intro to elec drives.pdf

7/24/2019 intro to elec drives.pdf

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ELECTRIC DRIVES

INTRODUCTION TO ELECTRIC

DRIVES

INTRODUCTION TO ELECTRIC DRIVES -MODULE 1

Electrical Drives

Drives are systems employed for motion control

Require prime movers

Drives that employ electric motors as

prime movers are known as Electrical Drives


Electrical Drives

About 50% of electrical energy used for drives

Can be either used for fixed speed or variable speed

75% -constant speed, 25% variable speed (expanding)

MEP 1522 will be covering variable speed drives


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Example on VSD application

motor pump

valve

Supply

Constant speed Variable Speed Dri ves

PowerIn

Power lossMainly in valve

Power out

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1


motor pump

valve

Supply

motorPEC pump

Supply


PowerIn

Power loss

Power out



Power outPowerIn


Power out

motor pump

valve

Supply

motorPEC pump

Supply




PowerIn

Power loss

PowerIn

Power out


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Conventional electric drives (variable speed)

Bulky

Ineff icient

inf lexible


Modern electric drives (With power electronic con verters)

Small

Efficient

Flexible


Modern electric drives

Inter-disciplinary

Several research area

Expanding

Machine design

Speed sensorless

Machine Theory

Non-linear control

Real-time control

DSP application

PFC

Speed sensorless

Power electronic converters

Utility interfaceRenewable energy


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Components in electric drives

e.g. Single drive - sensorless vector control from Hitachi



e.g. Multidrives system from ABB



Motors

DC motors - permanent magnet wound field

AC motors induction, synchronous (IPMSM, SMPSM),brushless DC

Applications, cost, environment

Power sources DC batteries, fuel cell, photovoltaic - unregulated

AC Single- three- phase utility, wind generator - unregulated

Power processor

To provide a regulated power supply

Combination of power electronic converters

More efficient

Flexible

Compact AC-DC DC-DC DC-AC AC-AC


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Control unit

Complexity depends on performance requirement

analog- noisy, inflexible, ideally has infinite bandwidth. digital immune to noise, configurable, bandwidth is smaller than

the analog controllers DSP/microprocessor flexible, lower bandwidth - DSPs perform

faster operation than microprocessors (multiplication in single

cycle), can perform complex estimations


Overview of AC and DC drives

Extracted from Boldea & Nasar



DC motors: Regular maintenance, heavy, expensive, speed limit

Easy control, decouple control of torque and flux

AC motors : Less maintenance, light, less expensive, high speed

Coupling between torque and flux variable

spatial angle between rotor and stator flux


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Before semiconductor devices w ere introdu ced (


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Elementary principl es of mechanics

M

v

Fm

Ff

dt

MvdFF fm

Newtons law

Linear motion, constant M

First order differential equation for speed

Second order differential equation for displacement

Ma

dt

xdM

dt

vdMFF

2

2

fm

x



First order differential equation for angular frequency (or velocity)

Second order differential equation for angle (or position)

2

2

m

le

dt

dJ

dt

dJTT

With constant J,

Rotational motion

- Normally is the case for electrical drives

dt

JdTT mle

Te , m

Tl

J


dt

dJTT mle

For constant J,

dt

dJ m

Torque dynamic present during speed transient

d

d m Angular acceleration (speed)

The larger the net torque, the faster the acceleration is.

0. 19 0. 2 0.21 0.22 0.23 0.24 0.25-200

-100

0

100

200

speed(rad/s)

0.19 0. 2 0.21 0.22 0.23 0.24 0.25

0

5

10

15

20

torque(Nm)

Elementary principles of mechanics


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dt

vdMFF le

Combination of rotational and translational motions

r r

Te,

Tl

Fl Fe

v

M

Te = r(Fe), Tl = r(Fl), v =r

dt

dMrTT 2

le

r2M -Equivalent moment inertia of the

linearly moving mass


Elementary principles of mechanics effect of gearing

Motors designed for high speed are smaller in size and volume

Low speed applications use gear to utilize high speed motors

Mot orTe

Load 1,

Tl1

Load 2,

Tl2J 1

J 2

mm1

m2

n1

n2


Mot orTe

Load 1,

Tl1

Load 2,

Tl2J 1

J 2

mm1

m2

n1

n2

Mot orTe

J equ

Equivalent

Load , Tlequ

m2

2

21equ JaJJ

Tlequ = Tl1 + a2Tl2

a2 = n1/n2

Elementary principles of mechanics effect of gearing


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Motor steady state torque-speed characteristic

Synchronous mch

Induction mch

Separately / shunt DC mch

Series DC

SPEED

TORQUE

By using power electronic converters, the motor characteristiccan be change at will


