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ac drive new

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rajesh thote

on 11 June 2013

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Transcript of ac drive new

AC Induction Motor Theory
Variable Frequency Drive

Pulse Width Modulation (PWM)
Variable Frequency Drives
Energy savings on most pump and fan applications.
Better process control and regulation.
Speeding up or slowing down a machine or process.
Inherent power-factor correction
Protection from overload currents
Safe Acceleration
The Purpose of VFD
Run a machine or process at a desired speed.

Produce adequate torque to handle the load.
Use power efficiently to produce the necessary torque at a given speed.

Effectively monitor the application or process.
VFD is used to:
AC Induction Motor – Squirrel Cage Design
AC Induction Motor Theory
Three-phase induction motors are the most common and frequently encountered machines in industry
simple design, rugged, low-price, easy maintenance
wide range of power ratings: fractional horsepower to 10 MW
run essentially as constant speed from no-load to full load
Its speed depends on the frequency of the power source
not easy to have variable speed control
requires a variable-frequency power-electronic drive for optimal speed control
Stator of IM
An induction motor has two main parts
A stationary stator
consisting of a steel frame that supports a hollow, cylindrical core
constructed from stacked lamination, having a number of evenly spaced slots, providing the space for the stator winding
A revolving rotor
composed of punched lamination, stacked to create a series of rotor slots, providing space for the rotor winding
one of two types of rotor windings
conventional 3-phase windings made of insulated wire (wound-rotor) » similar to the winding on the stator
aluminum bus bars shorted together at the ends by two aluminum rings, forming a squirrel-cage shaped circuit (squirrel-cage)
Two basic design types depending on the rotor design

Conducting bars laid into slots and shorted at both ends by shorting rings.

Complete set of three-phase windings exactly as the stator. Usually Y-connected, the ends of the three rotor wires are connected to 3 slip rings on the rotor shaft. In this way, the rotor circuit is accessible.
Slip rings
Wound rotor
Squirrel cage rotor
Balanced three phase windings, i.e. mechanically displaced 120 degrees form each other, fed by balanced three phase source
A rotating magnetic field with constant magnitude is produced, rotating with a speed

Synchronous Speed = 120*f/P

where ‘f’ is the supply frequency and
P is the no. of poles.
Rotating Magnetic Field
Synchronous speed
This rotating magnetic field cuts the rotor windings and produces an induced voltage in the rotor windings

Due to the fact that the rotor windings are short circuited, for both squirrel cage and wound-rotor, an induced current flows in the rotor windings

The rotor current produces another magnetic field

A torque is produced as a result of the interaction of those two magnetic fields
Principle of operation
At what speed will the IM run?
Can the IM run at the synchronous speed, why?
If rotor runs at the synchronous speed, which is the same speed of the rotating magnetic field, then the rotor will appear stationary to the rotating magnetic field and the rotating magnetic field will not cut the rotor. So, no induced current will flow in the rotor and no rotor magnetic flux will be produced so no torque is generated and the rotor speed will fall below the synchronous speed
When the speed falls, the rotating magnetic field will cut the rotor windings and a torque is produced
Induction Motor Speed
So, the IM will always run at a speed lower than the synchronous speed
The difference between the motor speed and the synchronous speed is called the Slip speed

where nslip= slip speed
nsync= speed of the magnetic field
nm = mechanical shaft speed of the motor
Induction Motor Speed
where s is the slip

Notice that : If the rotor runs at synchronous speed
s = 0
If the rotor is stationary
s = 1
Slip may be expressed as a percentage by multiplying the above eq. by 100.
The Slip
Speed Control of AC Motors

Change the number of poles in the motor; this means separate windings;

Change the slip characteristics of the motor; this is done with varying resistors, as done in a wound-rotor motor;

Varying the stator voltage; or

Change the frequency of the power supplied to the motor. This is the method of choice .
Typical torque-speed characteristics of induction motor
Torque-speed characteristics of IM
The induced torque is zero at synchronous speed.

The curve is nearly linear between no-load and full load.

There is a maximum possible torque that can’t be exceeded. This torque is called pullout torque and is 2 to 3 times the rated full-load torque.

The starting torque of the motor is slightly higher than its full-load torque, so the motor will start carrying any load it can supply at full load.
Torque-speed characteristics of IM
Torque-speed characteristics of IM
To maintain the constant torque, V/f method is used.

