DC Motor Braking MCQ Quiz in తెలుగు - Objective Question with Answer for DC Motor Braking - ముఫ్త్ [PDF] డౌన్‌లోడ్ కరెన్

Concept:

Plugging:

• It is also called reverse current braking in which the armature terminals or the supply polarity of a separately excited or shunt motor when running are reversed.
• In braking mode, as armature terminals are reversed so the motor tends to run in the opposite direction.
• Due to the reversal of armature connections, applied voltage (V) and back emf (E_b) start acting in the same direction around the circuit. That we can observe in the below figure

Figure: Motoring mode of operation

Figure: Braking mode of operation

Apply KVL in the supply loop of circuit regarding breaking mode, we get

V - I_aB (R_B + R_a) + E_b = 0

=> I_aB = \(\frac{{{\rm{V}} + {{\rm{E}}_{\rm{b}}}}}{{{{\rm{R}}_{\rm{a}}} + {R_B}}}\)

Where

R_a = armature resistance

R_B is braking or external resistance required to limit armature current under braking mode.

I_aB = braking current

V = terminal voltage

E_b = back emf

Braking torque T_aB = E_b I_aB / ω 

Note:

During plugging the electromagnetic torque opposes the rotation. Then the speed will be reduced gradually to zero. Even at zero speed, I_aB exists due to supply voltage. So there will be some torque due to this the motor will rotate in the opposite direction. So before coming near to zero supply has to be disconnected and mechanical brakes applied.

Calculation:

Given that

Supply voltage V = 200 V

Armature current I_a = 10 A

Armature resistance R_a = 2Ω 

Breaking current I_aB = 10 A

We know that, under motoring mode

Back emf E_b = V - I_aR_a = 200 - 10 × 2 = 180 V

I_aB = \(\frac{{{\rm{V}} + {{\rm{E}}_{\rm{b}}}}}{{{{\rm{R}}_{\rm{a}}} + {R_B}}}\)

10 = (200 + 180)/(2 + R_B)

=> 2 + R_B = 38

R_B = 36Ω 

∴ The external resistance required to limit the armature current at 10 A is R_B = 36Ω 

Advantages of Electrical Braking Over Mechanical Braking:

• In mechanical braking due to excessive wear on the brake drum, liner, etc., it needs frequent and costly replacement. This is not needed in electrical braking and so electrical braking is more economical than mechanical braking.
• Due to the wear and tear of the brake liner frequent adjustments are needed thereby making the maintenance cost.
• Mechanical braking produces metal dust, which can damage bearings. Electrical braking has no such problems.
• If mechanical brakes are not correctly adjusted it may result in shock loading of machine or machine parts in case of lift, trains which may result in discomfort to the occupants.
• Electrical braking is smooth.
• In mechanical braking the heat is produced at the brake liner or brake drum, which may be a source of failure of the brake. In electric braking, the heat is produced at a convenient place, which in no way is harmful to a braking system.
• In regenerative braking electrical energy can be returned back to the supply which is not possible in mechanical braking.
• The noise produced is very high in mechanical braking.
   

Note: Only disadvantage of electrical braking is that it is ineffective in applying holding torque.

There are three types of electrical braking done in a DC Motor

• Dynamic Braking
• Plugging
• Regenerative Braking

Dynamic Braking:

• It is also known as Rheostatic braking.
• In this type of braking, the DC motor is disconnected from the supply and a braking resistor Rb is immediately connected across the armature.
• The motor will now work as a generator and produces the braking torque.
• During electric braking when the motor works as a generator, the kinetic energy stored in the rotating parts of the motor and a connected load is converted into electrical energy.
• It is dissipated as heat in the braking resistance Rb and armature circuit resistance Ra.
• Dynamic Braking is an inefficient method of braking as all the generated energy is dissipated as heat in resistances.

Plugging:

• It is also known as reverse current braking.
• The armature terminals or supply polarity of a separately excited DC motor or shunt DC motor when running are reversed.
• Therefore, the supply voltage V and the induced voltage Eb i.e. back emf will act in the same direction.
• The effective voltage across the armature will be V + Eb which is almost twice the supply voltage.
• Thus, the armature current is reversed and a high braking torque is produced.
• Plugging is an inefficient method of braking because, in addition to the power supplied by the load, the power supplied by the source is wasted in resistances.

