DC Motor Drive - ENCONencon.fke.utm.my/nikd/skee4433/dc_m.pdfPower Electronics and Drives (Version...
Transcript of DC Motor Drive - ENCONencon.fke.utm.my/nikd/skee4433/dc_m.pdfPower Electronics and Drives (Version...
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
1
DC Motor Drive
• General Concept• Speed Control• SCR Drives• Switched-mode DC Drives
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
2
DC Motor
• Advantages of DC motor:– Ease of control– Deliver high starting torque– Near-linear performance
• Disadvantages:– High maintenance– Large and expensive (compared to induction
motor)– Not suitable for high-speed operation due to
commutator and brushes– Not suitable in explosive or very clean
environment
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
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DC Motor Drives
• The DC drive is relatively simple and cheap (compared to induction motor drives). But DC motor itself is more expensive.
• Due to the numerous disadvantages of DC motor (esp. maintenance), it is getting less popular, particularly in high power applications.
• For low power applications the cost of DC motor plus drives is still economical.
• For servo application, DC drives is still popular because of good dynamic response and ease of control.
• Future Trend? Not so bright prospect for DC, esp. in high power drives.
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
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Separately Excited DC Motor
• The field windings is used to excite the field flux.
• Armature current is supplied to the rotor via brush and commutator for the mechanical work.
• Interaction of field flux and armature current in the rotor produces torque.
va, Va
vf, If
La
Ra
Lf
Rf
ia, Ia if, If
Eg
ω
Td
J
B
TL
+
+
+
−
−
−
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
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Operation
• When a separately excited motor is excited by a field current of if and an armature current of iaflows in the circuit, the motor develops a back emfand a torque to balance the load torque at a particular speed.
• The if is independent of the ia .Each windings are supplied separately. Any change in the armature current has no effect on the field current.
• The if is normally much less than the ia.
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
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Field and armature equations
rad/sec)(in speedmotor theis and
rad/s)-V/A(in constant agemotor volt theis
:as expressed is voltage,speed asknown also is which emf,back motor The
ly.respective inductor, and
resistor armature theare and where
:current armature ousInstantane
lyrespective inductor, and
resistor field theare and where
:current field ousInstantane
ω
ω
v
fvg
ff
ga
aaaa
ff
fffff
K
iKe
LR
edtdi
LiRv
LRdt
diLiRv
=
++=
+=
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
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Basic torque equation
)(kg.mmotor theof inertia:
(N.m) torqueload: )(N.m/rad/s constant,friction viscous:
where
:i.e. inertia, andfriction theplus torqueload the toequal bemust
torquedeveloped theoperation, normalFor
:as written isit Sometimes
rad/s)-V/A(in
constant. torque theis )( where
:ismotor by the develped torqueThe
2J
T
B
TBdtd
JT
iKT
KK
iiKT
L
Ld
atd
vt
aftd
++=
=
=
=
ωω
φ
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
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Steady-state operation
fvaagaaa
fvg
fff
IKRIERIV
IKE
RIV
ω
ω
+=+=
=
=
circuit armature The
:bygiven is emfback The
circuit, fieldFor
saturated.not ismotor theAssuming zero. is sderivative time,operations state-steadyUnder
Rf
Ra
Ia
La
If
Eg
Lf
+
−
+ +
−
Va
−Va
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
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Steady-state torque and speed
ω
ω
ω
ω
dd
Laftd
fv
a
a
a
fv
aaa
TP
TBIIKT
IKV
I
R
IKRIV
=
+==
=
−=
:ispower required The
:is torquedeveloped The
tage.supply vol on theonly depends speedmotor theconstant,kept iscurrent field theif isThat
small, is i.e. loaded,lightly ismotor the
or when usual), is(which valuesmall a is If
:derivedeasily becan speedmotor The
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
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Torque and speed control
• From the derivation, several important facts can be deduced for steady-state operation of DC motor.
• For a fixed field current, or flux (If) , the torque demand can be satisfied by varying the armature current (Ia).
• The motor speed can be varied by:– controlling Va (voltage control)– controlling Vf (field control)
• These observations leads to the application of variable DC voltage to control the speed and torque of DC motor.
