Variable voltage variable frequency drive

Variable voltage variable frequency drives

1. Presented by: BHEEMANNA ANGADI [1OX13EE020 ] The Oxford college of engg-bangalore 1 VARIABLE VOLTAGE VARIABLE FREQUENCY DRIVES

2. CONTENTS INTRODUCTION DRIVE OPERATION PRINCIPLE SYSTEM DESCRIPTION VVVFD CONTROLLER SPWM TECHNIQUE POWER RATINGS ADVANTAGES & APPLICATIONS DRAWBACKS CONCLUSION

3. INTRODUCTION Earlier, IM have been used mainly in applications requiring a constant speed Because of conventional methods of speed control &expensive Nowdays , Due to certain advantages of IM, Ac drives are more popular Hence it is required to develop ac drives Availability of power transistors ,IGBT , GTO have allowed the development of variable speed induction motor drives

4. CONCEPT OF VVVF DRIVES  Is a type of adjustable-speed drive used in electro-mechanical drive systems to control AC motor speed and torque by varying motor input frequency and voltage also termed as  adjustable-frequency drive ,  variable speed drive ,  AC drive ,  micro drive  or  inverter drive

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. 5

6. Contd … Stator voltage V 1 =f Φ 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. Decrease in flux is also avoided to retain the torque capability of 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. 6

7. SYSTEM DESCRIPTION A VFD in a block diagram .

8. Rectifier: The rectifier in a VFD is used to convert incoming ac power into direct current (dc) power. DC bus: DC output of rectifier flows through the dc link to inverter input. Any ripple must be smoothed out before any transistor switches “on”. If not, this distortion will show up in the output to the Motor Inverter: The “Insulated Gate Bipolar Transistor” (IGBT) is a common choice in modern VFDs. The IGBT can switch on and off several thousand times per second and precisely control the power delivered to the motor. The IGBT uses a method named “pulse width modulation” (PWM) to simulate a current sine wave at the desired frequency to the motor.

9. VVVFD Controller . 9

10. SPWM TECHINIQUE FOR v/f V/F = Constant v=voltage and f=frequency, c= constant In sinusoidal pulse width modulation there are multiple pulses per half-cycle and the width of the each pulse is varied with respect to the sine wave magnitude corresponding to that duration.  In order to achieve the constant ratio ,we have to fire the (IGBT )in such way that the amplitude of output voltage waveform in a cycle will maintain the v/f ratio according the frequency change. Waveforms are shown on next slide 10

11. Pulse-width-modulated voltage and current waveforms

12. ADVANTAGES Large energy savings at lower speed. Increased life of rotating components due to lower operating speed. Reduced noise and vibration level. Reduction of thermal and mechanical stresses. Lower KVA Rating High power factor

13. APPLICATIONS Fans Blowers Mill run-out tablets Cranes Conveyers Traction etc……

14. DRAWBACKS Power converters for the control of ac motors are more complex. Power converter for ac drives are more expensive. Power converters for ac drives generate harmonics in the supply system and load circuit.

15. THANK YOU……..