DC Motor Drives - Single Phase And Three Phase Rectifier Controlled Drives

- DC motors are widely used in applications which require
good speed control, speed regulation, frequent starting,
braking & speed reversal etc
- E.g. for such applications are lifts, traction, cranes, hoists,
- E.g. for such applications are lifts, traction, cranes, hoists,
machine tools, printing presses, textile mills etc.
- Low power DC motors are used for position control
applications
- Commonly used DC motors are
a. Separately excited DC motors
a. Separately excited DC motors
b. Shunt motors
c. Series motors
d. Cumulatively Compound motors

1. DC MOTOR DRIVES –SINGLE PHASE &
THREE PHASE RECTIFIER CONTROLLED
DRIVES DRIVES

2. DC Motor Drive Systems
- DCmotors are widely used in applications which require
good speed control, speed regulation, frequent starting,
braking&speedreversaletc
-
E
.
g
.
for
such
applications
are
lifts,
traction,
cranes,
hoists,
-
E
.
g
.
for
such
applications
are
lifts,
traction,
cranes,
hoists,
machinetools,printingpresses,textilemillsetc.
- Lowpower DCmotors are used for position control
applications
- CommonlyusedDCmotorsare
a
.
Separately
excited
DC
motors
a
.
Separately
excited
DC
motors
b.Shuntmotors
c.Seriesmotors
d.CumulativelyCompoundmotors
2

3. 3

4. - Basic equations applicable to all DC motors are
V = E
b+ I
aR
a……. (1)
E
b= K
eφω
m……. (2)
T = K
eφI
a……. (3)
where V = applied voltage,
E
b
= back
emf
,
I
a
= armature current,
where V = applied voltage,
E
b
= back
emf
,
I
a
= armature current,
R
a
= armature resistance, φ= flux/pole, ω
m
= angular velocity,
T = torque produced by motor, K
e
= motor constant
From (1), E
b
= V –I
a
R
a
……. (4)
From (3),
I
a
= T/(
K
e
φ
) ……. (5)
From (3),
I
a
= T/(
K
e
φ
) ……. (5)
Substituting (2) & (5) in (4)
=> K
e
φω
m
= V –(T/(K
e
φ))R
a
i.e,
4
( )
)6( .......
2
T
K
R
K
V
e
a
e
mφ φ
ω
− =

5. Shunt & separately excited motor - Here field current is constant & flux can be
assumed to be constant, i.e, K
eφ= K, a constant
(2)
=> E
b= Kω
m
(3)
=> T = KI
a
(6)
=> ω
m= (V/K) –(R
a/K
2
)T
Series motor Series motor - Here flux is a function of armature current
i.e, φ∝I
a, φ= K
fI
a (3)
=> T = K
eK
fI
a
2
(6)
=> ω
m= (V/(K
eK
fI
a)) –(R
a/K
eK
f)
Compound motor
-
Here no load speed depends on strength of shunt
-
Here no load speed depends on strength of shunt field and slope of characteristics depends on
strength of series field
- Cumulatively compound motors are used in those
applications where drooping characteristics similar
to series motor is required & at the same time no
load speed limited to a safer value. E.g, lift
5

6. Speed control of DC motors
- According to the equation,
Motor speed can be controlled by following methods,
i) Armature voltage control
ii)
Field flux control
( )
T
K
R
K
V
e
a
e
m2φ φ
ω
− =
ii)
Field flux control
iii)Armature resistance control
- Speed-torque curves of DC motors for these methods of speed control
are shown below
i)
6

7. ii) iii)
7

8. -In armature voltage control , the applied voltage across
armature is varied to get the desired speed control
-But it can provide speed control only below rated speed
because armature voltage cannot be increased above rated
value
-For speed control above rated speed , field flux control is
employed
-In a separately excited DC motor, flux is controll ed by varying
voltage across field winding & in a series motor it is
controlled either by varying number of turns in fie ld winding
or connecting a diverter resistance across field wi nding or connecting a diverter resistance across field wi nding
-In armature resistance control , speed is controlled by
wasting power in external resistors that are connec ted in
series with armature
8

9. The maximum Torque & Power limitations of DC drives
is shown below.
9

10. Variation of Torque
- We know T ∝φI
a
. During armature voltage control, flux in the
machine is kept constant.
- i.e, T ∝I
a
. Also I
a
is limited to maximum rated current.
- So during this period, maximum torque that can be produced by a
motor remains constant. motor remains constant.
- For speed above rated speed, we have to go for flu x control.
- Here as flux (φ) changes, speed increases & T decreases
Variation of Power
- Power, P =VI. During armature voltage control, as ‘V’ varies, power
also varies. also varies.
- During flux control (speed above rated speed), ‘V’ is maintained
constant at rated value.
- So power∝current(I). Since maximum value of current is limit ed to
rated value, the maximum power that can be produced by the motor
remains constant
10

