26-04-2012, 05:05 PM
Automatic Stalled Rotor & Earth Fault Protection of Motor
Automatic Stalled Rotor & Earth Fault Protection of Motor.doc (Size: 358.5 KB / Downloads: 100)
Introduction
There are wide ranges of A.C. motor are used in industries, because of the numerous duties for which they are used. Due to their simple construction and easily operated they are used. For numerous used of A.C. motor to operation carried out in the industry. They will be well protected to gives the uninterrupted operation of unit or industry. Therefore All the motors that will installed in the industry they need a protection, but fortunately, the more fundamental problems affecting the choice of protection are independent of the type of motor and the type of load to which it is connected. The safety-oriented use of components is becoming increasingly important in process automation due to situations in which man and the environment need to be protected by the safe shutdown of the machine or plant at any time. As new regulations in safety technology are required, so we required to the design of a modern motor protection device must be adequate to cater for the protection needs of any one of the vast range of motor designs in service, many of the designs having no permissible allowance for overloads. The generally various types of faults are occurs in the motor they are give below:-
Theory of Faults in Motor
In motor generally no. of faults are occurs. View of my seminar the two i.e. Stall rotor fault or Earth fault are consider. Here to understand and to know about these two faults.
Start Rotor Faults:-
When a motor is started, it draws a current well in excess of full load rating throughout the period that the motor takes to run-up to speed. While the motor starting current reduces somewhat as motor speed increases, it is normal in protection practice to assume that the motor current remains constant throughout the starting period. The starting current will vary depending on the design of the motor and method of starting. For motors started DOL (direct-on-line), the nominal starting current can be 4-8 times full-load current. However, when a star-delta starter is used, the line current will only be 1/of the DOL starting current.
Excessive Start Time/Locked Rotor:-
A motor may fail to accelerate from rest for a number of reasons:
a. Loss of a supply phase
b. Mechanical problems
c. Low supply voltage
d. Excessive load torque, etc.
A large current will be drawn from the supply, and cause extremely high temperatures to be generated within the motor. This is made worse by the fact that the motor is not rotating, and hence no cooling due to rotation is available.
Stall Rotor fault:-
Should a motor stall when running or be unable to start because of excessive load, it will draw a current from the supply equivalent to the locked rotor current. It is obviously desirable to avoid damage by disconnecting the machine as quickly as possible if this condition arises. Motor stalling can be recognized by the motor current exceeding the start current threshold after a successful start – i.e. a motor start has been detected and the motor
Current has dropped below the start current threshold within the motor safe start time.
Conclusion
Relays control output circuits of a much higher power.
Safety is increased
Protective relays are essential for keeping faults in the system isolated and keep equipment from being damaged.