Every well-developed machine may be considered to consist of three important different parts: the prime mover (engine or motor), the transmission system, and the working machine proper. The function of the first two parts, the prime mover with its control system, and the transmission system, which may comprise a set of shafts, pulleys, belts, gears, etc., is to impart motion to and operate the working machine. Because of their function of maintaining the working machine in motion, the first two parts (the prime mover and the transmission system) are embraced by the common term drive. The simplest example of a drive is a hand-powered device. Devices of this class are still used in household appliances and some agricultural implements, though industrially they are of no importance today. Horse-powered drive which at one time replaced hand-powered drive by using animals for providing motive power also has no industrial importance today. These two means of motive power were eventually replaced by mechanical drives powered by windmills, waterwheels and turbines, steam engines, and internal combustion engines.
Nowadays, modern water and steam turbines are extensively used in electric power stations generating large amounts of electrical energy for the centralised industry supply. electrified traction facilities, agriculture, and domestic needs. In utilizing this source of energy, the principal means of driving all kinds of working machines is the electric motor. This makes it the predominant type of drive in modern conditions, with the term electric drive being applied to it.
NATURE OF ELECTRIC CIRCUIT
The electric supply available may be 3-phase AC, single-phase AC, or DC.
In case a three-phase AC supply is available, a polyphase induction motor, squirrel cage type for small ratings, and slip-ring type for higher ratings may be used, provided this suits the requirements of the load.
DC motors are not used as widely as AC ones. The reasons are given below:
1. At present a large percentage (in fact, the whole) of the electrical energy used for domestic and commercial purposes is generated in a form because of economic and technical reasons, i.e., supply available is usually ac. Additional equipment is, therefore, required for converting the existing AC supply into a de-supply.
On the contrary, AC motors have the advantages of possessing few working parts, requiring less maintenance and replacement of spares, and providing uninterrupted long service life.
These requirements are particularly significant in connection with services such as the operation of traction equipment, hoists, and elevators. In some cases, such as in electric excavators, steel mills, and cranes, the speed control is so important that the existing AC supply is converted into a DC supply in order to employ de motors.
TYPES OF DRIVES
Group Drive By group drive is a drive in which a single electric motor drives a line shaft by means of which an entire group of working machines may be operated. The aggregate of kW output ratings of all the individual motors, and further causes a reduction in cost.
If the machines are liable to short but sharp overloads, group drive is again advantageous, because 100% overload on an individual machine will cause hardly 10% overload when being driven by group drive.
Individual Drive. Examples of such a drive are single-spindle drilling machines, various types of electrical hand tools, and simple types of metalworking machine tools and mechanisms. Though it costs more than a group drive, each operator has complete control of his machine, which enables him to vary its speed if necessary and stop while not in use, thus eliminating no-load losses.
For driving heavy machines such as lifts, cranes, shapers, lathes, etc., and for purposes where constancy of speed and flexibility of control is required, such as in paper mills and the textile industry, the individual drive is essential.
For new factories individual drive is preferred, as it causes some savings in the cost of superstructure because of is much lighter and less expensive.
In the case of individual drives too, the energy is transmitted to the different parts of the same mechanism by means of mechanical parts (such as gears, pulleys, etc.). This drawback is overcome in the case of multi-motor drives.
Multimotor Drive
The use of individual and multi-motor drives has enabled the introduction of automation in production processes, which in turn has considerably increased the productivity of various industrial organizations.
FOUR QUADRANT OPERATION OF ELECTRIC MOTORS
These four quadrants of operation are feasible for any de or ac rotating machine but the de machine is much freer to transfer its operation between quadrants and operates satisfactorily at any point within the envelope.
The load torque of the hoisting mechanism may be taken to be constant (fe.. independent of speed); forces due to friction and windage being negligible in the case of low speed.
The speed-torque characteristic of a loaded hoist by means of the vertical line passing the first and fourth quadrants to that of the load presented by the loaded cage and hence the speed-torque curve of the unloaded hoist is represented by the vertical line passing through the second and third quadrants.
Hence, to drive the loaded hoist up, the developed torque in the motor M must act in the same direction as the speed of rotation ie, Ty should be of positive sign. Since the speed is also positive being an upward motion, the power will also have a positive sign, de, the drive is said to be motoring.
It is convenient to assume that both E and I are positive under this condition, Le, to assume the flux to be positive so that positive E corresponds to positive speed and positive 1 to positive torque. If in all circuits the direction of the current in the field winding is assumed to be the same, the flux is automatically assumed to be positive, Negative armature cur- rent then indicates negative torque, fe, the torque in the reverse, downward, or clockwise direction. Similarly negative emf indicates negative speed. The motor equation V = E+IR must, of course, be used consistently for all cases, and no attempt is made to prejudge the behavior by using the "generator equation" V = E-IR at any time.
