Part of Lecture series on EEE-413, Electrical Drives (DC Drives) delivered by me to students of VIII Semester B.E. (Electrical), Session 2018-19. Z. H. College of Engg. & Technology, Aligarh Muslim University, Aligarh. Missing materials will be uploaded shortly. Please comment and feel free to ask anything related. Thanks!

1. EEE-413N
Electric Drives
Lec. 11 & 12
Mohd. Umar Rehman
umar.ee.amu@gmail.com
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2. Braking of DC Motors
Braking is required to quickly stop the motor or limit the speed.
In applications like cranes or hoists the torque of the drive motor may
have to be controlled so that the load does not have any undesirable
acceleration, e.g. in the case of lowering of loads under the influence
of gravity.
Electric braking methods are superior than the mechanical methods
due to a number of advantages.
In electric braking the motor acts as a generator developing a
negative torque that opposes the motion.
Braking can be classified as follows:
(a) Regenerative Braking
(b) Dynamic Braking
(c) Plugging
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3. Regenerative Braking
In this method, the machine is made to run as a generator whose
input energy is the K. E. of the machine and load.
The power generated is fed-back to the source.
The source must have the capability to store the energy or to supply
the energy to other loads.
Regenerative Braking can be implemented in the following ways:
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4. Regenerative Braking: Method (i)
By lowering the armature voltage, such that V < E
This causes reversal of armature current and consequently the torque
developed also gets reversed.
Motor speed drops quickly because of the negative torque.
Armature voltage is lowered gradually so that the required braking
torque is generated.
The K. E. of the machine is converted into electrical energy and
supplied to the source.
By this method, braking may be obtained from base-speed to a very
low speed.
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5. Regenerative Braking: Method (ii)
For motors fed by a constant voltage source, regenerative braking is
possible if the motor speed increases beyond the rated no-load speed
(E > V ).
This is possible only if the motor drives some active load.
If speed increases beyond a certain limit, the armature current may
exceed the safe limit.
Field supply may be reduced to limit the armature current in such
cases.
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6. Regenerative Braking: Method (iii)
When the motor is fed by a constant voltage source, regenerative
braking is possible below the rated speed, when supplied through a
step-up chopper.
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7. Remarks
Regenerative braking cannot be used for DC series motor, because as
the armature current reverses, the field also reverses, reversing E and
causing a short-circuit condition.
Regenerative braking can only be used when there are extra loads
connected to the line and that their capacity is ≥ regenerated power.
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8. Regenerative Braking Characteristics
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9. Dynamic Braking
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10. Dynamic Braking...Contd
Armature of the motor is disconnected from the supply and the
armature circuit is closed by a suitable resistance RB.
The generated energy is dissipated in Ra & RB
Speed-Torque Equation during motoring:
ωm =
V
K
−
Ra
K2
T
Speed-Torque Equation during dynamic braking:
ωm = −
Ra
K2
T ( V = 0)
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11. Dynamic Braking...Contd
In motoring mode, E V , hence if the armature circuit terminals are
directly short-circuited, then the armature circuit that will flow in the
reverse direction will be Ia
V
Ra
which is nearly the same current
drawn by the motor when started DOL.
To limit this current to a safe value, resistance used during starting
may be connected to the armature.
Motor decelerates due to -ve torque, as the speed decreases, armature
current and hence braking torque also decreases.
To quickly stop the motor, the braking resistance may be gradually
decreased in a way similar to starting
Dynamic braking is an inefficient method as the generated energy is
wasted as heat in the braking resistor. However, it can be used to
quickly stop the motor during power failure.
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12. Dynamic Braking...Contd
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13. Plugging
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14. Plugging...Contd
In this method, armature terminals of the motor are suddenly
reversed.
The armature current also gets reversed and is given by:
Ia =
V + E
Ra
2V
Ra
This large value of current can be limited by inserting a suitable
resistance RB in the armature circuit. Speed-Torque equation now
becomes
ωm = −
V
K
−
Ra + RB
K2
T
The braking torque is enough even near zero speeds, therefore
variable braking resistance is not used.
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15. Plugging...Contd
Moreover, the supply must be disconnected when close to zero speed,
otherwise the motor will speed up in the reverse direction.
This braking method is also inefficient because the energy generated
as well as the energy supplied from the source is also wasted in the
resistance.
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16. Plugging...Contd
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17. Multi-quadrant Operation
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18. Multi-quadrant Operation...Contd
For a single quadrant operation (I or III), the source does not require
the capability of reversing the supply voltage or current.
For two quadrant operation (I & II), the source should have the
capability of supplying voltage of either polarity and allowing current
to flow in either directions.
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19. Task
1 Why a DC motor drive is essentially a 4 quadrant drive?
2 Name the power electronic converters that are capable of single
quadrant, two quadrant and four quadrant operation.
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