1. ELECTRIC DRIVES INTRODUCTION TO ELECTRIC DRIVES MODULE 1
Dr. Nik Rumzi Nik Idris
Dept. of Energy Conversion, UTM
2013

2. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Electrical Drives
Drives are systems that are used to perform some specific tasks, such as to move some loads or objects
Source of motion is from prime movers
Drives that employ electric motors as
prime movers are known as Electrical Drives

3. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
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 1523 will be covering variable speed drives

4. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Electrical Drives

5. Example on VSD application
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Example on VSD application
Constant speed
Variable Speed Drives
motor
pump
valve
Supply
Power In
Power loss
Mainly in valve
Power out

6. Example on VSD application
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Example on VSD application
Constant speed
Variable Speed Drives
motor
pump
valve
Supply
motor
PEC
pump
Supply
Power loss
Mainly in valve
Power out
Power In
Power In
Power loss
Power out

7. Example on VSD application
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Example on VSD application
Constant speed
Variable Speed Drives
motor
pump
valve
Supply
motor
PEC
pump
Supply
Power In
Power loss
Mainly in valve
Power out
Power loss
Power In
Power out

8. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Conventional electric drives (variable speed)
Variable speed drives without power electronics
3-phase
power supply AC
motor DC
Generator
Motor
Load
Bulky
Inefficient
inflexible

9. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Modern electric drives (With power electronic converters)
Input power
3-phase IM
Power
Electronic
Converters
Load
Unregulated
DC or AC
Feedback
Voltage, current,
speed, etc
References
Speed, torque,
position
Controller
Inter-disciplinary
Several research area
Expanding
Small
Efficient
Flexible

10. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Modern electric drives (With power electronic converters)
Power
Electronic
Converters
Configurations of Power Electronic Converters depend on:
Sources available
Drive Performance - applications
- Braking
- Response
- Ratings

11. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Modern electric drives (With power electronic converters)
Power
Electronic
Converters
Example of PE converters for high performance application:
AC – DC conversion
DC – AC conversion

12. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Modern electric drives (With power electronic converters)
Input power
3-phase IM
Power
Electronic
Converters
Load
Unregulated
DC or AC
Feedback
Voltage, current,
speed, etc
Controller
References
Speed, torque,
position

13. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Modern electric drives (With power electronic converters)
Controller

14. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Modern electric drives (With power electronic converters)
Controller
SCALAR CONTROL
Microprocessor/Microcontroller based
Less computational requirement
Low-medium performance
VECTOR CONTROL
Trend: DSP- based, FPGA
High computational requirement – real time torque, flux estimations
High performance

15. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Modern electric drives (With power electronic converters)
Input power
3-phase IM
Power
Electronic
Converters
Load
Unregulated
DC or AC
Feedback
Voltage, current,
speed, etc
sensors
Controller
References
Speed, torque,
position

16. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Modern electric drives (With power electronic converters)
Feedback
Voltage, current,
speed, etc
sensors

17. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Modern electric drives (With power electronic converters)
Feedback
Voltage, current,
speed, etc
sensors
Phase currents
Hall effect device
Control &
Protection
Observers
Torque
Torque sensor
Rotor Speed
Mech. speed sensor
DC link voltage
Hall effect device
Rotor
Speed
Control &
Protection
Torque

18. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Overview of AC and DC drives
Extracted from Boldea & Nasar

19. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Overview of AC and DC drives

20. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Overview of AC and DC drives
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

21. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
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 (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

22. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Overview of AC and DC drives
After semiconductor devices were introduced (1950s)

23. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
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

24. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Elementary principles of mechanics v x
Newton’s law Fm M Ff
Linear motion, constant M
First order differential equation for speed
Second order differential equation for displacement

25. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Elementary principles of mechanics 
Te , m Tl
Rotational motion
- Normally is the case for electrical drives J
First order differential equation for angular frequency (or velocity)
Second order differential equation for angle (or position)
With constant J,

26. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Elementary principles of mechanics

27. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Elementary principles of mechanics
For constant J,
Torque dynamic – present during speed transient
Angular acceleration
Larger net torque and smaller J gives faster acceleration

28. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Elementary principles of mechanics
Driving power
Load power
Change in KE
A step change in speed requires an infinite driving power
Therefore  is a continuous variable

29. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Elementary principles of mechanics
Integrating the equation with time and setting the initial speed (0) = 0, we obtain the following:

30. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Elementary principles of mechanics
A drive system that require fast acceleration must have
large motor torque capability
small overall moment of inertia
As the motor speed increases, the kinetic energy also increases. During deceleration, the dynamic torque changes its sign and thus helps motor to maintain the speed. This energy is extracted from the stored kinetic energy:
J is purposely increased to do this job !

31. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Torque-speed quadrant of operation  1 2
T -ve
+ve
Pm -ve
T +ve
+ve
Pm +ve
Quadrant of operation is defined by the speed and torque of the motor
Most rotating electrical machines can operate in 4 quadrants
Not all converters can operate in 4 quadrants T 3 4
T -ve
-ve
Pm +ve
T +ve
-ve
Pm -ve

32. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Torque-speed quadrant of operation  Te m m Te
Quadrant of operation is defined by the speed and torque of the motor
Most rotating electrical machines can operate in 4 quadrants
Not all converters can operate in 4 quadrants
Quadrant 2
Forward braking
Quadrant 1
Forward motoring T Te Te m m
Quadrant 3
Reverse motoring
Quadrant 4
Reverse braking

33. Elementary principles of mechanics
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Elementary principles of mechanics
Combination of rotational and translational motions r 
Te,  Tl Fl Fe v M
Te = r(Fe), Tl = r(Fl), v =r
r2M - Equivalent moment inertia of the
linearly moving mass

34. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
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
Motor Te
Load 1, Tl1
Load 2, Tl2 J1 J2 m
m1
m2 n1 n2

35. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Elementary principles of mechanics – effect of gearing
Motor Te
Load 1, Tl1
Load 2, Tl2 J1 J2 m
m1
m2 n1 n2
Motor Te
Jequ
Equivalent Load , Tlequ m
Tlequ = Tl1 + a2Tl2
a2 = n1/n2=2/1

36. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Motor steady state torque-speed characteristic (natural 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

37. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Load steady state torque-speed characteristic
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
SPEED
T~ C
Coulomb friction
T~ 2
Friction due to turbulent flow
T~ 
Viscous friction
TORQUE

38. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Load steady state torque-speed characteristic
Constant torque, e.g. gravitational torque (active load)
SPEED
TORQUE
Gravitational torque  TL Te
Vehicle drive FL gM
TL = rFL = r g M sin 

39. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Load steady state torque-speed characteristic
Hoist drive
Speed
Torque
Gravitational torque

40. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
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 Tl Te
Torque
r2
r3
r1
Steady state
speed r
Speed

41. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Torque and speed profile 10 25 45 60
t (ms)
speed
(rad/s)
100
Speed profile
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) ?

42. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Torque and speed profile 10 25 45 60
t (ms)
speed
(rad/s)
100
0 < t <10 ms Te = 0.01(0) (0) + 5 Nm = 5 Nm
10ms < t <25 ms Te = 0.01(100/0.015) +0.01( t) + 5
= ( t) Nm
25ms < t< 45ms Te = 0.01(0) (100) + 5 = 6 Nm
45ms < t < 60ms Te = 0.01(-100/0.015) ( t) + 5
= – 66.67t

43. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Torque and speed profile
speed
(rad/s)
100
Speed profile 10 25 45 60
t (ms)
Torque
(Nm)
72.67
torque profile
71.67 6 5 10 25 45 60
t (ms)
-60.67
-61.67

44. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Torque and speed profile
Torque
(Nm) 70
J = kg-m2, B = 0.1 Nm/rads-1 and Tload = 5 Nm. 6 10 25 45 60
t (ms)
-65
For the same system and with the motor torque profile given above, what would be the speed profile?

45. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
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

46. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
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
Ambient temperature, To p1
Thermal capacity, C (Ws/oC)
Surface A, (m2)
Surface temperature, T (oC) p2
Emitted heat power
(convection)
Input heat power
(losses)

47. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
Power balance:
Heat transfer by convection:
, where  is the coefficient of heat transfer
Which gives:
With T(0) = 0 and p1 = ph = constant ,
, where

48. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
Heating transient  t
Cooling transient t 

49. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
The duration of overloading depends on the modes of operation:
Continuous duty
Load torque is constant over extended period multiple
Steady state temperature reached
Nominal output power chosen equals or exceeds continuous load
Continuous duty
Short time intermittent duty
Periodic intermittent duty
Losses due to continuous load t
p1n 

50. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
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

51. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
Short time intermittent duty
p1s p1
p1n  t1 t

52. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
Short time intermittent duty  t1 t

53. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
Periodic intermittent duty
Load cycles are repeated periodically
Motors are not allowed to completely cooled
Fluctuations in temperature until steady state temperature is reached

54. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
Periodic intermittent duty p1
heating
coolling t

55. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
Periodic intermittent duty
Example of a simple case – p1 rectangular periodic pattern
pn = 100kW, nominal power
M = 800kg
= 0.92, nominal efficiency
T= 50oC, steady state temperature rise due to pn
Also,
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 = /180 = 35 minutes

56. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
Periodic intermittent duty
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,

57. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Ratings of converters and motors
For fast acceleration, motor torque has to be much larger than the load torque
Transient torque limit can be several times motor rated torque because of the large thermal capacity of the machine
Transient torque limit cannot exceed the device ratings because of the small thermal capacity of the device
Continuous torque limit is determined by the motor ratings
To protect motor from continuous overloading, thermal protection mechanism has to be used.

58. INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Ratings of converters and motors
Torque
Power limit for transient torque
Transient torque limit
Continuous torque limit
Power limit for continuous torque
Maximum
speed limit
Speed