Electric Drives

ELECTRIC DRIVES

INTRODUCTION TO ELECTRIC DRIVESMODULE 1

Dr. Nik Rumzi Nik IdrisDept. of Energy Conversion, UTM

2006

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

Example on VSD application

motor pump

valve

Supply

Constant speed Variable Speed Drives

PowerIn

Power lossMainly in valve

Power out



motor pump

valve

SupplymotorPEC pump

Supply


PowerIn

Power loss

Power out



Power outPowerIn


Power out

motor pump

valve

SupplymotorPEC pump

Supply




PowerIn

Power loss

PowerIn

Power out


Conventional electric drives (variable speed)

• Bulky

• Inefficient

• inflexible


Modern electric drives (With power electronic converters)

• Small

• Efficient

• Flexible


Modern electric drives

• Inter-disciplinary

• Several research area

• Expanding

Machine designSpeed sensorlessMachine Theory

Non-linear controlReal-time controlDSP applicationPFCSpeed sensorless Power electronic converters

Utility interfaceRenewable energy


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



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 controller’s • 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



Before semiconductor devices were introduced (<1950)• AC motors for fixed speed applications• DC motors for variable speed applications

After semiconductor devices were introduced (1950s)

• Variable frequency sources available – AC motors in variable speed applications

• Coupling between flux and torque control• Application limited to medium performance applications –

fans, blowers, compressors – scalar control

• High performance applications dominated by DC motors – tractions, elevators, servos, etc



After vector control drives were introduced (1980s)

• AC motors used in high performance applications – elevators, tractions, servos

• AC motors favorable than DC motors – however control is complex hence expensive

• Cost of microprocessor/semiconductors decreasing –predicted 30 years ago AC motors would take over DC motors


Classification of IM drives (Buja, Kamierkowski, “Direct torque control of PWM inverter-fed AC motors - a survey”,

IEEE Transactions on Industrial Electronics, 2004.


Elementary principles of mechanics

M

v

Fm

Ff

dtMvd

FF fm

Newton’s law

Linear motion, constant M

• First order differential equation for speed• Second order differential equation for displacement

Ma

dtxd

Mdtvd

MFF 2

2

fm

x



• First order differential equation for angular frequency (or velocity)• Second order differential equation for angle (or position)

2

2m

le dtd

Jdt

dJTT

With constant J,

Rotational motion

- Normally is the case for electrical drives

dtJd

TT mle

Te , m

Tl

J


dtd

JTT mle

For constant J,

dt

dJ m

Torque dynamic – present during speed transient

dt

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

spee

d (r

ad/s

)

0.19 0.2 0.21 0.22 0.23 0.24 0.250

5

10

15

20

torq

ue (

Nm

)




dtvd

MFF le

Combination of rotational and translational motions

r r

Te,

Tl

Fl Fe

v

M

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

dtd

MrTT 2le

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

MotorTe

Load 1, Tl1

Load 2, Tl2

J1

J2

mm1

m2

n1

n2


MotorTe

Load 1, Tl1

Load 2, Tl2

J1

J2

mm1

m2

n1

n2

MotorTe

Jequ

Equivalent Load , Tlequ

m2

221equ JaJJ

Tlequ = Tl1 + a2Tl2

a2 = n1/n2

Elementary principles of mechanics – effect of gearing


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 characteristic can 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 torque, e.g. gravitational torque (active load)

SPEED

TORQUE

Gravitational torque

gM

FL

TL = rFL = r g M sin



Hoist drive

Speed

Torque

Gravitational torque


Load and motor steady state torque

At constant speed, Te= Tl Steady state speed is at point of intersection between Te and Tl of the steady state torque characteristics

TlTe

Steady state speed

r

Torque

Speedr2r3r1


Torque and speed profile

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



10 25 45 60 t (ms)

speed (rad/s)

100

0 < t <10 ms Te = 0.01(0) + 0.01(0) + 5 Nm = 5 Nm

10ms < t <25 ms Te = 0.01(100/0.015) +0.01(-66.67 + 6666.67t) + 5 = (71 + 66.67t) Nm

25ms < t< 45ms Te = 0.01(0) + 0.01(100) + 5 = 6 Nm

45ms < t < 60ms Te = 0.01(-100/0.015) + 0.01(400 -6666.67t) + 5 = -57.67 – 66.67t

le TBdtd

JT



10 25 45 60

speed (rad/s)

100

10 25 45 60

Torque (Nm)

72.6771.67

-60.67

-61.67

56

t (ms)

t (ms)

Speed profile

torque profile



10 25 45 60

Torque (Nm)

70

-65

6

t (ms)

For the same system and with the motor torque profile given above, what would be the speed profile?

J = 0.001 kg-m2, B = 0.1 Nm/rads-1 and Tload = 5 Nm.


Thermal considerations

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



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 capacity, C (Ws/oC)

Surface A, (m2)Surface temperature, T (oC)Input heat power

(losses)

Emitted heat power(convection)

Ambient temperature, To



Power balance:

21 ppdtdT

C

Heat transfer by convection:

)TT(Ap o2

Cp

TC

Adt

Td 1

Which gives:

/th e1A

pT

AC

, where

With T(0) = 0 and p1 = ph = constant ,

, where is the coefficient of heat transfer



t

T

t

/te)0(TT

T

/th e1A

pT

Heating transient

Cooling transient

Aph

)0(T



The duration of overloading depends on the modes of operation:

Continuous duty Short time intermittent duty Periodic intermittent 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

Ap n1

p1n

Losses due to continuous load



Short time intermittent duty

Operation considerably less than time constant,

Motor allowed to cool before next cycle

Motor can be overloaded until maximum temperature reached

t1




Ap s1

maxT Ap n1

t

T

p1

p1n

p1s

t1




t

T

/ts1 e1A

pT

maxT Ap n1

/ts1n1 1e1A

pA

p /ts1n1

1e1pp1

/tn1

s1

te11

pp

1



Periodic intermittent duty

Load cycles are repeated periodically

Motors are not allowed to completely cooled

Fluctuations in temperature until steady state temperature is reached




p1

t

heating coollingcoolling

coolling

heating

heating




Example of a simple case – p1 rectangular periodic pattern

pn = 100kW, nominal powerM = 800kg= 0.92, nominal efficiencyT= 50oC, steady state temperature rise due to pn

kW911

pp n1

Also, C/W180

509000

Tp

A o1

If we assume motor is solid iron 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 twice the nominal,

0 0.5 1 1.5 2 2.5

x 104

0

5

10

15

20

25

30

35


Torque-speed quadrant of operation

T

12

3 4

T +ve +vePm +ve

T -ve +vePm -ve

T -ve -vePm +ve

T +ve -vePm -ve


4-quadrant operation

m

Te

Te

m

Te

m

Te

m

T

• Direction of positive (forward) speed is arbitrary chosen

• Direction of positive torque will produce positive (forward) speed

Quadrant 1Forward motoring

Quadrant 2Forward braking

Quadrant 3Reverse motoring

Quadrant 4Reverse braking


Ratings of converters and motors

Torque

Speed

Power limit for continuous torque

Continuous torque limit

Maximum speed limit

Power limit for transient torque

Transient torque limit


Steady-state stability

Electric Drives

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