5–48 469 MOTOR MANAGEMENT RELAY – INSTRUCTION MANUALCHAPTER 5: SETTINGSOverload Curve feature is tailored to protect these types of motors. Voltage is continuallymonitored during motor starting and the acceleration thermal limit curve is adjustedaccordingly.The Voltage Dependent Overload Curve is comprised of the three characteristic shapes ofthermal limit curves as determined by the stall or locked rotor condition, acceleration, andrunning overload. The curve is constructed by entering a custom curve shape for therunning overload protection curve. Next, a point must be entered for the accelerationprotection curve at the point of intersection with the custom curve, based on the minimumallowable starting voltage as defined by the minimum allowable line voltage. The lockedrotor current and safe stall time must also be entered for that voltage. A second point ofintersection must be entered for 100% line voltage. Once again, the locked rotor currentand the safe stall time must be entered, this time for 100% line voltage. The protectioncurve created from the safe stall time and intersection point will be dynamic based on themeasured line voltage between the minimum allowable line voltage and the 100% linevoltage. This method of protection inherently accounts for the change in motor speed asan impedance relay would. The change in impedance is reflected by motor terminalvoltage and line current. For any given speed at any given line voltage, there is only onevalue of line current.
