Industrial Drives : Concepts ( sai saikumar jn)

Industrial Drives : Concepts

Introduction of Industrial Drives:

Electric drives have inherent advantages over other prime movers. Special motors and control gears have been developed to suit every application. Induction motor is the veritable work horse of industry. Squirrel cage induction motor is used for all constant speed applications because of its low cost., rugged design and simple control gear. Wound rotor induction motor is used where one or more of the following consideration are involved:
(i) High starting torque
(ii) Low starting current
(iii) Speed control over a limited range

Synchronous motor is suitable for all constant speed application. It is generally more economical in rating above 100 kW particularly for slow speed drives because of high power factor, better efficiency and lower cost.

DC motors are invariably used where smooth and precise control over a wide range with or without quick speed reversals is needed. The type of electric drive and control gear for a particular application are determined by the following consideration:
(i) Duty ; whether heavy, medium light,
(ii) Starting torque,
(iii) Limitations on starting current,
(iv) Speed control range and its nature,
(v) Need for automatic control,
(vi) Environmental conditions.

Any production equipment used in a modem industry consists of three components. These are :
1. The prime mover along with its control equipment.
2. The motion transmitting device, and
3. The actual apparatus or equipment (load)

The function of the first two components is to impart motion to the production unit. The prime mover (which is an electric motor), the shaft transmitting motion, and the associated control equipment are together called the 'electric drive. The drive together with the load makes a complete drive system.

There are three types of industrial drives, indicating the trends in the form of advancement. These are the group drive, the individual drive and the multi-motor drive.

1. Industrial Drives > The group electric drive:

The group electric drive was used in the earlier days. It had a single motor of sufficient capacity to drive an entire group of machines used in a shop. The motor was connected to a line shaft and through the use of belts and pulleys all the machines were driven. This form of drive was very inefficient difficult to control, unsafe, and had many other objectionable features. This type of drive is not used now and is of historical interest only.

2. Industrial Drives > In the individual drive:

In the individual drive there is one motor for each working machine. The electric motor is an integral part of the machine and can be specially designed to the needs of that machine.

3. Industrial Drives > Multi-motor drive:

The third type of drive is the multi-motor drive. This type of drive has more than one motor for each working machine. Examples are metal cutting machine tools, paper making machines and rolling mills, etc.

Industrial Drives > CHARACTERISTICS OF DIFFERENT TYPES OF LOADS:

In electric drives the driving equipment is an electric motor. One of the essential requirements in the selection of a particular type of motor for driving a machine is the matching of speed- torque characteristic of the driven unit and that of motor. Therefore, the knowledge of how the load torque varies with speed of the driven machine is necessary. Different types of load exhibit different speed-torque characteristics. However, most of the industrial loads can be classified into the following four general categories:

1. Constant torque type load;
2. Torque proportional to speed (generator type load);
3. Torque proportional to square of the speed (fan type load);
4. Torque inversely proportional to speed (constant power type load).

Constant torque characteristic of industrial drive:

Most of the working machines that have mechanical nature of work like shaping, cutting, grinding or shearing, require constant torque irrespective of speed. Similarly, cranes during hoisting and conveyers handling constant weight of material per unit time also exhibit this type of characteristic. The speed-torque characteristic of this type of load is given by T=K and is shown in the Figure.

Industrial Drive > Torque proportional to speed (Generator type load):

Separately excited dc generators connected to a constant resistance load, eddy current brakes, and calendering machines have a speed-torque characteristic given by T=Kω. This type of characteristic is shown in Figure.

Industrial Drive > Torque proportional to square of the speed (Fan type load):

Another type of load met in practice is the one in which load torque is proportional to the square, of the speed. The typical examples are : fans, rotary pumps, compressors and ship propellers. The speed-torque characteristic of this type of load is given by T=Kω² and is shown in Figure.

Industrial Drive > Torque inversely proportional to speed (constant power type load):

Certain types of lathes, boring machines, milling machines, steel mill coiler, and electric traction load exhibit hyperbolic speed- torque characteristic. In such loads the torque is inversely proportional to speed or the load power remains constant. This type of characteristic is given by T=K/ω and shown in Figure.



Most of the loads require extra effort at the time of starting to overcome static friction. In power application it is known as breakaway torque and the control engineers call it 'stiction'. Because of stiction, the speed-torque characteristics of the loads are modified near the zero speed. For a constant torque type of load, the characteristic as modified due to stiction is shown in Figure.

