17253098 motor control and protection

8/3/2019 17253098 Motor Control and Protection

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Motors and Motor Control

Oladokun Sulaiman

Understand working principles of motor starters and various protection devices


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Objective

At the end of the lecture students will be able

to describe the working principles of motor

starters and various protection devices


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3

Motors A motor is basically a generator running in

reverse.

A current is passed through the coil, producing

a torque and causing the coil to rotate in the

magnetic field.

Once turning, the coil of the motor generates a

back emf, just as does the coil of a generator.

The back emf cancels some of the applied emf,

and limits the current through the coil.


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4

Motors and Back emf

The phrase back emfis used foran emf that tends to reduce theapplied current

When a motor is turned on, thereis no back emf initially

The current is very large becauseit is limited only by the resistanceof the coil


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Motor DC

AC

AC Motor

Induction motor Synchronous motor

Wound rotor motor

Operate based on Speed (S)= 120f/p F-Frequency

P-Number of poles


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6


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Control

What you want to control = what you can control

For DCmotors:

speed voltage

N

S

N SV

Ve back

emf

R

windingsresistance

e is a voltage generated by therotor windings cutting the

magnetic field

Control: getting motors to do what you wantthem to


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Needs for Motor Control1. Induction motor drawn 5-8x full-load current

(FLC) when starting2. Due to maximum flux cutting rate (s = 100%) inrotor- creating large induced rotor currents

3. Supply power factor very low i.e. 0.2 lagging atstarting, 0.5 lagging on no-load & 0.85 lagging

on full-load4. This starting surge current reduces as motoraccelerates up to rated speed

5. Operating at light loads with low power factor -inefficient as supply current higher causing

higher IR (copper) losses6. To improve - reduce supply voltage for lightloads motor

7. Achieved with electronic voltage controller i.e.soft-starter and/or energy manager - match

supply voltage to start-up & load conditions


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8. This will maintain operating power factor as highas possible - minimise supply current & powerlosses

9. Most induction motors have Direct-on-Line(DOL) - inexpensive & simple to operate &maintain provided current surge not cause

heating damage to motor10. When larger motors started by DOL can cause

voltage dip due to large starting current

11. May result in malfunction of others - lighting dip& flickering effects

12. To limit, motors started at reduced voltage- fullsupply reconnected when accelerated close torated speed - star-delta, auto transformer &electronic "soft" starter


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1000

100

10

1

0.1

0.01

10 100 10001

Timein

Seconds

Current in Amperes

Motor

Characteristics

Inrush Current

Normal Operating Current

Motor Inrush Curve


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300%

O ver lo

a

d

1000

100

10

1

0.1

0.01

10 100 10001

Timein

Seconds

Current in Amperes

Sho

r

tCircui t

1000

100

10

1

0.1

0.01

10 100 10001

Timein

Second

s

Current in Amperes


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0100

200

400

600

Current%

0Slip

%Auto transformer on60%

STARDELTA

STARTING

AUTO

TRANSFORMER

STARTING

DIRECT ON

LINESTARTING

COMPARISON OF STARTERS


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Contactor

Performing switching action to connect/disconnect

power supply to motor. Electromagnetically

operated 3-pole switch initiated from local, remote

stop/start push buttons. If current above rated,


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Direct on line

Simple arrangement, used for majority induction motor

Motor directly switched onto 3 phase AC power supplylines

Further circuit additions remote control & reversing(required extra contactor)

Short duration but large starting current Acceptable provided voltage dip < 10~15% duringstarting

For larger motor - unacceptable voltage dip at bus-bars -malfunctions of other consumers & possible drop out of

supply contactors If prolonged cause supply line & generator protection to

trip


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Power circuit operation Control circuit operation

Manual closing of fused isolator Q1 Control circuit voltage available (e.g.110V from control transformer)

Closing of line contactor KM1 Press start button I (local orremote)

KM1 contactor holds-in Auxiliary contact on KM1 latchescontactorRemote indicator lamp on

KM1 contactor drops out, motor

stops

Press stop button O (local or

remote) on overload the OCR tripsout the stop buttonOCR must be manually reset (afterthermal time delay)


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Star delta

If motor stator winding is star connected, only 1/3 ofstarting current required if motor start with deltaconnected

For small motors operated by manual c/o switch

For large motors - phase windings automaticallyswitched using timing controlled contactors

At initial starting, motor wont rotate, thus nomechanical output produced

Therefore, current taken by the motor will determineby supply voltage & impedance of motor phasewindings


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Manual closing of fused isolator Q1 Control circuit voltage available(e.g. 110V from control transformer)

Closing contact of KM1: star connection Press start button S2 to close KM1

Closing of KM2: motor supply KM1 closes KM2

Opening of KM1: star connection opens hold in of KM1 KM2 by KM2auxiliary

Closing of KM3: delta connection Opening of KM1 by KM2 auxiliaryClosing of KM3 by KM1 auxiliary

KM2 & KM3 contactors drop out, motorstops

Stop by S1 button or OCR trip F1

Note: KM2 has a pair of auxiliary contacts with a time delay action (typically 40ms) betweenthe operating of the N/C and the closing of the N/O contacts.


