CHAPTER XXIV
SELECTION AND INSTALLATION OF DYNAMOS AND MOTORS
=General Conditions Governing Selection.=--In any particular case, the voltage, current capacity, and type of dynamo selected will depend upon the system of transmission or distribution to which it is to be connected, and the character of the work which it is required to perform. The suitability of the different types of dynamo for various kinds of work has already been considered to some extent, but there are certain general conditions which are applicable to almost all cases, such as:
1. Construction; 2. Operation; 3. Cost; 4. Number and size of units.
=Construction.=--This should be as _simple_ as possible and of the most solid character. All parts should be interchangeable, and have a good finish. All machines should be provided with eye bolts or other means by which they can be lifted or moved, as a whole or in parts, easily and without injury. These features are so carefully attended to and guaranteed by the manufacturers as to leave little choice in this direction.
=Operation.=--The considerations relating to the operation of a machine involve an examination of the details of its construction, in order to determine the amount of attention it will require, the character of its regulating device, its _capacity_, _form_, and _weight_.
=Ques. What may be said regarding capacity?=
Ans. Dynamos and motors should not be overloaded, because the efficiency is greater when the working load does not exceed the rated capacity of the machine.
=Form.=--As a rule, there is not much choice in the matter of form between standard machines, as they are uniformly symmetrical, well proportioned and compact. It is a mistake, however, to select a light machine for stationary use, as the weight of a machine increases its strength, stability and durability.
=Cost.=--In some cases, the matter of first cost is important and deserves careful consideration. It should be remembered, however, that high grade electric machinery cannot be built out of low grade materials and with poor workmen; therefore, when necessity compels the selection of a cheap machine, it should not be expected that its service will be as satisfactory as that of a first class machine.
=Number and Size of Units.=--The best number and size of units for an electrical plant is usually governed by the requirements of the driving engines. As a rule, dynamos and motors are not much less efficient at quarter load than at full load, and the smaller dynamos are fully equal to the larger machines in this respect, therefore, a generating plant can be subdivided, and if so desired, without any detrimental results except those to a multiplicity of units.
=Ques. What is the important consideration with respect to efficiency?=
Ans. Efficiency at maximum load is not so important as efficiency at average load.
For instance, in the diagram, fig. 439, the rated efficiency of one dynamo as shown by the curve A, is 95 per cent., and that of another, as shown by curve B, is 91 per cent., but it will be observed that the average efficiency of B is much higher, being 75 per cent. at quarter-load, 89 per cent. at half-load, and 91 per cent. at three-quarter load, to 55, 77 and 89 per cent. of A, at the corresponding loads. In this case, A is higher than B only at full load, and as full load is a limit which should not be reached except in special cases, and then only for short intervals of time, the service rendered by B would be much more satisfactory in the long run. In order to avoid the difficulties possible under these conditions, a guarantee to carry 25 per cent. overload for two hours without injury should be required, and either this or the rated load be taken, as the full load, so as to give a factor of safety of 25 per cent.
=Ques. Upon what does the choice of field winding of a dynamo depend?=
Ans. The different classes of field winding have already been discussed, but in general the conditions governing selection are as follows: The series dynamo is used where a constant current at variable voltage is desired, as in series arc lamp circuits. A shunt dynamo is used on constant voltage circuits, where the distance from the machine to the load is not great, that is, where there is small line loss. With a compound dynamo there is compensation for line loss, that is, it can be constructed so that the voltage at its terminals, or at the load can be maintained constant or allowed to increase or decrease with a change in load. It can thus operate lamps at constant voltage though they be located at some distance, or the voltage at the end of the line can be made to increase with an increase of load, as is frequently the case in railway work.
=Ques. For what conditions of service are series motors adapted?=
Ans. They are used on constant current circuits, and also on constant voltage circuits as in railway work and similar purposes where an attendant is always at hand to regulate the speed.
=Ques. Name some advantages and disadvantages of series motors.=
Ans. They are easily started even under heavy loads, the winding is cheaper than the other types and the speed is nearer constant than shunt motors when operated on constant current circuits. When used on constant pressure circuits, such as is employed for incandescent lighting, the speed will depend on the load.
=Ques. What kind of circuit is suitable for shunt motors?=
Ans. They are used on constant voltage circuits.
=Ques. What are the advantages of shunt motors?=
Ans. The speed remains nearly constant for variable load.
=Ques. State the disadvantages.=
Ans. They start less easily under a heavy load than do series motors, and the speed cannot be varied through any wide range without considerable loss. The shunt motor requires more attention than the series type and is more liable to be burnt out.
=Location.=--The place chosen for the dynamo or motor should be dry, free from dust, and preferably where a cool current of air can be had. It should allow sufficient room for a belt of proper length when a belt drive is used.
