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Motor Protection
Overview
Protection Principles
Application Guide
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Introduction
 
Cutaway of a Typical 3-Phase AC Motor
 
 
Rotor of a Squirrel Cage Motor
 
 
 
 
Three phase motors can be classified into two types: induction and synchronous. An induction motor consists of two parts: the stator and the rotor. The stator core is built of sheet-steel laminations that are supported in a frame.

The windings are placed in the stator slots 120 electrical degrees apart. Windings may be connected in “star” (or wye) or delta configuration.

The rotor of the induction motor is made of a laminated core with conductors placed parallel to the shaft. The rotor conductors are embedded in the surface of the core, and are not insulated from the core, because rotor currents follow the “least resistance” path. The rotor conductors are shorted by end rings at both ends.Any motor failure will have the following cost contributors: repair or replacement, removal, installation and loss of production. Most of the motor failure contributors and failed motor components are related to motor overheating. Thermal stress can potentially cause the failure of all the major motor parts: Stator, Rotor, Bearings, Shaft and Frame.

Motor Protection Overview
There are two main risks for an overheated motor: Stator windings insulation degradation and rotor conductors deforming or melting. Insulation lifetime decreases by half if the motor operating temperature exceeds thermal limit by 10ºC. There are a number of conditions that can result in damage of three-phase motors. These damages are a result of operating conditions or internal or external faults. External faults and operating conditions include: undervoltage, asymmetricalloading, phase and ground faults on the motor feeder and overloading during starting and running operation. Internal faults include: ground faults, faults between windings and inter-turn faults.


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