Variable Frequency Drive(VFD)

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”.

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 and inductor to make it almost pure DC and then again turned it in to AC voltage of desired frequency with the help of transistors (Fig 1).
schematic diagram and block representation of vfd variable frequency drive
Fig 1

Animated video lecture on VFD




·         In case when a three Induction motor is used with VFD

When three phase supply is given to a 3 phase induction motor it produces a rotating magnetic field and the rotation speed of this field in depend up on the total number of poles and the frequency of the applied voltage, it is known as synchronous speed and is given by

Ns = 120 F/P

where F is the frequency and P is the number of poles.

Three phase induction always runs at speed which is less then the synchronous speed as it is the speed of the rotor so it can be represented as Nr

Nr = Ns (1-s)
where "s" is slip 

Let the motor is of 4 poles and for slip being 3% ie 0.03 in this case a table is drawn which is given below showing the speed of motor i.e Nr at different applied frequency by the VFD which shows that by varying frequency different speed can be achieved.



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%↓)

Some important components inside VFD (Fig 2)

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 the switching 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.

4.      Above three elements generally comes with the VFD as an inbuilt feature in case if not so, because it is depend on the model and make of VFD which we are using then they should be connected to the VFD externally in the vfd panel.

5.      The core unit of VFD consist Rectifier, LC filter and IGBT Based Inverter unit.

6.      Output of the VFD then connected to the motor whose speed to be controlled, multiple number of Motor can also be connected to the VFD but its depend on the model of VFD. 

Fig 2

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