ssd_unit_1.ppt

1
EE 1403 - SOLID STATE DRIVES
UNIT 1 - Fundamentals of Electric Drives

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)

Example on VSD application
motor pump
valve
Supply
Constant speed Variable Speed Drives
Power
In
Power loss
Mainly in valve
Power out

motor pump
valve
Supply
motor
PEC pump
Supply
Power
In
Power loss
Power out
Power loss
Mainly in valve
Power out
Power
In

Power loss
Mainly in valve
Power out
motor pump
valve
Supply
motor
PEC pump
Supply
Power
In
Power loss
Power
In
Power out

Conventional electric drives (variable speed)
• Bulky
• Inefficient
• inflexible

Modern electric drives (With power electronic converters)
• Small
• Efficient
• Flexible

BLOCK DIAGRAM OF ELECTRIC DRIVE

Components in electric drives
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

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Components in electric drives
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

AC-DC Converters or Rectifiers

AC-DC Converters or Rectifiers (Cont.)

VSI Controlled Inverter for IM Drive

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

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
characteristics can be changed 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
Tl
Te
Steady state
speed
r
Torque
Speed
r2
r3
r1

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
p2
p1
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:
2
1 p
p
dt
dT
C 

Heat transfer by convection:
)
T
T
(
A
p o
2 


C
p
T
C
A
dt
T
d 1





Which gives:
 





 /
t
h
e
1
A
p
T
A
C



, where
With T(0) = 0 and p1 = ph = constant ,
, where  is the coefficient of heat transfer

t

T

t






 /
t
e
)
0
(
T
T
T

 





 /
t
h
e
1
A
p
T
Heating transient
Cooling transient
A
ph

)
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
A
p n
1


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 reached

t1

A
p s
1

max
T

A
p n
1

t
T

p1
p1n
p1s

t1
 t
T

 





 /
t
s
1
e
1
A
p
T
max
T

A
p n
1

 






/
t
s
1
n
1 1
e
1
A
p
A
p  



 /
t
s
1
n
1
1
e
1
p
p
1
/
t
n
1
s
1
t
e
1
1
p
p
1



 


Load cycles are repeated periodically
Motors are not allowed to completely cooled
Fluctuations in temperature until steady state temperature is reached

p1
t
heating coolling
coolling
coolling
heating
heating

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
kW
9
1
1
p
p n
1 










 Also, C
/
W
180
50
9000
T
p
A o
1






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 10
4
0
5
10
15
20
25
30
35

Type of Loads
• Load torque can be of two types
• Active load torque:- Active torques continues to act in the
same direction irrespective of the direction of the drive. e.g.
gravitational force or deformation in elastic bodies.
• Passive load torque:- the sense of the load torque changes
with the change in the direction of motion of drive. e. g. torques
due to friction, due to shear and deformation of inelastic bodies

Type of Loads (Cont.)
• It is a passive load to the motor.
• Load torque is independent of the
speed of the motor.
• Characterized by the requirement
of an extra torque at very near
zero speed.
• It is also known as break away
torque or stiction.

• Torque is directly proportional
to the speed.
• Calendaring machines, eddy
current brakes and separately
excited dc generators feeding
fixed resistance loads have such
characteristics.
Viscous Friction Load

• Load torque magnitude is proportional
to some power of speed.
• Centrifugal pumps, propeller in ships
or aeroplanes, fan or blower type of
load has such characteristics.
• For fan,
Fan type Load

Types of Load (Cont.)
• Hyperbolic speed-torque
characteristics, where load
torque is inversely
proportional to speed or
load power is constant.
Certain type of lathes,
boring machines, milling
machines, steel mill coilers
etc are having this type of
load characteristics.
Constant Power Load

• Load torques that vary with time
Load variation with time can be periodic and repetitive in certain
applications.
• One cycle of the load variation is called a duty cycle.
• The variation of load torque with time has a greater importance in
the selection of a suitable motor.
Classification of loads that vary with time:
(a) Continuous, constant loads: Centrifugal pumps or fans operating
for a long time under the same conditions, paper making machines
etc.
(b) Continuous, variable loads: Metal cutting lathes, hoisting
winches, conveyors etc.

