Motor&Drive Basics
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Transcript of Motor&Drive Basics
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Drive and Motor Basics
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Induction Motor Advantages
Low cost (compared with DC)
Wide availability
Low maintenance - no brushes or commutator
Rugged design - can be used in harsh environments
Low inertia rotor designs
High electrical efficiency
Wide speed ranges
No separately-powered field windings
Good open-loop performance
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AC MOTOR SIZE
Frame size is directly related to base RPM,
for a given Horsepower
Examp le: 15 HP motors o f dif ferent base speeds
Base RPM
Frame Size
Torque
Amps
3600 (2-pole)
215
22.5 lb-ft
18.5
1800 (4-pole)
254
45 lb-ft
18.7
1200 (6-pole)
284
67.5 lb-ft
19.3
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AC MOTOR FORMULA
120 x Frequency
# of PolesSYNC RPM =
Example: 4-pole mo tor
SYNC RPM = 120 x 60 / 4po les = 1800 RPM
%SLIP =SYNC RPM - FULL LOAD RPM
SYNC RPMX 100
Examp le: 1750 RPM moto r% Slip = (1800 - 1750) / 1800 x 100 = 3% Slip
SYNCHRONOUS SPEED
MOTOR SLIP
VOLTS / HERTZ
V/Hz=Moto r Line Volts
Motor Frequency
Example: 460 V, 60 Hz moto r
V/Hz = 460/60 = 7.66 V/Hz
VOLTS FREQUENCY V/Hz
460 60 7.66
345 45 7.66
230 30 7.66
115 15 7.66
7.66 1 7.66
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Elements of an Induc tion Moto r: The Stator
Stator Core
Laminat ion stack
of n otched steel
plates
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Elements of an Induc t ion Motor: The Stator (4-po le)
t
The stator ind uces m agnetic l ines of
f lux acros s the air gap, into th e rotor
Rotat ing
magnet ic f ie ld
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Elements of an Indu ct ion Motor: The Rotor
Laminat ions ofhigh-s i l icon
con tent steel
Cast aluminum
rotor b ars
Cast aluminumend r ings
Low-eddy current loss
magnet ic medium
Electrica l ly jo ins rotor
bars at both motor ends
Carry induc ed current
(skewed bars show n)
No direct electr ical con nect ion s are made to the rotor . Al l forces are
magnet ical ly indu ced by the stator, via the air gap.
Rotor Bar Current
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Typical AC Induction Motor Speed / Torque Curve
Across-the-line operation @ 60 Hz, NEMA B motor
Full load op erating point (100%
current & torque)
1750 RPM (namep late)
Breakdown point : Maximum
torque motor can produce
before locking rotor
Synchronous no-load speed
1800 RPM
(50 rpm )
100
175
225
Start ing Torqu e
Pull-Up Torqu e
150
%T
Speed
SLIP
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Speed
AC Motor Speed / Torque Curve family on Inverter Power
Slip (50 rpm )
100
175
225
150
%T
Slip (50 rpm)
100% load torque
operat ing l ine
Moto r base speed:
1750 RPM
At any appl ied Frequency, an induct io n m otor wi l l s l ip a fixed RPM at rated load.
Peak Inv erter Torq ue
(150 -200%)
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Induc t ion Motor Equivalent Circui t
Stator Rotor
A ir
Gap
R1
XLR
XM
XR
RLOAD = R2/ Slip*
Al thou gh there is no phy sica l connect ion b etween rotor and stator, the
induc ed f ield causes the motor m odel to behave as i f there is.
Stator
Resistance
Leakage
Reactance
Magnetizing
Reactance
Rotor
Reactance
*(R2is rotor bar resistance)
V
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Motor Cur rent Vectors
Stator RotorA ir
Gap
R1XLR
XM
XR
RLOAD
Stator
Resistance
Leakage
Reactance
Rotor
Reactance
Total Curren t Magnetizing
Current
Torque
Current
Magnetizing
Current
Torque-Producing Current
Total Current is the Vector s um of
Magnet iz ing and Torque-prod ucing
current , which are at a r ight angle
to each oth er.
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Motor Cur rent Vectors
Magnetizing
Current
Torque-Producing Current
MagnetizingCurrent
Torque-
Producing
Current
Magnetizing
Current
Torqu e-Producin g Current
LIGHT
LOAD
MEDIUMLOAD
&
HEAVY
LOAD
High % of total current is magnetizing current
Magnetizing c urrent is reactive (low p.f.)
Measured (total ) motor c urrent is not a good
indicator o f load level.
Most of tota l current is
torque-producing
Motors run at high
pow er factor
Total motor current is
prop ort ional to load level .
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460
100
V
60
Hz
Hz60 120
120
Constant Horsepower
Constant Voltage
% T
& HP
Constant Torqu e
50
Field Weakened Range
Motor Operat ion above Base Speed
Torquea V/HzFrequency increases
above base speed, but
voltage levels of f .
The result is inc reased
speed with weakened
torque, or cons tant HP
operat ion.
