Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB

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CHAPTER VMotor Drives

• Motor drive systems definitions

• Review of motor principles

• Mechanical Requirements of Motor Drives

Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB

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• DEFINITION: Electric drives for motor is used to draw electrical energy from the mains and supply the electrical energy to the motor at whatever voltage, current and frequency necessary to achieve the desired mechanical output.

• General arrangement for variable speed drive.

Controller PowerElectronicsConverter

ProcessControl

Computer

Motor Process/Load

Power Source

Electric Motor Drives

Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB

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Example of Motor Drive System: Air Conditioning System

SystemController

PowerElectronicsConverter

Motor Airconditioner

Power Source

BuildingCooling

Desiredtemperature

Indoorsensors

Indoor temperatureand humidity

Temperature andhumidity

Desiredhumidity

Variable speed drive

• Three Main Elements of Motor Drives:– Electric Motor (DC, AC, SRM, Stepper)

– Load Type

– Control

• Requirements of Drives:– Variable speed

– Controllable Torque

Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB

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Motor: Review of general principles

wirecarryingcurrent oflength : Area;:

:Force

:DensityFlux

(I)Current Finger Middle )( (Flux) Field MagneticFinger Pointing

(F) Force Thumb Rule Hand"Left "

lA

IlBFA

B

FORCE

CURRENT

MAGNETIC FIELD

Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB

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Flux in C-core

++++++++++

----------

Leakage flux

iron

air-gap

+

-

Current into the paper

Current out of the the paper

NOTE: Use right hand "Screw Rule" to determine the direction of flux

NI

MMFFlux

R

VI

EMFCurrent

Reluctance

Resistance

analogy magnetic-Electric

Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB

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Air-gap flux densities

MMF=NI

Area A

g

1.8T)-(1.6iron theof saturation by the limited isdensity flux The

gap. theoflength theand (NI) coil theof MMF theknowingonly by calculated becan density flux gap-air that Note

,

gap),-air (in theair of Reluctance

o

o

o

g

NI

AB

g

NIAMMF

ThenA

g

Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB

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Evolution of motor geometry

++++++

++++

--

--

--

--

--

++

--

++++++++++

----------

C--Core Split air-gap

Simple motorgeometry

Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB

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Torque Production

++

++ + + + ++++

...........

Force

Force

+ current into the paper

. current out of the paper

TORQUE PRODUCTION

"SLOTTING"

Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB

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M

F

dt

dua

Madt

duMMu

dt

dFFF

M

leM

where

:motionLinear

Mechanical System requirements for drives: Linear motion

Mf e f L

x

Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB

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dt

dJT

dt

d

JT

J

rFT

m

m

Then,

:i.eon,accelaratiangular theis

,inertia ofmoment For

.

Rotating motion

r

f

torque

Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB

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Motor and load interaction

Motor Load

TL

Tem

Jeq

m

Jeq = combined load–motor moment of inertia

m = motor speed (rad/s)

TL = load torque

Tem = motor torque

eq

Jm

eq

Lem

LemJ

L

em

J

T

dt

d

J

TT

TTT

T

T

,accelerate toload theandmotor theof )( inertia

combined thecauses andbetween difference The

NetTorque:

(drag)resistance

windandfriction bearing theplus,LoadTorque:

motor by the produced eneticTorquElectromag:

Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB

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Example(1)

Using the motor-load structure as above, assume the motor has a combined inertia of 0.058kgm2. The load torque is negligible. Calculate the required electromagnetic torque if the speed is to be increased from standstill to 1,800rpm in 5 sec.

mNdt

dJT

radrotation

rpmtdt

d

meqem

mm

.19.17.37058.0

sec/7.372

sec60

min1

sec5

)01800( 2

Motor Load

TL

Tem

Jeq

m

Jeq = combined load–motor moment of inertia

m = motor speed (rad/s)

TL = load torque

Tem = motor torque

Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB

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Example(2)

The speed profile of the rotating system shown in previous example. (Jeq=0.058kgm2) is shown below. Assume the load torque is 5 N.m. Calculate and plot, as a function of time the electromagnetic torque required from the motor

0.2

0.1 0.3 0.4 0.5 t(s)

speed(rad/s)

300

–300

SectionB

Section A

Section C

Section D

Section E

Section F

Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB

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Torque and Power

rotation. of speedangular theis where

:as expressed becan power then , time

aldifferenti din place akesrotation t ldifferntia thisIf

:is donework

aldifferenti the, angle aldifferenti aby rotate ocylinder t

thecauses net torque a if system, rotationalIn

dt

d

Tdt

dT

dt

dWp

dt

TddW

T

m

m

Td

Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB

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Friction

• Friction within motor and load appose rotation

• Examples:– Bearings (to support rotating structure)

– Air (drag/windage)

– Ditction (friction at zero speed)

• Moving objects:– Coulomb friction (independent of speed)

– Viscous friction (increases linearly with speed)

Power Electronics and Drives (Version 3: 2003): Dr. Zainal Salam, UTM-JB

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• Centrifugal (squared) Torque– Load torque is a function of speed– Example: Fans

Load Types

• Constant Torque

– Load torque is independent of speed

– Example: Low speed hoist, elevator

Torque

Speed

Torque

Speed