11. Methods of armature voltage control
When supply is AC, we can go for
i) Ward-Leonard scheme
ii) Transformer with taps & uncontrolled rectifier
iii) Controlled rectifier
When supply is DC,
iv) Chopper control
i) Ward-Leonard scheme
11

12. -Here the speed of DC motor is controlled by armatu re voltage control
-The armature voltage is supplied by the separately excited DC
generator. So by controlling generated emfof genera tor, we can
control the speed of motor
-The generated emfcan be controlled by varying the flux in the
machine machine
-Due to high initial cost, maintenance & low effici ency, this system is
not using currently
ii) Transformer & uncontrolled rectifier
12

13. - Variable voltage for DC motor control can be obtai ned by either using
an autotransformer or a transformer with tapings f ollowed by an
uncontrolled rectifier
- Autotransformer is used for low power applications & for high power
applications a transformer with tapings is employed
-
Here to control the speed of the motor, the input A C voltage to
-
Here to control the speed of the motor, the input A C voltage to uncontrolled rectifier is varied
iii) Controlled rectifier
- Controlled rectifiers are used to get variable DC voltage from an AC
source of fixed voltage (Also known as static Ward- Leonard drives)
- Commonly used controlled rectifier circuits are,
i) 1
φ
Half wave controlled rectifier
i) 1
φ
Half wave controlled rectifier
ii) 1 φfully controlled rectifier
iii) 1 φhalf controlled rectifier
iv) 3φfully controlled rectifier
v) 3φhalf controlled rectifier
13

14. iv) Chopper - Used to get a variable DC voltage from a fixed vo ltage DC source
Single phase half wave controlled rectifier fed sep arately
excited DC motor drive
-
The
drive
circuit
is
shown
in
figure
-
The
drive
circuit
is
shown
in
figure
- Motorisshownbyitsequivalentcircuit
- Fieldsupplyisnotshown.Whenfield
controlisrequired,fieldisfedfroma
controlledrectifier.
- TheACinputvoltageisdefinedby,
V
=
V
Sinωt
V
s
=
V
m
Sinωt
- Herethethyristorcanbetriggered
onlywhenitisforwardbiased.This
happenswhensupplyvoltageisgreater
thanbackemf,E
14

15. - If ‘θ’ is the minimum firing angle below which thy ristorcannot be
turned on, then V
m
Sinωt = E or θ= Sin
-1
(E/V
m
)
- The maximum value of firing angle ‘θ’ = ( π-θ)
Working
- From ωt = 0 to ωt = α, thyristordoes not conduct & load voltage,
V
0
= E
-
Consider that the
thyristor
is triggered at an angle
ω
t =
α
, which is
0
-
Consider that the
thyristor
is triggered at an angle
ω
t =
α
, which is
greater than ‘θ’
- At ωt = α, thyristorstarts conduction & load volta ge = V
s
- During conduction, load current is decided by (V
s
-E) & it increases
from zero, reaches a maximum value & decreases
- At ωt = π, supply voltage = 0 & for ωt > π, it is negative
-
Even then load current does not become zero because of inductive
-
Even then load current does not become zero because of inductive effect
- When supply voltage become negative, inductor quic kly reverses its
polarity to maintain the current through it & energ y stored in it
during positive half cycle is released to resistor & back emfof motor
through thyristor
15

16. -Finally at ωt = β, energy stored
in the inductor is completely
released to load & load current
decays to zero. Now thyristoris
commutated.
-
β
is called extinction angle
-
β
is called extinction angle
-From ωt = βto ωt = 2π+ α,
thyristordoes not conduct.
So load voltage is E &
current = 0
-
At
ω
t =
2
π
+
α
,
thyristor
is
16
-
At
ω
t =
2
π
+
α
,
thyristor
is
triggered again & the cycle is
repeated

17. Average value of output voltage (Voltage applied to armature),








+ =
∫ ∫
+
β
α
απ
β
ω ω
π
2
2
1
t Ed t Sin V V
m avg
 

 

+ =
+
−] ]
2
2
1
t t Cos
E V
m
ω ω
απ
β
β
α
π
-Average value of load current = (Average value of voltage across
resistor/ Resistance value (R))
-
When the
thyristor
conducts, on applying KVL we can form an
[ ]
)7( .... ) 2( ) (
2
1
β α π β α
π
− + + − =E Cos Cos V V
m avg
-
When the
thyristor
conducts, on applying KVL we can form an
equation,
V
m
Sinωt = L(di/dt) + iR+ E
Voltage across resistor, iR= V
m
Sinωt -L(di/dt) -E
Average value of voltage across resistor =
17
∫π
π
2
0
2
1
dt iR

18. - i.e, Average load current,
(average value of voltage across inductor = 0, so t he term L(di/dt) can be neglected)
[ ]
∫
− = =
π
ω ω
π
2
0
2
1
tdEt Sin V
R R
V
I
m
res
avg
avg
( )
∫
− =
βα
ω ω
π
tdEt Sin V
R
m
2
1
- From (7) it is clear that by varying firing angle ‘α’, DC output voltage
of rectifier can be varied and by applying this var iable DC voltage to
∫ α
π
R
2
 

