The selection of a motor design (d.c., induction, synchronous) is a relatively simple task in the case of a continuous, unvarying load. This application does not call for speed control, so the best choice is a synchronous motor. The point is that a synchronous motor is started in the same manner as an induction motor, while it is smaller in size and more economical in operation than an induction motor of the same power rating (it has a higher power factor and a higher maximum torque).
In applications involving speed control, frequent starting and stopping, a varying load, and so on, however, the decision as to which motor design should be preferred must be taken by considering the operation of the drive and the speed-torque characteristics of various motors. Speed-torque characteristics may be classed into natural and controlled. A natural speed-torque characteristic is that which is obtained when a motor is started under its rated conditions, is connected in the circuit in the normal way, and there are no additional elements in the motor circuits. A controlled speed torque characteristic is that which is obtained when the motor circuits include some additional elements and the circuits are connected in some special manner. The natural speed-torque characteristics of several motors are compared in Fig 8.
An important consideration with regard to the speed-torque characteristics of motors is their flatness
a = ΔT/Δn
The flatness of a speed-torque characteristic may vary from region to region of operation.
In terms of flatness, it is customary to class the speed-torque characteristics of motors (Fig. 16.8) into absolutely flat (1) for which Δn = 0 and a = ∞ (such as in the case of synchronous motors); flat for which a = 10-40 (the linear portion of the speed-torque characteristic of an induction motor (2), the speed-torque characteristic of a shunt-wound motor (3)); and drooping for which a ≤ 10 (such as the speed-torque characteristic of a series-wound motor (4), the controlled speed-torque characteristic of a wound-rotor induction motor (5), and the controlled characteristic of a shunt-wound d.c. motor).
In some cases the degree of flatness of the speed-torque characteristic is a controlling consideration in the selection of a motor. For example, the motors used to drive hoisting and conveying machinery should preferably have a drooping' speed-torque characteristic, while the drive motors for cold-rolling steel mills should have a very flat characteristic.
In applications involving frequent starting and a varying load, preference is given to squirrel-cage induction motors as they are most reliable, simple to service, and are relatively inexpensive. Wound-rotor induction motors are more expensive, involve more servicing, are large in size, and have a lower power factor (because of the greater air gap). The advantages of wound-rotor induction motors in terms of starting torque are insignificant when compared with a double-cage motor. This is the reason why wound-rotor induction motors are used only in cases of special requirements for starting torque or starting current. (These requirements usually arise because the transformer substation can supply a limited power or because it also serves some special loads.) All in all, for loads in ratings not over 100 kW, with no speed control, the best choice is a squirrel-cage motor. For higher ratings, if a cage motor is ruled out, preference is given to wound-rotor motors.
Until quite recently, the speed of induction motors has been usually controlled by placing a resistance in the rotor circuit and by varying the number of pole pairs on the stator winding, both methods being fairly imperfect. The former is warranted only when the speed needs to be controlled between narrow limits, with the torque at the motor shaft remaining constant. The latter provides only stepped speed control and is practically limited to low-power metal-cutting machine-tools.
A more recent addition is speed control by thyristors. One of their advantages is that they can change the frequency of alternating current, so that the angular velocity of the rotating magnetic field can be varied continuously and, between broad limits. As a corollary, the speed of induction and synchronous motors can likewise be controlled at will.
Direct-current motors are more expensive, take more labour and time for servicing and wear out earlier than a.c, motors. Still, preference in many cases is given to d.c. motors because their speed can be varied between broad limits (in the ratios 3-to-1, 4-to-1 and more) by simple means. But even in such cases the Ward-Leonard system assuring a wide range of speed control is giving way to the more economical and compact thyristor speed control units.
Examples of applications where a d.c. motor drive is preferable are reversible rolling mills, adjustable nonreversible multi drive rolling mills, machinery operating in intermittent duty, blast furnace skip hoists, and special-purpose lathes.
Types of motor enclosure. The basic types of motor enclosure are as follows (It is to be stressed that the classification of motor enclosure types adopted in the USSR is different from those in force in the UK or the USA.-Translator's note).
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An open motor is one having ample openings in the end shields and the frame to permit passage of external cooling air over and around the windings of the machine. It is only suitable for few applications, because the machine might easily be fouled by dirt in an industrial environment and attending personnel runs the risk of an electric shock because the live parts of the machine are readily accessible.
A screen-protected (or guarded) machine is one in which all openings are covered by wire-mesh screens so constructed that rain drops, foreign matter, filings, saw-dust and the like cannot enter the machine, but particles of dust can. Motors with this type of enclosure may be installed outdoors.
A totally. enclosed machine is one so enclosed as to prevent the free exchange of air between the inside and the outside of the case. For better cooling, air is forced through the machine, for which purpose inlet and outlet air conduits may be provided. Totally enclosed machines are installed in locations where the atmosphere may contain dust, vapours, corrosive fumes, and the like.
An explosion-proof machine is a totally enclosed machine whose enclosure is designed and constructed to withstand an explosion of a specified gas or vapour which may occur within it and to prevent the ignition of the surrounding medium by sparks, flashes or explosions occurring inside the machine case.
Motors intended for use in damp locations have damp-proof or moisture-proof insulation.
Labels: Electricity and Magnetism