Motor Control And Protection

Understand working principles of motor starters and various protection devices

Motors and Motor Control Oladokun Sulaiman

Objective • At the end of the lecture students will be able to describe the working principles of motor starters and various protection devices

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. 3

Motors and Back emf •

The phrase back emf is used for an emf that tends to reduce the applied current • When a motor is turned on, there is no back emf initially • The current is very large because it is limited only by the resistance of the coil

4

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

6

Control Control: getting motors to do what you want them to What you want to control     =     what you can control For DC motors:

speed

voltage windings’ resistance

N

V

N

R ω

V

S

e

back emf

S

e

is a voltage generated by the rotor windings cutting the magnetic field

1. 2. 3. 4. 5. 6. 7.

Needs for Motor Control

Induction motor – drawn 5-8x full-load current (FLC) when starting Due to maximum flux cutting rate (s = 100%) in rotor- creating large induced rotor currents Supply power factor very low i.e. 0.2 lagging at starting, 0.5 lagging on no-load & 0.85 lagging on full-load This starting surge current reduces as motor accelerates up to rated speed Operating at light loads with low power factor inefficient as supply current higher causing higher I²R (copper) losses To improve - reduce supply voltage for light loads motor Achieved with electronic voltage controller i.e. soft-starter and/or energy manager - match supply voltage to start-up & load conditions

8. 9.

10. 11. 12.

This will maintain operating power factor as high as possible - minimise supply current & power losses Most induction motors have Direct-on-Line (DOL) - inexpensive & simple to operate & maintain provided current surge not cause heating damage to motor When larger motors started by DOL – can cause voltage dip due to large starting current May result in malfunction of others - lighting dip & flickering effects To limit, motors started at reduced voltage- full supply reconnected when accelerated close to rated speed - star-delta, auto transformer & electronic "soft" starter

Motor Characteristics

1000

100

Normal Operating Current

Time in Seconds

10

1

0.1

Inrush Current

0.01 1

10

100

1000

Current in Amperes

Motor Inrush Curve

1000

100

100

1

0.1

10

Time in Seconds

Time in Seconds

10

Short Circuit

300 % Overload

1000

1

0.1

0.01

0.01 1

10

100

1000

Current in Amperes

1

10

100

1000

Current in Amperes

COMPARISON OF STARTERS

Current %

60 0

DIRECT ON LINE STARTING

40 0

20 0 0 10 0

AUTO TRANSFORMER STARTING STAR DELTA STARTING Slip Auto transformer % on 60%

0

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,

Direct on line • Simple arrangement, used for majority induction motor • Motor directly switched onto 3 phase AC power supply lines • Further circuit additions – remote control & reversing (required extra contactor) • Short duration but large starting current • Acceptable provided voltage dip < 10~15% during starting • 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

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 or remote)

KM1 contactor ‘holds-in”

Auxiliary contact on KM1 ‘latches’ contactor Remote indicator lamp ‘on’

KM1 contactor drops out, motor stops

Press stop button ‘O’ (local or remote) on overload the OCR trips out the stop button OCR must be manually reset (after thermal time delay)

Star delta • If motor stator winding is star connected, only 1/3 of starting current required if motor start with delta connected • For small motors – operated by manual c/o switch • For large motors - phase windings automatically switched using timing controlled contactors • At initial starting, motor won’t rotate, thus no mechanical output produced • Therefore, current taken by the motor will determine by supply voltage & impedance of motor phase windings

Power circuit operation

Control circuit operation

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 KM2 auxiliary

Closing of KM3: delta connection

Opening of KM1 by KM2 auxiliary Closing of KM3 by KM1 auxiliary

KM2 & KM3 contactors drop out, motor stops

Stop by S1 button or OCR trip F1

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

Comparison if star & delta connection

Ratio of

I L (Y ) I L(∆)

VL

3.Z 1 = = 3 3.VL Z

Current surge from star to delta • Motors generate back emf against power supply when running • When supply removed, magnetic field does not collapse 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 starting period