 

− =
−)] ( )] (
2
1
t t Cos
E V
R
m
ω ω
β
α
β
α
π
[ ]
)8( .... ) ( ) (
2
1
αβ β α
π
− − − =E Cos Cos V
R
I
m avg
of rectifier can be varied and by applying this var iable DC voltage to armature we can control the speed of the motor acco rding to (6)
- This is a single quadrant drive (1
st
quadrant –forward motoring ,
Since the average value of output voltage is always positive)
18

19. Single phase fully controlled bridge rectifier fed separately
excited DC motor drive
-Thedrivecircuitisshowninfigure
- Motorisshownbyitsequivalentcircuit
-
Field
supply
is
not
shown
.
When
field
-
Field
supply
is
not
shown
.
When
field
controlisrequired,fieldisfedfroma
controlledrectifier.
- TheACinputvoltageisdefinedby,
V
s
=V
m
Sinωt
- Herethemotorcurrentcanbecontinuousordiscontinuous
-
If
‘θ
’
is
the
minimum
firing
angle
below
which
thyristor
cannot
be
-
If
‘θ
’
is
the
minimum
firing
angle
below
which
thyristor
cannot
be
turnedon,thenV
m
Sinωt=E or θ=Sin
-1
(E/V
m
)
19

20. Working–Continuouscurrentmode
(highL/Rratio)
- Duringpositivehalfcycleofsupplyvoltage,thyristorsT
1
&T
2
become
forwardbiasedandtheyaregatedatωt=α.Theyconduct&carrythe
loadcurrent.Theloadvoltageisthesupplyvoltage.
- From0<ωt<α,theloadvoltageisE
- Forωt>π,loadvoltageisnegative.ButthyristorsT
1
&T
2
continueto
conduct
because
of
load
inductance
.
T
&
T
conduct
until
ω
t
=
π
+
α
1
2
conduct
because
of
load
inductance
.
T
1
&
T
2
conduct
until
ω
t
=
π
+
α
- At ωt =π+α, T
3
& T
4
are gated & they are turned ON. At the same
instantT
1
&T
2
turnedOFF.T
3
&T
4
conductsuntilωt=2π+α
- At ωt = 2π+α, T
1
& T
2
are triggered again & the cycle is repeated
Working –Discontinuous current mode
(low L/R ratio)
- During positive half cycle of supply voltage, thyr istorsT
1
& T
2
become
forward biased and they are gated at
ω
t=
α
. They conduct & carry the
forward biased and they are gated at
ω
t=
α
. They conduct & carry the
load current. The load voltage is the supply voltag e.
- From 0 < ωt < α, the load voltage is E
-For ωt > α, load voltage is negative. But thyristo rsT
1
& T
2
continue to
conduct because of load inductance until ωt = β. T
1
& T2 are turned
OFF at β
20

21. 21

22. - From ωt=βto ωt=π+α, the load current remains zero & load voltage is
E.
- At ωt=π+α, T
3
& T
4
are gated. They conduct & load current flows
through them from ωt=π+αto ωt=π+β
- At ωt=π+β, load current is zero & hence T
3
& T
4
are turned OFF.
During
ω
t=
π
+
β
to
ω
t=2
π
+
α
no device conducts & load current is 0
.
During
ω
t=
π
+
β
to
ω
t=2
π
+
α
no device conducts & load current is 0
.
Load voltage = E
- At ωt=2π+α, T
1
& T
2
are turned ON & the cycle is repeated
For continuous current mode,
Average value of output voltage (Voltage across arm ature),


=
=
−
∫
+
+
]
1
t
Cos
m
m
av
V
t
d
t
Sin
V
V
ω
απ
απ
π
ω
ω
π
-For 0<α<90, V
av
is positive & it act as a rectifier. For 90<α<180, V
av
is
negative & it act as a line commutated inverter
22


=
=
−
∫
]
t
Cos
m
av
t
d
t
Sin
V
V
ω
α
α
π
ω
ω
π
)9....(
2
α
π
Cos
V
V
m
av
=

23. - Average value of load current = average value of v oltage across the
resistor/Resistance value
-When the thyristorconducts, on applying KVL we can form an
equation, V
m
Sinωt = L(di/dt) + iR+ E
Voltage across resistor,
iR
=
V
Sin
ω
t
-
L(
di
/
dt
)
-
E
Voltage across resistor,
iR
=
V
m
Sin
ω
t
-
L(
di
/
dt
)
-
E
Average value of voltage across resistor =
i.e, Average load current,
(average value of voltage across inductor = 0, so t he term L(
di
/
dt
) can be neglected)
∫π
π
0
1
iRdt
∫
− = =
π
ω ω
π
0
) (
1
tdEt Sin V
R R
V
I
m
res
avg
avg
(average value of voltage across inductor = 0, so t he term L(
di
/
dt
) can be neglected)
23
 

