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DRIVES
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AC Drive Basics
AC Drives - Basics & Beyond…
Copyright © 2005 Rockwell Automation, Inc. All rights reserved
28th June’2005 1
AC Drive Basics
How Does A Motor Work?
1. Current flowing through a conductor
2. The direction of the current
flow
3. Pass a conductor through a magnetic field
Basic Rules:
Magnetic flux around the conductor
Polarity of the magnet
A voltage or current flow
AC Drive Basics
Stator Magnetic Field
Rotor Field Created by InducedCurrent Flow in Rotor Conductors
N S
Elastic Nature of Magnetic Lines of Force Rotor
Torque Generated in a motor
N S
F
AC Drive Basics
Workhorse of modern Industry Found in virtually every phase of Manufacturing One of the Most Common Forms of rotating power in the World• Consists of two sets of electromagnets – One set ,the stator, housed in motor Frame and other rotor, free to rotate and supported by bearings and motor end bells.
Squirrel Cage Induction Motor
Rotor
Fan BladesRotor BarEnd Rings
StatorPerforated Lamination disks stacked together and through careful die casting process Aluminum or Cu – Brass Alloy is filled in the channels and then End rings are formed.
AC Drive Basics
Issue with IM Motor
To change Speed below and above base speed Smoothly
AC Drive Basics
1. Starting Current - Very High 2. Power Factor - Poor at light load3. Efficiency - Not so good 4. Speed control - A big Problem5. Protection - additional Equipments Required
Operational Issues
AC Drive Basics
Mechanical
Constant SpeedMotor
ACMotor
VariableSpeedShaft
Vary Pitch Diameter
Eddy current
Constant Speed Motor
ACMotor
ExciterCoupling Field
Solid state DC
DC Motor
DC OutputAC Input
Rotating DCField Control
DC Motor
AC Motor
DC Gen
Solid state AC
AC Motor
Fixed DC Bus
InverterDiode AC OutputVariableFrequencyRectifier
LC
Hydraulic
Pump
Valve
Hydraulic Motor
ACMotor
Constant Speed Motor
Typesof
AdjustableSpeed
AC Drive BasicsConstant
Speed MotorAC
Motor
Vary Pitch Diameter
Mechanical Variable Speed
Pump
Valve
HydraulicACMotor
Constant Speed Motor
Hydraulic
Variable SpeedShaft
AC Drive Basics
Variable Speed Shaft
Constant Speed Motor
ACMotor
ExciterCoupling Field
Eddy Current Coupling
Benefits Initial cost Allows torque control Simple control
Limitations Heat generation Size Power consumption
AC Drive Basics
These Methods have numerous disadvantages Like –
- No ENERGY SAVING,- No smooth speed variation,
- No power factor improvement….. etc ……..
AC Drive Basics
Frequency Drives
AC Drive BasicsDrives Classification - Voltage basis
DC Drives : Not in much use due to Maintenance related problems associated with DC Motors.
AC drives : Market is zooming up to big targets and getting new fields of application
AC Drive BasicsDC Drives :Operation and Algorithm
Field
Cur. Reg.Speed
Reg.
SCR
Control
Field Bridge
High Bandwidth Current regulator
Armature
Cur. Reg.
