ELECTRICAL DRIVES:

An Application of Power Electronics

Presented by

R.Reddy Prasad
Assistant professor
 Power Electronic Systems

What is Power Electronics ?

A field of Electrical Engineering that deals with the application of

power semiconductor devices for the control and conversion of
electric power

sensors
Input
Source Power Electronics Load
- AC Converters
- DC Output
- unregulated - AC
- DC
POWER ELECTRONIC
CONVERTERS – the
heart of power in a power
Reference Controller electronics system
 Power Electronic Systems

Why Power Electronics ?

sensors
Input
Source Power Electronics IDEALLY LOSSLESS
Load !
- AC Converters
- DC Output
- unregulated - AC
- DC

Reference Controller
 Modern Electrical Drive Systems

• 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)

• Variable speed drives typically used PEC to supply the motors

DC motors (brushed) AC motors

SRM - IM
BLDC - PMSM
 Modern Electrical Drive Systems

Classic Electrical Drive for Variable Speed Application :

• Bulky
• Inefficient
• inflexible
 Modern Electrical Drive Systems

Typical Modern Electric Drive Systems

Power Electronic Converters Electric Motor

Electric Energy Electric Energy Electric Mechanical
- Unregulated - - Regulated - Energy Energy

POWER IN Power
Electronic Motor Load
Converters

feedback

Reference
Controller
 Modern Electrical Drive Systems
Example on VSD application

Constant speed Variable Speed Drives

valve

Supply
motor pump

Power out

Power
In

Power loss
Mainly in valve
 Modern Electrical Drive Systems
Example on VSD application

Constant speed Variable Speed Drives

valve

Supply Supply
motor pump motor
PEC pump

Power out
Power out
Power
Power
In
In

Power loss
Power loss
Mainly in valve
 Modern Electrical Drive Systems
Example on VSD application

Constant speed Variable Speed Drives

valve

Supply Supply
motor pump motor
PEC pump

Power out
Power out
Power
Power
In
In

Power loss
Power loss
Mainly in valve
 Modern Electrical Drive Systems
Example on VSD application

Electric motor consumes more than half of electrical energy in the World.

Fixed speed Variable speed

Improvements in energy utilization in electric motors give large

impact to the overall energy consumption

HOW ?
Replacing fixed speed drives with variable speed drives
Using the high efficiency motors

Improves the existing power converter–based drive systems

 Modern Electrical Drive Systems

Overview of AC and DC drives

DC drives: Electrical drives that use DC motors as the prime mover

Regular maintenance, heavy, expensive, speed limit
Easy control, decouple control of torque and flux

AC drives: Electrical drives that use AC motors as the prime mover

Less maintenance, light, less expensive, high speed

Coupling between torque and flux – variable spatial angle

between rotor and stator flux
 Modern Electrical Drive Systems

Overview of AC and DC drives

Before semiconductor devices were introduced (<1950)

• AC motors for fixed speed applications
• DC motors for variable speed applications

After semiconductor devices were introduced (1960s)

• 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
 Modern Electrical Drive Systems

Overview of AC and DC drives

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
 Modern Electrical Drive Systems

Overview of AC and DC drives

Extracted from Boldea & Nasar

 Power Electronic Converters in ED Systems
Converters for Motor Drives
(some possible configurations)

DC Drives AC Drives

AC Source DC Source AC Source DC Source

DC-AC-DC DC-DC

AC-DC AC-DC-DC AC-DC-AC AC-AC DC-AC DC-DC-AC

Const. Variable NCC FCC

DC DC
 Power Electronic Converters in ED Systems

Converters for Motor Drives

Configurations of Power Electronic Converters depend on:

Sources available

Type of Motors

Drive Performance - applications

- Braking
- Response
- Ratings
 Power Electronic Converters in ED Systems
DC DRIVES

Available AC source to control DC motor (brushed)

AC-DC AC-DC-DC

Uncontrolled Rectifier
Single-phase Control
Control
Three-phase
Controlled Rectifier DC-DC Switched mode
Single-phase 1-quadrant, 2-quadrant
Three-phase 4-quadrant
 Power Electronic Converters in ED Systems
DC DRIVES
AC-DC
400

200

+ 2Vm
Vo  cos 
-200


-400
0.4 0.405 0.41 0.415 0.42 0.425 0.43 0.435 0.44

50Hz Vo 10

1-phase Average voltage

 over 10ms 5

0
0.4 0.405 0.41 0.415 0.42 0.425 0.43 0.435 0.44

500

50Hz
+ -500
3-phase 0.4 0.405 0.41 0.415 0.42 0.425 0.43 0.435 0.44
3VLL,m
Vo Vo  cos 

30

20

 Average voltage
over 3.33 ms 10

0
0.4 0.405 0.41 0.415 0.42 0.425 0.43 0.435 0.44
 Power Electronic Converters in ED Systems
DC DRIVES
AC-DC

