Rotor Voltage Control Of Three Phase Induction Motor

In a wound-rotor motor , an external three phase resistances may be connected to its slip rings . the developed torque may be varied by varying the resistance Rx .

This method increase the starting torque while limiting the starting current . This is an inefficient method and there would be imbalances in voltages and currents if the resistances in the rotor circuit  not equal .

 

The wound rotor induction motor is designed to have a low-rotor resistance so that  the running efficiency  is high and the full load slip is low .
The increase in the rotor resistance not affect on the value of the max. torque but increase the slip at max. torque .
The wound-rotor motors are widely used in applications requiring frequent starting and braking with large motor torques .

Because of the availability of rotor winding's for changing the resistance of the rotor , the wound rotor offers greater flexibility for control . However,it increases the cost and needs of maintenance due to slip rings and brushes .
The three-phase resistors may be  replace by three-phase diode rectifier and a DC converter .

Slip power is the portion of the air-gap power , which is not converted into mechanical power.The slip power of the rotor circuit may be returning to the supply by replacing the dc converter and resistances R by three-phase full converter .

By replacing the bridge rectifiers by three three-phase dual converters (or cyclo-converters). The slip PF in either direction is possible .

The static Kramer and scherbius  drives are used in large power pump and blower applications where , limited range of speed control is required.Because the motor is connected directly to the source, the power flow of these drives is generally high .

Stator Voltage Control Of three Phase Induction Motor !

As the stator voltage is reduced the air gap flux and torque also reduced .

The torque is proportional to the square of the stator supply voltage and a reduction of the stator voltage can produce a reduction in the speed . 

 

Where , b< 1 

The range of speed control depends on the slip for maximum torque Smax .

the induced voltage is proportional to the flux , frequency and r.m.s air gap flux .
For a low slip motor , the speed range is very narrow. 

This type of voltage control is not suitable for a constant torque load and is normally applied to applications requiring low starting torque and narrow range of speed control at retivily low slip .

 

where , Km is constant and depends on the number of turns of the stator winding.

• The stator voltage can be varied by :

1. AC voltage controller
2. Voltage-fed variable DC link inverter 
3. pulse width modulation (PWM) inverter 

الجر الكهربائى (Electric Traction )

أنواع الجر الكهربى :

1.   Self contained : هذا النوع مستقل ذاتيا حيث يشتمل على محرك ديزل لتشغيل مولد كهربى و عادة يكون من نوع  التيار المستمر و الذى بدوره يمد محركات الجر بالطاقة الكهربائية اللازمة .

2. Grid Supplied : و هو الذى تستمد فيه المركبة أو القاطرة , الطاقة الكهربائية اللازمة من شبكة توزيع ممتده من محطة أو محطات توليد .

مميزات الجر الكهربائى :

1.  النظافة التى تصاحب استخدام المحركات الكهربائية 
2. يوفر المحرك الكهربائى عزما كبيرا لبدء الحركة , مما يسمح بتسارع عالى القيمة عند بدء الحركة 
3. يوفر المحرك الكهربائى امكانية استخدام الفرامل الكهربية و هى تتيح اعادة الطاقة الى الشبكة الكهربائية 
4. الوقت اللازم لصيانة و اصلاح المركبة الكهربية أقل بكثير مما تحتاجه المركبات الأخرى 
5. تكاليف الصيانة و الاصلاح للمركبات الكهربائية تبلغ حوالى نصف تلك التكاليف فى المركبات الاخرى 
6. التخلص من الأدخنة و العادم الذى قد يحتوى على عناصر سامة , يعتبر من أهم مزايا الجر الكهربائى 

عيوب الجر الكهربائى :

1. تكلفة الإنشاء مرتفعة 
2. أى عطل فى الشبكة الكهربائية و لو لفتره وجيزة سوف يؤدى إلى شلل تام فى حركة السير قد يمتد لساعات طويلة 
3. التداخل الذى يحدث بين شبكة الجر الكهربى و خطوط الهاتف و الاتصالات 

نظم تغذية شبكات الجر الكهربى :

1. التيار المستمر 
2. التيار المتردد أحادى الأوجه 
3. التيار المتردد ثلاثى الأوجه 

Starting Transformer

Starting transformers are a special type of economy transformer. Starting transformers are used as current limiters for starting three phase induction motors , thus eliminate supply voltage dips. 

for example , with weak networks and larger motors .

regardless of whether a transformer is needed for lighting a runway or for a wind-power station or whether a miniature transformer or a transformer for high output is required , Eaton applies .

These transformers are designed to suit a specific application for input voltage ranges for many world wide systems .

The starting transformer can be dry type air cooled , open type for inclusion in a switch gear

cubic , enclosed type in a separate enclosure or liquid cooled transformer tank using mineral transformer oil or synthetic fluid.