Arm Bridge
M
PG
Ref
Voltage Fdbk
Current Fdbk
Current Fdbk
Speed Fdbk
AC Drive Basics AC Technology
Current Source Inverter
Variable Voltage Inverter
PWM Inverters� V/Hz Inverters� High Performance Vector Drive – Preferred over CSI & VVI
AC Drive BasicsCURRENT SOURCE INVERTER
Variable DC Bus
ControlledRectifier
AC Input
L Inverter
Benefits Regeneration Synchronous transfer
Limitations Limited speed range Poor input power factor is poor
varies with speed Poor multi-motor capability Limited ride through capability May require motor matching Increased Line Harmonics Poor Efficiency Sensitivity to line Transients
Uses Current Regulator
AC Drive BasicsCSI WAVEFORMS
0
0
Voltage(Line to Neutral)
Current(Line)
Unclipped voltage spikes can cause motor insulation breakdown Current wave shape creates cogging at low speeds
AC Drive Basics
Benefits Good speed range Multi-motor Simple regulator
Limitations Poor input power factor varies with speed Limited ride through Increased input harmonics Low speed cogging Poor efficiency (typically 95%) Sensitivity to line transients
VARIABLE VOLTAGE INPUT (VVI)
Variable DC Bus
ControlledRectifier
AC InputL InverterC
+
Uses Voltage Regulator
AC Drive BasicsVVI WAVEFORMS
0
0
Voltage(Line to Neutral)
Current(Line)
output voltage is frequently called a “six step” waveform cusps in the current waveform cause cogging at lower speeds
AC Drive BasicsPWM INVERTER
Fixed DC Bus
DiodeRectifier
AC InputL InverterC
+
Benefits Constant input P.F. Wide speed range High efficiency (97 - 98%) Ride through options Open circuit protection Common bus regeneration Smooth low speed operation Vector control performance (optional)
Limitations Extra hardware for regeneration to line Motor noise (solved with IGBT’s) Possible Cable length limitations (with IGBT’s)
AC Drive BasicsPower Circuit of PWM Drive
AC Drive BasicsRectifier Section - Conversion From AC to DC Positive Diodes
Negative Diodes
1
4
3
6
5
2
5&6
6&1
1&2
2&3
3&4
4&5
5&6
| One 360° Cycle of Power |
| |
| |
| |
| |
60°
| |
| |
| |
| |
| |
3 phase AC input to the converter
DC output
AC Drive Basics
To Filter Section
Precharging Arrangement - in low Rated Drives
AC Drive Basics
Rectified DC output
OV
Filtered DC output
Filter Section - Smoothing of Rippled DC
Vdc
Vdc = 1.35VL- L
Idc =(1.1 X 746 X Drive HP)/ Vdc
CRectified Input
FilteredOutput
DC Bus (+)
DC Bus(-)
OV
AC Drive BasicsInverter Section – Conversion From DC to AC
Positive DC Bus Line
Negative DC Bus Line
Filtered DCInput
PWM Output
IGBT fired in a sequence to produce a PWM output
AC Drive BasicsPWM WAVEFORMS
0
0
Voltage(Line to Neutral)
Current(Line)
AC Drive Basics
Triangle Generator
Modulation Generator
Generation of PWM WAVEFORMS
AC Drive BasicsSine Weighted PWM Pulses
AC Drive BasicsSine Weighted PWM
AC Drive Basics
RPM = 120 x Applied FrequencyNumber of Poles per Phase
e.g for a 4 POLE & 50 Hz FREQUENCY 120 X 50 RPM = ----------- = 1500 4
AC Motor
SPEED CONTROL
AC Drive Basics
Speed Speed
0 750 1440Base
1500Sync
60 RPMSlip
N
Rotor Speed = Synchronous Speed - Slip Speed
Slip
AC Drive Basics
T = K x x ILine
V
F
2
In an AC motor, torque varies by:
Where : V/F proportional to Motor Flux I is current drawn by the motor
PLAY WITH V/F RATIO TO GET HIGHER STARTING TORQUE
TORQUE Control
AC Drive BasicsWhy Voltage varies as frequency is changed
1). V/F proportional ФRated (Motor Rated Flux) If Ф > ФRated (What Happens?) If Ф < ФRated (What Happens?)
2). Impedance/Reactance Issue XL is dominant above 3-5
hertz
LZ XR Ss
22
Since I =V/Z; If we increase or decrease the applied frequency we must also increase or decrease the applied voltage accordingly.
fL 2XL
3). To keep the torque Constant as T α V/F Ratio
AC Drive Basics
0
% Currentand
% Torque
Speed (RPM)
Current
200
400
600
500 9001725
1800RPM
Torque
1750
100
Rated Torqueat rated FLA.
Synchronousspeed
Break down torque(peak Torque)
Basespeed
Motor Operating Characteristics
Starting torque(Break over Torque)
Starting Current
No LoadPull UP torque(DIP Torque)
AC Drive Basics
400
300
200
100
00 20 40 60 80 100
Perc
ent T
orqu
e
Percent Synchronous Speed
Breakdown Torque
Operating Range of Variable Frequency Drives
Torque & Speed Curve without Drive
AC Drive Basics
% Currentand
% Torque Current Rated Torqueat rated FLA.