T α I*Ф
where I = stator current
Ф = flux
V α d Ф /dt
Ф α ∫Vdt
Ф α V*T
Ф α V/f

By making flux constant by maintaining the V/f ratio constant provides control over
the torque.
The main objective of the VFD is to vary the speed of the motor by varying the supply frequency while providing the closest approximation to a sine wave for current.
Energy savings.
Increase life span of motors.
Provides protection to the motor against various faults.
Variable Frequency Drive
A VFD in a block diagram.
VFD Basics
Rectifier Circuit

All VFD’s need a power section that converts AC power into DC power.

This is called the converter bridge or a rectifier.

The converter is commonly a three-phase, full-wave-diode bridge.
VFD Basics
Rectifier is that special type of converter that converts AC to DC.
VFD Basics
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.

Ripples are removed using an LC filter at the DC bus.
VFD Basics
Simplified Inverter Section of a VFD
VFD Basics

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.

By switching the inverter-transistor devices on and off many times per half cycle, a pseudo sinusoidal current waveform is approximated.
VFD Basics
Transistors provide fast switching capability for a relatively low cost.
The general types of transistors are:
The bipolar transistor;
The gate turn off transistor (GTO)
The field-effect transistor (FET);
The insulated gate-field-effect transistor (IGFET);
The insulated gate-bipolar transistor (IGBT).

Mostly IGBT’s are used.
VFD Basics

The input voltage is rectified through the diodes, and a DC bus voltage should be present.

The DC + and DC – terminals will typically read approximately 325 volts DC on a 230 volts AC supplied drive and 650 volts DC on a 460 volts AC supplied drive.

This waveform, when viewed, is straight DC, possibly with some rippling effect from the AC input.
Input Waveforms
Pulse-width-modulated voltage and current waveforms
Pulse Width Modulation

The DC waveform looks more like an AC waveform but the voltage waveform is much different.

The power semi-conductors in the inverter section act as switches of the DC bus, and therefore, are pulsing the motor with some voltage.
Pulse Width Modulation

A typical square wave takes its shape on the square-wave look due to this switching function rather than a rotational, changing state of amplitude.

Thus, frequency is varied by varying the width of the output voltage.

This frequency and amplitude pattern is sometimes called the carrier frequency of a PWM drive.
Pulse Width Modulation
Components of VFD
Gate Drive Board (GDB)

A PCB containing the circuitry necessary for operating (gating) the output transistors of the VFD.

Can also monitor main circuit temperature, current and voltage.
Components of VFD
Control Card

A Control Card is a PCB that is the main interface component.

Used to connect external equipments and operator interface components to and from the VFD.
Components of VFD
Pre charge Resistor

Used to limit the inrush current while the capacitors begin to charge during starting.

Once the capacitor is charged to the target voltage, a contactor bypasses the pre charge resistor.
Components of VFD
Line Reactor

Consists of a conductor coiled around a magnetic core.
Change to current amplitude or direction is opposed
by the existing magnetic field.
Reduces the discontinuity of the current drawn by a VFD’s converter section, thus reducing harmonics.
Components of VFD
Line Choke & Filter Capacitor

A single reactor electrically placed ahead of the dc bus capacitor in a VFD.

Reduces harmonics created by the VFD.
Components of VFD
VFD Complete Circuit
Control methods
V/F method

Simple control method for AC induction motor
Ratio of voltage to frequency is the flux level in
the machine.
Dictates the amount of torque that the machine produces at a
given operating point.
The ratio is established in accordance with the base voltage
& motor base frequency ratings.
This ratio yields a linear pattern that the VFD follows
to produce rated motor torque.
Control methods
Open Loop Vector

Open Loop is actually a closed loop system, but the feedback loop comes from within the VFD itself instead of an external encoder.
Also called as “Sensorless Vector” drives.
The µP creates a mathematical "model" of the motor operating parameters and keeps it in memory.
The µP monitors the output current, compares it to the model and executes the necessary error corrections.
Only drawback is that as the motor gets slower, the ability of the µP to detect the changes in magnetics becomes more difficult.
Control methods
Closed Loop Vector

Uses a shaft encoder on the motor to give positive shaft position indication back to the microprocessor (µP).
For torque control, the feedback from PG encoder allows the µP to adjust the pattern so that a constant level of torque can be maintained regardless of speed.
A true closed Loop Vector Drive can also make an AC motor develop continuous full torque at zero speed that makes them suitable for crane and hoist applications.
Control methods
Functions & Features
Functions & Features
Functions & Features
Significant energy savings

Easy setup & programming


Better design
Jump, Skip or Critical Frequencies

Fault Logs and On-Board Diagnostics

Power Loss Ride Through

Slip Compensation

Speed search or Pick-Up a Spinning Load

Set-up Parameters

The Control Method

Acceleration or Accel-Ramp Rate

Automatic Restart

Stopping Method
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