Protection locked rotors

Synchronous-motor rotor frequency can be detected because the rotor field circuit is available. Special control schemes have been devised which take into account both speed and induced rotor current in providing locked-rotor and accelerating protection. 

When the locked rotor torque cannot be measured by the Table 11.7 Protection against contact with live or moving parts... 

Operating conditions that may overload a machine and raise its temperature beyond permissible limits may be called unfavourable. This overheating, however, will be gradual (exponential), unlike rapid (adiabatic) heating as caused during a locked rotor condition. The machine now follows its own thermal curve and therefore a conventional thermal protection device can be used to protect it from such conditions. These conditions may arise due to one or more of the following ... 

Stalling or locked rotor protection. This is also detected by the prolonged starting time as well as overheating of the machine. It is possible that the machine was already under operation and hot when it had stalled. Under such a condition, the rotor operates at a high freqitency and is more vulnerable to damage. Since it is not possible to create a replica of the rotor, separate... 

Motors are classified by size, apphcation, electrical type, NEMA design letter, and environmental protection and cooling methods. They are rated for special standard environmental and operating service conditions by performance and mechanical configuration voltage and frequency, locked-rotor kVA, service factor, horsepower, speed, torque, locked-rotor current, performance, temperature rise, duty cycle, and frame size. 

Thermistors Devices that sense temperature through changes in resistance. Signals from a thermistor may be amplified to interrupt the contactor holding coil to provide a degree of protection against motor locked-rotor conditions and running overloads. 

High-Accuracy Trip Curve Adjustments. Certain aspects of an overload relay s trip curves are critical for real motor protection. These include time-to-trip on locked rotor, motor overload conditions, and phase loss. 

Thermal overloads are hmited from approaching the ideal motor protection curves for these critical conditions. Because they do not directly measure current loading, but instead approximate it through use of heaters within the overload, they are subject to thermal overshoot (continued heat absorption after locked-rotor current is removed), residual heat remaining after reset, mechanical variables, and other difficulties in placing the various curves in optimum relationship with each other. 

Overload protection is recommended for all motors, especially coolant pump motors, and shall be provided for each motor rated at more than 0.5 kW. Continuously operating motors over 1 kW shall be protected against overloads and against a stalled rotor state with built-in thermal sensors typically employed (EN 60034-11). The use of appropriate protection devices for special-duty motors (e.g., rapid traverse, locking, rapid reversal) is recommended. For motors with ratings less than or equal to 2 kW, overload protection may not be required. 

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