=Foundations.=--It is most important to secure a good foundation for every dynamo, and great care should be taken to have them entirely separate from those of the walls of the building in which the machine is installed, and if the dynamo be directly driven, but not on the same bed plate as the engine, a foundation large enough for both together should be laid down. Stone or concrete may be used, or brick built with cement, having a large thick stone bedded at the top.
For small machines the holding down bolts may be set with lead or sulphur in holes in the stone top, but for large machines the bolts should be long enough to pass down to the bottom, where they should be anchored with iron plates.
=Setting up of Dynamos and Motors.=--In unpacking the machines care should be taken to avoid injury to any part, and in putting the parts together, each part should be carefully cleaned, and all the parts put together in exactly the right way. The shafts, bearings, magnetic joints, and electrical connection should receive especial attention and be thoroughly cleaned of every particle of dirt, grit, dust, metal clippings, etc.
=Ques. Who should preferably assemble the machines?=
Ans. Whenever possible, they should be assembled by someone thoroughly familiar with the construction; but if the services of such a person cannot be had, no one should attempt to put a machine together unless he has a drawing or photograph of the same for a general guide.
=Ques. What precaution should be taken with the armature?=
Ans. It should be handled carefully to avoid any injury to the wires of the winding and their insulation.
If it become necessary to lay the armature on the ground it should be laid on clean paper or cloth, but it is better to support it by the shaft on two wooden horses or other supports, and thus avoid any strain on the armature body or commutator.
=Connecting Up Dynamos.=--The manner in which the connections of the field magnet coils, brushes, and terminals, are connected to one another depends entirely upon the type of machine. The field magnet shunt coils of shunt and compound wound dynamos, are invariably arranged in series with one another, and then connected as a shunt to the brushes or terminals of the machine. The series coils of series and compound wound machines are arranged either in series or in parallel with one another, according to conditions of operation, and then connected in series to the armature and external circuit.
=Coupling Up Field Magnet Coils.=--In coupling up the coils of either salient or consequent pole field magnets, assume each of the pole pieces to have a certain polarity (in bipolar dynamos two poles only, a north and south pole respectively, are required; in multipolar dynamos the poles must be arranged in alternate order around the armature, the number of N and S poles being equal), then apply Flemming's rule as given under fig. 132, to each of the coils, and ascertain the direction in which the magnetizing current must flow in each in order to produce the assumed polarity in each of the pole pieces. Having marked these directions on the coils, they can be coupled up in either series or parallel connection according to requirements, so that the current flows in the proper direction in each.
=The Drive.=--Various means are employed to connect the engine or other prime mover with the dynamo, or the motor with the machinery to driver. Among these may be mentioned the following:
1. Direct drive; 2. Belt drive; 3. Rope drive; 4. Gear drive; 5. Friction drive.
=Ques. What is a direct drive?=
Ans. One in which the driving member is connected direct to the driven member, without any interposed gearing.
Fig. 443 shows a direct connected unit, which is an example of direct drive.
=Ques. What may be said with respect to direct drive?=
Ans. It is the simplest method and the space required is less than with belt drive. With direct drive the engine and dynamo must run at the same speed; this is a disadvantage because the desirable speeds of the two machines may not agree.
Since the usual engine speeds are slower than dynamo speeds, direct drive involves the use of a larger dynamo for a given output than would be necessary with belt connection, and involves a corresponding increase in cost and greater friction loss due to the rotation of larger and heavier parts.
=Ques. Mention some of the features of belt drive.=
Ans. Greater flexibility in the original design of a plant is possible and new arrangements of old apparatus can be made at any time. It gives conveniently any desired speed ratio and permits the use of high speed dynamos and motors.
=Ques. State some of the disadvantages of belt drive.=
Ans. Considerable space is required and the action is not positive. Belts exert a side pull on the bearings which results in wear, also loss of power by friction.
=Ques. Give a rule for determining the proper size of belt.=
Ans. _A single belt travelling 1,000 feet per minute will transmit one horse power per inch of width; a double belt will transmit twice this amount._
EXAMPLE.--What size of double belt is required to transmit 50 horse power at 4,000 ft. speed, and what diameter pulley must be used for 954 revolutions per minute at 4,000 ft. speed of belt?
The horse power transmitted per inch is
4,000 ----- × 2 = 8 1,000
accordingly, the width of belt required to transmit 50 horse power is
50 ÷ 8 = 6.25, say 6".
For 4,000 ft. per minute belt speed, the distance _in inches_ travelled by the belt _per revolution_ of the pulley.
