Motors fault

Dijferential Protection Differential protection is apphed to detect internal motor faults quickly and hmit damage. The cost of this protection is justified on large motors [II19 kW (1500 hp) and above]. For which hmiting the motor damage may save the cost of this additional protection many times over. 

All these considerations apply to HV motors, particularly if they are fed directly from the main generator switchboard. LV motors can often be grouped together and considered as one large equivalent motor. It is sometimes possible to ignore the contributions from LV motors because their circuits often have a low X-to-R ratio, which causes the motor contribution to decay very fast. Also, the connected cables, busbars and transformers in the circuit will tend to attenuate the motor fault contribution. 

Gao, X.Z. Ovaska, S.J. 2001. Soft computing methods in motor fault diagnosis. Applied Soft Computing 1(1) 73-81. 

Kowalski, C.T. Orlowska-Kowalska, T. 2003. Neural networks application for induction motor faults diagnosis. Mathematics and Computers in Simulation 63 435-448. 

Protection of the electrical supply system in the event of a motor or motor-feeder short circuit. The fault must be cleared from the rest of the system to prevent further trouble,... 

High-voltage contactor-type motor controls depend on power fuses for short-circuit protection. The fuses are coordinated with the overload relays to protect the motor circuit over the full range of fault conditions from overload conditions to solid maximum-current short circmts. 

FIG. 29-8 Typical high-voltage ac motor starter illiistrating several protective schemes fuses, overload relays, ground-fault relays, and differential relays with the associated current transformer that act as fault-current sensors. In practice, the differential protection current transformers are located at the motor, hut the relays are part of the starter. 

Both differential and ground relaying detect ground faults. Ground-fault protection is located at the starter and protects the cable and the motor differential CTs are located at the motor and protect the motor only. Economic priorities indicate ground-fault protec tion first, adding differential protection when justified by potential savings in downtime and repair costs. 

These conditions may not occur esen on a fault in the motor. 

Yes because of lower capacity of motor, cables and switchgears and a low fault level... 

The motor makes rumbling noise and the stator eurrent fluctuates. The rotor circuit may be broken and should be repaired. If the motor also overheats, there may be an inter-turn fault or a short-circuit between the phases. Detect these and rectify if possible, otherwise rewind the motor. 

An electric motor must be adequately protected against all unfavourable operating conditions and internal or external faults. We have classified these conditions into three categories to identify the most suitable protection ... 

Frequent starts It is not a fault condition, but rapid heating of motor s stator and rotor due to frequent starts will be no less severe than a fault condition, hence considered in this category. 

It trips quickly on a fault condition and relieves the motor from the prolonged stresses of the fault, which may cause excessive thermal and electrodynajiiic stresses. 

The latest practice for up to 6.6 k V motors is to use an HT load break, fault-making isolator in conjunction with the appropriate type and size of HRC fuses, a vacuum contactor and a motor protection relay. The contactors... 

Lythall, R.T., AC Motor Control (on earth fault protection and thermistor protection). 

Interrupting the equipment such as an induction motor, a transformer, a cable or an overhead line on a fault, such as on a short-circuit or a ground fault. 

In the above situation, even an overspeeding motor on a fault elsewhere would feed back the supply source and require such protection. The protective scheme isolates the faulty source from being fed by the healthy sources. Figure 21,18 illustrates a simple power circuit provided with a directional G/F relay. In the event of a fault in system B, source B alone would isolate. Source A would not feed the fault as relay b would trip the breaker B and eliminate /g. The relays are necessarily set at lower settings and at lower tripping times than the non-directional... 

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