Load steady state torque-speed characteristic

SPEED

TORQUE

Frictional torque (passive load) Exist in all motor-load drive

system simultaneously

In most cases, only one or two

are dominating

Exists when there is motion

T~ C

Coulomb friction

T~

Viscous friction

T~2

Friction due to turbulent flow

TL

Te

Vehicle drive



Constant torq ue, e.g. gravitational torque (active load)

SPEED

TORQUE

Gravitational torque

gM

FL

TL = rFL = r g M sin


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Hoist drive

Speed

Torque

Gravitational torque


Load and motor steady state torque

At constant speed, Te= TlSteady state speed is at point of intersection between Te and Tl of the

steady state torque c haracteristics

TlTe

Steady state

speed

r

Torque

Speedr2r3 r1


Torque and speed profil e

10 25 45 60 t (ms)

speed

(rad/s)

100

The system is described by: Te Tload = J(d/dt) + B

J = 0.01 kg-m2, B = 0.01 Nm/rads-1 and Tload = 5 Nm.

What is the torque profile (torque needed to be produced) ?

Speed profile


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Torque and speed profil e

10 25 45 60 t (ms)

speed

(rad/s)

100

0 < t


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Thermal con siderations

Unavoidable power losses causes temperature increase

Insulation used in the windings are classified based on the

temperature it can withstand.

Motors must be operated within the allowable maximum temperature

Sources of power losses (hence temperature increase):

- Conductor heat losses (i2R)

- Core losses hysteresis and eddy current- Friction losses bearings, brush windage


Thermal considerations

Electrical machines can be overloaded as long their temperature

does not exceed the temperature limit

Accurate prediction of temperature distribution in machines is

complex hetrogeneous materials, complex geometrical shapes

Simplified assuming machine as homogeneous body

p2p1 Thermal capacit y, C (Ws/oC)Surface A, (m2)

Surface temperature, T (oC)Input heat power

(losses)

Emitted heat power

(convection)

Amb ient tem perat ure, To



Power balance:

21 ppdt

dTC

Heat transfer by convection:

)TT(Apo2

C

pT

C

A

dt

Td 1

Which gives:

/th e1A

pT

A

C

, where

WithT(0) = 0 and p1 = ph = constant ,

, where is the coefficient of heat transfer


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t

T

t

/te)0(TT

T

/th e1A

pT

Heating transient

Cooling transient

A

ph

0T



The duration of overloading depends on the modes of operation:

Continuous duty

Short time intermittent duty

Periodic i ntermittent duty

Continuous duty

Load torque is constant over extended period multiple

Steady state temperature reached

Nominal output power chosen equals or exceeds continuous load

T

t

A

p n1

p1n

Losses due to continuous load




Operation considerably less than time constant,

Motor allowed to cool before next cycle

Motor can be overloaded until maximum temperature reac hed


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t1




A

p s1

maxT A

p n1

t

T

p1

p1n

p1s

t1




t

T

/ts1 e1A

pT

maxT A

p n1

/ts1n1 1e1A

p

A

p /ts1n1 1e1pp1

/tn1

s1

te1

1

p

p1



Periodic intermittent duty

Load cycles are repeated periodically

Motors are not allowed to completely cooled

Fluctuations in temperature until steady state temperature is reached


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Periodic intermittent duty

p1

t

heating coollingcoolling

coolling

heating

heating




Example of a simple case p1 rectangular periodic pattern

pn = 100kW, nominal powerM = 800kg

= 0.92, nominal efficiency

T= 50oC, steady state temperature rise due to pn

kW911

pp n1

Also, C/W180

50

9000

T

pA o1

If we assume motor is solid iro n of specific heat cFE=0.48 kWs/kgoC,thermal capacity C is given by

C = cFE M = 0.48 (800) = 384 kWs/ oC

Finally , thermal time constant = 384000/180 = 35 minutes




Example of a simple case p1 rectangular periodic pattern

For a duty cycle of 30% ( period of 20 mins), heat losses of twic e the nominal,

0 0.5 1 1.5 2 2.5

x 104

0

5

10

15

20

25

30

35


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Torque-speed quadrant of operation

T

12

3 4

T +ve +ve

Pm +ve

T -ve +ve

Pm -ve

T -ve

-ve

Pm +ve

T +ve

-ve

Pm -ve


4-quadrant operation

m

Te

Te

m

Tem

Te

m

T

Direction of positive (forward)

speed is arbitrary chosen

Direction of positive torque will

produce positive (forward) speed

Quadrant 1

Forward motoringQuadrant 2

Forward braking

Quadrant 3

Reverse motoring

Quadrant 4

Reverse braking


Ratings of con verters and motors

Torque

Speed

Power limit forcontinuoustorque

Continuous

torquelimit

Maximum

speedlimit

Power limit for

transient torque

Transient

torquelimit


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Steady-state stability

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intro to elec drives.pdf