A variable frequency drive (VFD) controls the speed and torque of an AC electric motor by varying the frequency of the power supplied to the motor. A VFD consists of a rectifier that converts AC power to DC, a DC bus, and an inverter that converts the DC back to AC at a variable frequency. VFDs help reduce energy consumption by only providing the power needed based on the motor's load, unlike standard constant speed motors. A VFD allows a motor to operate in constant torque mode at lower speeds for constant power output, and constant power mode at higher speeds where torque decreases but power remains constant.
2. CONTENTS
• What is a VFD?
• Purpose of the VFD
• VFD Basics
• Variable Frequency Control
3. What is a VFD?
– Variable Frequency Drive (VFD)
– A VFD can be used to control both the speed and
torque of a standard induction AC electric motor.
– It varies both the frequency of the AC waveform
being delivered to the motor saving money in
electricity.
– Basic components of a VFD:
• Input section, draws AC electric power from the
utility, Rectifier section, converts the AC into DC
power.
4. Purpose of the VFD
• VFDs help to limit demand and electrical
consumption of motors by reducing the amount of
energy they consume.
– Standard motors are constant speed and when they
are energized they run at a 100% no matter the load.
– Soft Start
– Only use energy you need
6. converter
VFD Basics
All VFD’s need a power section that converts AC
power into DC power.
This is called the converter bridge.
Sometimes the front end of the VFD, the converter
is commonly a three-phase, full-wave-diode
bridge.
7. DC Bus
The DC bus is the true link between the converter
and inverter sections of the drive.
Any ripple must be smoothed out before any
transistor switches “on”.
If not, this distortion will show up in the output to
the motor.
The DC bus voltage and current can be viewed
through the bus terminals.
VFD Basics
8. Inverter
The inverter section is made up primarily of
modules that are each made up of a transistor
and diode in combination with each other which
inverts the DC energy back to AC.
The power semi-conductors in the inverter section
act as switches.
Inverters are classified as voltage-source, current-
source of variable-voltage types. This has to do
with the form of DC that the inverter receives
from the DC bus.
VFD Basics
42. Variable Frequency Control
• The block diagram for a voltage source inverter
drive for asynchronous motor is as shown in fig
43. • The voltage source inverter shown in the figure
generates a 3 phase voltage waveform the
frequency of which is variable right from 0 HZ.
• The output voltage of the inverter can be either six
step or PWM in order to reduce the harmonic
contents.
44. Modes of Operation
• The synchronous motor can operate in one of the two
following modes of operation at a given time :
1. Constant torque mode
2. Constant power mode
1) Constant torque mode :
• As shown in fig. , upto the base speed i.e. rated speed,
the synchronous motor operates in the constant
torque mode.
• The torque produced by the motor remains constant,
as airgap flux is maintained constant & the airgap flux
remains constant due to the fact that the ratio of
stator voltage to stator frequency (v/f) is constant
upto the base speed.
45. • The (v/f) ratio is maintained constant in order to
avoid the saturation of core. The core saturation
may take place if the ratio (v/f) exceeds a
particular value.
• At the low speeds the characteristics in fig is not
linear. This boost in the stator voltage at low
speeds is essential to overcome the effect of stator
resistance at low frequency.
47. Output voltage V
Constant torque Constant power
V
boost
Constant
(v/f)
Rated frequency Output frequency
f
Stator voltage-stator frequency
48. 2) Constant power mode :
• Above the base speed i.e. rated speed, the synchronous
motor is operated in the constant power mode as shown
in fig1 .
• At base speed, the output voltage of the inverter reaches
the rated value of stator voltage, therefore it is not
possible to increase this voltage further. Thus the (v/f)
ratio does not remains constant after this point, infact it
decreases with increase in the stator frequency.
• This will weaken the air gap flux and hence will decrease
the torque producing capacity of the motor as shown in
fig1.
• The mechanical power developed by the motor is equal to
the product of Torque & angular velocity 𝜔 i.e. P=T× 𝜔.
49. • Therefore as torque reduces with increase in the
speed, the power output remains constant.