Break down torque(peak Torque)
0
Speed (RPM)
200
500 900
17251800RPM
Torque
100
75RPM Slip75RPM Slip
Torque / Speed with drive applied
AC Drive Basics0 to Twice Base Speed Operation
Torque
Speed (Multiple of Base) Base
Peak
Rated
1.25 1.5 1.75 2
1 N2
.64
.44.33 .25
CT range CHP
Tpeak =
AC Drive Basics
Motor or Torque Control Technologies
AC Drive BasicsMotor Control Technologies
Conveyor MixerExtruder
Volts / Hertz SensorlessVector
FieldOriented
Speed Regulator Speed Regulator Torque RegulatorSpeed Regulator
Current LimitAdvanced
Current Limit
High BandwidthCurrent Regulator
Converting
FluxVector
Speed Regulator Torque Regulator
Current Regulatorbased on
Estimated Values
Converting
Parameter Selectable – (Application Based)
AC Drive Basics
MOTOR
INVERTER
Voltage Feedback
CURRENTLIMIT Frequency Ref
Electrical Frequency
+ VOLTAGECONTROL
V RefV/Hz Gate SignalsElec. Freq
Slip Frequency
+
SLIPESTIMATOR
V/Hz Control
Current Feedback
Speed RefCURRENT
LIMIT
Current Limit monitorsmotor current and altersfrequency command
Slip Compensation alters frequencyreference during load changes
takes a speed reference and varies Voltage and FrequencyVolts/Hertz Control
The drive monitors total current - no current resolver, Cannot distinguish between Id from Iq (Torque Capability limited)
AC Drive BasicsVolts/Hertz Control
Notice that the ability of the drive to maintain high torque output at low speeds drops off significantly below 3 Hz
AC Drive BasicsVolts/Hertz Control
Can not keep the shaft speed at desired level when subjected to shock loads.
However Drives with slip compensation feature correct for extra load but the dynamics are somewhat limited.
Shock Load
AC Drive BasicsVector Definition ?
To know the direction as well as magnitude of a certain quantity
Vector Drive DefinitionThe ability to independently identify and control the flux and
torque producing components of current in a motor for the purpose of accurate torque and power control.
ITotal = √ (IReal) 2 + (IReactive) 2
AC Drive Basics
Ia = Torque Producing Current (Armature Current)If = Flux Producing Current (Field Current)
DC
IaIa If
Z Z
DC
Ia
If
Load 1
Load 2
a*sin (d)
DC Motor Model
T = K X Ф X Ia X Sin (d)d = 900
AC Drive Basics
Iq = Torque Producing CurrentId = Flux Producing CurrentIs = Total Stator Current
ACIs
IdIq
Z
Z Z
AC
Id
Iq
Load 1
Load 2
Is
Is
K Id * Iq * sin(d)
T = K X Ф X Ia X Sin (d)d = Varies
AC Motor Model
AC Drive Basics
Conveyor MixerExtruder
Volts / Hertz SensorlessVector
FieldOriented
Speed Regulator Speed Regulator Torque RegulatorSpeed Regulator
Current LimitAdvanced
Current Limit
High BandwidthCurrent Regulator
Converting
FluxVector
Speed Regulator Torque Regulator
Current Regulatorbased on
Estimated Values
Converting
AC Drive Basics
Voltage Feedback
MOTOR
INVERTER
VOLTAGECONTROL
V RefV/Hz Gate SignalsElec. Freq
Slip Frequency
+
SLIPESTIMATOR
Speed RefCURRENT
LIMIT+ Frequency Ref
V/Hz Control
CURRENTRESOLVER Current FeedbackTorque I Est.
Current Feedback - Total
FLUXVECTOR
CONTROLV Vector
Torque I E
st.
Torque I Est.
Sensorless Vector Control
A current resolver that separates flux producing current from torque producing currentProduces Tmax /Amps of motor Current keeping Flux Current Constant
AC Drive BasicsSensorless Vector Control
High Starting (Maximum) Torque about 250%
AC Drive BasicsSensorless Vector Control
Shock Load
Better Dynamic Response to shock loads
AC Drive Basics
Conveyor MixerExtruder
Volts / Hertz SensorlessVector
FieldOriented
Speed Regulator Speed Regulator
Torque Regulator
Speed Regulator
Current LimitAdvanced
Current Limit
High BandwidthCurrent Regulator
Converting
FluxVector
Speed Regulator Torque Regulator
Current Regulatorbased on
Estimated Values
Converting
AC Drive Basics
MOTOR
INVERTER
Voltage Feedback
PG
CURRENTREG.
