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).
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