Transcript Basics of Motor Drives

Basics of Motor Drives
Prepared By:Muhammad Atta-ul-Ghaffar
(Sr.Electrical Engineer)
CENTURY PAPER & BOARD MILLS LTD.
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Drives used for the control of electric
motors speed. AC drive used for the
controlling of AC motors speed while DC
drives used for DC motors. AC drive is
also called Variable Frequency Drive.
What is Drive?
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A variable-frequency drive (VFD) is a system for
controlling the rotational speed of an AC motor
by controlling the frequency of the electrical
power supplied to the motor. Variable-frequency
drives are also known as adjustable-frequency
drives (AFD), variable-speed drives (VSD), AC
drives, adjustable-speed drives (ASD), microdrives or inverter drives. They operate by varying
the frequency of the AC voltage supplied to the
motor using solid state electronic devices.
Variable Frequency Drive
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AC Adjustable Speed Drives used as
electrical control devices that change the
operating speed of a motor.
 ASD’s are able to vary the operating
speed of the motor by changing the
electrical frequency input to the motor.
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Principle of Operation
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VFD
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The speed an AC induction motor operates is
given by the following equation:
Synchronous Speed =
Where:
Frequency = Electrical frequency of power
supply in Hz
No. of Poles = Number of electrical poles in
motor stator
Principle…
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Motors speed can be changed by altering
the electrical frequency, the # of poles, or
both.
 Motor speed can be changed by altering
the # of poles in a motor from 4 to 2:
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4 pole motor operating on 60 hertz = 1800 rpm.
2 pole motor operating on 60 hertz = 3600 rpm.
Principle….
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Motor speed can be changed by altering
the frequency of the electrical supply:
4 pole motor operating on 50 hertz = 1500 rpm.
4 pole motor operating on 40 hertz = 1200 rpm
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By varying frequency, we can adjust the
speed over a wide range or vary the
speed precisely using precise changes in
the electrical frequency input to the
motor.
Principle…
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ASD's actually control both frequency and
voltage simultaneously to maintain a
constant volts/hertz ratio which keeps
current flow similar to full speed
conditions
 This allows the motor to draw full current
at any speed and produce full torque as
motor speed changes.
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Power & Torque
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What happens to the Horsepower when
we lower the speed and torque using
frequency?
Horse Power =
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Reduced Horsepower = Reduced Energy Use = Energy Savings!!!!!!!!!!!
Power & Torque
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Advantages:.
 Energy Savings.
 Improved Process Control.
 Reduced Starting Voltage.
 Lower System Maintenance.
 Bypass Capability.
 Multi-motor Control.
ASD Advantages & Disadvantages
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Initial Cost.
Motor Heating at Low Speed.
Maintenance.
Output Harmonics.
Induced Power Lines Harmonics.
Disadvantages.
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A typical adjustable speed drive using a
silicon controller rectifier (SCR) power
conversion' section. The SCR, (also
termed a thyristor) converts the fixed
voltage alternating current (AC) of the
power source to an adjustable voltage,
controlled direct current (DC) output
which is applied to the armature of a DC
motor.
DC Drives…Principle of Operation.
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SCR's provide a controllable power output
by "phase angle control", so called
because the firing angle (a point in time
where the SCR is triggered into
conduction) is synchronized with the
phase rotation of AC power source. If the
device is triggered early in half cycle,
maximum power is delivered to the
motor; late triggering in the half cycle
provides minimum power .
DC Drive
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Nonregenerative DC Drives:Nonregenerative DC drives are the most
conventional type in common usage. In their
most basic form they are able to control
motor speed and torque in one direction only.
Regenerative DC Drives:Regenerative adjustable speed drives, also known as
four-quadrant drives, are capable of controlling not
only the speed and direction of motor rotation, but
also the direction of motor torque.
Types of DC Drives
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Types of DC Drives
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Converter Types : The power conversion or rectified power
section of a DC drive is commonly called
the converter.
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Types of DC Drives
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There are three different types of ASD's
on the market that primarily differ in the
type of rectification they use to convert
AC to DC and back to AC.
VVI—Variable Voltage Input.
CSI---Current Source Input.
PWM---Pulse Width Modulated.
Types of ASD’s
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The VVI is the oldest AC drive technology and
was the first AC drive to gain acceptance in
the industrial market.
The VVI is sometimes called a “six-step
drive” due to the shape of the voltage
waveform it sends to the motor.
VVI drives are fairly economical between 25
and 150 horsepower for ranges of speed
reduction from 15 to 100% (about 10 to 60
Hertz).