SPEEDREG.
Torque Current ReferenceSpeed Ref
Speed Feedback
Electrical Frequency
VOLTAGECONTROL
VOLTAGEVECTOR
Gate Signals
Auto TuneParameters
V/Hz Control
CURRENTRESOLVER
Flux I EstimateCurrent Feedback
Torque I Estimate
V Mag
V Angle
Flux Vector Control w/ Feedback
Uses Speed & Current Regulator ,Has a Current Resolver that separates Torque and Flux Currents Auto - tuning is a must
AC Drive BasicsFlux Vector Control w/ Feedback
2
1
Torq
ue
Speed (Hz)1 2 5 10 20 30 40 50 60
Flux Vector Control – Torque Vs Speed
Much better response at low speeds – 2Hz Torque is greater than 150%
AC Drive BasicsFlux Vector Control w/ Feedback
Shock Loads
Shock load dynamic Response much faster
Encoder FOCLoad Removed
Sensorless FOC Load Removed
FOC,FV & SVLoad applied
V/Hz Load
Applied
0.9
0.875
0.85
0.8250 1 2 3 4 5
Time (seconds)
Per U
nit Q
uant
ities
AC Drive Basics
AC Drive BasicsFORCE-FIELD ORIENTED CONTROL
Excellent Technology to produce as high as 400% Maximum torqueProduces 100% torque at zero speed – Key for Hoist Application
AC Drive BasicsFORCE-FIELD ORIENTED CONTROL
FORCE-FIELD ORIENTED CONTROL – Torque Vs Speed
Excellent response at low speeds – At 1Hz Torque is greater than 250%
AC Drive BasicsFORCE - FIELD ORIENTED CONTROL
Shock load dynamic Response is Excellent
Encoder FOCLoad Removed
Sensorless FOC Load Removed
FOC,FV & SVLoad applied
V/Hz Load
Applied
0.9
0.875
0.85
0.8250 1 2 3 4 5
Time (seconds)
Per U
nit Q
uant
ities
AC Drive BasicsVector Vs Field Oriented Control
Vector Control� Acknowledges that motor current is the vector sum of the torque and
flux currents and uses this information to provide better control of motor speed/torque.
Field Oriented Control� The ability to independently control the flux and torque in a motor for the
purpose of accurate torque and power control.
AC Drive Basics
Benefits from Drives What Benefits ?
AC Drive Basics
Saves Energy costsImproves Process operation by ‘Smooth’ speed control
Improves INPUT Power Factor
Increase Efficiency and Life of mechanical equipment (due to
‘soft starting’)
Lower chances of System disruptions (by lowering current
inrush from 600% to 100-150%)
Benefits of VFD
AC Drive Basics
275
15075
0
0120
240
360
480
600
%Torque
%Current
150%-260% 180%
115%
150%
600%
480%
600%
ACDrive
Full Voltage Starter
Reduced Voltage Starter
Solid State* (SMC)
ABC
C
B
A
* maximum shown,adjustable via digital switches
180%
Flexibility in Starting Current
Lower Investment in DG setsNo Penalty From Electricity Board
Benefits of VFD
AC Drive BasicsBenefits of VFD
ACCELERATION RANGE
0 15 3060 300Seconds
Full Voltage
AC Drive (Adjustable)
(Load Dependent)
(Not Adjustable)
Reduced Voltage
Solid State
600
Smooth Start and perfect Control
Acceleration Time =
Where:WK2 = Moment of inertia (lb-ft 2)
h N = Change in motor RPM T = Torque required308 = Constant
WK2 x hN 308 x T
AC Drive Basics
STOPPING CHOICES
CoastTime
Speed
SoftRamp Stop
Dynamic Braking
DC Injection
Coast Soft Ramp D.B. DC Inj.AC Drive X X X X XSolid StateX XR.V. XF.V. X
Benefits of VFD
AC Drive Basics
Complete Protection for motorAgainst Over voltageOverloadMotor StallingI2t Protection to Motor
& so on ……….Just Name….
Protections ??
Benefits of VFD
AC Drive Basics
Drives Save Energy ?
