Motors and Motor Control

Oladokun Sulaiman

Understand working principles of motor starters and various protection devices

Objective

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

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.

4

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

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

What you want to control = what you can control

For DC motors:

speed voltage

N

S

N SV

V e back emf

R

windings’ resistance

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

emf: electromagnetic force

Control: getting motors to do what you want them to

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

(FLC) when starting2. Due to maximum flux cutting rate (s = 100%) in

rotor- creating large induced rotor currents3. Supply power factor very low i.e. 0.2 lagging at

starting, 0.5 lagging on no-load & 0.85 lagging on full-load

4. This starting surge current reduces as motor accelerates up to rated speed

5. Operating at light loads with low power factor - inefficient as supply current higher causing higher I²R (copper) losses

6. To improve - reduce supply voltage for light loads motor

7. Achieved with electronic voltage controller i.e. soft-starter and/or energy manager - match supply voltage to start-up & load conditions

8. This will maintain operating power factor as high as possible - minimise supply current & power losses

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

10. 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- full supply reconnected when accelerated close to rated speed - star-delta, auto transformer & electronic "soft" starter

1000

100

10

1

0.1

0.0110 100 10001

Tim

e in

S

econ

ds

Current in Amperes

Motor Characteristics

Inrush Current

Normal Operating Current

Motor Inrush Curve

300 %

Overlo

ad

1000

100

10

1

0.1

0.0110 100 10001

Tim

e in

S

econ

ds

Current in Amperes

Sh

ort C

ircu

it1000

100

10

1

0.1

0.0110 100 10001

Tim

e in

S

econ

ds

Current in Amperes

0100

200

400

600

Cu

rren

t %

0Slip %Auto transformer

on 60%

STAR DELTA STARTING

AUTO TRANSFORMER STARTING

DIRECT ON LINE STARTING

COMPARISON OF STARTERS

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, contactor will tripped out automatically by OCR, disconnecting motor from supply.

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 buttonOCR 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 auxiliaryClosing 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

3

1

.3

.3

)(

)(

ZV

ZV

I

I

L

L

L

YLRatio of

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 double-

wound 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 KM1Interlocking of KM3 by KM1Closing KA1 by KM1

Closing KM2: motor supply via transformer

Closing of KM2 by KA1Hold in of KM2

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

Closing KM3: direct supply to motor Closing KM3 by KM1Interlocking 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

• 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

Auto transformer - operation

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

DC motor model

V e

R

• The back emf depends only on the motor speed.

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

e = ke

= k I

kke

Controlling speed with voltage

DC motor model

V e

R

• The back emf depends only on the motor speed.

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

e = ke

= k I

• Consider this circuit’s V: V = IR + eIstall = V/Rcurrent when

motor is stalledspeed = 0

torque = max

How is V related to

V = + ke R k

- or -

= - + R ke V

Speed is proportional to voltage.

speed vs. torque

torque

speed

ke V

at a fixed voltage

R kV

max torque when stalled

no torque at max speed

speed vs. torque

torque

speed

ke V

at a fixed voltage

R kV stall torque

no torque at max speed

Linear mechanical power Pm = F v

Rotational version of Pm =

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

Motor specs

Electrical Specifications (@22°C)For motor type 1624   003S 006S 012S 024

-------------------------- -------- -------- -------- --------- -------nominal supply voltage (Volts) 3 6 12 24armature resistance (Ohms) 1.6 8.6 24 75maximum power output (Watts) 1.41 1.05 1.50 1.92maximum efficiency (%) 76 72 74 74no-load speed (rpm) 12,000 10,600 13,000 14,400no-load current (mA) 30 16 10 6friction torque (oz-in) .010 .011 .013 .013stall torque (oz-in) .613 .510 .600 .694velocity constant (rpm/v) 4065 1808 1105 611back EMF constant (mV/rpm) .246 .553 .905 1.635torque constant (oz-in/A) .333 .748 1.223 2.212armature inductance (mH) .085 .200 .750 3.00

ke

k

Back to control

Basic input / output relationship:

How to change the voltage?

We want a particular motor speed .

We can control the voltage applied V.

V = + ke R k

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

Open-loop vs. Close-loop Control

Open-loop Control:

actual speed

desired dV

Motor

a

actual speed a

- compute V from the 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

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

ProtectionMotor 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 Protection1. 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 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 analysis- of motor and coupling

Check noise

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

Brass cap

Tinned wire copper

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: 1. Electromagnet 2. Armature that can be attracted by the

electromagnet 3. Spring 4. Set of electrical contacts

Case study 1: How a relay works?

Case study 2: Under voltage trip

UV relay coil

Fuse

Circuit breaker

Generator

3-ph 440V bus bars

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

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

LL

PH II

I 577.03

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 single-

phasing - normal feature

• 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

• 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

Effect of single phasing (cont/..)

61

Willas-Array Solution for Motor Control

Control Unit

With Motor Control Cell

Feedback Signal

(Speed / Positon)

Power Management

Power Stage = 6 x IGBT or MOSFET

IPM Module3 x Driver IC

Gate Driver

Gate Driver

Gate Driver

Block Diagram 1

Others

Hall Sensor

(Inside Motor)

BLDCMotor

PWM Signal

AC Input

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