These drives are also used widely on
specialty high speed applications (400 to
3000 Hertz).
Variable Voltage Input (VVI)
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Advantages:
 Good Speed Range.
 Multiple motor control from one unit.
 Simple Control Regulator.
Disadvantages:
Power Factor decreases with decreasing speed.
 Low Speed Motor Cogging (shaft pulsing/jerky
motion)
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VVI…
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The CSI is very similar to the VVI except
that it is more sensitive to current as
opposed to a VVI drive which is more
sensitive to voltage.
 CSI drives are usually lower cost above 50
horsepower than VVI drives for pumps
and fan applications.
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Current Source Input (CSI)
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Advantages: High Efficiency
 Inherent Short Circuit Protection
Disadvantage:
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Power Factor decreases with decreasing speed.
Low Speed Motor Cogging (shaftpulsing/jerky motion)
Cannot test drive without motor connected.
Requires Isolation Transformer on Input Side.
Large physical size of Drive due to internal power
components
CSI…
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CSI Circuit and Waveforms
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These drives are the newest technology and
use sophisticated power electronics to
accomplish the same frequency and voltage
control.
They provide good efficiency with very little
motor heating associated with the other
types of drives.
Pulse Width Modulated or PWM drives provide
the best output current to operate the motor
and are becoming very popular for adjustable
speed applications.
Pulse Width Modulated
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High Efficiency.
Wide controllable speed range.
Constant Power Factor regardless of
speed.
Multi motor operation from one drive.
No cogging problems.
Competitive Price.
PWM Advantages.
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Extra Hardware required for line
regenerative capability.
 Complexity of equipment is high
compared to VVI and CSI.
 Some PWM drives produce significant
audible noise.
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PWM Disadvantages
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PWM Circuit & Waveforms
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There are minimal differences between AC
Drives and DC Drives regarding their
controllability, AC Drives do in fact
present more advantages and cost
savings in the long run. Other differences
between AC Drives and DC Drives are set
out below.
AC Drives are used for AC Motor speed control,
whereas DC Drives are used for DC Motor speed control.
Difference Between AC & DC Drive
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AC Drives are used to control AC output, while
DC Drives are used to control DC output.
AC Drives have both a converter and an
inverter, whereas DC Drives have a converter.
In AC Drives, speed control is done by changing
the frequency, while in DC Drives speed control
is done by armature control (below base speed)
and field control (above base speed).
AC Drives are more expensive than DC Drives.
Difference
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Block diagram of DC Drive
Difference
Block diagram of AC Drive
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Parameter:A variable which has a constant value for a
specific application is called Parameter.
Parameter Types:Function Parameter (can be read & write)
Visualization Parameter (can only be read)
BICO parameters (can be read and write)
Parameterization
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Function Parameter.
The response of a function block is
determined by function parameters. Typical
examples of function parameters are:
Normalization of an input signal.
Acceleration or deceleration times in the
ramp-function generator.
Proportional gain (Kp) and integral time (Tn)
in the speed controller.
(combination of parameters is called function block)
Parameterization
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BICO Parameters.
With BICO parameters, you can determine the
sources of the input signals of a function block.
Visualization Parameters.
Visualization parameters are used for visualizing
internal quantities (e.g. applicable output
current). These parameters are only displayed
and cannot be changed by you.
(BICO---BInector COnnector---These connect one function
to another)
Parameterization
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Faults and alarms messages show
on the drives display when any
abnormality in the system occurred.
These can be reset or diagnose by
consulting the product manual provided
by the drive manufacturer.
Faults & Alarms in Drives
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Current:Electricity is the flow of free electrons in
a conductor from one atom to the next
atom in the same general direction. This
flow of electrons is referred to as current.
Electrons move through a conductor at
different rates and electric current has
different values.
Basics of Electricity
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Current
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Electricity can be compared with water
flowing through a pipe. A force is required to
get water to flow through a pipe. This force
comes from either a water pump or gravity.
Voltage is the force that is applied to a
conductor that causes electric current to
flow.
(Voltage is the Cause, Current is the
Effect)
Voltage
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Voltage
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In an electrical circuit, voltage applied to a
conductor will cause electrons to flow. Voltage is
the force and electron flow is the motion. The
rate at which work is done is called power and is
represented by the symbol “P”.
Power is measured in watts and is represented
by the symbol “W”. The watt is defined as the
rate work is done in a circuit when 1 amp flows
with 1 volt applied.
Power
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