This paper expands on an unclassified talk given by Mr. Scott Forney to the Nautilus Chapter of Naval Submarine League on March 24th 2000. Mr. Forney is program manager of the electric drive project for the Electric Boat division of General Dynamics. He is responsible for the installation of electric drive systems in Los Angeles, Seawolf and Virginia class submarines. Any errors of/act are mine not his. Author.
A couple of years ago, several major corporations banded together to form a research consortium to:
- design and build an entirely new family of electric motors based on a completely different approach than the traditional induction motor.
- develop an integrated electric power system for all naval platforms regardless of whether gas or steam turbines were the primary source of power. Separate ships service generators will be eliminated. All electric power would be provided by either the main steam or gas curbine.
- develop an electric drive system that would, in the case of submarines, eliminate the need for reduction gears, propeller shafts and shaft alleys. A major hull opening (for the shaft) would be eliminated.
The Consortium, consisting of the Electric Boat division of General Dynamics, Westinghouse Electric, Newport News Shipbuilding and at least one other company now are seeing the light at the end of the runnel.
Each of the above facets are discussed separately below.
The New Family of Electric Motors
The new design is called the Permanent Magnet Radial Field Motor. It is commonly ref erred to as the permanent magnet motor.
The research consortium considered axial field, induction and radial field machines. Axial field machines didn’t provide the efficiency needed. Induction motors, commonly used for pumping applications on submarines, proved to be too big, heavy, expensive and didn’t provide the efficiency being sought. So, as a starting point, the consortium looked at a transverse flux permanent magnet motor developed by Rolls Royce.
The permanent magnet motor emerging from consortium research has multi-pole permanent magnets embedded in the motor rather than being surface mounted. Moreover, the permanent magnets are embedded in the rotor rather than being mounted on the stator.
To overcome the brittleness normally associated with permanent magnets, Mr. Forney said it was necessary to “break the magnets into chunks and shot qualify them” during rebuilding.
Early on, the new design showed promise. In its second iteration, the permanent magnet motor was half the size of an induction motor of comparable power and gave a fourfold improvement in performance.
The current iteration is the “most torque dense ever built. It produces the same torque at one RPM as it does at 3,000 RPM”. It weighs slightly more than a quarter of the weight of an induction motor with comparable power rating, is cheaper to build and operate and provides a large improvement in performance at peak loads.
According to Mr. Forney, two different topologies have been developed and patented in the development of the permanent magnet motor. These topologies are:
- a modular-based, pulse width modulated, 3,000 horsepower single phase portion of the drive. Six of these modules are used.
- an advanced soft switching technology.
Using both topologies, power densities of 14 KW /cubic foot have been achieved. Mr. Forney said: “In today’s submarines power densities are nowhere near that. Both topologies, when tested, produced a very quiet electric motor. Acoustic issues of the motors of the past are gone.”
Another improved feature of the new motor is the reduction of voltage stress, an undesirable characteristic of today’s electric motors. “The new motor has 1,000 times less stress.”
Mr. Forney mentioned one small example of the innovative thinking that has gone into the new generation of electric motors. The bulky cooling unit, customarily accompanying large induction motors has been built in the frame of the permanent magnet motor, saving precious space.
The test prototype of the permanent magnet motor has completed testing at Westinghouse and will undergo shipboard testing on the LSV-2, a quarter-scale model of the Virginia class submarine, under construction (April 2000) al the Newport News shipbuilding facility.
The Integrated Power System
The new family of permanent magnet motors will become the starting point for a new integrated power supply system.
At present, electric power aboard ship is provided by separate turbogenerator systems. This is true across all types of ship platforms.
In almost all our nuclear submarines, propulsion is provided by steam turbines connected to reduction gears and a single propeller shaft. A portion of the steam output of the reactor is diverted to the ships service turbogenerators to produce the necessary electric power to run the boat.
This arrangement has reached its limits according to Admiral Bowman, Director of Naval Reactors.(1) About 75 percent of available reactor output is being used for main propulsion. Yet, at the same time, new applications, in addition to existing needs, require more power than is available from the current system. These inadequacies pointed to the need for a new power system.
Under the new integrated power system, all available steam output from the reactor plant is used to generate electricity. Main steam turbines and attached reduction gears are eliminated. Indeed, the long-familiar propeller shaft and the shaft alley also are eliminated. Electric propulsion will replace the present direct rurbine drive.
According to Mr. Forney, with the new system, as much as 80 percent of the power now going into propulsion could be diverted to other purposes for varying periods of time. The flexibility inherent in being able to divert power to whatever purpose is necessary is one of the major goals of the new integrated power supply system.
The same approach of an integrated power system is being sought across other platform types. Destroyers, for example, will have their gas rurbine plants only producing electricity. As in submarines, they will have a rurbine electric system. Again, an integrated power system will vastly increase system flexibility, providing whatever electric power levels are required (within the capacity of the plant.)
Moreover, commonality of power system components in integrated power systems across platform types implies a great reduction in system costs (presumably because of the possibility of ordering components in larger quantities than would be the case now). These cost savings are in addition to the savings from the new family of permanent magnet electric motors that are much more efficient, lighter, smaller and cheaper (both to operate and build) than the induction motor family currently in use in the fleet.
Mr. Forney stated that research findings show that fossil-fueled ships would save an estimated 23 percent in operating costs from the introduction of integrated power systems alone, without taking into account the savings from the new permanent magnet motors.
The ability to combine computer commercial off-the-shelf technology (COTS) with a modularized, integrated electric power system and a new family of better electric motors has made possible significant advances in other areas.
In damage control, for example, it becomes possible to envision reacting to system failures in real time. Failure in a system component can be detected almost immediately because computer monitoring of the system thousands of times a second is now feasible. Immediately, long before a systems operator becomes aware of the system failure, the failed unit has been swapped out of the system and been replaced by a redundant unit.
The implications are revolutionary: Ships with an integrated power system will be more robust and with have greatly enhanced survivability, partly as a result of vastly improved and automated damage control procedures.
Submarines, as well as surface vessels using the new technology, will eliminate reduction gears and propeller shafting. The implications are clear: Engine room maintenance is greatly simplified; hull openings for propeller shaft alleys will not be needed; all ships so equipped will have stronger hulls.
The Electric Drive
The electric drive, according to Mr. Forney, involves wrapping a permanent magnet electric motor around the propeller.
Beyond pointing out that the propeller shaft and shaft alley normally connecting vessel and propeller is eliminated in the electric drive system, Mr. Forney did not discuss in detail the electrical connection between ship and drive nor any details of the drive.
Mr. Forney did state that the permanent magnet motor driven propulsion system ” … weighed 40 percent less (than the current system) and produced 40 percent more flow area.” More importantly, he pointed out that stealth is greatly enhanced and that propeller cavitation should not be the problem it has been in the past.
The resulting drive has been tested to depths of 110 feet in increments of ten feet to determine its performance and acoustic behavior. These tests were completed in December 1999.
In conclusion, Mr. Forney did point out that commercial cruise ships have already begun to use electric motor driven propulsion systems because of their obvious advantages.
(1) Speech by Adm. F. L. (Skip) Bowman on 18 Feb. 1999 at a meeting of the Long Island Council of Navy League.