• (c) Pulsating loads: Reciprocating pumps and compressors, frame
saws, textile looms and generally all machines having crank shaft.
(d) Impact loads: Apparent, regular and repetitive load peaks or
pulses which occurs in rolling mills, presses, shearing machines,
forging hammers etc. Drives for such machines will have heavy fly
wheels.
(e) Short time intermittent loads: Almost all forms of cranes and
hoisting mechanisms, excavators, roll trains etc.
(f) Short time loads: Motor generator sets for charging batteries,
servo motors used for remote control of clamping rods of drilling
machines.
Loads of the machines like stone crushers and ball mills are
characterized by frequent impact of small peaks so they are
classified as continuous variable loads rather than the impact loads

• One and the same machine can be represented by a load
torque which either varies with the speed or with the time.
• For example, a fan load whose load torque is proportional to
the square of the speed, is also a continuous, constant load.
• Load torque of a crane is independent of the speed and also
short time intermittent nature.
• Rocking pumps for petroleum have a load which vary with
angular position of the shaft, but also be classified as a
pulsating load.

Type of Load (Cont.
High speed Hoist Traction Load
(Constant torque; but with viscous friction)

Power requirement for different
Loads
• Mine Hoist Polishing
Machine

Loads (Cont.)
• Sheering machine for cutting Textile loom

Loads (Cont.)
• Planing Machine

Loads (Cont.)
• Drilling Machine Grinding
Machine

Dynamics of Motor-Load
Combination
• The motor and the load that it drives are
represented by the rotational system.
• The basic equation of the motor-load system
is,

Dynamics of Motor-Load Combination
• where is motor and load torque respectively in Nm, J is
• the moment of inertia and is the angular velocity in rad/sec.
• Motor torque is the applied torque and load torque is the resisting torque.
• Different states at which an electric drive causing rotational motion are
(i) :- The drive will be accelerating, in particular,
picking up speed to reach rated speed.
(ii) :- The drive will be decelerating and particularly, coming
to rest.
(iii) :- The motor will continue to run at the same speed, if it
were running or continue to be at rest, if it were running.

Quadrant diagram of Speed-Torque
Characteristics
• The speed is assumed to be positive if the direction of
rotation is anticlockwise or in such a way to cause an
‘upward’ or forward motion of the drive. For reversible drive
positive direction of the speed can be assumed arbitrarily
either clockwise or anticlockwise.
• The motor torque is positive if it produces increase in speed
in the positive sense. The load torque is assigned the
positive sign when it is directed against the motor torque.
• Plot of speed torque characteristics of the load/ motor for all
four quadrant of operation is known as quadrantal diagram.

Four Quadrant Operation
• Motor is driving a hoist consisting of a cage with or without load, a rope wound on to
a drum to hoist the cage and a balance weight of magnitude greater than that of the
empty cage but less than that of the loaded cage.
• The arrow in the figure indicates the actual directions of the motor torque, load torque
and motion in four quadrants.
• The load torque of the hoisting mechanism is of active type and assumed to be
constant due to negligible friction and windage for low speed hoist.
• Speed torque curve of the hoist is represented by vertical line passing through two
quadrants. Loaded hoist characteristics in first and fourth and unloaded in second
and third quadrants.
• In the first quadrant the load torque acts in the opposite direction to that of rotation.
Hence to drive the loaded hoist up, the motor developed torque must be in the
direction of the rotation or must be positive. The power will also be positive so, this
quadrant is known as ‘forward motoring quadrant’.

