Electric Drives MCQ Quiz in தமிழ் - Objective Question with Answer for Electric Drives - இலவச PDF ஐப் பதிவிறக்கவும்

The main components of electric drive are:

• Power source
• Power modulator
• Electrical motors
• Load
• Control unit
• Sensing unit

Positive feedback is not the main component of electrical drives.

The suitable side of the motor can be calculated with the formula of 

L = \(\sqrt\frac{{L_1}^2 \times{t_1} + {L_2}^2 \times{t_2} + .....}{T}\)

where t₁, t₂ ...t_n are the time for the individual loads

L₁ , L₂ ....are loads

T = total time period for one cycle

​Putting the value of L1 , L2 , t1, t2 and T in the formula we get

L = \(\sqrt\frac{{100}^2 \times{10} + {50}^2 \times{10} + .....}{30}\)

= 64.65 KW

which is nearly equal to 67 KW.

• Conveyor belts are used power plant industries as well as in any assembly line
• Huge load is driven by the conveyor belt so the motor needs very high starting torque greater than their running torque
• Three phase induction motors are used for these systems

The motoring or braking mode of operation of the electrical drive is decided by the power modulator.

• An electrical drive can be defined as an electromechanical device for converting electrical energy into mechanical energy to impart motion to different machines and mechanisms for various kinds of process control.
• In other words, the system which is used for controlling the motion of an electrical machine, such type of system is called an electrical drive.
• The block diagram of the electrical drive is shown in the figure below.

Ward Leonard controlled D.C. drives:

The speed control of DC motor accomplished by means of an adjustable voltage generator is called the Ward-Leonard system. If it is desired to have wide and very sensitive speed control, then this system is used.

This system is commonly employed for elevators, heavy-duty excavators, hoists and main drives in steel mills as this method can give unlimited speed control in either direction.

Concept:

Worm gear:

• A worm gear is a gear including a shaft with a spiral section that engages with and runs a toothed wheel.
• Worm gears are a traditional form of gear and a sort of one of the six simple machines.
• Fundamentally, a worm gear is a screw butted up in front of what looks like a normal spur gear with slightly curved and angled teeth.

Worm Gear Uses:

1. The first one is the great reduction ratio. A worm gear can have a high reduction ratio with small effort – all one should do is add margins to the wheel. Thus, you can utilize it to either greatly improve the torque or highly decrease the speed. It will commonly take several reductions of a conventional gearset to obtain the same reduction rate of a simple worm gear – meaning users of worm gears have fewer moving components and fewer spaces for failure.
2. The second reason to employ a worm gear is the inability to reverse the power direction. Due to the friction between the wheel and the worm, it is effectively impossible for a wheel with the load applied to it to begin the worm moving.
3. On a normal gear, the output and input can be changed independently when enough load is applied. This makes adding a backstop to a normal gearbox necessary, further raising the complication of the gear configuration.

Transmission of Power to Drive:

There are many ways of transmitting mechanical power developed by a motor to the driven machine.

1. Direct Drive:

• In this case, the motor is coupled directly to the driven machine with the help of solid or flexible coupling.
• Flexible coupling helps in protecting the motor from sudden jerks.
• Direct drive is nearly 100% efficient and requires minimum space but is used only when the speed of the driven machine equals the motor speed.
   

2. Belt Drive:

• Flat belts are extensively used for line-shaft drives and can transmit a maximum power of about 250 kW.
• The minimum distance between the pulley centers should be 4 times the diameter of the larger pulley with a maximum ratio between pulley diameters of 6 : 1.
• The power transmitted by a flat belt increases in proportion to its width and varies greatly with its quality and thickness.
• There is a slip of 3 to 4 percent in the belt drive.
   

3. Rope Drive:

• In this drive, a number of ropes are run in V-grooves over the pulleys.
• It has a negligible slip and is used when the power to be transmitted is beyond the scope of belt drive.
   

4. Chain Drive:

• Though somewhat more expensive, it is more efficient and is capable of transmitting larger amounts of power.
• It is noiseless, slipless, and smooth in operation.
   

5. Gear Drive:

• It is used when a high-speed motor is to drive a low-speed machine.
• The coupling between the two is through a suitable ratio gearbox.
• In fact motors for low-speed drives are manufactured with the reduction gear incorporated in the unit itself.

Kiln Drives:

Rotary kiln drive is used in cement processing mills. The requirement of kiln drive depends on the type of Cement Mill Process (wet or dry). A variable-speed motor drives the pinion. 

The requirements of a kiln motor are the following:

• Power requirement is high.
• The speed control ratio is 1:10. Low creeping speeds of 1 rpm may be required.
• Starting torque should be in the range of 200 to 250% of full load torque.
• The acceleration of the drive should be completed in about 15 s.
• For small periods an overload capacity of 200-250% may be required.
• The motor must have suitable control for inching and spotting during maintenance.

The motors that meet the above requirements are as follows:

1. A.C. motors with speed control.
2. Slip ring induction motor.
3. Ward Leonard controlled the D.C motor.
4. Three-phased shunt-wound commutator motor.
5. Cascade controlled A.C. motor.
6. D.C. motor with transformer step switch control.

Conclusion: Since the squirrel cage induction motor starting torque is low as compared to the required torque, and also speed control is somewhat difficult. Hence, we can't use the Squirrel cage induction motor.

Plugging:

• In this 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

Regenerative braking:

• In this method, no energy is drawn from the supply during the braking period and some of the energy fed back to the supply system
• The braking torque generated in the motor is in the opposite direction of rotation of the rotor
• It can be applied to both DC and AC motors

Dynamic braking:

For dynamic braking, the armature is disconnected from supply, and external resistance is connected so it is acting as a separately excited generator. Its stored kinetic energy converted into electrical energy and dissipated into the braking resistor.