 

− = − =
+ +
+
− ∫)( )] (
1
) (
1
t t Cos
E V
R
tdEt Sin V
R
I
m m avg
ω ω
απ
α
απ
α
απ
α
π
ω ω
π
)10 ....(
2
,.
R
E V
RE
Cos
R
V
I ei
avg m
avg
−
= − =
α
π

24. Discontinuous current mode From ωt=αto ωt=β, the load voltage is V
m
Sinωt and from ωt=βto
ωt=α+π, the load voltage is E.
So average value of output voltage (Voltage across the armature),
)
(
1
ω
ω
ω
π
βαπ


+
=
∫
∫+
t
Ed
t
d
t
Sin
V
V
m
avg
Average value of load current,
[ ]
)11 ....( ) ( ) (
1
)
(
β α π β α
π
ω
ω
ω
π
α β
− + + − =


+
=
∫
∫
E Cos Cos V V
t
Ed
t
d
t
Sin
V
V
m avg
m
avg
∫
− =
βα
ω ω
π
tdEt Sin V
R
I
m avg
) (
1
24
∫ α
π
R
[ ]
)12 ....( ) ( ) (
1
,.
α β β α
π
− − − =E Cos Cos V
R
I ei
m avg

25. This drive can operate in 2 quadrants
1. Quadrant I operation (forward motoring)
for (0<α<90), the average output voltage of rectifi er is positive &
V
a
>E. Now the machine will work as a motor
2. Quadrant IV operation(reverse braking)
for (90<α<180), the average output voltage of recti fier is negative &
V
a
<E. Now the motor will work in regenerative braking mode by
supplying energy to source & rectifier will work as an inverter
25

26. -for regenerative braking operation, αshould be adj usted so that V
a
is
negative & E should be reversed by any of following method
i) The load coupled to motor should drive the motor in reverse
direction
ii) Field current may be reversed with motor runnin g in forward
direction direction iii) Armature terminal connections may be reversed while motor
running in forward direction
-The variation of motor input voltage as a function of αis shown below
26

27. Single phase half controlled bridge rectifier fed
separately excited DC motor drive
-Thedrivecircuitisshowninfigure
-Motorisshownbyitsequivalentcircuit
-
Field
supply
is
not
shown
.
When
field
-
Field
supply
is
not
shown
.
When
field
controlisrequired,fieldisfedfroma
controlledrectifier.
-TheACinputvoltageisdefinedby,
V
s
=V
m
Sinωt
-
Here
the
motor
current
can
be
continuous
or
discontinuous
-
Here
the
motor
current
can
be
continuous
or
discontinuous
- If ‘θ’ is the minimumfiring angle belowwhich thyristor cannot be
turnedon,thenV
m
Sinωt=E or θ=Sin
-1
(E/V
m
)
27

28. Working–Continuouscurrentmode
(highL/Rratio)
- During positive half cycle of supply voltage, Thyristor T
1
& Diode D
1
areforwardbiasedandT
1
isgatedatωt=α.Theyconduct&carrythe
loadcurrent.Theloadvoltageisthesupplyvoltage.
- For ωt >π, load voltage is negative. But Thyristor T
1
continue to
conduct
because
of
load
inductance
.
When
supply
voltage
reverses,
conduct
because
of
load
inductance
.
When
supply
voltage
reverses,
D
1
become reverse biased and turned off. At the same time D
2
becomeforwardbiased,startsconduction.SoT
1
&D
2
conductuntilωt
=π+α
- At ωt =π+α, T
2
is gated & turned ON. At the same instant T
1
gets
turnedOFF.T
2
&D
2
conductsuntilωt=2π
-
For
ω
t
>
2
π
,
load
voltage
is
positive
.
But
Thyristor
T
2
continue
to
-
For
ω
t
>
2
π
,
load
voltage
is
positive
.
But
Thyristor
T
2
continue
to
conduct because of load inductance. When supply voltage rev erses,
D
2
become reverse biased and turned off. At the same time D
1
becomeforwardbiased,startsconduction.SoT
2
&D
1
conductuntilωt
=2π+α
- At ωt = 2π+α, T
1
is triggered again & the cycle is repeated
28

29. Working –Discontinuous current mode
(low L/R ratio)
- During positive half cycle of supply voltage, Thyristor T
1
& Diode D
1
areforwardbiasedandT
1
isgatedatωt=α.Theyconduct&carrythe
loadcurrent.Theloadvoltageisthesupplyvoltage.
- For ωt >π, load voltage is negative. But Thyristor T
1
continue to
conduct because of load inductance. When supply voltage rev erses,
D
become
reverse
biased
and
turned
off
.
At
the
same
time
D
D
1
become
reverse
biased
and
turned
off
.
At
the
same
time
D
2
becomeforwardbiased,startsconduction.SoT
1
&D
2
conductuntilωt
=β
- Atωt=β,loadcurrentbecomezero.SoT1&D2getsturnedOFF
- Fromωt=βtoωt=π+α,nodevicesconduct&loadvoltage=E
- Atωt=π+α,T
2
isgated&turnedON.T
2
&D
2
conductsuntilωt=2π
-
For
ω
t
>
2
π
,
load
voltage
is
positive
.
But
Thyristor
T
2
continue
to
-
For
ω
t
>
2
π
,
load
voltage
is
positive
.
But
Thyristor
T
2
continue
to
conduct because of load inductance. When supply voltage rev erses,
D
2
become reverse biased and turned off. At the same time D
1
becomeforwardbiased,startsconduction.SoT
2
&D
1
conductuntilωt
=π+β
- At ωt = 2π+α, T
1
is triggered again & the cycle is repeated
29

