Overload Protection

	A. Cold running O/L     B. Hot running O/L     C. Cold locked rotor	D. Hot locked rotor curve E. Acceleration curve @80% F. Acceleration curve @100%
Motor Thermal Limits & Acceleration Curves
K Factor Used for Negative Sequence Protection
RTD Connection for Thermal Protection & Biasing

Three-phase motors are designed in such a way that overloads must be kept below the machine thermal damage limit. The motor thermal limits curves consist of three distinct segments, which are based on the three running conditions of the motor: the locked rotor or stall condition, motor acceleration and motor running overload. Ideally, curves should be provided for both hot and cold motor conditions. For most motors, the motor thermal limits are formed into one smooth homogeneous curve.

The acceleration curves are an indication of the amount of current and associated time for the motor to accelerate from a stop condition to a normal running condition. Usually, for large motors, there are two acceleration curves: the first is the acceleration curve at rated stator voltage while the second is the acceleration at 80% of rated stator voltage (soft starters are commonly used to reduce the amount of inrush current during starting). Starting the motor on a weak system can result in voltage depression, providing the same effect as a soft-start).

The primary protective element of the motor protection relay is the thermal overload element and this is accomplished through motor thermal image modeling. This model must account for all thermal processes in the motor while motor is starting, running at normal load, running overloaded and if motor is stopped. The algorithm of the thermal model integrates both stator and rotor heating into a single model. If the motor starting current begins to infringe on the thermal damage curves or if the motor is called upon to drive a high inertia load such that the acceleration time exceeds the safe stall time, custom or voltage dependent overload curves may be required.

Negative sequence currents (or unbalanced phase currents) will cause additional rotor heating that will not be accounted for by electromechanical relays and may not be accounted for in some electronic protective relays. The main causes of current unbalance are: blown fuses, loose connections, stator turn-to-turn faults, system voltage distortion and unbalance, as well as external faults.

Thermal models can have following enhancements and additions: motor start inhibit; standard, custom and voltage dependant overload curves; thermal model biasing by measured current unbalance and RTD’s; separate thermal time constants for running and stopped motor conditions; independent current unbalance detector; acceleration limit timer; mechanical jam detector; start and restart supervision.