DC Motor Braking MCQ || DC Motor Braking Questions and Answers

Answer:4. All of the above

Explanation: 

• In the Plugging method, the motor is reconnected to the supply in such a way that it has to develop torque in opposite direction to the movement of the rotor, it can be done by reversing the connections of the armature.
• The motor will decelerate until the speed is zero and then accelerate in opposite direction; Immediately, it is necessary to disconnect the motor from the supply as soon as the system comes to rest.
• The kinetic energy of the rotating parts of the motor is wasted and an additional amount of energy from the supply is required to develop the torque in the reverse direction, i.e. the motor should be connected to the supply during braking.
• It can be applied to both DC and AC motors.

The Plugging is commonly used for the following applications:

• • Controlling elevators
  • Rolling Mills
  • Printing Presses
  • Machine tools

Answer: 4. Electric Braking

Explanation: 

In Electric braking, the kinetic energy of the rotating parts of the motor is converted into electrical energy which in turn is dissipated as heat energy in resistance, or in sometimes, electrical energy is returned to the supply. Here. no energy is dissipated in brake shoes.

The electric braking method is the best among all due to the following advantages

• The electric braking is smooth, fast, and reliable.
• Higher speeds can be maintained: this is because the electric braking is quite fast. This leads to the higher capacity of the system.
• Electric braking is more economical; this is due to excessive wear on brake blocks or brake lining that results in frequent and costly replacement in mechanical braking
• The heat produced in electric braking is less and not harmful but the heat produced in mechanical braking will cause the failure of brakes.
• Electric braking, sometimes, it is possible to feedback electric energy during the braking period to the supply system. This results in a saving in the operating cost. This is not possible in the case of mechanical braking.

Answer:3. For low current applications

Explanation: 

• Dynamic braking is also known as rheostat breaking.
• In this method, the armature of the running motor is disconnected from the supply and is connected to a variable resistor.
• During the braking motor act as a generator and dissipates its available energy to the rheostat in the form of heat.
• After reduction of energy motor will come to a halt.
• This type of braking is used extensively in connection with the control of cranes, hoists, elevators, and in the applications where frequent starting, stopping and reversing are required.
• This method cannot be used for low current applications because due to low current heat generated will be less and the motor will take more time to stop.

Answer:1. Dynamic Braking

Explanation: 

• Dynamic braking is also known as rheostat breaking.
• In this method, the armature of the running motor is disconnected from the supply and is connected to a variable resistor.
• During the braking motor act as a generator and dissipates its available energy to the rheostat in the form of heat.
• After reduction of energy motor will come to a halt.
• This type of braking is used extensively in connection with the control of cranes, hoists, elevators, and in the applications where frequent starting, stopping and reversing are required.
• This method cannot be used for low current applications because due to low current heat generated will be less and the motor will take more time to stop.

Answer: 4. All of the above

Explanation: 

• The DC motor is used in tractions because of its excellent braking characteristics and ability to smoothly transition from the motor to the generator mode and vice versa.
• During the braking period, the motor is operated as a generator and the kinetic or gravitational potential energy (cranes or hoists) is dissipated in resistors (plugging) or returned to the supply (regenerative braking).
• In earlier days, the DC motor is suited for traction because of the high starting torque and having the capability of handling overloads.
• In addition to the above characteristics, the speed control of the DC motor is very complicated through semiconductor switches.

Answer: 2. Generator

Explanation: 

Regenerative braking: In this type of braking back emf (E 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

Answer:4. Eddy current

Explanation:

Type of electric Braking method

Regenerative braking: In this type of braking back emf (E 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

Note:- Eddy current is the electrical effect or response of the system, which is reflected mechanically at brakes to reduce the speed of the motor. Thus, eddy current is not an electrical brake, it is a mechanical one. As eddy currents flow through the conductor, heat is generated, referred to as Joule heating.

Answer:4. Plugging mode

Explanation: 

• In the Plugging method, the motor is reconnected to the supply in such a way that it has to develop torque in opposite direction to the movement of the rotor, it can be done by reversing the connections of the armature
• The motor will decelerate until the speed is zero and then accelerates in opposite direction; Immediately, it is necessary to disconnect the motor from the supply as soon as the system comes to rest
• The kinetic energy of the rotating parts of the motor is wasted and an additional amount of energy from the supply is required to develop the torque in the reverse direction, i.e. the motor should be connected to the supply during braking
• It can be applied to both DC and AC motors.

Answer: 2. Reversal of armature connections

Explanation: 

• Plugging braking occurs when the motor is rotating in the forward direction under the actions of external torque or due to inertia, but the winding is connected for the reverse direction.
• In the plugging method, the motor reversal can be achieved by reversing the polarity of the applied voltage given to either the armature or the field winding of the motor.
• Polarity reversal of the armature is used more often because the polarity of field reversal provides a long braking time due to the relatively high inductance of the field winding.
• Because of the problem of interrupting highly inductive field current and the time needed for the field current to build up in opposite direction, it is a common practice to reverse armature connections.

Answer:3. Capacitor Motor

Explanation: 

• In Rheostatic braking, the electric motor is disconnected from the supply during the braking period and is reconnected across the same electrical resistance.
• But field winding is continuously excited from the supply in the same direction.
• Thus, during the starts working as a generator during the braking period and all the kinetic energy of the rotating parts is converted into electric energy and is dissipated across the external resistance.
• In principle, a synchronous motor may be stopped by either plugging or rheostatic braking.
• But, the plugging method has certain disadvantages. Synchronous motors usually are of a large power rating and the line disturbances that occur during plugging are beyond the permissible limits.
• Also, since the plugging torques arise from the damper winding and unless it is specially designed for the purpose, the torques developed during plugging are smaller in magnitude than those obtained by rheostatic braking.
• Hence, rheostatic braking is almost exclusively used for braking synchronous motors.

As long as the field excitation is maintained, rheostatic braking can easily be applied to synchronous motors by disconnecting the armature from the supply and connecting it to resistances so that the machine acts as an alternator.