts to ensure the control circuit is wired correctly. --- ### ### Wiring Diagram Overview The relay is usually wired in series with the motor power lines and integrated into the control logic as follows: ```mermaid graph TD Power[L1, L2, L3] --> Contactor[AC Contactor] Contactor --> JR28[JR28 Thermal Relay] JR28 --> Motor[Inductive Motor] subgraph Control Circuit 95[Terminal 95 - NC] --> 96[Terminal 96 - NC] 96 --> Coil[Contactor Coil A1] end ``` --- ### ### Applications * **Motor Protection:** Safeguards against overheating due to mechanical jams. * **Phase Loss:** Detects when one phase is missing, preventing motor burnout. * **Industrial Automation:** Common in pump stations, conveyor belts, and HVAC fans.