Electrical 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 when one or more of the following consideration are involved:
- High starting torque
- Low starting current
- 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:
- Duty, whether heavy medium or light.
- Starting torque.
- Limitations on starting current.
- Speed control range and its nature.
- Need for automatic control.
- Environmental conditions.
Any production equipment used in a modern industry consists of three components. These are:
- The prime mover along with its control equipment.
- The motion transmitting device.
- The actual equipment (load).
The function of the first two components is to impart motion to the production unit. The prime mover, 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.
- The group of 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.
- 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 the needs of the machine.
- 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.
Characteristics Of Different Types Of Load
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 the 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.
- Constant torque type load.
- Torque proportional to speed (Generator type load).
- Torque proportional to square of the speed (Fan type load).
- Torque inversely proportional to speed (Constant power type load).
Constant Torque Characteristics
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 conveyors banding constant weight of material per unit time also exhibit this type of characteristic. The speed-torque of this type of load is given by T=K.
Torque Proportional To Speed
Separately excited DC generators connected to constant resistance load, eddy current brakes and calendaring machines have a speed-torque characteristic given by T=Kω.
Torque Proportional To Square Of The Speed
Another type of load met in practice is one in which load torque is proportional to the square of the speed. The typical examples are fans, rotary pumps and compressors. The speed-torque characteristic of this type of load is given by T=Kω2.
Torque Inversely Proportional To Speed
Certain type of machines like boring machines, milling machines 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.
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.
Both AC and DC electrical drives are used for different operations in a crane. The preferred drives on consideration of economy and utility are indicated below:
|Operation||Type Of Drive|
|Hoisting And Lowering||AC slip ring motor. Ward Leonard controlled DC shunt motor and DC compound motor.|
|Crane Travel||AC Slip ring motor.|
|Trolley Travel||AC Slip ring motor.|
|Slew And Swing Action||AC Slip ring motor or DC shunt motor.|
|Boom hoist||AC Slip ring motor.|
DC motors are used for 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.
Rolling Mills Drives
The following type of drives are used for rolling mills:
- DC Motors
- 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 are 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.
Call for a starting torque of about 250% in addition to the speed control feature. The commonly used are:
- Slip Ring Induction Motor
- Three Phase Shunt Wound Commutator Motor
- Cascade Controlled AC Motor
- Ward Leonard Controlled DC Motor
- DC Motor With Transformer Step Switch Control
Textile Industry Drives
The textile industry requires special type of drives for
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, 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 HP’s. These motors are normally run at 750-1000 rpm.
The spinning mills use one of the following of these three types of drives:
- A 4-pole or 6-pole squirrel cage induction motor.
- A pole amplitude 4/6 or 6/8 poles induction motor.
- 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.
Paper Industry Drives
In a paper industry, the drives are required for
- Pulp Making
- Paper Making
In the pulp making process, the logs of wood are either ground in mechanical grinders or else they are chemically treated with alkalies 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.
Normal starting current, high starting torque (double cage) squirrel cage motors with direct-on-line starters are used for conveyors drives because they have often to start with full load.
Wound rotor induction motors and synchronous motors are generally used on large size machines. Squirrel cage motors are used for small compressors only.
The squirrel cage induction motors and synchronous motors are used for driving blowers and fans.
Centrifugal pumps are driven by squirrel-cage induction motors or synchronous motors. Reduced voltage starters can be used because of low starting torque requirements.
Squirrel cage motors are normally used for them.
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 clear ventilating air can be ensured even industry dusty atmosphere.