Electric Braking:

Sometimes it is required to stop a motor quickly in case of emergency or to save time if the motor is being used for frequently repeated operations.

The motor and its load may be brought to rest by either mechanical (friction) braking or electric braking.

In mechanical braking, the motor is stopped due to the friction between the moving parts of the motor and the brake shoe i.e. kinetic energy of the motor is dissipated as heat.

Mechanical braking has several disadvantages including non-smooth stops and greater stopping time.

In electric braking, the kinetic energy of the moving parts (i.e., motor) is converted into electrical energy that is dissipated in resistance as heat.

Types of Braking in DC Motor:

(i) Rheostatic or Dynamic braking
(ii) Plugging
(iii) Regenerative braking

Regenerative braking: In this type braking back emf (E_b) is greater than the supply voltage (V), which reverses the direction of the motor armature current. The motor begins to operate as an electric generator.

Dynamic braking: In this type of braking, the DC motor is disconnected from the supply and a braking resistor Rb is immediately connected across the armature. The motor will now work as a generator and produces the braking torque.

Plugging: In this method, the terminals of supply are reversed, as a result of the generator torque also reverses which resists the normal rotation of the motor and as a result the speed decreases

Regenerative method of braking is the moment where back e.m.f. of rotor is more than the applied voltage.

Electric Braking:

• Sometimes it is required to stop a motor quickly in case of emergency or to save time if the motor is being used for frequently repeated operations.
• The motor and its load may be brought to rest by either mechanical (friction) braking or electric braking.
• In mechanical braking, the motor is stopped due to the friction between the moving parts of the motor and the brake shoe i.e. kinetic energy of the motor is dissipated as heat.
• Mechanical braking has several disadvantages including non-smooth stops and greater stopping time.
• In electric braking, the kinetic energy of the moving parts (i.e., motor) is converted into electrical energy that is dissipated in resistance as heat.
• Alternatively, it is returned to the supply source (Regenerative braking).

Regenerative braking

• In regenerative braking, the motor is run as a generator. As a result, the kinetic energy of the motor is converted into electrical energy and returned to the supply.
• Here the back e.m.f. of rotor is more than the applied voltage
• By means of that Regenerative braking results in Energy saving opportunities in the motor.

Method 1:

In this method, the field winding is disconnected from the supply, and the field current is increased by exciting it from another source as shown below.

• It results in, induced EMF (E) exceeds the supply voltage V and the machine feeds energy into the supply.
• Thus braking torque is provided up to the speed at which induced EMF and supply voltage are equal.
• As the machine slows down, it is not possible to maintain induced EMF at a higher value than the supply voltage.
• Therefore, this method is possible only for a limited range of speed.
   

Method 2:

• In this method, the field excitation does not change but the load causes the motor to run above the normal speed (e.g., descending load on a crane or train on downward slope or hills).
• In that case, the induced EMF (E) becomes greater than the supply voltage V
• The direction of armature current I, therefore, reverses but the direction of shunt field current If remains unaltered as shown.

Hence the torque is reversed and the speed falls until E becomes less than V.

Electric Braking:

• Sometimes it is required to stop a motor quickly in case of emergency or to save time if the motor is being used for frequently repeated operations.
• The motor and its load may be brought to rest by either mechanical (friction) braking or electric braking.
• In mechanical braking, the motor is stopped due to the friction between the moving parts of the motor and the brake shoe i.e. kinetic energy of the motor is dissipated as heat.
• Mechanical braking has several disadvantages including non-smooth stops and greater stopping time.
• In electric braking, the kinetic energy of the moving parts (i.e., motor) is converted into electrical energy that is dissipated in resistance as heat.
• Alternatively, it is returned to the supply source (Regenerative braking).
   

Regenerative braking

• In regenerative braking, the motor is run as a generator. As a result, the kinetic energy of the motor is converted into electrical energy and returned to the supply.
• By means of that Regenerative braking results in Energy saving opportunities in the motor.
   

