electric drives professor mohammed zeki khedher lecture one
TRANSCRIPT
Electric Drives
Professor Mohammed Zeki Khedher
Lecture One
Introduction
In some countries nearly 65% of the total electric energy produced is consumed by electric motors.
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
1. ENERGY CONVERSION IN ELECTRIC DRIVES
1.1. ELECTRIC DRIVES - A DEFINITION
Figure 1.1. Constant speed electric drive
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 1.2. Variable speed electric drive
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
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Overview of AC and DC drives
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.
Electric Machines
“An engineer designing a high-performance drive system must have intimate knowledge about machine performance and Power Electronics”
Electric Machines (cont’d)
• 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
Electric Machines (cont’d)
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.
Selection Criteria for Electric Machines
• Cost• Thermal Capacity• Efficiency• Torque-speed profile• Acceleration• Power density, volume of motor• Ripple, cogging torques• Peak torque capability
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)
Example on VSD application
motor pump
valve
Supply
Constant speed Variable Speed Drives
PowerIn
Power lossMainly in valve
Power out
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Example on VSD application
motor pump
valve
SupplymotorPEC pump
Supply
Constant speed Variable Speed Drives
PowerIn
Power loss
Power out
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Power lossMainly in valve
Power outPowerIn
Power lossMainly in valve
Power out
motor pump
valve
SupplymotorPEC pump
Supply
Constant speed Variable Speed Drives
Example on VSD application
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
PowerIn
Power loss
PowerIn
Power out
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Conventional electric drives (variable speed)
• Bulky
• Inefficient
• inflexible
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Modern electric drives (With power electronic converters)
• Small
• Efficient
• Flexible
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
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
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Controllers
Controllers embody the control laws governing the load and motor characteristics and their interaction.
Controller
Torque/speed/position commands
Torque/speed/ position feedback
Thermal andother feedback
Vc, fc, start,shut-out, signals, etc.
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INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Components in electric drives
e.g. Single drive - sensorless vector control from Hitachi
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Components in electric drives
e.g. Multidrives system from ABB
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 controller’s • DSP/microprocessor – flexible, lower bandwidth - DSPs perform
faster operation than microprocessors (multiplication in single cycle), can perform complex estimations
26
DC Motors
• Limitations:
• Advantage: simple torque and speed control without sophisticated electronics
• Regular Maintenance • Expensive motor
• Heavy motor • Sparking
27
DC DRIVES Vs AC DRIVES
DC drives:
Advantage in control unit
Disadvantage in motor
AC Drives:
Advantage in motor
Disadvantage in control unit
28
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.
29
Costs
• Power Electronics Controller costs approximately 2 to 5 times AC motor
• Cost decreases with bigger size
30
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
31
Figure 1.6. Primary energy consumption for P.E.C. / motor / pump systems
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Power consumption with flow
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
x=1 for frictional systems (e.g. feed drives)
x=2 for fans and pumps
34
General Torque EquationTranslational (linear) motion:
dt
dJT
Rotational motion:
dt
dvMF
F : Force (Nm)M : Mass (Kg )v : velocity (m/s)
T : Torque (Nm)J : Moment of Inertia (Kgm2 ) : angular velocity ( rad/s )
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Torque Equation: Motor drives
dt
dJTTor
dt
dJTT LeLe
0 Le TT Acceleration
0 Le TT Deceleration
0 Le TT Constant speed
Te : motor torque (Nm) TL : Load torque (Nm)
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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 onlythe load torque but also dynamic torque, ( Jd/dt ).
During deceleration, the dynamic torque, ( Jd/dt ), hasa negative sign. Therefore, it assists the motortorque, Te.
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
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
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Elementary principles of mechanics
• 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
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
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
)
Elementary principles of mechanics
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Elementary principles of mechanics
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
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
MotorTe
Load 1, Tl1
Load 2, Tl2
J1
J2
mm1
m2
n1
n2
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
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
43
1.6. MOTION / TIME PROFILE MATCH
Figure 1.8. Motion / time profilea.) speed b.) position c.) required load torque
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Figure 1.9. Required speed / time profile
T t J t T te r L
Example 1.2. The direct drive torque / time curveA 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.
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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rr
atrad smax .
./
376 8
0 21884 2
T
Nm
Nm
Nme
1884 0 02 10 37 68 10 47 68
0 10 10
1884 0 02 10 37 68 10 27 68
. . . ;
;
. . . ;
for 0 t 0.2s
for 0.2 t 0.8s
for 0.8 t 1s
r 0 0.
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:
46
Figure 1.10. Motor torque / time requirements
The motor torque / time requirements are shown on figure 1.10.