30. For continuous current mode, Average value of output voltage
(Voltage across armature),
t
d
t
Sin
V
V
m
avg
ω
ω
π
π∫
=
1
t
d
t
Sin
V
V
m
avg
ω
ω
π
α∫
=
) 1( ,.
α
π
Cos
V
V ei
m
avg
+ =
30

31. For Discontinuous conduction mode
,
Average value of output voltage (Voltage across arm ature),
31








+ =
∫ ∫
+
π
α
απ
β
ω ω ω
π
tdE tdt Sin V V
m avg
1
[ ]
β α π α
π
− + + + =( ) 1(
1
,.E Cos V V ei
m avg

32. - Here the average output voltage
of the converter is always positive
- So regenerative braking is not possible
and we can go for plugging (motor will
work as generator during braking and
electrical energy produced is wasted in electrical energy produced is wasted in an external resistance connected in series
with armature during braking)
Note
- When compared to fully controlled converter, half controlled
converter is cheap (using only 2 controlled devices ), produces less converter is cheap (using only 2 controlled devices ), produces less ripple and got better power factor
- It gives control only in first quadrant
32

33. 3 Phase fully controlled bridge rectifier fed
separately excited DC motor Drive
-A 3 phase fully controlled bridge rectifier fed s eparately excited DC
motor drive is shown in figure
-Contain 6 thyristors
T
1
, T
2
, T
3
, ….T
6
-The supply voltage is
sinusoidal 3 phase AC
voltage with phase
sequence ABC
-
The output of rectifier is
-
The output of rectifier is fed to the armature of DC motor
-Here 2 thyristorsconducts at a time, one in the up per group and one in
the lower group
-Each thyristorconducts for a duration of 120◦
33

34. Working –for α= 30◦ - Consider that T
5& T
6are already conducting, load voltage is V
CB
- The voltage V
AB become more positive for ωt>60
◦
and hence T
1is triggered at
ωt=(60
◦
+α), it starts conduction & T
5gets turned OFF
- T
1& T
6conducts from ωt=(60
◦
+α) to ωt=(120
◦
+α), load voltage is V
AB
-
At
ω
t=(120
◦+
α
), T
2
is triggered & is turned ON & T
6
gets turned OFF
-
At
ω
t=(120
+
α
), T
2
is triggered & is turned ON & T
6
gets turned OFF
- T
1& T
2conducts from ωt=(120
◦
+α) to ωt=(180
◦
+α), load voltage is V
AC
- At ωt=(180
◦
+α), T
3is triggered & is turned ON & T
1gets turned OFF
- T
3& T
2conducts from ωt=180 ◦+αto ωt=(240
◦
+α), load voltage is V
BC
- At ωt=(240
◦
+α), T
4is triggered & is turned ON & T
2gets turned OFF
- T
3& T
4conducts from ωt=(240
◦
+α) to ωt=(300
◦
+α), load voltage is V
BA
- At ωt=(300
◦
+α), T
5is triggered & is turned ON & T
3gets turned OFF
-
T
& T
conducts from
ω
t=(300
◦
+
α
) to
ω
t=(360
◦+
α
), load voltage is V
-
T
5
& T
4
conducts from
ω
t=(300
◦
+
α
) to
ω
t=(360
◦+
α
), load voltage is V
CA
- At ωt=(360 ◦+α), T
6is triggered & is turned ON & T
4 gets turned OFF
- T
5& T
6conducts from ωt=(360 ◦+α) to ωt=(420 ◦+α), load vo ltage is V
CB
- The cycle is then repeated
34

35. α= 0
◦
α= 30
◦
35

36. α
= 90
◦
α
= 90
Average value of output voltage (Voltage applied to armature),
( i.e, by varying α,average value of output voltage can
be varied and hence the spee d of motor)
tdt Sin V V
mL avg
ω ω
π
α
π
α
π
∫
+
+
=
)
3
2
(
)
3
(
)3/(
1
)] (
)
3
2
(
)
3
(
3
t Cos
mL
V
ω
α
π
α
π
π
−
+
+
=
α
π
Cos
V
V ei
mL
avg
3
,.=
36