Auto transformer • Starting large motor with prolong run-up period demand very high current surge from supply generator even for few seconds • 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 doublewound transformer • Meant for operation of short starting period only • Only applicable to large motor drives due to initial cost

Power circuit operation

Control circuit operation

Manual closing of fused isolator Q1

Control circuit voltage available (e.g. 110V from control transformer)

Closing KM1: star connection of transformer

Press start button S2 to close KM1 Interlocking of KM3 by KM1 Closing 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- Hold in of KM3 KM3) Opening of KM2 by KA1 KM3 contactors drop out, motor stop

Stop by S1 button or OCR trip F1

Auto transformer - operation • 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 & then full supply voltage rapidly reconnected to motor – open transition • Danger with open-transition - very large surge current can flow after transition from reduced to full voltage

Soft starter (additional)

Conclusion • DOL starter - simple & cheap but causes large starting 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

Controlling speed with voltage • The back emf depends only on the motor speed.

e = ke ω

• The motor’s torque depends only on the current, I.

τ = kτ I

R V

e

DC motor model

Controlling speed with voltage • The back emf depends only on the motor speed.

e = ke ω

• The motor’s torque depends only on the current, I.

τ = kτ I

Istall = V/R

current when motor is stalled speed = 0 torque = max

V = IR + e How is V related to ω ?

τ R V =         + ke ω   kτ 

R V

• Consider this circuit’s V:

e

­ or ­

 V  R ω = ­         τ +    ke   kτ ke  DC motor model

Speed is proportional to voltage.

speed vs. torque at a fixed voltage speed ω

 V    ke 

no torque at max speed

max torque when stalled

torque τ 

 kτV    R 

speed vs. torque at a fixed voltage speed ω

 V    ke 

no torque at max speed

Linear mechanical power  Pm =  F • v  Rotational version of Pm =  τ • ω 

torque τ 

 kτV    R 

stall torque

speed vs. torque at a fixed voltage speed ω

 V    ke 

Linear mechanical power  Pm =  F • v  Rotational version of Pm =  τ • ω 

max speed

power output

speed vs. torque

torque τ 

 kτV    R 

stall torque

Motor specs

Electrical Specifications (@22°C) For motor type 1624  

003S

006S

012S

024

nominal supply voltage armature resistance maximum power output maximum efficiency no-load speed no-load current friction torque stall torque velocity constant back EMF constant torque constant armature inductance

3 1.6 1.41 76 12,000 30 .010 .613 4065 .246 .333 .085

6 8.6 1.05 72 10,600 16 .011 .510 1808 .553 .748 .200

12 24 1.50 74 13,000 10 .013 .600 1105 .905 1.223 .750

24 75 1.92 74 14,400 6 .013 .694 611 1.635 2.212 3.00

­­­­­­­­­­­­­­­­­­­­­­­­­­

   ke 

­­­­­­­­

(Volts) (Ohms) (Watts) (%) (rpm) (mA) (oz-in) (oz-in) (rpm/v) (mV/rpm) (oz-in/A) (mH)

­­­­­­­­

­­­­­­­­

­­­­­­­­­         ­­­­­­­

 kτ

Back to control Basic input / output relationship: τ R V =         + ke ω   kτ 

We can control the  voltage applied V. We want a particular  motor speed ω .

How to change the voltage?

V is usually controlled via PWM -- “pulse width modulation”

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 to the motor

Open­loop vs. Close­loop Control Open­loop Control:  desired speed ω

Controller  solving for V(t)

V(t) Motor

ω actual speed

If desired speed ωd  ≠ actual speedSo ωa, what?

Closed­loop Control:  using feedback ωd  − ωa

desired ωd

­

compute V from  the current error

V

PID controller actual speed

ωa

ωa Motor

Speed control: • • • •

Stator voltage control Supply frequency control Rotor resistance control Pole changing

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 frequency converter-> 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 drive Backdrop- complexity

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

Different Categories of Overload

Protection Motor enclosure • Totally enclosed , non ventilation • Splash –proof type • Totally enclosed fan cooled • Drip proof type Name plate- rating, supply , connection ,frame type and size,permisible temperature,rpm, enclosure type,# of pole.