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Comparison if star & delta connection

3

1

.3

.3

)(

)(==

Z

V

Z

V

I

I

L

L

L

YL

Ratio

of


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Current surge from star to delta

Motors generate back emf against power supply whenrunning

When supply removed, magnetic field does notcollapse immediately

Motor will slow down but still generate emf When supply reconnected, supply voltage & motor

emf are not in phase

Thus each time the starter is operated, different current

surge will produced To overcome auto transformer is used where the

supply is eventually never disconnected during startingperiod


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Auto transformer

Starting large motor with prolong run-up period demand very

high current surge from supply generator even for fewseconds

Will causes severe voltage dip - affects other loads Reduced voltage starting will limit starting surge current

One method step it down using transformer When motor accelerated up to almost rated speed, then

reduced voltage will resume to normal

Special transformer uses one winding for input & output

Thus, cheaper, smaller & lighter than equivalent double-wound transformer

Meant for operation of short starting period only

Only applicable to large motor drives due to initial cost


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Manual closing of fused isolator Q1 Control circuit voltage available(e.g. 110V from control transformer)

Closing KM1: star connection oftransformer

Press start button S2 to close KM1Interlocking of KM3 by KM1Closing KA1 by KM1

Closing KM2: motor supply via

transformer

Closing of KM2 by KA1

Hold in of KM2

Opening KM1: star connection opens Opening KM1 by KA1 (after time delay)

Closing KM3: direct supply to motor Closing KM3 by KM1

Interlocking KM1 by KM3

(Note the mechanical interlock of KM1-KM3)

Hold in of KM3Opening of KM2 by KA1

KM3 contactors drop out, motor stop Stop by S1 button or OCR trip F1


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Supply voltage connected across complete winding & motor

connected to reduced voltage tapping Number of tapping available - giving output voltage ranging from

50% ~ 80% of main supply If 60% tap supplied at 440 V, output will be 60% x 440 = 264 V

Multiple tapping - to match motor current demand to supply

capability Autotransformer can be use in both open & closed transition

switching sequence between start & run conditions

Star delta - reduced voltage initially supplied, disconnected & thenfull supply voltage rapidly reconnected to motor open transition

Danger with open-transition - very large surge current can flow aftertransition from reduced to full voltage

Auto transformer - operation


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Soft starter (additional)


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Conclusion

DOL starter - simple & cheap but causes largestarting surge

Star delta starting reduces surge but more

complex require 3 contactors & timer Auto transformer - can arranged to match

motor surge current & run-up period with

suitable voltage but the most expensive one


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Controlling speed with voltage

DC motor model

V e

R

The back emf depends only on the motor speed.

The motors torque depends only on the current, I.

e = ke

= k

I


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k

ke

Controlling speed with voltage

DC motor model

V e

R

The back emf depends only on the motor speed.

The motors torque depends only on the current, I.

e = ke

= k

I

Consider this circuits V: V = IR + eIstall = V/Rcurrent when

motor is stalled

speed = 0

torque = max

How is V related to ?

V = + ke R

k

- or -

= - +R

ke

V

Speed is proportional to voltage.


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speed vs. torque

torque

speed

ke

V

at a fixed voltage

R

kV

max torque whenstalled

no torque at maxspeed


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speed vs. torque

torque

speed

ke

V

at a fixed voltage

R

kV

stall torque

no torque at maxspeed

Linear mechanical power Pm = F v

Rotational version of Pm =


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speed vs. torque

torque

speed

ke

V

at a fixed voltage

R

kV

stall torque

max speed

Linear mechanical power Pm = F v

Rotational version of Pm =

power output

speed vs.torque


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Motor specs

Electrical Specifications (@22C)

For motor type 1624 003S 006S 012S 024

-------------------------- -------- -------- -------- --------- -------nominal supply voltage (Volts) 3 6 12 24

armature resistance (Ohms) 1.6 8.6 24 75

maximum power output (Watts) 1.41 1.05 1.50 1.92

maximum efficiency (%) 76 72 74 74

no-load speed (rpm) 12,000 10,600 13,000 14,400no-load current (mA) 30 16 10 6

friction torque(oz-in) .010 .011 .013 .013

stall torque (oz-in) .613 .510 .600 .694

velocity constant (rpm/v) 4065 1808 1105 611

back EMF constant (mV/rpm) .246 .553 .905 1.635torque constant (oz-in/A) .333 .748 1.223 2.212

armature inductance (mH) .085 .200 .750 3.00

ke

k


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Back to control

Basic input / output relationship:

How to change the voltage?