37. This drive can operate in 2 quadrants
1. Quadrant I operation (forward motoring)
for (0<α<90), the average output voltage of rectifi er is positive &
V
a
>E. Now the machine will work as a motor
2. Quadrant IV operation(reverse regenerative braking)
for (90<α<180), the average output voltage of recti fier is negative &
V
a
<E. Now the motor will work in regenerative braking mode by
supplying energy to source & rectifier will work as an inverter
37

38. 3 Phase half controlled bridge rectifier fed separa tely
excited DC motor Drive
-A 3 phase half controlled bridge rectifier fed se parately excited
DC motor drive is shown in figure
-Contain 3 thyristors &
3 Diodes 3 Diodes T
1, T
2, T
3 & D
1, D
2, D
3
-The supply voltage is
sinusoidal 3 phase AC
voltage with phase
sequence ABC
-
The output of rectifier is
-
The output of rectifier is fed to the armature of a
DC motor
-Here 1 thyristor& 1 Diode conducts at a time
-Each thyristorconducts for a duration of 120◦
38

39. Working –for α= 30
◦
- The voltage V
AB become more positive for ωt>60
◦
and hence T
1is triggered at
ωt=(60
◦
+α), it starts conduction & T
5gets turned OFF. Diode D
1is also forward
biased & it conducts
- T
1& D
1conducts from ωt=(60
◦
+α) to ωt=120
◦
, load voltage is V
AB
- At ωt=120
◦
, V
ACbecome more positive than V
AB. Hence D
1gets reverse biased and
D
gets forward biased.
D
2
gets forward biased.
- T
1& D
2conducts from ωt=120
◦
to ωt=(180
◦
+α), load voltage is V
AC
- At ωt=(180
◦
+α), T
2is triggered & is turned ON & T
1gets turned OFF
- T
2& D
2conducts from ωt=180
◦
+αto ωt=240
◦
, load voltage is V
BC
- At ωt=240
◦
, V
BA become more positive than V
BC. Hence D
2gets reverse biased and
D
3gets forward biased.
-
T
2
& D
3
conducts from
ω
t=240
◦
to
ω
t=(300
◦+
α
), load voltage is V
BA
-
T
2
& D
3
conducts from
ω
t=240
to
ω
t=(300
+
α
), load voltage is V
BA
- At ωt=(300
◦
+α), T
3is triggered & is turned ON & T
2gets turned OFF
- T
3& D
3conducts from ωt=(300
◦
+α) to ωt=360
◦
, load voltage is V
CA
- At ωt=360 ◦, V
CBbecome more positive than V
CA. Hence D
3gets reverse biased and
D
1gets forward biased.
- T
3& D
3conducts from ωt=360
◦
to ωt=(420
◦
+α), load voltage is V
CB
- The cycle is then repeated
39

40. For α= 30◦ Average value of output voltage (Voltage across arm ature), Average value of output voltage (Voltage across arm ature),








− + =
∫ ∫
+
+ 3
2
3 3
2
)
3
(
3
2
1
π
α
π
απ
π
ω
π
ω ω ω
π
td t Sin V tdt Sin V V
m m avg










+






=
− − −
+
+
)]
3
( ]
3
2
3
2
3
2
3
2
3
π
ω ω
απ
π
π
α
π
π π
t Cos t Cos
m m
V V
40

41. i.e, V
avg
(i.e, by varying α, the average value of
output voltage applied to armature can be
controlled & hence speed of the motor)
) 1(
23
α
π
Cos
V
m
+ =
α= 60
◦
41

42. - This is a single quadrant drive ( I
st
quadrant –forward
motoring)
- Here the average output voltage of the converter i s always
positive
-
So regenerative braking (
IV
th
quadrant operation) is not
-
So regenerative braking (
IV
quadrant operation) is not
possible and we can go for plugging (motor will wor k as
generator during braking and electrical energy prod uced is
wasted in an external resistance connected in seri es with
armature during braking)
42

43. Dual converter fed Separately excited DC motor Driv e
- A dual converter consist of two fully controlled r ectifiers connected in
anti parallel across the armature of DC motor
- For power rating upto10kW, single phase fully cont rolled rectifiers are
used
-
For higher ratings, 3 phase fully controlled rectif iers are used
-
For higher ratings, 3 phase fully controlled rectif iers are used
- Here 4 quadrant operation is possible
- A 3 phase dual
converter fed DC
motor drive system
is shown in figure is shown in figure
43

44. - The operation of 3 phase & 1 phase dual converter remains same. The
difference is in the rectifier configuration and su pply
- Here Rectifier A which provides a positive motor c urrent & voltage in
either direction, allows motor control in quadrants I & IV
- Rectifier B provides a negative motor current & vo ltage in either
direction, allows motor control in quadrants III & II direction, allows motor control in quadrants III & II
- There are 2 modes of control for a Dual converter
1. Non simultaneous or Non circulating current contr ol method
- Here one rectifier is operated at a time.
- Consequently no circulating current flows & induc tors L
1
& L
2
are not
required
- This eliminates losses associated with circulating current & weight &
volume associated with inductors
- Here automatic control of firing pulses is done & control is
complicated
44