Motor Protection

1. Short-circuit protection of stator windings 2. Stator-overheating protection 3. Rotor-overheating protection 4. Under voltage protection

Protection Measurement • Temperature • Voltage and current• Insulation resistance winding resistance • Vibration • Speed •

Testing: No load test Full load test k

Failures: • Insulation failure • Rotor bar failure • Mechanical problem Maintenance Periodic inspectionAccurate 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 analysis- of motor and coupling

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 electrical contact • 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

The Magnetic Trip Block

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 - fuse rating Transparent casing Tinned wire copper

Brass cap

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 & same rating • Position - between supply and the circuit – fuses removal means total isolation for the circuit • Two main types: – Cartridge fuse – High rupturing capacity (HRC) fuse

Checking Fuses: Visual inspection

Relays are amazingly simple devices.

There are four parts in every relay: • Electromagnet • Armature that can be attracted by the electromagnet • Spring • Set of electrical contacts

Case study 1: How a relay works?

Case study 2: Under voltage trip 3-ph 440V bus bars

Circuit breaker Fuse

Generator

UV relay coil

Case study 3: Single phasing

Normal Condition 1.4 A

0A

1.4 A

M

1.4 A 208V 1/3 HP Motor

What happened?

2.4 A

2.4 A 40 C

F.L.A. = 1.4 Amperes

(173%)

M

(173%)

Bi-metallic Single-phasing Protection (differential action)

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 motor contactor • For star connected motor – phase & line currents are equal, thus OCR has no problem in sensing correct winding current • For delta connected – uneasy task, therefore, normally line current will divides phasorally between 2 phases of motor windings

I PH =

IL 3

= 0.577 I L

Single phasing

Healthy condition (balanced)

Single phasing fault condition (unbalanced)

% of rated FLC % of rated FLC

IL2 and IL3

IA and IB

IC

60

102

62

131

70

130

79

161

100

243

129

185

Facts of single phasing • When one line open circuited, balanced condition will no longer exists • Note that current C is higher than others • At 60% of full load, due to single-phasing, line currents are 102% 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 singlephasing - normal feature

Effect of single phasing • If single-phasing occurs on light load, motor will keep running unless protection trips contactor • If motor stopped, it won’t restart • When contactor closed, motor will take large starting current 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 delay will result rapid & severe overheating • Worse case - if operator makes several restart, motor will burn out

Effect of single phasing (cont/..) • If motor fails to start – investigate first • UV protection - disconnected consumers from supply if total voltage loss / black-out, prevent restarting together resulting 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 won’t restart until contactor coil energised – require operator to reset manually • For essential services – restart automatically after certain delay is utmost important

Willas-Array Solution for Motor Control Block Diagram 1

AC Input

BLDC Motor

PWM Signal

Hall Sensor (Inside Motor)

Power Management

Control Unit With Motor Control Cell

Gate Driver Gate Driver Gate Driver 3 x Driver IC

Power Stage = 6 x IGBT or MOSFET

IPM Module Others

61

Feedback Signal (Speed / Positon)

Inspection & maintenance • Moving contacts in control gear - ‘wipe’ phenomenon i.e. if fixed part need to removed, moving part would follow on • Rolling / sliding action of contactor - to remove any oxide, 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 & pitted contacts • Contact restorer - helps reduce mechanical wear, but excess oil / grease encourages burning & pitting • Silver-faced & carbon contacts shouldn’t be lubricated

Inspection & maintenance (cont/…) • 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, rust removed using fine emery • Moving parts – free, no undue wear at pivots, magnets bedding properly & no filing on magnetic faces • Enclosure – dirt/rust accumulations, corroded parts, starter fixing bolts & earth bonding connection • Contactors & relays – signs of overheating & loose connections, dust/grease from insulating components

Inspection & maintenance (cont/…) • Contacts – excessive pitting & roughness • NEVER file silver alloy contacts or remove silver oxide good conductor • If need to replace, always replace both fixed & moving contacts 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 of emergency stop & auto restart