It is the purpose of this article to give an idea, of the operation of electric drive with so much of a description of the Jupiter's installation as is necessary to reach that end. Its simplicity is often remarked upon by those who have viewed it in port, the operation is equally simple and must be seen with the ship underway to be fully appreciated, " Bells "can be answered as fast as it is possible to give them and with perfect ease. There is nothing new in the Jupiter's installation of machinery except the main drive, the auxiliaries are of common types and require the usual attention. The main points that commend it to the operating engineer are the ease of upkeep ease of operation and reliability.
The main propelling machinery of the Jupiter consists, of the following; One turbo-generator; two induction motors; two water-cooled rheostats; one main switchboard; one exciter switchboard and three exciters (one used for exciting main generator and the others as ship's lighting and power plant); also the usual auxiliary machinery.
The turbine is the General Electric Curtis type. Inside the casing are nine wheels upon the shaft, making nine stages. Upon the periphery of each of these wheels is one row of blades with the exception of the first stage wheel, which has two rows of blades and in between these two rows is a row of stationery blades which extends for a distance of about one-quarter of the periphery. Fitted in between the wheels are diaphragms, which are made in halves, upper and lower, and are fitted to the turbine casing. In the periphery of these diaphragms are the nozzles, which extend from one set of moving blades to the next set of moving blades and entirely around the turbine. The first-stage nozzles are bolted on the turbine head and extend for only about one-quarter of the periphery, the same distance as the stationery blades of the first stage. When the turbine is running, the throttle is wide open to the steam chest. There are eight passages leading from the steam chest to the first-stage nozzles, each of which is closed by a spring-loaded control valve. A cam shaft extends the length of the steam chest with a cam for.each valve, so designed that the valves will be opened in succession. On the end of the cam shaft is a pinion with a rack Which gears into it, the rack being on an extension of the piston rod of the hydraulic cylinder.
The operating governor, which is entirely mechanical and geared to the turbine shaft, operates a pilot valve, which allows oil to enter the hydraulic Cylinder under about 75 pounds pressure, which moves the piston up or down, thereby opening and closing the control 'valves. Thus if the load is increased the governor will immediately open more control valves, allow more steam to enter turbine and keep turbine from slowing down; and in case of a decrease in load it will cut off steam to keep turbine from speeding up. The turbine governor is entirely responsible for holding speed constant at the valve set, as the speed of the ship depends entirely upon the governor, since With increase in turbine speed the motors' will speed tip. It operates so well that even in the gale, experienced by this vessel last December, 'no perceptible change in turbine speed or motor speed could be detected With the ship pitching the propellers out of water.
The steam passes from the main throttle to the steam chest through control valves and passages; first-stage nozzles; first-stage blading, consisting of two moving rows and part of a stationery row; second-stage nozzles; second-stage blading, and so through the nine stages to the exhaust space and exhaust trunk to condenser. The condenser is located above turbine.
The moving part of the -turbine is carried on two large bearings, with oil cooling pipes cast in the Babbitt Metal. Salt water circulates through these pipes to keep beatings cool. Forced lubrication is supplied on starting up and Stopping by means of an auxiliary oil pump, which also supplies pressure for operating the hydraulic governor cylinder, the pressure used on the cylinder is about .75 pounds, which is 'reduced to 25 or 30 pounds for the bearings. When turbine is up to speed, oil under pressure is furnished by a gear pump, geared to the turbine shaft, and the auxiliary pump is stopped automatically. In the turbine base is located the oil reservoir from which oil is taken by the oil pumps, circulated through the hearings, the oil cooler, strainer and back to the tank again.
There is only one essential difference between this turbine and the turbines of the same type in power plants ashore and that is in the governor. In turbo-generator sets on shore it is necessary to have but one speed, the normal running speed,and the governor is set permanently so as to give that speed. On this ship, speed of the ship is controlled by changing the speed of the turbine, and it is therefore necessary to be able to set the governor for any desired speed. This is accomplished by making one point movable in the
governor: mechanism which is ordinarily fixed. A rod runs from this point down under neatly the floor plates and comes up in a stand in front of switchboard, where by means of a hand wheel the position of a lever in the governor mechanism may be changed, thus changing the speed at which the turbine will run.
The turbine is limited in speed by the emergency governor which, is also of the centrifugal type. It is set for 2300 r.p.m. and at that speed trips the throttle, which is of quick closing type, and shuts steam off entirely. ...The throttle may also be closed by hand, by means of a wire pull from the operating switchboard.
Directly connected to the turbine is the generator, running at the speed of the turbine, situated upon the same base, with three bearings for the set. The generator is the revolving field type, two poles; normal - volts 2300 ; normal amperes 1370; rated capacity 5450 k.w. and normal frequency 33.2 cycle's. The frequency varies with the speed of the turbine, and the motor's run a speed depending upon the frequency and therefore upon the speed of the turbine. The frequency,
(f)= 1 p.m. (generator) x No. pairs poles (generator) / 60.
Since the generator has one pair of poles the frequency in this case is equal to the revolutions per second of the turbine. As the turbine goes from 900 r.p.m. to 2280 r. p.m. the frequency would vary from 15 to 38 cycles per second.
