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1. • UNIT-II: CONVERTER CONTROL:
Principle of phase control – Fundamental
relations; Analysis of series and separately
excited DC motor with single-phase and three-
phase converters – waveforms, performance
parameters, performance characteristics.
Continuous and discontinuous armature current
operations; Current ripple and its effect on
performance; Operation with freewheeling
diode; Implementation of braking schemes;
Drive employing dual converter.

2. INTRODUCTION
• Controlled rectifiers are used to get dc voltage from an ac
source of fixed voltage.
• As thyristors are capable of conducting current only in one
direction ,all these rectifiers are capable of providing current
only in one direction.
• Fully controlled rectifiers provide control of dc voltage in
either direction and allow motor control in quadrants 1 and 4.
• Half controlled rectifiers allow dc voltage in only one direction
and allow motor control in quadrant 1 only.

3. •

4. Single phase fully controlled rectifier control of
dc separately excited motor

5. • In a cycle of source voltage, thyristors T1 and T3 are
given gate signals from 𝛼 to 𝛱 and thyristors T2 and
T4 are given gate signals from 𝜋 + 𝛼 to 2𝜋.
• When the armature current does not flow
continuously ,the motor is said to operate in
discontinuous conduction mode.
• When the armature current flows continuously ,it
operates in continuous conduction mode.

6. • In the discontinuous conduction mode,
For 𝛼 ≤ 𝜔𝑡 ≤ 𝛽 ,motor is connected to
source. Hence ,𝑣𝑎 = 𝑣𝑠 .
For 𝛽 ≤ 𝜔𝑡 ≤ 𝛱 + 𝛼
When 𝑖𝑎 = 0 , 𝑣𝑎 = 𝐸.

7. • For discontinuous conduction mode,
Voltage across the armature is given by
Motor speed is given by

8. • For continuous conduction,
Armature Voltage is given as
Motor speed is given by

10. • Under the assumption of continuous conduction, dc output
voltage of the rectifier varies with the α as shown in
fig.5.28(a).[in the next slide]
• When working in quadrant 1, 𝜔𝑚 is positive and α≤90° and
the polarities of 𝑉
𝑎 𝑎𝑛𝑑 𝐸 𝑎𝑟𝑒 𝑎𝑠 𝑠ℎ𝑜𝑤𝑛 𝑖𝑛 𝑓𝑖𝑔. 5.28(𝑏).
• For 𝛼 > 900,
E, 𝑉
𝑎 ,𝜔𝑚 reverse as shown in fig.5.28(c).
Since armature current is in the same direction, machine
works as generator producing breaking torque.
• Since generated power is supplied to the source in this
operation , it is called regenerative breaking.

12. Single phase half controlled rectifier control of
dc separately excited motor

13. In the discontinuous conduction mode,
armature current freewheels through the path formed by D1 and D2
between the interval 𝜋 ≤ 𝜔𝑡 ≤ 𝛽.Hence the armature terminal
voltage becomes zero. So there will be no negative voltage unlike in
the fully controlled rectifier.
• For discontinuous conduction mode,

14. For continuous conduction,

17. Three phase fully controlled rectifier control of
dc separately excited motor

18. • Thyristors are fired in the sequence of their numbers
with a phase difference of 60° by gate pulses of 120°
duration.
• Each thyristor conducts for 120°, and two thyristors
conduct at a time-one from upper group and other
from lower group applying respective line voltage to
the motor.
• Here the discontinuous conduction is neglected
because it occurs in a narrow region of operation.

19. • For continuous conduction,
where Vm is the peak line voltage.

21. Rectifier control of dc series motor
• Single phase controlled rectifier fed dc series motors are
employed in traction
• As back emf decreases with armature current, discontinuos
conduction occurs only in narrow range of operation. Hence,
it can be neglected here.

23. Motor operation is described by the following
equations
𝐸𝑎 = 𝑘𝑎𝜔𝑚

25. Drive employing dual converter
• A Dual-converter consists of two fully controlled rectifiers
connected in anti-parallel across the armature as shown.

26. • Rectifier A ,which provides positive motor current
and voltage in either direction, allows motor control
in quadrants 1 and 4.
• Rectifier B provides motor control in quadrants 3 and
2 ,because it gives negative motor current and
voltage in either direction.

27. • In order to avoid the dc circulating current
between rectifiers , they are operated to
produce same dc voltage across the motor
terminals. Thus
𝑣𝐴 + 𝑣𝐵 = 0
which yields the condition
𝛼𝐴 + 𝛼𝐵 = 1800

28. • Although control of firing angle according to
the above relation prevents dc circulating
current , ac current does circulate due to
difference between instantaneous output
voltages of the two rectifiers.
• Inductors L1 and L2 are added to reduce ac
circulating current.

29. Reference
• Fundamentals of Elecrtical Drives
-Gopal K.Dubey

30. THANK YOU