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
11
Example 1• Consider a 500V, 10kW , 20A rated- DC motor
with armature resistance of 1 ohm. When supplied at 500V, the UNLOADED motor runs at 1040 rev/min, drawing a current of 0.8A (ideally current is zero at no-load).
– Estimate the full load speed at rated values – Estimate the no-load speed at 250V.
rad/sec)strictly equation thisreality,in :(Note
rev/min 51948.0
)1(8.0250
250V,at voltageand load-noAt
rev/min 100048.0
)1(20500
value,rated and load fullAt
48.01040
)1(8.0500
=−=−=
+=
=−=−=
=−=−=
+=+=
fv
aaa
fvaaa
fv
aaafl
aaafv
fvaagaaa
IKRIV
IKRIV
IKRIV
RIVIK
IKRIERIV
ω
ω
ω
ω
ω
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
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Variable speed operation
• Family of steady-state torque speed curves for a range of armature voltage can be drawn as above.
• The speed of DC motor can simply be set by applying the correct voltage.
• Note that speed variation from no-load to full load (rated) can be quite small. It depends on the armature resistance.
500 Speed (rev/min)
Torque
1000750250
500V375V250V125V
Rated torque)
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
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Base Speed and Field-weakening
• Base speed:ωbase– the speed which correspond to the rated Va, rated Ia
and rated If.
• Constant Torque region (ω > ωbase, )– Ia and If are maintained constant to met torque
demand. Va is varied to control the speed. Power increases with speed.
• Constant Power region (ω > ωbase, )– Va is maintained at the rated value and if is reduced to
increase speed . However, the power developed by the motor (= torque x speed) remains constant. Known as field weakening.
ωbaseω
Torque
Power
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
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Four quadrant operation
1
23
4
A
C
D
TORQUE
SPEED
FORWARDMOTORING
REVERSEMOTORING
REVERSEGENERATING
B
FORWARDGENERATING
vaeg
ia
ia = +; Te = +va = +; ω = +
ia = − ; Te = −va = + ; ω m= +
ω m
Te
eg va
ia = − ; Te = −va = − ; ω m= −
ia
ia = +; Te = +va = −; ω = −
ia
vaegeg va
ia
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
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Regenerative Braking (in Q2)
• Say the motor running at position A. Suddenly va is reduced (below eg). The current ia will reverse direction.Operating point is shifted to B.
• Since ia is negative, torque Te is negative.
• Power is also negative, which implies power is “generated” back to the supply.
• In other words, during the deceleration phase, kinetic energy from the motor and load inertia is returned to the supply.
• This is known as regenerative braking-an efficient way to brake a motor. Widely employ in electric vehicle and electric trains. If we wish the motor to operate continuously at position B, the machine have to be driven by mechanical source.
• The mechanical source is a “prime mover”. • We must force the prime mover it to run faster so
that the generated eg will be greater than va.
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
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Drive types
• SCR “phase-angle controlled” drive – By changing the firing angle, variable DC
output voltage can be obtained.– Single phase (low power) and three phase (high
and very high power) supply can be used– The line current is unidirectional, but the output
voltage can reverse polarity. Hence 2- quadrant operation is inherently possible.
– 4-quadrant is also possible using “two sets” of controlled rectifiers.
• Switched-mode drive– Using switched mode DC-DC converter. Dc
voltage is varied by duty cycle.– Mainly used for low to medium power range.– Single-quadrant converter (buck): 1- quadrant– Half bridge: 2-quadrant– Full bridge: 4-quadrant operation
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
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Thyristor/SCR drives
• Mains operated.
• Variable DC voltages are obtained from SCR firing angle control.
• Slow response.
• Normally field rectifier have much lower ratings than the armature rectifier. It is only used to establish the flux.
Speedreference
Three/single phase supply Single phase supply
ControlandSCRfiring
Currentsensor TachometerCurrent
Speed
M
T
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
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Continuous/Discontinuous current
• The key reason for successful DC drive operation is due to the large armature inductance La.
• Large La allows for almost constant armature current (with small ripple) due to “current filtering effect of L”. (Refer to notes on Rectifier).
