UTILIZATION OF ELECTRICAL ENERGY

CHAPTER 3: Control of Electric Drives

L-3-2

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 Electric Braking

• If the load is removed from an electric motor and supply to it be disconnected,

it will continue to run for some time due to inertia.
• The time elapsing before it stops will be especially long if the motor is massive
and has run at high speed.
• In many cases that the motor and its driven machine to be stopped quickly. In
fact, quick stopping of a motor is more essential than quick starting.
• Delay in starting up a motor only causes the machinery to stand idle; a delay in
stopping a motor may result in heavy damage to equipment or to the
manufactured products and even the loss of human life.
• The braking should be quick and reliable in action & the braking torque must
be controllable.

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 Contd.....

• In mechanical braking, the stored energy is dissipated as heat by brake shoes or

brake lining which rubs against the brake drum. In electric braking, the stored
energy of rotating part is converted to electrical energy & dissipated by
resistance in the form of heat or returned to supply.
Advantages of Electrical Braking over Mechanical Braking
 Lower maintenance cost
 In some cases part of energy is returned to supply
 In mechanical braking enormous heat is produced at brake shoes which leads to
failure of brakes while electrical braking there is minimal failure

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 Contd.....

Disadvantages & Limitations

 Electrical Braking though almost stop the machine but it cannot keep it
stationary, for which mechanical brakes is required additionally.
 Selection of motor is limited as motor has to behave as both generator during
braking & motor during motoring.

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 Types of Electrical Braking

1. Dynamic Braking or Rheostatic Braking

2. Plugging or Reverse Current Braking
3. Regenerative Braking

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 1. Dynamic (Rheostatic) Braking
• In this case, during braking, the motor is operated as a generator so that the mechanical energy is converted
into electrical energy which is dissipated as heat in the resistance connected as an electrical load.
• In dynamic braking, motor armature is disconnected from the supply & connected across the Braking
Resistance, RB . The generated energy is dissipated in (Ra+RB).
Case – I: Separately excited DC Motor
The connection diagram for motoring & braking are shown in the figure aside.
The supply to the field wind is kept as it is & it must be remembered that supply
to the field must not fail during dynamic braking otherwise there will be no
braking effect. This problem can be overcome by providing series winding in the
armature circuit.

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 Contd....

The speed torque curve for motoring & braking are:

∗
wm = - (Motoring)
∗ ∗
∗( )
wm = - (Braking)
∗
The slope of the characteristics depends on
the total resistance in the armature circuit.

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 Contd.....

Case – II: DC Series Motor

For DC series motor disconnecting the supply will not produce braking effect
because current through the field winding will also reverse. Thus, field winding
connection needs to be altered.

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 2. Plugging (Reverse Current Braking)
• Plugging involves reconnecting the power supply to the motor so that it tends to drive in the opposite
direction. It is obvious that, left to itself, the system will come to rest & then accelerate in the reverse
direction. In the case where it is required to bring the drive system to rest, it is necessary to include a relay
to disconnect the supply exactly at the instant when motor stops.
• This is the most inefficient technique of electric braking since, in addition to dissipating the electrical
energy converted from mechanical energy, in resistance of the circuit, the electrical energy drawn from the
supply is wasted.
For DC Shunt Motor
∗
wm = - (Motoring)
∗ ∗
∗( )
wm = - (Plugging)
∗ ∗

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 Contd.....

For DC series Motor

In case of DC series motor it should be ensured that the flow of current in the field
winding remains unchanged when the current flow in the armature winding is
reversed.

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 Contd.....

• In order to reverse the torque developed it is necessary to reverse the magnetic

field or the armature current (but not both at the same time).
• Since the field winding usually has a large time constant due to large
inductance armature current is reversed.
• When the armature current is reversed the back EMF will no longer oppose the
applied voltage but will aid it, hence additional resistance RB is inserted in the
armature circuit, simultaneously with the reversing of the connection of the
armature.

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 3. Regenerative Braking

• In case of regenerative braking the energy is supplied to the source and for this
to happen Eb>V and in which case the armature current is negative. Since Eb=
kbφwm and flux cannot be increased above the rated value because of
saturation, the only possibility for back EMF to exceed applied voltage is by
increasing the speed above the rated speed.
• Regeneration in DC series motor is not possible in an ideal manner. In case of
DC series motor both field current & armature current are same and that makes
field flux and armature current directly proportional. An increase in speed is
followed by decrease in armature current & thus field flux. Back EMF being
directly proportional to field flux falls & thus back EMF in DC series motor
cannot be greater than the supply voltage. Thus in that case regeneration would
not occur.

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Contd.....

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 Summary

Rheostatic Braking: Remove Supply & dump energy in Resistor

Plugging: Reverse the supply & produce braking torque. Since back emf supports
supply, add RB to limit the current.
Regenerative : Make Eb>V, to supply energy back to source.

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 Example

A 220V 970 RPM 100A dc separately excited motor has an armature resistance of
0.05 Ω it is braked by plugging from an initial speed of 1000 RPM. Calculate:
a. Resistance to be placed in the armature circuit to limit the braking current to
twice the full load current.
b. Braking Torque
c. Torque when speed has fallen to zero.

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 Example
A 220V 970 RPM 100A dc separately excited motor has an armature resistance of 0.05 Ω it is braked by plugging from an initial
speed of 1000 RPM. Calculate:
a. Resistance to be placed in the armature circuit to limit the braking current to twice the full load current.
b. Braking Torque
c. Torque when speed has fallen to zero.
Solution:
Ia =
Or, 100*0.05 = 220 – Eb1
Or, Eb1 = 220 – 5 = 215 V ; N1 = 970 rpm
We have, Eb ∝ N
= ; Eb2 = 215*1000/970 = 221.65 V
Hence, Iplugging = 2* Ifull = 2*100 = 200 A
Iplugging = ; 200*(0.05+RB) = 220+221.65; RB = 2.15825 Ω [a]

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 Example
A 220V 970 RPM 100A dc separately excited motor has an armature resistance of 0.05 Ω it is braked by plugging from an initial
speed of 1000 RPM. Calculate:
a. Resistance to be placed in the armature circuit to limit the braking current to twice the full load current.
b. Braking Torque
c. Torque when speed has fallen to zero.
Solution:
. ∗
Braking Torque = = = 423.32 Nm [b]
/
When speed has fallen to zero, Eb = 0
Hence, Ia = (V+Eb)/(Ra+RB) = 220/(0.05+2.15825) = 99.626 A
T ∝ φIa ; T ∝ Ia (Separately excited)
=
.
=
.
T2 = Torque when speed has fallen to zero = 210.86 Nm [c]

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End of L-3-2

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