motor&drive basics

7/30/2019 Motor&Drive Basics

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Drive and Motor Basics


2/35

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


3/35

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


4/35

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


5/35

Elements of an Induc tion Moto r: The Stator

Stator Core

Laminat ion stack

of n otched steel

plates


6/35

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


7/35

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


8/35

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


9/35

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%)


10/35

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


11/35

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.


12/35

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 .


13/35

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.


14/35

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)


15/35

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


16/35

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


17/35

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


18/35

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


19/35

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


20/35

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 .


21/35

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


22/35

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


23/35

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


24/35

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%


25/35

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


26/35

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


27/35

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.


28/35

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:


29/35

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


30/35

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


31/35

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


32/35

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


33/35

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


34/35

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


35/35

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

motor&drive basics

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