Electric Drives

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Electric Drives

• Professor Mohammed Zeki Khedher
• Lecture One

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Introduction

• In some countries nearly 65 of the total
  electric energy produced is consumed by electric
  motors.

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Some Applications of Electric Drives

• Electric Propulsion
• Pumps, fans, compressors
• Plant automation
• Flexible manufacturing systems
• Spindles and servos
• Appliances and power tools
• Cement kilns
• Paper and pulp mills textile mills
• Automotive applications
• Conveyors, elevators, escalators, lifts

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1. ENERGY CONVERSION IN ELECTRIC DRIVES
1.1. ELECTRIC DRIVES - A DEFINITION
About 50 of electrical energy produced is used
in electric drives today. Electric drives may run
at constant speed (figure 1.1) or at variable
speed (figure 1.2).
Figure 1.1. Constant speed electric drive
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Figure 1.2. Variable speed electric drive
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1.2. APPLICATION RANGE OF ELECTRIC DRIVES A
summary of main industrial applications and power
range of electric drives is shown on figure 1.3.
Figure 1.3. Electric drives - variable speed
applications
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Overview of AC and DC drives
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Energy/Cost Savings

• System efficiency can be increased from 15 to
  27 by introducing variable-speed drive operation
  in place of constant-speed operation.
• For a large pump variable-speed drive, payback
  period 3-5 years whereas operating life is 20
  years.

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Electric Machines

• An engineer designing a high-performance drive
  system must have intimate knowledge about machine
  performance and Power Electronics

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Electric Machines (contd)

• DC Machines - shunt, series, compound, separately
  excited dc motors and switched reluctance
  machines
• AC Machines - Induction, wound rotor synchronous,
  permanent magnet synchronous, synchronous
  reluctance, and switched reluctance machines.
• Special Machines - switched reluctance machines

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Electric Machines (contd)

• All of the above machines are commercially
  available in fractional kW to MW ranges except
  permanent-magnet, synchronous, synchronous
  reluctance, and switched reluctance which are
  available up to 150 kW level.

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Selection Criteria for Electric Machines

• Cost
• Thermal Capacity
• Efficiency
• Torque-speed profile
• Acceleration
• Power density, volume of motor
• Ripple, cogging torques
• Peak torque capability

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• 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)

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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Example on VSD application
Variable Speed Drives
Constant speed
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Example on VSD application
Variable Speed Drives
Constant speed
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Example on VSD application
Variable Speed Drives
Constant speed
Power In
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Conventional electric drives (variable speed)

• Bulky
• Inefficient
• inflexible

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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Modern electric drives (With power electronic
converters)

• Small
• Efficient
• Flexible

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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Modern electric drives

• Inter-disciplinary
• Several research area
• Expanding

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Controllers

• Controllers embody the control laws governing
  the load and motor characteristics and their
  interaction.
• Controller

Torque/speed/ position commands
Vc, fc, start, shut-out, signals, etc.
Torque/speed/ position feedback
Thermal and other feedback
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Components in electric drives
e.g. Single drive - sensorless vector control
from Hitachi
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Components in electric drives
e.g. Multidrives system from ABB
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Components in electric drives

• Motors
• DC motors - permanent magnet wound field
• AC motors induction, synchronous , 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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• 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 controllers
  
• DSP/microprocessor flexible, lower bandwidth -
  DSPs perform faster operation than
  microprocessors (multiplication in single cycle),
  can perform complex estimations

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• DC Motors

• Advantage simple torque and speed control
  without sophisticated electronics

• Limitations

• Regular Maintenance

• Expensive motor

• Heavy motor

• Sparking

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DC DRIVES Vs AC DRIVES

• DC drives
• Advantage in control unit
• Disadvantage in motor
• AC Drives
• Advantage in motor
• Disadvantage in control unit

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1.3. ENERGY SAVINGS PAYS OFF RAPIDLY
Consider a real case when a motor pump system of
15kW works 300 days a year, 24 hours a day and
pumps 1200m3 of water per day. By on/off and
throttling control, only, the system uses
0.36kWh/m3 of pumped water to keep the pressure
rather constant for variable flow rate. Adding a
P.E.C., in the same conditions, the energy
consumption is 0.28kWh/m3 of pumped water, with a
refined pressure control. Let us consider that
the cost of electrical energy is 40fils/kWh. The
energy savings per year S is
S 1200 300 (0.36 -0.28) 0.04 /year 1152
JD/year Now the costs of a 15kW PWM - P.E.C. for
an induction motor is less than 4000JD. Thus, to
a first approximation, the loss savings only pay
off the extra investment in less than 4 years.
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Costs

• Power Electronics Controller costs approximately
  2 to 5 times AC motor
• Cost decreases with bigger size