+
3-
phase 3-phase
Vt supply
supply


Q2 Q1

Q3 Q4
T
 Power Electronic Converters in ED Systems
DC DRIVES
AC-DC

F1 R1

3-phase
supply
+ Va -
R2 F2

Q2 Q1

Q3 Q4
T
 Power Electronic Converters in ED Systems
DC DRIVES
AC-DC

Cascade control structure with armature reversal (4-quadrant):

iD

ref + Speed
iD,ref + Current Firing
controller Controller Circuit
_
_

iD,ref
Armature
iD, reversal
 Power Electronic Converters in ED Systems

Drawbacks of DC motor Controls

• Very high cost

• Heavy weight
• Losses are high due to commutation and brushes
• Regular Maintenance Required
• We need a converter to convert ac-dc
• Converter controller was costlier
• Low efficiency
 Modeling and Control of Electrical Drives
Modeling of the Power Converters: IM drives

INDUCTION MOTOR DRIVES

Scalar Control Vector Control

Const. V/Hz is=f(r) FOC DTC

Rotor Flux Stator Flux Circular Hexagon DTC

Flux Flux SVM
 Introduction
Scalar control of ac drives produces good steady state
performance but poor dynamic response. This manifests itself
in the deviation of air gap flux linkages from their set values.
This variation occurs in both magnitude and phase.

Vector control (or field oriented control) offers more precise

control of ac motors compared to scalar control. They are
therefore used in high performance drives where oscillations
in air gap flux linkages are intolerable, e.g. robotic actuators,
centrifuges, servos, etc.
 Modelling and Control of Electrical Drives
Current controlled converters in DC Drives - PI-based

i*a +
PI PWM

Converter
i*b +
PI PWM

i*c + PWM
PI

• Sinusoidal PWM

Motor
• Interactions between phases  only require 2 controllers
• Tracking error
 Modelling and Control of Electrical Drives
Current controlled converters in AC Drives - PI-based

i*a +
PI PWM

Converter
i*b +
PI PWM

i*c + PWM
PI

Motor
Modelling and Control of Electrical Drives

• Perform the 3-phase to 2-phase transformation

- only two controllers (instead of 3) are used

• Perform the control in synchronous frame

- the current will appear as DC
 Modelling and Control of Electrical Drives
Current controlled converters in AC Drives - PI-based

i*a

PI
SVM Converter
i*b
3-2 2-3
PI
i*c

3-2

Motor
 Modelling and Control of Electrical Drives
Current controlled converters in AC Drives - PI-based

va*
id* + PI
controller
 id
vb*
SVM
dqabc
or SPWM IM
iq* + VSI
PI vc*

 iq controller
s

Synch speed
s
estimator

abcdq
 Modelling and Control of Electrical Drives
Current controlled converters in AC Drives - PI-based

Stationary - ia Stationary - id
4 4

2 3

0 2

-2 1

-4 0
0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01 0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02

4 Rotating - ia Rotating - id
4

2 3

0 2

-2 1

-4 0
0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01 0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02
 Modelling and Control of Electrical Drives
Modeling of the Power Converters: IM drives

Control of induction machine based on steady-state model (per phase SS

equivalent circuit):

Is Lls Llr’
Rs Ir ’

+
+
Lm
Vs Rr’/s
Eag
– Im –
 Modelling and Control of Electrical Drives
Modeling of the Power Converters: IM drives
Te

Pull out
Torque Intersection point
(Tmax) (Te=TL) determines the
Te
steady –state speed

Trated TL

sm rotors
rated r
s
 Modelling and Control of Electrical Drives
Modeling of the Power Converters: IM drives

Given a load T– characteristic, the steady-state speed can be

changed by altering the T– of the motor:

Variable voltage (amplitude), variable

Pole changing
frequency (Constant V/Hz)
Synchronous speed change with no.
Using power electronics converter
of poles
Operated at low slip frequency
Discrete step change in speed

Variable voltage (amplitude), frequency

fixed
E.g. using transformer or triac
Slip becomes high as voltage reduced –
low efficiency
 Modelling and Control of Electrical Drives
Modeling of the Power Converters: IM drives

Variable voltage, fixed frequency

e.g. 3–phase squirrel cage IM
600
V = 460 V Rs= 0.25 
500 Rr=0.2  Lr = Ls =
0.5/(2*pi*50)
400
Lm=30/(2*pi*50)
Torque

300
f = 50Hz p=4

200 Lower speed  slip

higher
100
Low efficiency at low
speed
0
0 20 40 60 80 100 120 140 160
w (rad/s)
 Modelling and Control of Electrical Drives
Modeling of the Power Converters: IM drives
Constant V/Hz