Abov e 2:1 , moto r
torque drops s harp ly
& operat ion is not
recommended.
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Typical AC Induction Motor Current & Torque Curves
Across-the-line operation @ 60 Hz, NEMA B motor
100
175
225
150
% T
Speed
% I
650
400
Lin ear range: 40-150%load(operatingrange in which c urrent is
propor t ional to torque)
Start ing ( inrush) current
Breakdown current :
maximum level when motor
locks rotor (sta lls)
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AC DRIVE BASICS
All AC Drives convert fixed voltage and frequency into variable voltage
and frequency, to run 3-phase induc t ion motors.
LINE INPUT
MOTOR
OUTPUT
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Types of AC Drives
In todays marketplace, there are
3 basic AC Drive categories:
Open loop Volts / Hz Drives
Open loop Sensorless Vector
Drives
Closed loop Flux Vector Drives
A ll are Pulse-Width-Modu lated (PWM)
Some manufacturers offer 2-in-1 & 3-in-
1 Drives, combining these attributes.
V/Hz
SENSOR-
LESS
VECTOR
FLUXVECTOR
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Volts / Hz Drives
Vol
ts
230
460
30 60 HzRPM*
900 1800(Base)
0
Moto r voltage is var ied l inear ly with frequenc y
No compensat ion for motor & load dy namics
Poor sh ock load respon se characteris t ics
*( 4-pole mo tor)
Motor Nameplate V/Hz
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AC Motor Torque & HP vs. Speed
50
100
30 60
900 1800
0
T& HP
%
Torque
HP
Hz
RPM
Motor Torque is constant to base speed
HP var ies p roport ion al ly to speed
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AC
Input
DC
BusCaps
AC to DC
Rectif ier
Pulse-Width-Modulated Inverter
DC FilterDC to AC
Inverter
IGBTs
AC
Output
All PWM inv erters (V/Hz, Vecto r & Senso rless Vecto r) share sim ilar pow er circu it
topo log ies .
AC is converted to DC, fi l tered, and inv erted to variable frequency, variable
vol tage AC.
M
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PWM Power Circuit:AC to DC Converter Sect ion
AC
Input
DC
Bus
Caps
AC to DC
Rectif ier DC Filter
+
-
Input Reactor
(opt ion)
DC Reacto r
The AC input is rectified and filtered into fixed-voltage DC
Certain manufacturers units contain an integral DC reactor (choke)
as part o f the DC fi l ter.
Add ing an external AC inpu t reactor wi l l y ie ld sim i lar benef i ts .
Bo th reduce harmo nics, smoo th and lower peak current .
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PWM Power Circuit:DC to AC Inverter Sect ion
DC Filter DC to ACInverter
IGBTs
AC
Output
M
AnIGBT(Insulated Gate Bipolar Transistor ) is a high -speed pow er semicon ductor s witch .
IGBTs are pulse-width m odulated with a specif ic f i r ing pat tern, chopping the DC voltage into 3-
phase AC voltage of the proper frequency and v oltage.
The result ing m otor c urrent is near-sinus oidal, due to motor ind uctance.
Imotor
Vu-v
U
V
W
IGBT Fir in g
Signals
+
-
Basic V/HZ Control Circuit: Input Feedback and Control
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IGBT Fir in g
Signals
PWMmicroprocessor
controller
Operator
Interface
V
f
Basic V/HZ Control Circuit: Input, Feedback and Control
Signals
Motor current &
voltage feedbackDC Bus c urrent &
voltage feedback
Speed reference
Flux Vector Control Elements: Input Feedback Control Signals
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IGBT Gating
Signals
PWM
microprocessorcontroller with
Vector algorithm
Man-
mach ine
Interface
Flux Vector Control Elements: Input,Feedback, Control Signals
Encod er Feedback
Motor current &
voltage feedbackDC Bus vol tage
feedback
Speed and / or
Torque reference
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Variable Torque Applications: Centr i fugal Pumps & Fans
Load varies with the square of
the speed
HP varies with the cu be of the
speed
Ideal ly suited for AC Drives
Energy savings benef i ts: only
50% power required at 80% f low
AC Drives replace ineff icient
dampers, guide vanes and valves
Speed
Flow,Torque&H
orsepower100%
100%
T = K x (RPM)2
HP = K x (RPM)3
80%
50%
80%
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4-Quadrant Operation of AC Motors on Inverter Power
FORWARD
MOTORING
REVERSE
MOTORING
REVERSE
REGENERATING
FORWARD
REGENERATING
+ RPM- RPM
ClockwiseTORQUE
Counter-
Clockwise
TORQUE
Conditions for Regenerating on an AC Motor
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WEIGHT
PULLROTATION
Conditions for Regenerating on an AC Motor
AC Motors regenerate when pu l led faster than th eir
syn c speed at the appl ied frequency.
At 60 Hz, if a mo tor is pul led faster th an 1800 RPM*,
the moto r wi l l b ehave as aninduc t ion g enerator.
