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 VFD are variable speed drive,
adjustable speed drive, adjustable frequency drive, AC drive and micro drive.
Frequency is directly related to motor’s
speed. In other words, the faster the frequency the faster will be the speed of
motor. If an application does not require an electric motor to run at full
speed, the VFD can be used to ramp down the frequency and voltage to meet the
requirements of the electric motor’s load. As the application’s moor speed
requirements change, the VFD can simply turn up or down the motor speed to meet
the speed requirements.
A VFD is a power conversion device. The
VFD converts a basic fixed frequency, fixed voltage sine wave power to a
variable frequency, variable voltage output used to control speed of induction
motors.
WORKING
PRINCIPLE:
The first stage of a Variable
Frequency AC Drive or VFD, is the converter. The converter is comprised of six
diodes, which are similar to check valves used in plumbing systems. They allow
current t flow in only one direction: the direction is shown by arrow in figure
shown below.
For example, whenever A-phase
voltage (voltage is similar to pressure in plumbing systems) is more positive
than B or C phase voltage, than that diode will open and allow current to flow.
When B-phase becomes more positive than A-phase, then the B-phase diode will
open and the A-phase diode will close.
The
same is true for the 3 diodes on the negative side of the bus. Thus, we get six
current “pulses” as each diode opens and closes. This is called a “SIX pulse
VFD” which is the standard configuration for current Variable Frequency Drives.
Let us assume that the drive is
operating on a 480V power system. The 480V rating is “RMS” or root-mean squared.
The peaks on a 480V system are 679V. As
you can see, the VFD DC bus has a DC voltage with an AC ripple. The voltage
runs between approximately 580V and 680V.
We can get rid of the AC ripple on
the DC bus by adding a capacitor. A capacitor operates in a similar fashion to
a reservoir or accumulator in a plumbing system. This capacitor absorbs the AC
ripple and delivers a smooth DC voltage. The AC ripple on the DC bus is
typically less than 3 volts. Thus, the voltage on the DC bus becomes
approximately 650V DC. The actual voltage will depend on the voltage level of
the AC line feeding the drive, the level of voltage unbalance the power system,
and any reactors or harmonic filters on the drive.
The diode bridge converter that converts AC to DC is sometimes
just referred to as a converter. The converter that converts the DC back to AC
is also a converter, but to distinguish it from the diode converter, it is
usually referred to as an “inverter”. It has become common in the industry to
refer to any DC to AC converter as an inverter.
Note
that in real VFD the switches shown would actually be transistors. When we
close one of the top switches in the inverter, that phase of the motor is
connected to the positive DC bus and the voltage on that phase becomes
positive. When we close one of the bottom switches in the converter, that phase
is connected to the negative DC bus and becomes negative. Thus, we can make any phase on the motor
become positive or negative at will and can thus generate any frequency that we
want. So, we can make any phase be positive, negative or zero.
The sine wave is shown for
comparison purposes only. The drive does not generate this sine wave.
Notice that the output from VFD is a
rectangular waveform. VFD’s do not produce a sinusoidal output. This
rectangular waveform would not be a good choice for a general purpose
distribution system, but is perfectly adequate for a motor.
PROJECT
WORK:
We have selected this project as our
semester project due to its bundle of advantages and versatilities in implementation.
At start it proves to be a challenging
task for us but at the end of the day we have completed it successfully. We
have decided to make a VFD interfaced with a microcontroller
The circuit is divided in two parts i.e.
microcontroller circuit and electronic circuit. Both the circuits are shown
below clearly:
MICROCONTROLLER CIRCUIT:
ELECTRONIC CIRCUIT:
WORKING PRINCIPLE:
The microcontroller is interfaced with a
KEYPAD and takes frequency as an input that at how much frequency the motor is
needed to run. After input from keypad the microcontroller will provide gate
signals to the MOSFETS connected further. These MOSFETS are used to operate an
electronic circuit which is used to run the motor on desired frequency.
SR
No:
|
Keypad
Button
|
Delay(ms)
|
Frequency(Hz)
|
01.
|
0
|
0
|
0
|
02.
|
1
|
10
|
100
|
03.
|
2
|
20
|
50
|
04.
|
3
|
30
|
33.33
|
05.
|
4
|
40
|
25
|
06.
|
5
|
50
|
20
|
07.
|
6
|
60
|
16.667
|
08.
|
7
|
70
|
14.285
|
09.
|
8
|
80
|
12.5
|
10.
|
9
|
90
|
11.11
|
11.
|
*
|
16.667
|
60
|
12.
|
#
|
5.555
|
180
|
COMPONENTS DETAILS:
We have used a number of components
in our project which are very clear from the circuit diagrams provided above.
Some of them are AT89C51 microcontroller
a 3×4 keypad IR2110 IC’s, MOSFETS a
number of diodes, resistors and capacitors are also used.
ADVANTAGES
OF Variable Frequency Drive [VFD[:
Suppose
that if we have an application that does not need to be run at full speed, then
you can cut down energy costs by controlling the motor with a variable
frequency drive which is one of benefits of variable frequency drive.
VFD’s allow you to match the speed
of the motor-driven equipment to the load requirement. There is no other method
of AC electric motor control that allows you to accomplish this.
Electric motor systems are
responsible for more than 65% of power consumption in industry today,
optimizing motor control systems by installing or upgrading to VFD’s can reduce
energy consumption in your facility by as much as 70%. Additionally, the
utilization of VFD’s improves product quality, and reduces production costs.
Combining energy efficiency tax incentives, and utility rebates, returns on
investment for VFD installations can be as little as 6 months.
By operating motors at the most efficient
speed for any application, fewer mistakes will occur and thus, production
levels will increase, which earns the company higher revenues. On conveyers and
belts jerks on startup could be eliminated by allowing high through put.
Equipment will last longer and will have less
downtime due to maintenance when its controlled by VFD’s ensuring optimal motor
application speed. Because of the VFD’s optimal control of the motor’s
frequency and voltage, the VFD will offer better production for your motor from
issue such as electro thermal overloads, phase protection, over voltage, under
voltage etc. When a load is started with a VFD one will not subject the motor
or driven load to the instant shock of across the line starting but can start
smoothly, thereby eliminating belt, gear and bearing wear. It also is an
excellent way to reduce or eliminate water hammer since we can have smooth
acceleration and deceleration cycles.
VFD’s are best solution for saving energy and
better process control in addition to that they provide us following benefits:
· A VFD may be used for control of process temperature, pressure or flow
without use of a separate controller. Suitable sensors and electronics are used
to interface driven equipment with VFD.
· Maintenance costs can be lowered, since lower operating speeds results
in longer life for bearing and motors.
· Eliminating throttling valves and dampers also does away with
maintaining these devices and all associated controls.
· A soft starter for motor is no longer required.
· Controlled ramp-up speed in a liquid system can eliminate water hammer
problems.
· Ability of a VFD to limit torque to user selected level can protect
driven equipment that cannot tolerate excessive torque.
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