19-03-2012, 03:04 PM
Full Voltage Techniques
assgnmnt.docx (Size: 22.52 KB / Downloads: 35)
Direct-On-Line (DOL)
DOL is the traditional and simplest method of motor starting, and most other methods
are base lined against it. It is also often called across the line start. This method is the
Direct connection of the terminal voltage to the motor stator with no additional
components, and also for this reason is most economical in terms of installation cost and
ease of use. It is also one of most reliable and robust methods.
The disadvantage with this method is that it gives the highest possible starting current of all the method. A normal value is between 6 to 7 times the rated motor current but values of up to 9 or 10 times the rated current exist. Besides then starting current there also exists a current peak that can rise up to 14 times the rated current since the motor is not energized from the first moment when starting.
Shunt Capacitors
Connecting a capacitor in parallel to the motor can help compensate the reactive demand from the motor during startup by supplying a leading current and thus improve the power factor while still achieving high starting torque because of the full voltage. This provides some relief to the supply source. The shunt capacitance may be left connected if they are properly rated so as to provide power factor correction or removed as the motor approaches rated speed. the shunt capacitance that is left connected should never be sized larger than the motor’s magnetizing current. Capacitor starting may be used in conjunction with other starting methods. In general, capacitor starting is used for relatively large motors that need fast starting or improved efficiency
during startup. it was found that capacitor starting could reduce up to half the maximum voltage dip as compared to the DOL start and still maintain other adequate starting characteristics. Capacitor starting is a reliable and robust method for motor starting on weak electrical system .
Electromechanical Reduced Voltage Techniques
The electromechanical reduced voltage methods can be implemented through
conventional circuitry, such as resistors and transformers. During these reduced voltage
startup methods, the thermal capability due to the increased time to reach rated speed
must be taken into consideration. Transient current surges are produced from open circuit
transitions, but with closed circuit transitions these switching transients are avoided.
Star-Delta
This is a starting method that reduces the starting current and starting torque. The components normally consist of three contactors, an overload relay and a timer for setting the time in the star-position (starting position). The motor must be delta connected during a normal run, in order to be able to use this starting method. The received starting current is about 30 % of the starting current during direct on line start and the starting torque is reduced to about 25 % of the torque available at a D.O.L start. The transition from star to delta transition usually occurs once nominal speed is reached, but is sometimes performed as low as 50% of nominal speed .This starting method only works when the application is light loaded during the start.