Regeneration cond i t ions:
Overhaul ing loads
Fast d ecelerat ion o f hig h in ert ial loads
Stopping on a t imed-ramp
Cycl ic loads o r eccentr ic shaf t loading* 1750 RPM base
speed at 60 Hz
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AC Drive Regeneration
AC
Input
DC
BusCaps
IGBTs M
ONE - WAY TWO - WAYEnergy Flow :
+
_
Current f lows b ack into the DC bus, via the IGBT switchin g & back diodes.
AC Drive front-end rect i f ier is un id i rect ional ; energy canno t f low b ack into the AC
line.
Some returned energy is diss ip ated in lo sses in the capaci tors, switches, and
mo tor win din gs (10-15%).
Excessive regeneration can cause prob lems, such as DC Bus Overvol tage.
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Dynamic Braking on AC Drives
AC
Input
DC
Bus
Caps
M+_
DB
R
DYNAMIC
BRAKING
CONTROL
VDC
Feedb
ack
SIG
NAL
DYNAMIC BRAKING is typ ical ly an option for AC DrivesA seventh IGBT, integral ly mounted, is m odulated wh en DC Bus vol tage is excessive.
Resistor Grid s (external on rat in gs 5 HP & abov e) dis sipate the excess energy.
DB is duty-cycle l imi ted to a set num ber of stopping operat ions
DB is ACTIVE when:
Motor has an overhaul ing load
Fast decel of high -inert ial load
Stopping in ramp -to-rest m ode
DB isNOT ACTIVEwhen:
Decelerat ing a fr ict io nal load
Stopping in c oast-to-rest mode
Drive is disabled or i f pow er
is removed
ynam c ra ng on r ves:
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y g
Application Considerations
DB is no t fai lsafe:i f the dr ive faul ts or p ower is remo ved, DB w i l l not
funct ion .
DB on ly operates when the dr ive is running :in co ast-rest or stand -by,
DB is inact ive.
DB should no t be used in EMERGENCY STOPPING:the dr ive wil l
con t inue on a t imed ramp , produ cing torque the ent i re t ime.
DB is sui table for intermit tent o perat ion o nly:oth er regenerat ive
solut ion s exist for long -term overhaul ing loads
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AC
DRIVEAC
DRIVEAC
DRIVE
AC Drives on a Common DC Bus: Typic al Conn ection Diagram
THERMAL- MAG
BREAKER
INPUT LINE
REACTOR
SEMICONDUCTORFUSES
INTERLOCKED
DC CONTACTOR
DC DRIVE BASICS
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DC DRIVE BASICS
DC Drives co nvert AC line vo ltage into var iable DC voltage with an SCR
phase-control led br idg e rect i f ier, to pow er the DC motor ARMATURE. A
separate f ield supply p rovid es the moto r with DC FIELD excitat ion .
LINE INPUT MOTOR OUTPUT
Armature
Field
A1
A2
F1
F2
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Power SwitchesThe SCR: (Si l icon Con trol led Rect i f ier)
a.k.a. -Thyristor
ANODE CATHODE
GATE
Extremely robust solid-state switch / 40+ year proven track record
Key element in DC Drive power circuit
Simple pulse gating turns on current flow
Device has self-turn-off when reverse biased
Stud-mount, hockey-puck and encapsulated 2-, 4- and 6-pack types
available in certain sizes and ratings.
TRIGGER
-
+
Application Issues: AC Line Notching
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Application Issues: AC Line Notching
on DC Drives
AC
Input
Commutation notches are caused by the
transfer of current from one SCR to another.
The notches can cause misfiring on drives
common to the same power line.V ph-ph
Solu tion : Installat ion of a small (25-50 uH range), 3-ph ase reactor on each
DC contro l ler wil l prevent cros s-talk and oth er related prob lems.
Elements of a DC Drive:Non-regenerative type
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Elements of a DC Drive:Non regenerative type
AC
Input
SCR Fir ing
Signals
Microprocessor
controller
Operator
Interface
SE
Q
REFLO
CAL
AC MOTOR DRIVE
0.75
KW
200 V v1.3
HEALTH
L
R
PROGE
M
RUN
F
WD
REV
JOG
RESET
STOP
RESET
Speed or Torqu e
Reference
Field
Contro l
Signals
A1
A2
F1
F2
Tachometer
Feedback
(closed-loop)
Motor vol tage
feedback
Line current
feedback
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Elements of a DC Drive:Regenerative type
AC
Input
SCR Fir ing Signals
Microprocessor
controller
Operator
Interface
S
EQ
REFLO
CA
L
AC MOTOR DRIVE
0.75KW
200 V v1.3
HEALTH
L
R
PROGE
M
RUN
F
WD
REV
JOG
RESET
STOP
RESET
Speed or Torqu e
Reference
Field
Contro l
Signals
A1
A2
F1
F2
Tachometer
Feedback
(closed-loop)
Motor vol tage
feedbackFWD/MOT REGEN/REV
F
F F F
F FR
R
R
R
R
R
Line current
feedback