The voltage is varied depending upon the speed, the normal exciting current is 250 amp., which is furnished by one of three 5 k.w. d.c. turbo-generator sets which run at 3600 r.p.m. The field of the exciter can be varied from the main switchboard and can also be opened from that board.
On each end of the revolving field is a fan which takes in air from the bilges, circulates it through the generator, and discharges to an air duct at the top of the generator which, leads to the suction to the forced draft blowers. When under forced draft, this, heated air is used in the boilers. The generator is totally enclosed with the exception of the opening to the bilge for the intake of Air. The three terminals of the three phases lead out to one side of the generator and underneath the floor plates to bus-bars. These leads arc heavily insulated with mica and asbestos and insulated from the structure of the ship with porcelain insulators.
The motors, two in number, one on each shaft, are of the induction type. They are rated at 2750 h.p. each, and for a normal speed of 10 r.p.m. The 'outside, diameter of the stator is to feet. The stator is wound for 36 poles and therefore the synchronous speed of the motors is equal to 60 x f / No. pairs poles (motor), and since f=r.p.m. (generator) x No. pairs x No. pairs poles (generator) / 60, we have that r.p.m. (motor)= r.p.m. (generator)/18. This would be the speed of motors in case there were no load upon them, but since there is always some load upon them the speed is always a little less than the above.
The speed 60 x f / No. pairs poles (motors) is the synchronous speed. As stated before the motor will never run at this speed but always very close to it. The amount it varies from this synchronous speed in per cent is known as the " slip." The " slip " will vary from practically zero at light load to about 1.5 per cent at full load.
In a motor of this type, in order to obtain high torque at starting and reversing, it is necessary to introduce a certain amount of resistance' in series with the rotor. The rotor windings terminate in three slip rings on the shaft, and it is by means of these rings that the additional resistance is connected. With the resistance cut out these three slip rings are short' circuited, which allow the induced currents in the rotor to circulate in the rotor windings only. By the movement of a lever the slip rings are changed from the short circuited state to a condition where the resistance is in circuit. This resistance consists of three calorite coils, which are enclosed in cylindrical headers through which sea water is circulated to
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carry away the heat. This is the only resistance connected with the motors and has nothing to do with controlling speed or absorbing energy. They are simply provided to give the motor torque for starting or reversing. Running with them in decreases the normal speed of the motor, i. e., increases the slip. It also decreases the efficiency. It has been found that with resistances in the slip will increase so much that only about 9 knots speed can be obtained. However, as stated, the resistance is only to give starting torque so that it is not necessary after motors are turning over and is therefore cut out. It can be cut in when underway very quickly, but requires a little more time to cut out. However, this is probably the only installation that will have, this feature, as the design of the motors for future installations will be so as to do away with any outside resistances.
As can be seen from the diagrammatic sketch of power circuits, there are three cables from the generator. One of these runs to a point in between the motors where it divides into two cables, and one goes to the stator of each motor. The other two leads run behind the switchboard to bus-bars, and from each of these two bus-bars one lead is taken to each motor through oil switches as indicated. In order to change the direction of rotation it is only necessary to reverse the current in two of the three phases, and therefore one is carried to the motors direct. The current from the generator is carried in this way to the stators of the motors and produces a rotating field. Currents are induced in the rotor which either simply circulate around in the rotor windings, or, with the resistance cut in, they circulate through the rotor windings and resistance.
Mounted upon the front of the switchboard are the levers for operating the four oil switches, one ahead and one astern for each motor; two recording watt-hour meters which record the kilowatt hours used by each motor; two ammeters which show the current taken by each motor; one voltmeter which indicates the voltage of the main generator; one indicating watt meter which shows the total instantaneous power; one ammeter and one voltmeter which show the volts and amperes of the exciter, and a rheostat handle for controlling voltage of exciter and a knife switch in exciter field circuit; one frequency meter with double scale, the top scale being the speed of the generator and the bottom that of motors. Leads to all the instruments are taken from the power leads by current and potential transformers. The back of the switchboard is enclosed by a screen, and at no time is anyone liable to accidentally come in contact with any high voltage. In front of this switchboard is a pedestal on the top of which is a small wheel by which the speed of turbine is controlled. One man can handle the switchboard, control both motors and the turbine and then have very little to do in comparison with other forms of propulsion.
The following points are worthy of note in regard to electric drive:
1. The turbine always runs in the same direction and at high speed, no backing turbine necessary.
2. There is no starting and stopping of turbine in maneuvering.
3. Turbine is small and easier to repair.
4. Installations having two turbo-generator sets can run all propellers with one set up to its maximum load.
5. There is absolutely no racing under any conditions.
6. Both motors, if running, must run at the same speed. One may be stopped and one running, or one may be backing and one going ahead, but always at the same speed.
7. The engine-room watch is reduced.
8. Full backing power is always available.
9. Occupies less space and weighs less than any other drive.
10. Upkeep is small.
11. Very easy to operate. Signals can be answered quickly, making it reliable in an emergency.
12. Saves coal,—water rate of turbine per S. H. P. on full power trial, 11.68 lbs.