We want a particular

motor speed .

We can control thevoltage applied V.

V = + ke R

k

V is usually controlled via PWM -- pulse widthmodulation

PWM


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PWM

PWM -- pulse width modulation

Duty cycle:

The ratio of the On time and the Off time in one cycle

Determines the fractional amount of full power delivered tothe motor


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Open-loop vs. Close-loop Control

Open-loop Control:

actual speed

desired d

V

Motor

a

actual speed

a

-

compute V fromthe current error

d a

Closed-loop Control: using feedback

desired speed Controller

solving for V(t)

V(t)

Motor

If desired speed d actual speed a,So what?

PID controller


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Speed control:

Stator voltage control

Supply frequency control

Rotor resistance control Pole changing


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VSD Conventional control of supply frequency and terminal change

of phase to minimize losses counter current /pluging+regenerative +dynamic

Development in speed and torque control

From ward leornard system -> thyristor controlled DC drive

->PWM AC variable voltage regulation ->variable frequencyconverter-> AC VSD or inverter

Cost effective method of speed control+ application to high

power+relibaility+maintainability+save energy+ improve

efficiency+ match speed and torque of drive with process driveBackdrop- complexity


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Component-

Motor

drive control unit-power source to motor, increase and decrease

motor set point at operator panel+ feed back loop give the driv

the actual speed+Power modulation control the speed , torque

and power along with direction of motor and machine- i.e

converter, inverter, cycloconverter.

+sensing unit

+operator unit


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Different Categories of Overload


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Protection

Motor enclosure

Totally enclosed , non ventilation

Splash proof type Totally enclosed fan cooled

Drip proof type

Name plate- rating, supply , connection ,frametype and size,permisible temperature,rpm,

enclosure type,# of pole.


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Motor Protection1. Short-circuit protection of stator windings

2. Stator-overheating protection

3. Rotor-overheating protection

4. Under voltage protection


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Protection

Measurement

Temperature

Voltage and current-

Insulation resistance

winding resistance

Vibration

Speed

Testing:

No load test

Full load test k


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Failures:

Insulation failure

Rotor bar failure

Mechanical problemMaintenance

Periodic inspection-

Accurate shaft alignment or belt tension

Check motor heating@ heating- check and clean air filter

Keep motor clean and free from dirt

Keep motor dry - Check for dampness around and inside motor

Check bearing regularly- lubrication at right quantity

Vibration anal sis- of motor and cou lin


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Circuit Breaker Safely & interrupt prospective short circuit fault

current expected in circuit Will trips but can be reset & reused Link mechanism provided, closes main contacts

under spring pressure & wipes the surface of

fixed contact points - ensuring good electricalcontact Main contact open rapidly with snap action Resulting arc transferred to special arcing

contacts above the main contact Arc chutes with arc splitter quickly stretch &cool the arc till it snaps

Circuit breaker is open when the arc quenched


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The MagneticTrip Block


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Fuse Protect circuit from damage faults & over

current Designed to blow rapidly before circuit damage

takes place

Many types and sizes, marked with size of steady

current can be carried without blowing - fuserating Transparent casing

Brass cap

Tinned wire copper


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Fuse Rating Important correct rating for normal current

flowing in circuit it protects Lower rating - every time switch on, fuse will blow Higher rating promoting positive dangerous

circuit with over current flowing without blowing

fuse - overheat & can cause fire If fuses blow, must replaced by same type & samerating

Position - between supply and the circuit fusesremoval means total isolation for the circuit

Two main types: Cartridge fuse High rupturing capacity (HRC) fuse


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Checking Fuses: Visual inspection


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Relaysare amazingly simple devices.There are four parts in every relay:1. Electromagnet2. Armature that can be attracted by the

electromagnet

3. Spring4. Set of electrical contacts


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Case study 1: How a relay works?


