What is variable frequency drive & how it works
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This document discusses variable frequency drives (VFDs) which vary the frequency and voltage supplied to electric motors to control their speed. It describes the key components of a VFD including the rectifier, DC bus, and inverter. The rectifier converts AC power to DC, the DC bus stores and filters it, and the inverter converts it back to AC of variable frequency to control motor speed. VFDs can operate in scalar or vector control modes, and their parameters like frequency and voltage settings must be configured for the specific motor. VFDs allow controlling motor speed without a mechanical transmission and provide braking methods like DC injection to slow motors.
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What is variable frequency drive & how it works
- 2. Understanding “VFD Variable Frequency Drives Xubair khan 19092 (BE)-Electronics
- 3. Contents • Introduction • Block Diagram • Building blocks • Modes of operation • VFD Parameters • Braking Technique • Start/Stop Command methods
- 4. Introduction – A Variable Frequency Drive (VFD) is a type of motor controller that drives an electric motor by varying the frequency and voltage supplied to the electric motor. Other names for a VFD are variable speed drive, adjustable speed drive, adjustable frequency drive, or an AC drive. – Frequency (or hertz) is directly related to the motor’s speed (RPMs)
- 5. Energy Usage
- 6. Electric Motor Facts • Electric motors account for more than two- thirds of all energy used in industry • 63% of the energy consumed is linked to air & liquid flow • A typical 50HP industrial motor, with an efficiency of 90%, costs over $25,000 to operate continuously over a year.
- 7. – Standard motors are constant speed and when they are energized they run at a 100% speed no matter the load. – What if the speed of the driven machine (Fan, Pump) is to be changed?
- 8. What is a VFD? – Variable Frequency Drive (VFD) – Equation of motor speed Speed= 120 x f /P • P=No. of poles • F=Line Frequency – How to change line frequency? Constant =50Hz
- 9. Block Diagram VFD Fundamentals 50 Hz Power Electrical Energy ABB Variable Frequency To Motor VFD RECTIFIER (AC - DC) INVERTER (DC - AC) AC DC AC VFD Variable Frequency50 Hz
- 10. VFD Explored First, the Converter (usually a diode rectifier) converts three-phase AC power to DC power. Next, the DC Bus stores and filters the DC power in a large bank of capacitors. Last, the Inverter (usually a set of six IGBTs) switches or inverts the DC power in a Pulse Width Modulated (PWM) AC waveform to the motor.
- 11. Rectifier • Basic Building blocks of rectification – Diodes (Uncontrolled)
- 12. Thyristors • Controlled • Output voltage can be controlled Gate Pulse Conduction after Gate Pulse
- 14. Output Voltage • Output Voltage (dc) = 3× 3×𝑉𝑚 ∏ x cos 𝐴 –Vm=Peak Value of voltage –A= Firing Angle • Firing at zero gives maximum output dc voltage
- 15. Is it a perfect Direct Current? • Conversion of AC into DC a perfect process? – Ripples • How to eliminate the ripples? – Filters DC bus in VFD
- 16. Inverter Action • Switching DC voltage ON and OFF will make it AC • Filtered output from DC bus is sent to inverter in VFD
- 17. RECTIFIER Positive DC Bus Negative DC Bus + - INVERTER How switching can convert DC into AC?
- 22. Pulse width modulation • Such a waveform is not acceptable – Nowhere near Sine wave • Contains harmonics – Multiples of fundamentals • Solution – Pulse width modulation
- 23. RECTIFIER Positive DC Bus Negative DC Bus + - INVERTER How Often You Switch From Positive Pulses To Negative Pulses Determines The Frequency Of The Waveform Frequency Voltage
- 24. Area Under the Curve
- 25. VFD Control Mode • Scalar mode • Vector mode
- 26. V/F Control Mode • Scalar mode • Drive is unaware of what is happening in the motor Example: • A 400V scalar drive is told to run a 400V, 50 Hz motor at 50% speed Following V/F pattern, Voltage applied by the drive will also be half. • Perfect when at no load. • After loading, motor will run at less than 50% speed • Drive is unaware of it Solution Vector Control
- 27. MotorDrive Speed setting=50% Speed at no load=50% Load increase
- 28. MotorDrive Speed setting=50% Speed=40% No idea what is happening
- 29. Vector Control Mode • Vector control with sensor – Feedback through encoder – Better speed regulations up to 0.01% – Faster response to load variations
- 31. VFD input Parameters • Max./Base frequency setting • Motor rated output • Motor rated voltage • Motor rated current
- 32. VFD input Parameters • Frequency Reference setting methods • Stop Command method • Start frequency • Stop frequency (DC Braking starts) • Torque Boost
- 33. Frequency Reference Setting Methods • Potentiometer • 0-10V input voltage • 4-20(mA)
- 34. Frequency Reference Setting through Potentiometer Min Max Min Max Frequency
- 36. VFD drives ……….
- 37. Stop Command methods • Coast to stop • Ramp to stop
- 38. Coast to stop Coast to Stop Frequency Run Command Motor speed Command is removed
- 39. Ramp to stop Frequency Run Command Command is removed DC Brake Ramp to Stop
- 40. Auto Tuning • Drive familiarizing itself with motor VFD here I am M
- 41. Auto Tuning MotorDrive Start of Auto tuning Primary resistanceLeakage reactanceDC Brake Voltage Torque Boost Voltage Slip compensation
- 42. DC Injection Braking • No mechanical Contact • DC is applied at the stator winding • DC causes stator to be become a magnet with constant field • A voltage is induced inside the rotor causing current to flow • According to Lens’s law, this current will cause rotor to stop