• Average value of the ripple current is zero. No significant effect on the torque.
• If La is not large enough, or when the motor is lightly loaded, or if supply is single phase (half-wave), discontinuous current may occur.
• Effect of discontinuous current: Output voltage of rectifier rises; motor speed goes higher. In open-loop operation the speed is poorly regulated.
• Worthwhile to add extra inductance in series with the armature inductance.
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
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Basic single-phase drive
fm
f
ga
gaa
am
a
VV
ER
EVI
VV
απ
απ
cos2
: voltageField
emfback theis ;
:iscurrent (DC) Armature
cos2
:is voltagearmature current, continuousFor
=
−=
=
+vs
_
Ia
Ta1
Ta2
Ta3
+
+
−
Va
Ra
Ta4
La
Eg −
If
+vs
_
Tf1
Tf2
Tf3+
−
Lf
Tf4
Lf
Vf
ARMATURE FIELD
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
20
Basic three-phase drive
fm
f
Ea
Eaa
aLLm
a
VV
VR
VVI
VV
απ
απ
cos2
:fieldfor used is phase single If
emfback theis ;
:current (DC) Armature
cos3
: voltageArmature
,
=
−=
= −
Ia
Ta1
+
+
−
Va
Ra
La
Eg−
Ta3
Ta2
Ta6
_ vcn +
n_ vbn
+
_ van +
Ta5
Ta4
+
−
Lf
Lf
Vf
If
ARMATURE FIELD
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
21
Example 2
o
fafm
a
faf
am
af
fEE
Eaaa
am
a
f
V
KIRKIT
V
KIRKITV
IKIT
KIVV
VRIV
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KI
32.62
60200
25.225.2
6024022
cos
2cos
cos2
and5.2
i.e emf,back theis Where
And
cos2
current, continuousFor
.continuous iscurrent theAssumerpm. 200at opearte motor to for the angle g triggerinthe
Calculate ohm. 2 is resistance armature theand 2.5motor theofconstant field The supply. ac 240V a toconnected
converter wave-full sby driven ismotor The Nm. 60 ofload orqueconstsnt t a hasmotor DC excited saperatelyA
1
1
=
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=
==
+=
=
=
−
−
ππ
ωπα
ωαπ
ωω
απ
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
22
Example 3A rectifier-DC motor drive is supplied by a three-phase, full controlled SCR bridge 240Vrms/50Hz per-phase. The field is supplied by a single-phase 240V rms/50Hz, with uncontrolled diode bridge rectifier. The field current is set as maximum as possible.The separately excited DC motor characteristics is given as follows:Armature resistance:Ra = 0.3 ohmField resistance: Rf =175 ohmMotor constant: KV =1.5 V/A-rad/sAssume the inductance of the armature and field circuit is largeenough to ensure continuous and ripple-free currents. If the delay angle of the armature converter (αa) is 45 degrees and the required armature current is 30A,
• a) Calculate the developed torque, Td.• b) Speed of the motor, ω (rad/s)• c) If the polarity of the field current is reversed, the motor
back emf will reverse. For the same armature current of 30A, determine the required delay angle of the armature converter.
NmIIKT
AV
R
VI
VV
Va
afvdf
ff
fm
f
58.5530235.15.1
235.1175
216
2160cos24022
cos2
)(
0. maximum, iscurrent fiels theSince
=××==
===�
=×==
=
πα
π
α
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
23
Example 3 (cont)
oLLm
aa
aLLm
a
aaga
g
aaag
oa
LLma
a
aaag
fv
g
VV
VV
AlsoVRIEV
VE
VVRIVE
VV
V
RIVEIK
E
4.132240233
)5.378(cos
3cos
cos3
,3.3783.0303.387
and3.387
thenreversed, is field ofpolarity theNow (c)
sec/rad 06.209235.15.1
3.3873.3873.0303.396
3.39645cos240233
cos3
, 45 with phase-by three supplied is armature The
speedMotor (b)
1
,
1
,
,
o
=
�
���
�××
−×= �
����
� ×=
=
−=×+−=+=
−=
=×
=�
=×−=−==×××==
=−=
=
−
−
−
−
−
ππα
απ
ω
πα
π
α
ω
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
24
Reversal
• DC motor in inherently bi-directional. Hence no-problem to reverse the direction. It can be a motor or generator.