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Example 1.3. gear - box drive torque / time curveLet 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 = 0.8.Let us calculate the total inertia (reduced to motor shaft), torque and power without and with counterweight.First the motor angular speed m is:
rad/s 22.16260
15502n2 nm (1.12)
The gear ratios may be defined as speed ratios - t /m for J4+J5 and d /m for J6 (figure 1.11).Consequently the inertia of all rotating parts Jr, reduced to the motor shaft, (figure 1.11), is:
2
m
2d
62m
2t
54321r JJJJJJJ
222
kgm 062.2522.162
5.78
22.162
5.22005.02815
(1.13)
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For the cabin and the counterweight, the inertia, reduced to motor shaft (Je) is:
22
2
2m
2
cwce kgm 07238.022.166
18001200
ummJ
(1.14)
Thus the total inertia Jt is:
2ert kgm 135.2507238.0062.25JJJ (1.15)
In absence of counterweight the la of energy conservation leads to:
ugmT cmem (1.16)
Consequently the motor torque, Tem, yields:
Nm 71.908.022.162
181.91200Tem
(1.17)
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The motor electromagnetic power Pem is:
W 1471522.16271.90TP memem (1.18)
On the other hand in presence of counterweight (1.16) becomes:
ugmm'T cwcmem (1.19)
Nm 71.30
8.022.162
181.98001200'Tem
(1.20)
So the motor electromagnetic P’em is:
W 490522.16271.30'T'P memem (1.21)
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
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
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
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
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Load steady state torque-speed characteristic
Constant torque, e.g. gravitational torque (active load)
SPEED
TORQUE
Gravitational torque
gM
FL
TL = rFL = r g M sin
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Load steady state torque-speed characteristic
Hoist drive
Speed
Torque
Gravitational torque
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
TlTe
Steady state speed
r
Torque
Speedr2r3r1
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
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
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) + 0.01(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
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Torque and speed profile
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
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Torque and speed 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.
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Torque Equation: Graphical
Te
Forwardrunning
Speed
Forwardbraking
Reverseacc.
Reverserunning
Reversebraking
Forwardacc.
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Load Torque
Load torque, TL, is complex, depending on applications.
SPEED
TORQUE
TL = k2TL = k
TL = k
In general:
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
63
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Steady-state stability
64
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
loadfrictioner
t TTTdt
dJ
(1.22)
WVCSfriction TTTTT (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):
rV 'BT 2
rW CT
(1.24)
(1.25)
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Figure 1.13. Mechanical characteristics:a.) d.c. brush motor with separate excitation b.) induction motor
c.) synchronous motor
Figure 1.12. Components of friction torque, Tfriction
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IV (Induction)I (Synchronous)
II (dc Shunt/Separately excited)
III (dc series excited)
TmaxTorque
Speed
Various Motor Characteristics
68
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 r = 2TL. We calculate the speed and torque for the steady state point and find out if that point is stable.Solution:Let us first draw the motor and load (generator) torque speed curves on figure 1.14.
Figure 1.14. D.C. brush motor load match
69
The steady state point, A, corresponds to constant speed and B = 0 in (1.27). Simply the motor torque counteracts the generator braking torque:
eL TT (1.37)
Using the two torque speed curves we find:
21.0200 rA
rA
(1.38)
and thus:rad/s 476.190
2/1.01
200rA
(1.39)
andNm 238.95
2
476.190
2TT rA
LAeA
(1.40)
The static stability is met if:
Ar
L
Ar
e TT
(1.41)
In our case from the two torque / speed curves:
2
110 (1.42)
and thus, as expected, point A represents a situation of static equilibrium.
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
p2p1 Thermal capacity, C (Ws/oC)
Surface A, (m2)Surface temperature, T (oC)Input heat power
(losses)
Emitted heat power(convection)
Ambient temperature, To
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
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
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
t
T
t
/te)0(TT
T
/th e1A
pT
Heating transient
Cooling transient
Aph
)0(T
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
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
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
t1
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
Short time intermittent duty
Ap s1
maxT Ap n1
t
T
p1
p1n
p1s
t1
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
Short time intermittent duty
t
T
/ts1 e1A
pT
maxT Ap n1
/ts1n1 1e1A
pA
p /ts1n1
1e1pp1
/tn1
s1
te11
pp
1
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
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
Periodic intermittent duty
p1
t
heating coollingcoolling
coolling
heating
heating
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
Thermal considerations
Periodic intermittent duty
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
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,
0 0.5 1 1.5 2 2.5
x 104
0
5
10
15
20
25
30
35
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
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
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
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
INTRODUCTION TO ELECTRIC DRIVES - MODULE 1
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
1.8. MULTIQUADRANT OPERATION
These possibilities are summarised in Table 1.1 and in figure 1.16.
Table 1.1.
Mode ofoperation
Forwardmotoring
Forwardregenerative braking
Reversemotoring
Reverseregenerative braking
Speed, r + + - -
Torque, Te + - - +
Electricpower flow
+ - + -
4Q OPERATION
SPEED
TORQUE
I
III
II
IV
TeTe
Te Te
FMFB
RMRB
F: FORWARD R: REVERSE M : MOTORING B: BRAKING
4Q OPERATION: LIFT SYSTEM
Counterweight Cage
Motor
Positive speed
Negative torque
4Q OPERATION: LIFT SYSTEM
Convention:
Upward motion of the cage: Positive speed
Weight of the empty cage < Counterweight
Weight of the full-loaded cage > Counterweight
Principle:
What causes the motion?
Motor : motoring P =T= +ve
Load (counterweight) : braking P =T = -ve
4Q OPERATION: LIFT SYSTEM
You are at 10th floor, callingfully-loaded cage from gnd floor
You are at gnd floor, callingempty cage from 10th floor
You are at 10th floor, callingempty cage from gnd floor
You are at gnd floor, callingFully-loaded cage from 10th floor
Torque
Speed
FMFB
RM RB
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
Figure 1.16. Electric drives with four quadrant operation