Variable frequency drives (VFDs) are used in automation and programmable logic control systems to run AC motors at variable speeds and provide smooth motor starts. A VFD adjusts the frequency and voltage supplied to the motor to control its speed. It converts the input AC voltage to DC, filters it with capacitors, and converts it back to AC at the desired output frequency using transistors to control the motor speed. VFDs allow motors to run at speeds between 50-100% of their rated speed by varying the frequency supplied.

1. Variable Frequency
Drive(VFD)
• Used in Automation and Programmable Logic Control (PLC)
• Used for running an ac motor at variable speed
• Also used to give smooth start to ac motors
• It is usually called by simple name “Drive”
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2. Variable Frequency
Drive(VFD)
How it
works?
• It adjusts the frequency as well as magnitude of
the impressed voltage to the motor to control the
speed
To change the frequency first it converts input AC
voltage in to DC Voltage, filter it through capacitor
to make it almost pure DC and then again turned it
in to AC voltage of desired frequency with the help
of transistors.
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3. Variable Frequency
Drive(VFD)
Frequency
Provided by
VFD
Field
Rotation
Ns
Motor
Speed
Nr
Line to
Line
Voltage
VL-L
50 Hz 1500 Rpm 1455 Rpm 400 V
40 Hz (20%↓) 1200 Rpm
(20%↓)
1164 Rpm
(20%↓)
320 V
25Hz
(50%↓)
750 Rpm
(50%↓)
727 Rpm
(50%↓)
200 V
(50%↓)
Ns = 120 X F / P
• For a 4 pole induction motor and at 3% slip
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5. Variable Frequency
Drive(VFD)
1) ELCB/RCCB generally inbuilt in the VFD is to protect VFD against overload, short circuit and earth leakage
2) Reactor coil is used to smooth out disturbances caused due to fast switching elements of VFD circuit and to protect this
harmonics reaching out toward power source
3) Magnetic Contactor is used to provide remote switching or time delay switching like facilities for the purpose of
PLC/Automation.
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6. Variable Frequency
Drive(VFD)
4) Core unit of VFD consist Rectifier, LC filter and IGBT Based Inverter unit
5) Output of the VFD then connected to the motor whose speed is to be controlled, multiple number of Motors can also be
connected to the VFD but its depend on the model of VFD.
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7. Thank You
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