basics of motor drives

Basics of Motor DrivesBasics of Motor DrivesPrepared By:-

Muhammad Atta-ul-Ghaffar(Sr.Electrical Engineer)

CENTURY PAPER & BOARD MILLS LTD.

What is Drive?What is Drive?

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.

Muhammad Atta ul Ghaffar Sr.Elect.Engr. 2

Variable Frequency DriveVariable Frequency Drive

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), micro-drives or inverter drives. They operate by varying the frequency of the AC voltage supplied to the motor using solid state electronic devices.


Principle of OperationPrinciple of Operation

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.


VFDVFDMuhammad Atta ul Ghaffar Sr.Elect.Engr. 5

Principle…Principle…

The speed an AC induction motor operates is given by the following equation:

Synchronous Speed =

Where: Frequency = Electrical frequency of power

supply in HzNo. of Poles = Number of electrical poles in

motor stator


Principle….Principle….

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:

4 pole motor operating on 60 hertz = 1800 rpm. 2 pole motor operating on 60 hertz = 3600 rpm.


Principle…Principle…

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 rpmBy 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.


Power & TorquePower & Torque

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.


Power & TorquePower & Torque

What happens to the Horsepower when we lower the speed and torque using frequency?

Horse Power =


Reduced Horsepower = Reduced Energy Use = Energy Savings!!!!!!!!!!!

ASD Advantages & DisadvantagesASD Advantages & Disadvantages

Advantages:.Energy Savings.Improved Process Control.Reduced Starting Voltage.Lower System Maintenance.Bypass Capability.Multi-motor Control.


Disadvantages.Disadvantages.

Initial Cost.Motor Heating at Low Speed.Maintenance.Output Harmonics.Induced Power Lines Harmonics.


DC Drives…Principle of Operation.DC Drives…Principle of Operation.

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 DriveDC Drive

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 .


Types of DC DrivesTypes of DC Drives

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 DrivesTypes of DC DrivesMuhammad Atta ul Ghaffar Sr.Elect.Engr. 16

Types of DC DrivesTypes of DC Drives

Converter Types :-The power conversion or rectified power

section of a DC drive is commonly called the converter.


Types of ASD’sTypes of ASD’s

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.


Variable Voltage Input (VVI)Variable Voltage Input (VVI)

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).


VVI…VVI…

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)


Current Source Input (CSI)Current Source Input (CSI)

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.


CSI…CSI…

Advantages:-High EfficiencyInherent Short Circuit ProtectionDisadvantage:- 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 Circuit and WaveformsCSI Circuit and WaveformsMuhammad Atta ul Ghaffar Sr.Elect.Engr. 24

Pulse Width ModulatedPulse Width Modulated

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.


PWM Advantages.PWM Advantages.

High Efficiency.Wide controllable speed range.Constant Power Factor regardless of

speed.Multi motor operation from one drive.No cogging problems.Competitive Price.


PWM DisadvantagesPWM Disadvantages

Extra Hardware required for line regenerative capability.

Complexity of equipment is high compared to VVI and CSI.

Some PWM drives produce significant audible noise.


PWM Circuit & WaveformsPWM Circuit & WaveformsMuhammad Atta ul Ghaffar Sr.Elect.Engr. 28

Difference Between AC & DC DriveDifference Between AC & DC Drive

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.


DifferenceDifference

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.


DifferenceDifferenceMuhammad Atta ul Ghaffar Sr.Elect.Engr. 31

Block diagram of DC Drive Block diagram of AC Drive

ParameterizationParameterization

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)



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)



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)


Faults & Alarms in DrivesFaults & Alarms in Drives

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.


Basics of ElectricityBasics of Electricity

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.


CurrentCurrentMuhammad Atta ul Ghaffar Sr.Elect.Engr. 37

VoltageVoltage

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)


VoltageVoltageMuhammad Atta ul Ghaffar Sr.Elect.Engr. 39

PowerPower

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.


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