Variabe voltage and variable frequency drives

DRIVE CONTROL

1. Variable Voltage And Variable Frequency Drives 9/21/2015 1 Dept. Of Electrical Engg .

2. Contents 9/21/2015 2 Dept. Of Electrical Engg . Introduction Operating principle Example VVVFD system description VVVFD controller VVVFD Operation Conclusion  

3. Introduction A variable voltage and variable frequency drive (VVVFD) is a system for controlling the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor. A variable frequency drive is a specific type of adjustable-speed drive. Variable-frequency drives are also known as adjustable-frequency drives (AFD), variable-speed drives (VSD), AC drives or inverter drives 9/21/2015 3 Dept. Of Electrical Engg .

4. Speed Control Following methods are employed for speed control of Induction motors: - Pole changing Stator voltage control Supply frequency control Rotor resistance control 9/21/2015 4 Dept. Of Electrical Engg .

5. Operating Principle Variable frequency drives operate under the principle that the synchronous speed of an AC motor is determined by the frequency of the AC supply and the number of poles in the stator winding, according to the relation: Where, RPM = Revolutions per minute f = AC power frequency (Hertz) p = Number of poles (an even number) Synchronous motors operate at the synchronous speed determined by the above equation. The speed of an induction motor is slightly less than the synchronous speed. 9/21/2015 5 Dept. Of Electrical Engg .

6. Contd … Voltage induced in stator is proportional to the product of supply frequencies and flux. Any reduction in supply frequencies without change in terminal voltage causes an increase air gap flux. The increase in flux will saturate the motor. Therefore the variable frequency control below the rated frequency is generally carried out at rated air-gap flux by varying the terminal voltage with frequency so as to maintain (V/F) ratio constant at rated value. 9/21/2015 6 Dept. Of Electrical Engg .

7. The variable frequency control provides a good running and transient performance because of following features:- Speed control and braking operations are available from zero to above base speed operations. During transient the operation can be carried out at the max. torque while reduced current giving good dynamic response. Drop in speed from no lad to full load is small. 9/21/2015 7 Dept. Of Electrical Engg .

8. 9/21/2015 8 Dept. Of Electrical Engg . Fig. Shows the torque-developing characterstics of every motor: the Volts per Hertz ratio (V/F). Hertz 460/60 =7.67 (v/ hz ) If supply is 230 volts 230/60 =3.83 (v/ hz ) Volts

9. VVVFD Controller 9/21/2015 9 Dept. Of Electrical Engg .

10. Switching Bus with IGBTS 9/21/2015 10 Dept. Of Electrical Engg . Drive Output Waveform

11. 9/21/2015 11 Dept. Of Electrical Engg .

12. VVVFD Operation When a VVVFD starts a motor, it initially applies a low frequency and voltage to the motor. The starting frequency is typically 2 Hz or less. Starting at such a low frequency avoids the high inrush current. When a VVVFD starts, the applied frequency and voltage are increased at a controlled rate to accelerate the load without drawing excessive current . 9/21/2015 12 Dept. Of Electrical Engg .

13. Conclusion Due to many advantages offered by ac drives like automatic control, closed loop control, economical cost etc. ac motors are being replaced in fields which where totally capitalized by dc motors like traction, some industrial applications etc.   9/21/2015 13 Dept. Of Electrical Engg .

14. References 1.Clayton,A.E,The performance and design of direct current machines. 2.Chillikin,M.,Electric drives,Mir Publisher,Moscow . 3.Pillai,S.K,A first course on electrical drives,Wiley Eastern Ltd.,1982. 4.Bedford,B.D and R.G. Hoft,Principle of inverter circuit,John Wiley and sons, New York,1964. 5.Subrahmanyam,Vedam,Thyristor control of electrical drives,Tata McGraw-Hill Publishing,New Delhi 1988. 9/21/2015 Dept. Of Electrical Engg . 14