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Case study 2: Under voltage trip

UV relay

coilFuse

Circuitbreaker

Generator

3-ph 440V bus bars


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Normal ConditionNormal Condition

M

1.4 A

1.4 A

1.4 A

208V 1/3 HP Motor 40 C

F.L.A. = 1.4 Amperes

M

0 A

2.4 A

2.4 A

(173%)

(173%)

What happened?What happened?

Case study 3: Single phasing


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Bi-metallic Single-phasing

Protection (differential action)

Single phasing


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Single phasing

Occurs when one of three back-up fuses blows or if one of

contactor contacts is open-circuited Effect current increase in two remaining lines

Cause noisy motor uneven torque produced in rotor

Will detect by OCR unequal heating of bi-metal strips

causes differential movement, initiate OCR to trip motorcontactor For star connected motor phase & line currents are equal,

thus OCR has no problem in sensing correct windingcurrent

For delta connected uneasy task, therefore, normally linecurrent will divides phasorally between 2 phases of motorwindings

L

L

PH

II

I 577.0

3

==


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Single phasing


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Healthycondition

(balanced)

Single phasing fault condition(unbalanced)

% of ratedFLC

% of rated FLC

IL2 and IL3 IA and IB IC

60 102 62 131

70 130 79 161

100 243 129 185


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Facts of single phasing

When one line open circuited, balanced condition will no longerexists

Note that current C is higher than others

At 60% of full load, due to single-phasing, line currents are102% of full-load value but current C is 131%

102% may not activate OCR, thus motor remains connected However, local overheating in winding C will quickly get

damage Differential type relay used to protect motors against this

condition i.e. trips out with unbalanced currents

For most modern thermal OCR - protection against single-phasing - normal feature


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If single-phasing occurs on light load, motor will keeprunning unless protection trips contactor

If motor stopped, it wont restart

When contactor closed, motor will take large startingcurrent but develop no rotating torque

OCR - set to allow starting current at prolong period sufficient for start up period

With no ventilation on stationary motor - time delaywill result rapid & severe overheating

Worse case - if operator makes several restart, motorwill burn out

Effect of single phasing

Eff t f i l h i ( t/ )


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If motor fails to start investigate first

UV protection - disconnected consumers from supply iftotal voltage loss / black-out, prevent restarting togetherresulting huge current surge, tripping generator again

For LV motors UV provided by spring loaded motor

contactor For large HV motor - UV covered by relay separate from

OCR function or part of special motor relay

Motor wont restart until contactor coil energised

require operator to reset manually For essential services restart automatically after certain

delay is utmost important

Effect of single phasing (cont/..)

Willas Array Solution for Motor Control


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61

Willas-Array Solution for Motor Control

Control

UnitWith

Motor

Control

Cell

Feedback Signal

(Speed / Positon)

Power

Management

Power Stage

= 6 x IGBT

or MOSFET

IPM Module

3 x Driver IC

Gate Driver

Gate Driver

Gate Driver

Block Diagram 1

Others

Hall Sensor

(Inside Motor)

BLDCMotor

PWM Signal

ACInput

Inspection & maintenance


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Inspection & maintenance

Moving contacts in control gear - wipe phenomenon i.e.

if fixed part need to removed, moving part would followon

Rolling / sliding action of contactor - to remove anyoxide, ensure good metal-to-metal contact

Frequently operate contact subject to worn, bad contact,wipe lost, reduction in contact pressure & overheating regular inspection & cleaning

Rough contact surface could lower contact resistance -file used sparingly & only on badly burned & pittedcontacts

Contact restorer - helps reduce mechanical wear, butexcess oil / grease encourages burning & pitting

Silver-faced & carbon contacts shouldnt be lubricated

Inspection & maintenance (cont/)


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Closed copper contacts with long periods tend to build

oxide film - cause overheating Operated contact several times to clean surfaces

Magnet faces - kept clean & free from grease/oil, rustremoved using fine emery

Moving parts free, no undue wear at pivots, magnetsbedding properly & no filing on magnetic faces

Enclosure dirt/rust accumulations, corroded parts,starter fixing bolts & earth bonding connection

Contactors & relays signs of overheating & looseconnections, dust/grease from insulating components

Inspection & maintenance (cont/ )


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Contacts excessive pitting & roughness NEVER file silver alloy contacts or remove silver oxide -

good conductor

If need to replace, always replace both fixed & movingcontacts in pairs

Connections power & control connections for tightness,overheating, fraying & brittleness flexible leads

OCR - proper size (relate to motor FLC),dirt/grease/corrosion & freedom of movement

Control operation sequence during start-up, control &shut-down, excessive contact sparking, functioning ofemergency stop & auto restart

17253098 motor control and protection

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