4,000 × 12 ---------- = 50.31 inches 954
This is equal to the circumference of the pulley, and the corresponding diameter is
50.31 ---------- = 16.1, say 16 inches. π
=Ques. What is the proper speed for a belt?=
Ans. From 3,000 to 5,000 feet per minute, depending on conditions.
Points Relating to Belts.
1. The amount of power that a belt of given size can transmit is not a very definite quantity. The rule just given is conservative and will give an amply large belt for ordinary conditions.
2. A belt should make a straight run through the air and over the pulleys without wabbling; it should maintain an even and perfect contact with that part of the pulley with which it comes in contact. In order to do this it should be kept soft, pliable, and have no abrasions or rough places.
3. When belt fasteners give way there is too much strain upon belt. The greatest amount of slack in a belt is found where it leaves the driving pulley, hence the tightener should be near the driving pulley, as it takes up the slack, prevents vibration and diminishes strain on belts and bearings. More than 100 degrees of heat is injurious to belts.
4. Double belts should always run with the splices, and not against them. Quarter turn belts should be made of two ply leather, so as to diminish the side strain.
5. Friction is greatest when the pulleys are covered with leather. Friction depends upon pressure, but adhesion depends upon surface contact; the more a belt adheres to pulley surface without straining, through too much tightening, the better the driving power. Slipping occurs on wet days because the leather absorbs dampness.
6. A leather covered pulley will produce more resistance than polished or rough iron ones. A good belt dressing makes a smooth, resisting surface, and as it contains no oils which create a slippery surface to belts, it increases belt adhesion. The friction of leather upon leather is five times greater than leather upon iron.
7. Moisture and water distend the fibres, change the properties of the tanner's grease and softening compounds. Repeated saturation and drying will soon destroy leather. Leather well filled with tanner's grease or animal oil, if allowed to hang in a warm room for several months without handling, will dry out, become harsh, and will readily crack.
8. A running belt is stretched and relaxed at different times and unless there be perfect elasticity in all its parts there will not be uniform distension.
9. There should be 25 per cent. margin allowed for adhesion before a belt begins to slip.
=Rules for Calculating Speed and Sizes of Pulley.=--When two pulleys are working together connected by a belt, the one which communicates the motion is called the _driver_ and the other which receives it, the _driven pulley_.
=To Find the Size of the Driving Pulley:= Multiply the diameter of the driven pulley by its required number of revolutions, and divide the product by the revolutions of the driver. The quotient will be the diameter of the driver.
=To Find the Number of Revolutions of the Driven Pulley:= Multiply the diameter of the driver by its number of revolutions, and divide by diameter of driven. The quotient will be the number of revolutions of the driven.
=To Find the Diameter of the Driven that shall Make a Given Number of Revolutions, the Diameter and Revolutions of the Driver Being Given:= Multiply the diameter of the driver by its number of revolutions, and divide the product by the number of revolutions of the driven pulley. The quotient will be the diameter of the driven pulley.
=Rope Drive.=--In this method of power transmission, rope is run in V-shaped grooves in the rims of the pulleys; this form of drive, in some cases, is more desirable than others.
=Ques. What are some of the advantages of rope drive?=
Ans. More power can be transmitted with a given diameter and width of pulley, on account of the increased grip in the grooves. Rope drive can be employed for long or short distances by reason of its lightness and the action of the grooves.
=Gear Drive.=--This method is used where a positive drive is desired, as for elevator or railway motors. It admits of any degree of speed reduction without attending difficulties as would be encountered with belt drive.
Thus, with the worm type of gear as used on elevator motors a great reduction in velocity can be made without incurring the expense of countershaft as with a belt.
=Friction Drive.=--This is a very simple mode of transmitting power and has the advantages of simplicity and compactness. In operation, the driving wheel is pressed against the wheel to be driven, transmitting motion to the latter by the frictional grip. The drive is thrown out of gear by slightly moving the machine on its sliding base. In construction, the friction may be increased by making one wheel of the pair of wood, compressed paper, or leather.
=Electrical Connections.=--Circuits for dynamos and motors should be carefully planned so as to secure the simplest arrangement, and to avoid unnecessary expense and delay, the wiring should be installed in accordance with the requirements of the National Electrical Code.
=Ques. What may be said with respect to exposed and concealed wiring?=
Ans. Exposed wiring is cheap and accessible; a short circuit or ground is easily located and repaired. Concealed wiring, especially when placed under the floor, has the advantage of being out of the way, and thus protected from injury.
=Ques. In wiring a dynamo what are the considerations with respect to size of wire?=
Ans. All conductors, including those connecting the machine with the switchboard, as well as the bus bars on the latter, should be of ample size to be free from overheating and excessive loss of voltage. The drop between the generator and switchboard should not exceed one-half per cent. at full load, because it interferes with proper regulation and adds to the less easily avoided drop on the distribution system.