AC Drive Basics
Phase A
Phase B
Phase C
PWM VFDPWM VFDAC Input AC Input
PowerPowerAC Output AC Output
PowerPower
• IInput Current is less than Output since Source Voltage is Constantnput Current is less than Output since Source Voltage is Constant
• LLower demand on distribution systemower demand on distribution system
• PPower Consumption is proportional to motor speedower Consumption is proportional to motor speed
Real & Reactive Real & Reactive motor currentmotor current
Real Current OnlyReal Current Only
Energy Saving Concepts
AC Drive Basics
Auto-Economizer - Extra Saving
“Idle Mode” Energy Saver
Reduced Load
Automatically fold back Voltage to reduce motor flux
Energy Saving Concepts
A key in Press Applications - 3-5% Saving
AC Drive Basics
Energy Savings
High Efficiency (>97% )Improves Power Factor to 0.98Payback within 12 to 24 months for reduced energy consumption
An investment which pays back immediately !
Energy Saving Concepts
AC Drive BasicsTypes of Loads & Energy Savings
Constant Torque (CT)
Variable Torque (VT)
Constant Power (CHP)
AC Drive Basics
HP PRESSURE * FLOW
PRESSURE
SPEEDFLOW
HP SPEED3
SPEED2
Variable Torque
AFFINITY LAW
Power α (Speed)3
Torque α (Speed)2
AC Drive BasicsVariable Torque
P2 = P1 X (N2 / N1)3
P2 = 100HP X (1200 / 1440)3
P2 = 58HPPower Saved = P1 – P2 = 100-58 = 42HP
20 % REDUCTION IN SPEED REDUCES 45 % ENERGY CONSUMPTION
Multiply this quantity (42HP) by Time of operation and Cost/HP and get the energy saving on a variable torque applications Instantly.
AC Drive Basics
To reduce the flow from 100% to 60% , input power requirements are reduced from
100 % to 62 %
Variable Inlet Vane ID Fan application
Saves 38%
Variable Torque
AC Drive Basics
To reduce the flow from 100% to 60% , input power requirements are reduced only from
100 % to 86 %
Outlet Damper
Saves 14%
Variable Torque
ID Fan application
AC Drive Basics
Reduction of flow from 100% to 60% , results into Input power requirements reduction from
100% to 22%
The fan curve is changed by changing the motor speed
Variable Speed DriveEnergySaved
Saves 78% of Energy
ID Fan application
Variable Torque
AC Drive Basics
20 % SPEED REDUCTION SAVES 20 % ENERGY
Power α (Speed)Torque is Constant
P = 2 X Π X N X T
Constant Torque
AC Drive Basics
20 % SPEED REDUCTION SAVES 20 % ENERGY
Constant Torque
P2 = P1 X (N2 / N1)P2 = 100HP X (1200 / 1440)P2 = 84HPPower Saved = P1 – P2 = 100-84 = 16HP
Multiply this quantity (16HP) by Time of operation and Cost/HPand get the energy saving on a Constant torque applications Instantly.
AC Drive Basics
Compressors Application Operates at Lower Avg. Pressure
Proportional Savings No Unloading / Loading Cycles Less Leakages
Constant Lower Pressure Process Feedback
Better Motor EfficiencyHigher Power Factor
Controlled Accel / Decel TimeLower Starting Torque Lower Starting Current
Constant Torque
AC Drive Basics
Time
Reqd. Pr.
Pressure
Ksc
{
Savings Due to Pressure Reduction
6.1 6.0
With VFD6.5
Constant TorqueCompressors Application
AC Drive BasicsConstant Power
Power Remains Constant while Torque falls down as speed is exceeded base valueNo Energy SavingAttention needed for operation as greater speed can damage motor bearings and drive may not be able to generate required torque at high speed - may lead to motor cogging condition
AC Drive BasicsBest Features in our Drives
Ambient Temperature of 50 Degree C. Flexible performance for V/F, SVC or FVC Technology
with embedded ForceTM Feature 400% maximum torque can be achieved in closed loop As per IEEE 519 -1992 Built in DC Chokes to abate input Harmonics All Power Devices of 1600V PIV Built in Software to control the effect of reflected wave Built in EMC Filters Excellent Feature of Zero Stacking Internal family of communications modules Flexible Human Interface Module
AC Drive Basics
7th IGBT – Chopper is internally Provided Excellent Feature of Zero Stacking 6KV input transient protection 150% Overload for 60Seconds /200% for 3 Seconds Complete Output Short Circuit Protection Compact Size
Best Features in our Drives
AC Drive Basics