Method 1:

In this method, the field winding is disconnected from the supply, and the field current is increased by exciting it from another source as shown below.

• It results in, induced EMF (E) exceeds the supply voltage V and the machine feeds energy into the supply.
• Thus braking torque is provided up to the speed at which induced EMF and supply voltage are equal.
• As the machine slows down, it is not possible to maintain induced EMF at a higher value than the supply voltage.
• Therefore, this method is possible only for a limited range of speed.
   

Method 2:

• In this method, the field excitation does not change but the load causes the motor to run above the normal speed (e.g., descending load on a crane or train on downward slope or hills).
• In that case, the induced EMF (E) becomes greater than the supply voltage V
• The direction of armature current I, therefore, reverses but the direction of shunt field current If remains unaltered as shown.

Hence the torque is reversed and the speed falls until E becomes less than V.

Voltage (V) = 400 V

Load current (I_L) = 30

Armature resistance (R_a) = 1Ω

Shunt resistance (R_sh) = 250Ω

Field current \(\left( {{I}_{f}} \right)=\frac{V}{{{R}_{sh}}}=\frac{400}{250}=1.6~A\)

Armature current (I_a) = I_L – I_sh

= 30 – 1.6 = 28.4 A

Back emf (E_b) = V – I_aR_a

= 400 – (28.4) (1)

= 371.6 V

In plugging mode,

\(V+{{E}_{b}}={{I}_{ab}}\left( {{R}_{a}}+{{R}_{b}} \right)\)

\({{R}_{a}}+{{R}_{b}}=\frac{400+371.6}{1.5\times {{I}_{a}}}\)

Electric Braking:

• Sometimes it is required to stop a motor quickly in case of emergency or to save time if the motor is being used for frequently repeated operations.
• The motor and its load may be brought to rest by either mechanical (friction) braking or electric braking.
• In mechanical braking, the motor is stopped due to the friction between the moving parts of the motor and the brake shoe i.e. kinetic energy of the motor is dissipated as heat.
• Mechanical braking has several disadvantages including non-smooth stops and greater stopping time.
• In electric braking, the kinetic energy of the moving parts (i.e., motor) is converted into electrical energy that is dissipated in resistance as heat.
• Alternatively, it is returned to the supply source (Regenerative braking).
   

Regenerative braking

• In regenerative braking, the motor is run as a generator. As a result, the kinetic energy of the motor is converted into electrical energy and returned to the supply.
• By means of that Regenerative braking results in Energy saving opportunities in the motor.
   

Method 1:

In this method, the field winding is disconnected from the supply, and the field current is increased by exciting it from another source as shown below.

• It results in, induced EMF (E) exceeds the supply voltage V and the machine feeds energy into the supply.
• Thus braking torque is provided up to the speed at which induced EMF and supply voltage are equal.
• As the machine slows down, it is not possible to maintain induced EMF at a higher value than the supply voltage.
• Therefore, this method is possible only for a limited range of speed.
   

Method 2:

• In this method, the field excitation does not change but the load causes the motor to run above the normal speed (e.g., descending load on a crane or train on downward slope or hills).
• In that case, the induced EMF (E) becomes greater than the supply voltage V
• The direction of armature current I, therefore, reverses but the direction of shunt field current If remains unaltered as shown.

Hence the torque is reversed and the speed falls until E becomes less than V.

Braking:

• Mechanical braking is used in the breaking of battery-operated scooters
• Plugging is used where a series motor is used
• Rheostatic braking is used where shunt motor is used
• Regenerative braking is used in shunt motors where a load is in overhauling condition

Important Points

Regenerative braking: In this type of braking back emf Eb is greater than the supply voltage V, which reverses the direction of the motor armature current. The motor begins to operate as an electric generator.

Dynamic braking: In this type of braking, the DC motor is disconnected from the supply and a braking resistor (R) is immediately connected across the armature. The motor will now work as a generator and produces the braking torque.

Plugging: In this method, the terminals of supply are reversed, as a result of the generator torque also reverses which resists the normal rotation of the motor, and as a result, the speed decreases. Plugging has the highest braking torque.