Applications of Industrial Drive

Applications of Industrial Drive > TRAVELLING CRANES:

Both AC and DC drives are used for the different operations in a crane. The preferred drives on consideration of economy end utility are indicated below:

Operation	Type of drive
(i) Hoisting and lowering	AC slip ring motor. Ward Leonard . controlled DC shunt motor and DC compound motor.
(ii) Crane travel	AC slip ring motor.
(iii) Trolley travel	AC slip ring motor.
(iv) Slew and swing action	AC slip ring motor or DC shunt motor.
(v) Boom hoist	AC slip ring motor.

DC motors are used for load hoisting and lowering where smooth, precise and at the same time fast speed control is required as in the case of cranes used in steel plants, power houses and concrete dams.

Applications of Industrial Drives > ROLLING MILL DRIVES:

The following types of drives are used for rolling mills:
(i) DC motors,
(ii) AC slip ring motors with speed control.



The DC motors, because, of their inherent characteristics, are best suited for the rolling mills. Speed control is effected either through Ward Leonard system or by grid controlled mercury arc rectifiers. AC slip ring motors are suitable for roughing and re-rolling mills where very precise speed control is not required Their efficiency is low because of the power wasted in the rotor resistance. There is also abrupt rise in motor speed when the material leaves the rolling stands.

Applications of Industrial Drive > KILN DRIVES:

Call for a starting torque of about 250% in addition to the speed control feature. The commonly used drives are:

(i) Slip ring induction motor
(ii) Three phase shunt wound commutator motor
(iii) Cascade controlled AC motor
(iv) Ward Leonard controlled DC motor
(v) DC motor with transformer step switch control.

Applications of Industrial Drive > TEXTILE INDUSTRY:

The textile industry requires special types of drives for
(i) weaving, and
(ii) spinning.

(i) Applications of Industrial Drive > Weaving:

The motors used in weaving mills must have good cooling capacity to keep their temperatures within limits in the presence of large power losses. The rating of the motors and the cooling facility must be properly selected, because these motors are used in conditions where high moisture content is present along with lot cage induction motors with high rotor resistance, totally enclosed, fan cooled and having high temperature insulation are used to drive looms. For light fabrics like cotton, silk, nylon etc. small motors of less than 1 hp may be sufficient. For heavy fabrics such as wool, the rating of these motors may be 2-3 hps. These motors are normally run at 750-1000 rpm.

(ii) Applications of Industrial Drive > Spinning:

The spinning mills use one of the following three types of drives:
(i) A 4-pole or 6-poIe squirrel cage induction motor,
(ii) A pole amplitude 4/6 or 6/8 poles induction motor,
(iii) Two separate motors to be runs at 1500/1000 or 1000/750 rpm. But whatever may be the types of motor used, the motor must be started with controlled torque.

Applications of Industrial Drive > PAPER INDUSTRY:

In a paper industry, the drives are required for
(i) Pulp making, and
(ii) Paper making.

In the pulp making process, the logs of wood are either ground in mechanical grinders or else they are chemically treated with alkalis and simultaneously beaten up to turn them into soft pulp. In the mechanical method of pulp making, the electrical power requirement is very high because the wood is hard. Since the mechanical grinders operate at a constant speed of about 200-300 rpm,the motors can be started on no load. Thus synchronous motors are used for these drives. These motors normally run at 1000-1500 rpm and gears are used to reduce the speed to 200-300 rpm.

In the chemical method of pulp making, the logs of wood are continuously beaten by the beaters at the time of treatment with alkali. The power requirement of the beater motors is less than those of grinder motors but these motors require high starting torque. Therefore, slip ring induction motors with gears are used to drive these beaters at about 150-200 rpm.

Applications of Industrial Drive > BELT CONVEYRS:

Normal starting current, high starting torque (double cage) squirrel cage motors with direct-on-line starters are used for conveyr drives because they have often to start with full load.

Applications of Industrial Drive > Compressors:

Wound rotor induction motors and synchronous motors are generally used on large size machines. Squirrel cage motors are used for small compressors only.

Applications of Industrial Drive > Blowers-fans:

The squirrel cage induction motors and synchronous motors are used for driving blowers and fans.

Applications of Industrial Drive > Pumps:

Centrifugal pumps are driven by squirrel-cage induction motors or synchronous motors. Reduced voltage starters can be used because of low starting torque requirements.

Applications of Industrial Drive > Machine tools:

Squirrel cage motors are normally used for them.

Applications of Industrial Drive > Jaw crushers:

Belted slip ring induction motor is almost invariably used as, very often, the motor has to start against heavy load or a stuck crusher. Pipe ventilated motors should be used so that supply of cool and clean ventilating air can be ensured even industry dusty atmosphere.

from ur's -- Bellapuri saikumar
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