Electric drive ( Electrical Engineering) part-2

Topic includes

- Methods of power transfer by devices

- examples of selection of motors for particular loads

- application of drive

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Visit above link for part -1 

Methods of power transfer by devices

Transmission of drive

Mechanical power available at the motor shaft has to be transmitted the driver machine or machines. 

Transmission refers to the whole drivetrain, including clutch, gearbox, propshaft, differential and final drive shafts....

often, a transmission has multiple gear ratios with the ability to switch between them as speed varies.

Belt drive

- Belt is a loop of flexible material used to link two or more rotating shafts mechanically most often parallel.

- this belt drives least expensive drive. These are employed where a speed change is desired in the transmission of power and where it is not absolutely essential to maintain a fix the ratio between the driving and driven shaft.

The belt drivers are of two types flat belt drive and v belt drive

Flat belt drive

It is a long distance drive with unlimited degree of sleep.

The bells used for drive are made of hair, woven cotton, nylon, leather etc..

Out of which leather belt drive is the most economical type of drive.

Advantages and disadvantages of flat belt drive

Such advantages 

- Greater flexibility in the original design of a plant.

-It desires conveniently any desired speed ratio and permits the use of high speed motors, which are cheaper than low speed motors.

Such disadvantages

- large power transmission is not possible.

- to maintain the correct velocity ratio is difficult.

V-belt drive

It is essentially a short center, silent drive with a negligible slip. Such type of drive is suitable for motors of rating upto 450kw.

These are commonly used in industrial applications because of their relative low cost and wide range of sizes.

Such advantages

- The V belt drive gives compactness due to the small distance between the centres of pulleys.

- the drive is positive because this slip between the belt and pulley groove is negligible.

Such disadvantages

- the v belt drive cannot be used with large centre distances.

- the v belt drives are not so durable.

Gear Drive

It is short Central positive drive.

it was the first form of power transmission in the early steam drive Mills.

gear are mechanisms that mesh together via teeth and are used to transmit rotary Motion from one state to another.

Need of gear drive

most of the high speed or heavy load applications gear drive requires full lubrication, where the gears are impressed in lubricating oil.

Pulley

it is a wheel with a grooved rim around which a cord passes, which acts to change the direction of force applied to the cord and is used to raise heavyweight. 

Examples of selection of motors for particular loads

Synchronous motor

it operates at constant speed, speed control is not possible.

it is used in clocks all kinds of timing devices, recording instruments, sound recording, etc.

Single phase series motor

the motor possesses high starting torque which is about three times the full load current.

It is used in vacuum cleaners, refrigerators and in electric Traction .

Repulsion motor

Just like single phase series motor, repulsion motor also possesses high starting torque which is about three times full load torque with the starting current of twice full load current.

It is used where large starting torque is required but constant speed just as in coil winding machines.

Capacitor start induction motor

The motor possesses moderately high starting torque.

It operates at 0.7 to 0.9 power factor lagging.

It is used for refrigerators compressors and other drives which involve high starting torque.

Capacitor start capacitor run induction motor

the motors operating characteristics are similar to the capacitor start induction motor except that this motor operates at a better power factor than other motors due to additional capacitor.

It is used for drives requiring quite operation.

Repulsion start induction motor

The motor possesses high starting torque viz 2.5 to 4 times full load torque with a starting current of 3.5 times full load current. 

It requires less maintenance and it operates at 0.8 to 0.9 power factor

it can be used for refrigerator compressor and other drives where high starting torque is required.

Shaded pole induction motor

It is quite simple, single phase with self starting mechanism.

It has low starting torque.

it is used for small finance, blower slide and movie projectors.

Hysteresis motor

It runs at a synchronous speed.

The size varies from few watts to fraction of a horsepower.

It is used in electric clock or timing device.

Universal motor

this is a series motor which can be worked on DC or AC supply

It is also known as fractional horsepower motor

It is used in sewing machines and mixtures.

Application of drive for particular services

Textile mills

three phase AC motors are used, as their speed remains constant duty dependence on supply frequency.

 Paper industry

constant speed motors are used to give uniform thickness to the paper, so either three phase induction motors or synchronous motors are used.

Iron and steel industry

Motors used in mil shops are DC compound or induction motors.

speed control in case of DC motors is obtained by shunt field regulator

Printing press

Squirrel cage induction motors are used for constant speed work.

for rolling presses ,where a slow forward stroke and quick return stroke is required, to squirrel cage motors one of each stroke is used.

Crane

DC series motor or DC compound motors are preferred hair due to their high starting torque and easy speed controls..

Lifts

DC compound bound aur aise slip ring induction motors are used

To get speed control shunt field regulator method is used.