To maintain V/Hz constant

Approximates constant air-gap flux when Eag is large

+ +
V Eag Eag = k f ag
_ _

 ag = constant 
E ag

V
f f
Speed is adjusted by varying f - maintaining V/f constant to avoid
flux saturation
 Modelling and Control of Electrical Drives
Modeling of the Power Converters: IM drives
Constant V/Hz

900

800

50Hz
700

30Hz
600

500
Torque

10Hz
400

300

200

100

0
0 20 40 60 80 100 120 140 160
 Modelling and Control of Electrical Drives
Modeling of the Power Converters: IM drives
Constant V/Hz

Vs

Vrated

frated f
 Modelling and Control of Electrical Drives
Modeling of the Power Converters: IM drives
Constant V/Hz

Rectifier
3-phase VSI
supply
C IM

f
Ramp Pulse
V Width
s* +
Modulator
 Modelling and Control of Electrical Drives
Modeling of the Power Converters: IM drives
Constant V/Hz

In1 Out 1

Subsystem
isd
Va
isq
Out1
ird
speed
0.41147 In 1 Out2 Vb
Vd
Scope
Step Slider Rate Limiter irq
Out3
Gain 1 Vq
Vc
Constant V /Hz Te

speed
Induction Machine

To Workspace 1
torque

To Workspace

Simulink blocks for Constant V/Hz Control

 Modelling and Control of Electrical Drives
Modeling of the Power Converters: IM drives
Constant V/Hz

200

100
Speed
0

-100
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

400

200

0
Torque

-200
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

200

100

0
Stator phase current

-100
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
 Modelling and Control of Electrical Drives
Modeling of the Power Converters: IM drives

Problems with open-loop constant V/f

At low speed, voltage drop across stator impedance is significant

compared to airgap voltage - poor torque capability at low speed

Solution:
1. Boost voltage at low speed
2. Maintain Im constant – constant ag
 Modelling and Control of Electrical Drives
Modeling of the Power Converters: IM drives

700

600 50Hz

500 A low speed, flux falls below

the rated value
400
30Hz
Torque

300

10Hz
200

100

0
0 20 40 60 80 100 120 140 160
 Modelling and Control of Electrical Drives
Modeling of the Power Converters: IM drives

With compensation (Is,ratedRs)

700

• Torque deteriorate at low

600 frequency – hence
compensation commonly
500
performed at low
frequency
400
Torque

• In order to truly
300
compensate need to
measure stator current –
200
seldom performed
100

0
0 20 40 60 80 100 120 140 160
 Modelling and Control of Electrical Drives
Modeling of the Power Converters: IM drives

With voltage boost at low frequency

Vrated

Linear offset

Non-linear offset – varies with Is

Boost

frated
 Modelling and Control of Electrical Drives
Modeling of the Power Converters: IM drives

Problems with open-loop constant V/f

Poor speed regulation

Solution:
1. Compesate slip
2. Closed-loop control
 Modelling and Control of Electrical Drives
Modeling of the Power Converters: IM drives

Rectifier
3-phase VSI
supply
C IM

f
Ramp Pulse
+ Width
s* + V
Modulator
+ +

Vboost
Slip speed
calculator

Vdc Idc
 Modelling and Control of Electrical Drives
Modeling of the Power Converters: IM drives

A better solution : maintain ag constant. How?

ag, constant → Eag/f , constant → Im, constant (rated)

Controlled to maintain Im at rated

Is Lls
Rs Llr’ Ir ’

+ +
Lm Rr’/s
Vs Eag
maintain at rated
Im
– –
 Modelling and Control of Electrical Drives
Modeling of the Power Converters: IM drives

Constant air-gap flux

900

800

50Hz
700

30Hz
600

500
Torque

10Hz
400

300

200

100

0
0 20 40 60 80 100 120 140 160
 Modelling and Control of Electrical Drives
Modeling of the Power Converters: IM drives

Constant air-gap flux

Rr
j L lr   r 
Im  s Is j slip  Tr  1
R
j (L lr  L m )  r  1 r 
s Is  Im,
j slipTr  1
Rr
j L r 
Im  s Is
   R • Current is controlled using current-
j  r L r  r controlled VSI
 1  r  s

• Dependent on rotor parameters –

jslipTr  1 sensitive to parameter variation
Im  Is ,
  
jslip  r Tr  1
 1  r 
 Modelling and Control of Electrical Drives
Modeling of the Power Converters: IM drives

Constant air-gap flux

3-phase VSI
Rectifier
supply
C IM

Current
controller

* + slip |Is|
PI
-
+
s
r
+
 Brushless DC Motor

• No Commutators

• Position of Coils with respect to the magnetic

field is sensed electronically.

• Current is commutated through electronic

switches to appropriate phases.
SENSORED BLDC MOTOR
How it Works

• Halls Sensors sense the

position of the coils

• The Decoder Circuit turns

appropriate switches on
and off

• The voltage through the

specific coils turns the
motor
THANK YOU