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1.4. GLOBAL ENERGY SAVINGS THROUGH P.E.C. DRIVES
So far the energy savings produced by the P.E.C.
in variable speed drives have been calculated for
the drive only - P.E.C. and motor.
Figure 1.5. Primary energy consumption for
throttle / motor / pump system
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Figure 1.6. Primary energy consumption for P.E.C.
/ motor / pump systems
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Power consumption with flow
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Load

• The motor drives a load that has a
  characteristic torque vs. speed requirement.
• In general, load torque is a function of speed
  and can be written as
• Tl ? ?mx
• x1 for frictional systems (e.g. feed drives)
• x2 for fans and pumps

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General Torque Equation
Translational (linear) motion
F Force (Nm) M Mass (Kg ) v velocity (m/s)
Rotational motion
T Torque (Nm) J Moment of Inertia (Kgm2
) w angular velocity ( rad/s )
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Torque Equation Motor drives
Te motor torque (Nm)
TL Load torque (Nm)
Acceleration
Deceleration
Constant speed
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continue
Drive accelerates or decelerates depending on
whether Te is greater or less than TL
During acceleration, motor must supply not
only the load torque but also dynamic torque, (
Jdw/dt ).
During deceleration, the dynamic torque, ( Jdw/dt
), has a negative sign. Therefore, it assists the
motor torque, Te.
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Elementary principles of mechanics
v
x
Fm
M
Ff
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Elementary principles of mechanics
Rotational motion
- Normally is the case for electrical drives
J
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Elementary principles of mechanics
For constant J,
Torque dynamic present during speed transient
Angular acceleration (speed)
The larger the net torque, the faster the
acceleration is.
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Elementary principles of mechanics
Combination of rotational and translational
motions
Te r(Fe), Tl r(Fl), v r?
r2M - Equivalent moment inertia of the linearly
moving mass
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• 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
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Elementary principles of mechanics effect of
gearing
Tlequ Tl1 a2Tl2
a2 n1/n2
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1.6. MOTION / TIME PROFILE MATCH
Figure 1.8. Motion / time profilea.) speed b.)
position c.) required load torque
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Example 1.2. The direct drive torque / time
curve A direct drive has to provide a speed /
time curve such as in figure 1.9. against a
constant load torque of TL 10Nm, for a motor
load inertia J 0.02 kgm2.
Figure 1.9. Required speed / time profile
Neglecting the mechanical losses let us calculate
the motor torque (Te) / time requirements. The
motion equation for a direct drive is
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For the linear speed / time (acceleration -
deceleration) zones the speed derivative is
For the constant speed (cruising) zone
. Consequently the torque requirements from
the motor for the three zones are
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The motor torque / time requirements are shown
on figure 1.10.
Figure 1.10. Motor torque / time requirements
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Example 1.3. gear - box drive torque / time
curve Let us consider an electric drive for an
elevator with the data shown in figure 1.11.
Figure 1.11. Elevator electric drive with
multiple mechanical transmissions and
counterweight
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The motor rated speed nn 1550rpm. The
efficiency of gearing system is h 0.8. Let us
calculate the total inertia (reduced to motor
shaft), torque and power without and with
counterweight. First the motor angular speed wm
is
(1.12)
The gear ratios may be defined as speed ratios -
Wt /wm for J4J5 and Wd /wm for J6 (figure
1.11). Consequently the inertia of all rotating
parts Jr, reduced to the motor shaft, (figure
1.11), is
(1.13)
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For the cabin and the counterweight, the inertia,
reduced to motor shaft (Je) is
(1.14)
Thus the total inertia Jt is
(1.15)
In absence of counterweight the la of energy
conservation leads to
(1.16)
Consequently the motor torque, Tem, yields
(1.17)
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The motor electromagnetic power Pem is
(1.18)
On the other hand in presence of counterweight
(1.16) becomes
(1.19)
(1.20)
So the motor electromagnetic Pem is
(1.21)
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Motor steady state torque-speed characteristic
By using power electronic converters, the motor
characteristic can be change at will
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• 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
TORQUE
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Load steady state torque-speed characteristic
Constant torque, e.g. gravitational torque
(active load)
TL rFL r g M sin ?
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

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

Torque and speed profile
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) ?
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Torque and speed profile
0 lt t lt10 ms Te 0.01(0) 0.01(0) 5 Nm 5
Nm 10ms lt t lt25 ms Te 0.01(100/0.015)
0.01(-66.67 6666.67t) 5 (71
66.67t) Nm 25ms lt tlt 45ms Te 0.01(0)
0.01(100) 5 6 Nm 45ms lt t lt 60ms Te
0.01(-100/0.015) 0.01(400 -6666.67t) 5
-57.67 66.67t
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