Four Quadrant Operation (Cont.)
•
Speed torque curve of the hoist is represented by vertical line passing through two quadrants. Loaded
hoist characteristics in first and fourth and unloaded in second and third quadrants.
• In the first quadrant the load torque acts in the opposite direction to that of rotation. Hence to drive the
loaded hoist up, the motor developed torque must be in the direction of the rotation or must be
positive. The power will also be positive so, this quadrant is known as ‘forward motoring quadrant’.
• The hoisting up of the unloaded cage is represented in the second quadrant. As the counterweight is
heavier than the empty cage, the speed at which hoist moves upwards may reach a very high value.
To avoid this, the motor torque must act in the opposite direction of rotation or motor torque must be
negative. The power will be negative though the speed is positive, so this quadrant is known as
‘forward braking quadrant’.
• The third quadrant represents the downward motion of the empty cage. Downward journey will be
opposed by torque due to counterweight and friction at the transmitting parts, move cage downwards
the motor torque should must be in the direction of the rotation. Electric machine acts as a motor but
in the reverse direction compared to first quadrant. The torque is negative as speed is increased I the
negative direction, but the power is positive, this quadrant is known as ‘Reverse motoring quadrant’.

Four Quadrant Operation (Cont.)
• Fourth quadrant has the downward motion
of the loaded cage. As loaded cage has
more weight than the balanced weight to
limit the speed of the motion, motor torque
must have opposite polarity with respect to
rotation and acts as a brake. The motor
torque sign is positive, but as speed has
negative direction; the power will be
negative, this quadrant is designated as
‘Reverse braking quadrant’

Torque-speed quadrant of operation

T
1
2
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
Te
m
Te
m

T
• Direction of positive (forward)
speed is arbitrary chosen
• Direction of positive torque will
produce positive (forward) speed
Quadrant 1
Forward motoring
Quadrant 2
Forward braking
Quadrant 3
Reverse motoring
Quadrant 4
Reverse 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 of an
Electric Drive
• The drive is said to be in equilibrium if the torque developed by the motor is exactly equal
to the load torque.
• If the drive comes out of the state of equilibrium due to some disturbance, it comes back to
steady state for stable equilibrium but for unstable equilibrium the speed of the drive
increases uncontrollably or decreases to zero. When the drive coming out of the state of
equilibrium preserves it steady state at different speed (lying in small range), it is said to be
in neutral range.
• The stability of the motor load combination is defined as the capacity of the system which
enables it to develop forces of such a nature as to restore equilibrium after any small
departure therefore.
• Equilibrium state of the drive mainly disturbs because of the following two types of
disturbances,
1.Changes from the state of equilibrium takes place slowly and the effect of either the
inertia or the inductance is insignificant – Steady state stability.
2.Sudden and fast changes from the equilibrium state so effect of both inertia and
inductance can not be neglected- Dynamic or transient stability

Electric Drive (Cont.)
• Criteria for steady state stability:-
• Let the equilibrium of the torques and
speed is and the small deviations
are After the displacement from the
equilibrium state the torque equation
becomes,

• Considering the small deviation, changes can be
expressed as a linear function of change in speed,
• From the torque equation, where all quantities are
expressed in terms of their deviations from the
equilibrium,

• Solution is,
• Where, is the initial value of the deviation
in speed. For the stable system the exponent must
be negative, so speed increment will disappear
with time. The exponent will always be negative if,

• Criteria for the steady state stability
is for a decrease in the speed the
motor torque must exceeds the
load torque and for increase in
speed the motor torque must be
less than the load torque.
• Load torque results in a
stable equilibrium point, and the
load torque results in an
unstable situation.

• To check the stability at an operating point of
the motor, if an increase in speed brings
greater increase in load torque than the motor
torque, the speed will tend to decrease and
return to its original value, so operating point
will be a stable point else operating point will
be an unstable point.
• Cases (a), (b) and (c) represents stable
operation of drive.
• Cases (d), (e) and (f) represents unstable
operation of drive.
• Case (g) represents indeterminate condition.
Various Speed and
Torque Curves of Motor
and Load

Steady-state stability

CURRENT SENSOR
88
V.MOHAN HOD/EEE EGSPEC

CLOSED LOOP SPEED CONTROL USING PLL
89
V.MOHAN HOD/EEE EGSPEC

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