The **JR28-R-4MDP21** is a thermal overload relay commonly used in industrial automation and motor control circuits. It is designed to protect electric motors from overheating, phase failure, and prolonged stalls. --- ## 1. Technical Specifications The following table summarizes the key electronic and mechanical parameters of this specific model: | Parameter | Specification | | :--- | :--- | | **Model Type** | Thermal Overload Relay | | **Setting Range (Current)** | 1.6A to 2.5A (Typical for 4MDP21 series) | | **Rated Voltage (Ue)** | Up to 690V AC | | **Auxiliary Contacts** | 1 NO (Normally Open) + 1 NC (Normally Closed) | | **Tripping Class** | Class 10A (Standard trip within 10s at 7.2x current) | | **Compatibility** | Direct mounting on CJX2 series contactors | | **Reset Mode** | Manual or Automatic (Switchable) | --- ## 2. Core Electronic Components The device functions through several internal mechanisms that react to electrical changes: ### A. Bimetallic Strips The heart of the relay consists of three bimetallic strips (one for each phase). As current flows through these strips, they heat up. Because the two metals have different expansion coefficients, the strip bends. If the current is too high, the bend triggers the internal linkage. ### B. Phase Failure Mechanism The JR28 series includes a differential mechanism. If one phase loses current, the strips expand unevenly. The relay is designed to detect this imbalance and trip faster than a standard overload, protecting the motor from "single-phasing." ### C. Auxiliary Circuit * **NC Contact (95-96):** Usually wired in series with the contactor coil. When the relay trips, this opens, cutting power to the contactor and stopping the motor. * **NO Contact (97-98):** Used for signaling. It closes upon tripping to turn on a fault light or send a signal to a PLC. --- ## 3. Control Interface The front face of the JR28-R-4MDP21 includes several user-adjustable parts: 1. **Current Adjustment Dial:** A rotary scale used to set the exact full-load current (FLC) of the motor. 2. **Test Button:** Manually simulates a trip to verify that the auxiliary contacts and control wiring are working. 3. **Stop Button:** When pressed, it opens the NC contact but does not "trip" the mechanical latch. 4. **Reset Button/Selector:** A blue button that can be pushed to reset after a trip, or turned to toggle between **Manual (H)** and **Automatic (A)** reset modes. --- ## 4. Typical Wiring Diagram Below is a conceptual representation of how the electronic contacts are integrated: ```text L1 L2 L3 (Power In) | | | [ Contactor ] | | | [ JR28 Relay ] ----> [ Motor ] | | | | +-- (95) -- [ STOP Button ] -- (96) -- [ Contactor Coil ] | +-- (97) -- [ Fault Lamp ] -- (98) -- [ Neutral/Common ] ``` ---