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

Torque and speed profile
Torque (Nm)
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J 0.001 kg-m2, B 0.1 Nm/rads-1 and
Tload 5 Nm.
6
45
25
10
60
t (ms)
-65
For the same system and with the motor torque
profile given above, what would be the speed
profile?
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Torque Equation Graphical
Te
Speed
Forward running
Forward braking
Reverse acc.
Reverse running
Reverse braking
Forward acc.
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Load Torque
Load torque, TL, is complex, depending on
applications.
In general
TORQUE
TL k
TL kw
TL kw2
SPEED
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• 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
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Steady-state stability
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1.5.1. Typical load torque / speed
curves Typical load torque / speed curves are
shown on figure 1.7. They give a strong
indication of the variety of torque / speed
characteristics. Along such curves the mechanical
power required from the motor varies with speed.
Figure 1.7. Typical load speed / torque, speed /
power curves
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1.7. LOAD DYNAMICS AND STABILITY
(1.22)
(1.23)
where TS is the static friction torque (at zero
speed) TC is Coulomb friction torque (constant
with speed) TV is viscous friction torque
(proportional to speed) and TW is windage
friction (including the ventilator braking
torque, proportional to speed squared)
(1.24)
(1.25)
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Figure 1.12. Components of friction torque,
Tfriction
Figure 1.13. Mechanical characteristics a.) d.c.
brush motor with separate excitation b.)
induction motor c.) synchronous motor
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Various Motor Characteristics
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Example 1.4. D.C. brush motor drive stability. A
permanent magnet d.c. brush motor with the torque
speed curve drives a d.c. generator which
supplies a resistive load such that the generator
torque / speed equation is Wr 2TL. We calculate
the speed and torque for the steady state point
and find out if that point is stable. Solution Le
t us first draw the motor and load (generator)
torque speed curves on figure 1.14.
Figure 1.14. D.C. brush motor load match
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The steady state point, A, corresponds to
constant speed and B 0 in (1.27). Simply the
motor torque counteracts the generator braking
torque
(1.37)
Using the two torque speed curves we find
(1.38)
and thus
(1.39)
and
(1.40)
The static stability is met if
(1.41)
In our case from the two torque / speed curves
(1.42)
and thus, as expected, point A represents a
situation of static equilibrium.
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• 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)
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• 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
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Thermal considerations
t
Cooling transient
t
?
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Thermal considerations
The duration of overloading depends on the modes
of operation
Continuous duty Short time intermittent duty
Periodic intermittent duty
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• 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
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Thermal considerations
Short time intermittent duty
p1
t
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Thermal considerations
Short time intermittent duty
t
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• 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
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Thermal considerations
Periodic intermittent duty
t
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• 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
cFE0.48 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
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• 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,
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Torque-speed quadrant of operation
1
2

• T -ve
• ve
• Pm -ve

• T ve
• ve
• Pm ve

3
4

• T -ve
• -ve
• Pm ve

• T ve
• -ve
• Pm -ve

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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

4-quadrant operation

• 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
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• INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Ratings of converters and motors
Torque
Continuous torque limit
Power limit for continuous torque
Maximum speed limit
Speed
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1.8. MULTIQUADRANT OPERATION
These possibilities are summarised in Table 1.1
and in figure 1.16. Table 1.1.
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4Q OPERATION
F FORWARD R REVERSE M MOTORING B BRAKING
SPEED
w
w
Te
Te
FM
FB
I
II
TORQUE
RB
III
IV
w
w
Te
Te
RM
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4Q OPERATION LIFT SYSTEM
Positive speed
Negative torque
Motor
Counterweight
Cage
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4Q OPERATION LIFT SYSTEM
Convention
Upward motion of the cage Positive speed
Weight of the empty cage lt Counterweight
Weight of the full-loaded cage gt Counterweight
Principle
What causes the motion?
Motor motoring P Tw ve
Load (counterweight) braking P Tw -ve
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4Q OPERATION LIFT SYSTEM
Speed
You are at 10th floor, calling fully-loaded cage
from gnd floor
You are at 10th floor, calling empty cage from
gnd floor
FM
FB
Torque
RM
RB
You are at gnd floor, calling empty cage from
10th floor
You are at gnd floor, calling Fully-loaded cage
from 10th floor
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DC MOTOR DRIVES
Principle of operation
Torque-speed characteristic
Methods of speed control
Armature voltage control
Variable voltage source
Phase-controlled Rectifier
Switch-mode converter (Chopper)
1Q-Converter
2Q-Converter
4Q-Converter
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Figure 1.16. Electric drives with four quadrant
operation
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