• But the rectifier is unidirectional, because the SCR are unidirectional devices.
• However, if the rectifier is fully controlled, it can be operated to become negative DC voltage, by making firing angle greater than 90 degrees,
• Reversal can be achieved by:– armature reversal using contactors (2-
quadrant)– field reversal using contactors (2-quadrant)– double converter (full 4-quadrants)
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
25
Reversal using armature or field contactors
FIELD
DRIVE REVERSING USING ARMATURE OR FIELD CONTACTORS
CONTACTOR
CONTACTOR AT THE ARMATURESIDE (SINGLE PHASE SYSTEM)
Vs
Va Eg Va Eg Va Eg
1
2
1
2
CONTACTOR AT
1POSITION
(MOTORING)
CONTACTOR AT
2POSITION
(BRAKING/GENERATION)
CONTACTOR AT
2POSITION
(RESERVE)
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
26
Reversing using double converters
converter 1 converter 2
FIELD
Vs
Principle of reversal
Practical circuit
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
27
Switched–mode DC drives
• Supply is DC (maybe from rectified-filtered AC, or some other DC sources).
• DC-DC converters (coppers) are used.
• suitable for applications requiring position control or fast response, for example in servo applications, robotics, etc.
• Normally operate at high frequency– the average output voltage response is
significantly faster – the armature current ripple is relatively less
than the controlled rectifier
• In terms of quadrant of operations, 3 possible configurations are possible: – single quadrant,– two–quadrant – and four–quadrant
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
28
Single-quadrant drive
• Unidirectional speed. Braking not required.
fv
a
a
gaa
ont
a
IKV
R
EVI
DVT
tVdt
TV
Tt
on
=
−=
===
<<
�
ω
:as edapproximat becan speed and
;
:iscurrent (DC) Armature
1
:statesteady at voltagearmature The ,0For
0
ia
va
ton T
iaTorque ( ia)a)
Q4 Q1
Q2Q3
ω (va)
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
29
2 Quadrant DC drives
• FORWARD MOTORING (T1 and D2 operate)– T1 on: The supply is connected to motor terminal.– T1 off: The armature current freewheels through
D2.– Va (hence speed) is determined by the duty ratio.
• REGENERATION (T2 and D1 operate)– T2 on: motor acts as a generator– T2 off:, the motor acting as a generator returns
energy to the supply through D2.
Q1
Q2Q3
Q4
Torque
ω
+va
–
T2 D2
D1T1
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
30
4 Quadrant DC drives
• A full-bridge DC-DC converter is used.
+ va –
T1 T3
T2T4
D1
D4
D3
D2
Q1
Q2Q3
Q4
Torque
ω
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
31
4-quadrant: Forward motoring
• T1 and T2 operate; T3 and T4 off.
• T1 and T2 turn on together: the supply voltage appear across the motor terminal. Armature current rises.
• T1 and T2 turn off: the armature current decay through D3 and D4
+ va –
T1 T3
T2T4
D1
D4
D3
D2
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
32
Regeneration
• T1, T2 and T3 turned off.
• When T4 is turned on, the armature current rises through T4 and D2.
• When T4 is turned off, the motor, acting as a generator, returns energy to the supply through D1 and D2.
+ va –
T1 T3
T2T4
D1
D4
D3
D2
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
33
Reverse motoring
• T3 and T3 operate; T1 and T2 off.
• When T3 and T4 are on together, the armature current rises and flows in reverse direction.
• Hence the motor rotates in reverse direction.
• When T3 and T4 turn off, the armature current decays through D1 and D2.
+ va –
T1 T3
T2T4
D1
D4
D3
D2
Power Electronics and Drives (Version 3-2003):
Dr. Zainal Salam, UTMJB
34
Reverse generation
• T1, T3 and T4 are off.
• When T1 is on, the armature current rises through T2 and D4.
• When Q2 is turned off, the armature current falls and the motor returns energy to the supply through D3 and D4.
+ va –
T1 T3
T2T4
D1
D4
D3
D2