45. When rectifier A operates alone ,
- If firing angle α
A
is varied from 0 to π/2, the rectifier A acts as a
rectifier itself & machine operates as motor in for ward direction
(Quadrant I operation)
- If firing angle α
A
is varied from π/2 to π, the rectifier A acts as an
inverter & machine operates as generator in reverse direction inverter & machine operates as generator in reverse direction (Quadrant IV operation)
When rectifier B operates alone,
- If firing angle α
B
is varied from 0 to π/2, the rectifier B acts as a
rectifier itself & machine operates as motor in rev erse direction
(Quadrant III operation)
-
If firing angle
α
is varied from
π
/2 to
π
, the rectifier B acts as an
-
If firing angle
α
B
is varied from
π
/2 to
π
, the rectifier B acts as an
inverter & machine operates as generator in forward direction
(Quadrant II operation)
45

46. Speed reversal is carried out as follows,
- When operating in Quadrant-I, rectifier A will be supplying the motor
& B will not be operating
- Firing angle of rectifier A is set at the highest value
- Now rectifier A work as an inverter & forces the a rmature current to
zerozero
- After zero current is sensed, a dead time of 2 to 10msec is provided
to ensure turn OFF of thyristorsof rectifier A
- Now firing pulses are given to rectifier B
- At first firing angle is set at the highest value, so that motor brakes,
speed reduces to zero
- As speed reduces control loop adjusts firing angle continuously so
that motor accelerates in the reverse direction
46

47. 2. Simultaneous or Circulating current control meth od - Here both the rectifiers are controlled together
- In order to avoid circulating current between rect ifiers, they are
operated to produce same DC output voltage across t he motor
terminal
Thus V
A
+V
B
= 0,
i.e
, (3V
m
/
π
)Cos
α
A
+
(3V
m
/
π
)Cos
α
B
= 0
Thus V
A
+V
B
= 0,
i.e
, (3V
m
/
π
)Cos
α
A
+
(3V
m
/
π
)Cos
α
B
= 0
i.e, Cosα
A
+Cosα
B
= 0 i.e, α
A
+α
B
= 180
- If the above relation is satisfied, DC circulating current will be zero
- But due to difference in instantaneous output volt age of rectifiers,
there will be an AC circulating current
-
Inductors L
1
& L
2
are added in the circuit to reduce AC circulating
-
Inductors L
1
& L
2
are added in the circuit to reduce AC circulating
current
Operation
- When firing angle α
A
is varied from 0 to π/2 and firing angle α
B
is
varied from π/2to π, Rectifier A operates as a rectifier & Rectifier B
operates as an inverter.
47

48. The machine will either operates as a motor in Quad rant I by taking
power from Rectifier A or as a generator in quadran t II by feeding
back to supply through Rectifier B
- When firing angle, α
A
is varied from π/2to πand firing angle, α
B
is
varied from 0 to π/2, Rectifier A operates as an inverter & Rectifier B
operates as a rectifier. The machine will either op erates in Quadrant operates as a rectifier. The machine will either op erates in Quadrant IV or III. The machine will either operates as a mo tor in Quadrant III
by taking power from Rectifier B or as a generator in quadrant IV by
feeding back to supply through Rectifier A
The speed reversal is carried out as follows
- When operating in Quadrant I, rectifier A will be rectifying & rectifier
B will be inverting. B will be inverting.
- For speed reversalα
A
is increased & α
B
is decreased. The motor back
emfexceeds V
A
& V
B
- The armature current shifts to rectifier B & motor operates in
Quadrant II
- The control loop controls α
A
& α
B
instantaneously
48

49. Closed loop control of separately excited DC motor
drive
- The closed loop speed control above & below base s peed can be
explained by fig below

50. - The drive employs an inner current control loop & an outer speed
control loop
- This drive can operate at a constant field current & variable armature
voltage below base speed and at a constant armature voltage &
variable field current above base speed
-
Both armature & field are supplied from fully contr olled rectifiers
-
Both armature & field are supplied from fully contr olled rectifiers
Operation below base speed
- Here he firing angle of field rectifier is maintai ned at zero, applying
rated voltage to field
- This ensures rated field current and excitation fo r motor operation
below base speed
-
When actual speed is low compared to reference spee d, the speed
-
When actual speed is low compared to reference spee d, the speed error is high
- As a result current limiter saturates & sets the c urrent reference at
maximum permissible value
- Now the drive accelerates at maximum available cur rent & torque.

