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الطرق المختلفه لبدء تشغيل المحرك الحثي Induction Motor
Different Ways For Induction Motor Starting

من المعروف ان في بدايه  تشغيل المحرك Induction Motor يجب أن يتحمل مرور تيار عالي للتغلب على القصور الذاتي للكتلة الميكانيكية مما يستلزم عزم Torque كبير فى البداية ، ويبلغ التيار فى بداية التشغيل حوالي 6 مرات قدر تيار التحميل ، وقد يسبب التيار العالي هبوطا فى الجهد الكهربي المغذي للشبكة مما يؤدي إلى فصل بعض الأحمال ، لذلك يجب ألا يزيد الهبوط فى جهد المصدر أثناء بدء تشغيل المحرك عن 4 % ويتطلب ذلك أن يكون جهاز بدء التشغيل قادرًا على تحميل المحرك عدة مرات خلال الساعة الواحدة دون مشاكل للشبكة . لذلك هناك اكثر من طريقه مختلفه تستخدم لبدء تشغيل  Induction Motor  ومنها :-

1 - Direct on-line starting (DOL)   التشغيل المباشر من خط التغذية

This is the simplest mode, where the stator is directly connected to the mains supply. The motor starts with its own characteristics. When it is switched on, the motor behaves like a transformer with its secondary, formed by the very low resistance rotor cage, in short circuit. There is a high induced current in the rotor which results in a current peak in the mains supply:
Current on starting = 5 to 8 rated Current.
The average starting torque is:
T on starting = 0.5 to 1.5 rated T.
In spite of its advantages (simple equipment, high starting torque, fast start, low cost), direct on-line starting is only suitable when:
- the power of the motor is low compared to that of the mains, which limits interference from inrush current,
- the machine to drive does not need to speed up gradually or has a damping device to limit the shock of starting,
- the starting torque can be high without affecting machine operation or the load that is driven.


2 - Star/Delta   بدء التشغيل بإستخدام طريقة نجمة – دلتا

This method requires both connections for each phase (six in all) to be taken to the starter. Three contactors are used to first connect the motor in star and then to delta after a given time. Connecting the motor in star reduces the voltage applied to each winding to about 60% of the line voltage. This reduces the starting torque and current (typically 3.5 x FLC). After a given time the motor is switched to delta connection and then runs as if direct-on-line. Its main advantages are that it is relatively simple and low cost. The major problem with this method is that the reduced voltage level is in a single stage and is fixed. sometimes this voltage is not ideal, the torque it produces (65% of full load torque) may be too small and the motor stalls or does not give complete acceleration, or if it is too great the motor still starts with a pronounced snatch. The star/delta transition will produce a second current and torque peak which is almost the equivalent of having two direct-on-line starts. On some loads the motor sometimes almost stalls during this transition time. This method of starting does however have the advantage of being a low cost and simple solution if its limitations can be tolerated .


Advantages :
1- Low cost and simple
Disadvantages
1- Torque too high – causes snatch
2- Torque too low – motor stalls
3- Transition peak up to 20 x In
4- Motor can stall in transition

3 - Auto Transformer

This method uses transformer action to reduce the voltage applied to the motor and current seen by the supply. An improved torque/amp ratio is achieved and starting current is typically 3 x FLC, depending on the voltage rapping selected. Normally the voltage is applied to the motor in voltage steps through the transformer with the taps being selected through contactors. Typical tappings are 50%, 70%, followed by full voltage being applied to the motor. The major disadvantages are size and cost, and of course the mechanical snatch at switch on is not controllable and can still cause problems. Also once the tappings have been selected, it may be necessary to change them according to changes in load parameters.


Advantages
1- Simple operation
Disadvantages
1- Poor controllability
2- Bulky
3- Very Expensive

4 - Soft Starters

The soft start is designed to apply an adjustable voltage to the motor and increase this voltage gradually over a user-selectable acceleration period. The acceleration time being dependent on the application and desired characteristics. The added advantage of this method of reduced voltage control is that the motor can also be stopped gradually by slowly reducing the output voltage to the .‘Soft Stop’ feature offers a smooth stop in many process industries such as pumps, where fast stops can result in ‘water hammer’ and mechanical damage


Advantages
1- Reduced starting current
2- Reduced starting torque
3- Less mechanical stress
4- Improved control of acceleration and deceleration



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