51. - When actual speed reaches close to reference speed , current limiter
desaturates& speed settles down to reference speed
Operation above base speed
- The firing angle of armature rectifier is reduced to increase V
a
.
- The motor accelerates, E increases, e
f
decreases & field current
reduces reduces
- Speed increases & I
f
decreases until motor speed becomes equal to
reference speed
DC series motor drive for traction applications
Electric traction Electric traction - Electric traction –used to transport men & materia l from one place
to another
- It involve electric train, electric buses, trams & trolleys

52. Electric trains
- Electric trains run on fixed rails
- They are classified into main line train & suburban t rain
- Suburban train is used for transporting men within a city or
between cities located at small distances
-
All other services come under main line service
-
All other services come under main line service
- In an electric train, the driving motor & power mo dulators are
housed in the locomotive
- An overhead transmission line is used to supply power to the
train
-
Electric traction is classified into single phase AC & D C
-
Electric traction is classified into single phase AC & D C depending on the supply
- In India, 25kV, 50Hz AC supply is used for AC tractio n
- 1500V DC traction is used in Bombay –Igatpurisection
- The schematic of an electric locomotive is shown in n ext slide

53. - A pantograph is used to collect power from contact wire
- Inside locomotive, a transformer will step down th e voltage & fed to
power modulator which in turn powers the driving mo tors ( AC or DC
motors)
- Locomotive power rating can be as high as 6000HP & more
-
Powering such large 1 phase load creates unbalances in the system
-
Powering such large 1 phase load creates unbalances in the system

54. - To avoid unbalances, track supply is divided into different sections &
connected to different phases
- If 3 phase system has high capacity, the unbalance s are not affected
Nature of traction load
- When a train runs a number of frictional forces li ke bearing friction,
friction between wheel & rail, air friction etc. op poses the motion friction between wheel & rail, air friction etc. op poses the motion
- All these frictional forces together called train resistance
- The variation in train resistance with speed is sh own in figure
- In train resistance, strictionhas a large value & influence of air
friction is high, which varies as the
square of speed square of speed
- When deciding torque requirements of
driving motors, torque components
required to provide acceleration & to
overcome gravity must be considered

55. Important features of traction drives
- Large torque is required during start & acceleration in ord er to
accelerateheavymass
- Themotorissubjectedtotorqueoverloadsduringacceleration
-
Because
of
economic
reasons,
single
phase
supply
is
used
in
AC
traction
-
Because
of
economic
reasons,
single
phase
supply
is
used
in
AC
traction
- Thelocomotiveratingcanbe6000HP&higher
- Thesupplyhassharpvoltagefluctuations
- Theharmonicsinjectedintothesource,bothinDC&ACtractioncan
causemaloperationofsignals&interferenceintelephonelines
- Dynamicbrakingiswidelyused
-
Regenerative
braking
is
used
when
the
energy
saved
is
large
enough
-
Regenerative
braking
is
used
when
the
energy
saved
is
large
enough
tojustifytheadditionalcostofdrive&transmissionlines
- Wheelslipshouldbeavoided
- Suburban trains have more than one motor coach. Electrical
interconnectionisprovidedbetweenthetwosothatthedriveforall
motor coaches can be controlled fromthe master controller in the
motorcoachinthefront

56. Conventional DC & AC traction drives
-In India, conventionally DC series motors are used in traction drives
-Conventional DC/AC traction drives are generally c lassified into two,
1. DC/AC traction without controlled converters
2. DC/AC traction with controlled converters
1.
DC/AC traction without controlled converters
1.
DC/AC traction without controlled converters
- Here the traction drives does not use any controll ed converters
a) DC traction drive employing resistance control
-The DC supply is usually 1500V/750V.
-Uses DC series motors with rating 750V/375V
-Two series motors are permanently connected in ser ies to form one
pair pair
-The arrangement is shown in the figure
-Here, to get a speed variation from 0 to half rate d speed, the motor
pairs are connected in series as shown in figure (a )
-The speed variation is achieved by varying the res istances

57. - To get a speed variation from 0 to rated speed, th e motor pairs are
connected in parallel as shown in figure (b)
- The speed variation is achieved by varying the res istance
- This method got poor efficiency & require frequent maintenance due
to the presence of moving contacts

58. b) AC traction drive using transformer with on load tap changer
-Here a step down transformer with suitable tapings are used
-The transformer secondary is connected to a diode rectifier
-DC output of rectifier is fed to DC series motors
-The speed control is achieved by varying the input AC voltage of diode
rectifier by changing rectifier by changing transformer taps
-This method got high
efficiency, high initial
cost, frequent
maintenance. maintenance.

59. 2. DC/AC traction with controlled converters - Here controlled converters are used for controllin g the drive
Advantages
- High efficiency
- Low maintenance
- Higher acceleration & deceleration
- Flexible control
- Longer life
a) DC traction using chopper
-here chopper is used to get a variable DC supply f rom a fixed DC input -
variable DC from chopper is fed to DC series motor
-
variable DC from chopper is fed to DC series motor

60. b) AC traction using controlled rectifiers - Here controlled rectifiers are used to convert inp ut AC to DC
- Now this DC is fed to DC series motor
- DC output voltage of rectifier is controlled by va rying firing angle of
rectifier
-
A transformer is
-
A transformer is used to step down
the input AC (25kV)
voltage to a
suitable value