Glebov V.V.1, Klimov V.F.1, Volosnikov S.A.1, a
DOI 10.2412/mmse.83.5.981 provided by Seo4U.link
Keywords: tracked platforms, electromechanical transmission, electromechanical traction drives, electric machines.
ABSTRACT. The article gives an estimation of possible using the hybrid electromechanical transmission performed as a series circuit in tracked vehicle of 50-tonnes weight category using the series-manufactured components of hybrid electric drive system. As components of electric drive (motor-generators and traction electric motors) it is invited to use AC induction motors with squirrel-cage rotor, that has no moving contacts and can work both in motor and generator modes, and energy storage buffer is made on the basis of consecutively connected Lithium-ion batteries.
Introduction. As smaller and more powerful electric machines, namely motor-generators (MG) and traction electric motors (TEM), appear, as well as relatively small power converters, we can observe the raise of interest towards the use of electromechanical transmission for combat tracked platforms.
Electromechanical traction drives are relatively new devices to be used for combat tracked platforms, that is why the basic principles of their design considering the peculiarities of their application are not yet finalized. This would require rethinking of many of the principled approaches. Attempts to switch from finalization of source data to concrete parameters of devices with the purpose to use them for combat tracked platforms are currently based on the experience of design of similar devices for other fields and ranges of application.
In case of using sequential system development, the electric power generated by motor-generator is distributed via flexible electric cables and thus: MG, TEM, controllers and power converters can be placed irrespective of each other, without strict kinematic connection, providing the designers of combat tracked platforms with the possibility to create various design layouts. Total electric power in the range of 800-900 kW generated by MG shall be distributed accurately and effectively for quick and precise execution of commands input by driver to control tractive effort as well as power taken off for steering and braking. Also, the concept of hybrid combat tracked platform requires control of power flow distribution to on-board consumers of electric power, such as weapon station, protection, control systems, air conditioning, etc.
Problem Definition. There are currently no validated and standard procedures of selection of basic parameters of electromechanical devices (type of electric motor, supply frequency, energy parameters, etc.) to be used on combat tracked platform. Moreover, there are no quality criteria for design of such systems for tracked platforms. These circumstances largely constrain the development of electric machines (MG and TED) and electric drives on their basis for use in future samples of combat tracked platforms.
Analysis of recent achievements and publications. Currently, in developed countries experts in the development of military equipment are working on the creation and implementation of electromechanical transmissions for the tracked platforms. One of the main advantages of electromechanical transmission is the effect of "continuously variable transmission", the absence of the clutch mechanism and gear shifting with continuous flow of power to the tracks, and the TEM power is supplied directly to the final drives of the sprocket wheels.
German company Magnet Motor implemented electric transmission for Marder tracked infantry fighting vehicle [1,2]. Electric drive is an AC-DC-AC type system (alternating current – direct current – alternating current) and has six levels of power transmitted through the transmission. The motor-generator is connected with the diesel engine MV-833Еа500 by MTU company with 440 kW capacity. The power of the AC motor-generator was 420kW (at 2250rpm), and the power of two on-board AC traction electric motors was 750kW (at 3500rpm) each, and they were connected to the tracks via final drives. At the same time, electric motors can do the power recuperation from the lagging track to the leading one during the turn, in case the electric motor of the lagging side is switched to generator mode. The source noted that the Marder IFV with electric transmission with the weight of 29.5 t had a maximum speed of 72km/h, which corresponds to the parameters of the serial vehicle, the vehicle is also highly mobile and manoeuvrable.
For the development of GCV combat tracked vehicle, BAE Systems used hybrid electric drive using the on-board energy buffer storage . The source noted that indicative weight of GCV tracked platform is within 70 tonnes and capacity of its motor-generator is about 1,100 kW, hybrid systems provide this product more speed, stealth, and fuel efficiency compared with similar vehicles of this weight category, that use mechanical transmission.
Fig. 1. GCV Combat Tracked Vehicle by BAE Systems.
The source  reports on the development of a prototype of BMP-3 type tracked platform with side electric transmission. 320kW generator is connected directly to the diesel engine, and two BLDC servomotors (320kW each) are connected to the sprocket wheels via final drive. This provides the possibility for the diesel engine to operate in optimal modes for various driving conditions of the tracked platform with the most efficient use of the power plant capacity.
Object of the Article. The object of the article is to assess the possibility to use hybrid electromechanical transmission on combat tracked platforms of up to 50 tonnes weight, as well as to select the type of applied electric machines and other components of the electrical drive system.
Main Data. In the movement process of a tracked platform when turning with a radius of less free, the leading track being the leading one in relation to the hull, provides its movement and rolling of the lagging track. Therefore, the main part of the power N2 of the leading track is used to overcome external resistance, and a part of it is driven to the lagging track via the platform hull. If the lagging track is not braked, the mobile platform will turn with free turning radius and power N1 of the lagging track (small one) will be spent for the track idling. Brake the lagging track to make a sharp turn with radius less than free. Therefore, power N1 will be partially or completely be spent to overcome brake friction. If the brake of the lagging track were installed on the sprocket wheel of the tank, the supplied power would be ( multiplier that takes into account power loss at the track bypassing). This is the way the power is distributed at turning when kinematic connection between leading and lagging tracks is broken, for example, when final drives and steering clutches are used. Therefore, the most efficient kinematic patterns are the patterns when the power is transmitted from the lagging track to the leading one, i.e. with the power recuperation. And only part of the power of the lagging track is consumed by spinning friction device. Such steering mechanisms require less energy and evidently have better dynamic performance.
Transmission of T-64 family of armoured vehicles is fitted with steering device that keeps the speed of linear movement V0 when turning along the axis of symmetry of the leading track (Fig. 2a).
Fig. 2. Speed distribution for various steering mechanisms.
Such steering mechanism requires relatively less power, but when the vehicle enters the turn the speed of its centre of mass is getting reduced, thus creating moment turning the car around its centre of mass. It is obvious that at high movement speed such steering mechanism can cause skidding and spontaneous pivoting when making a turn.
Also, there are steering mechanisms for tracked platforms the turning speed of which is Vc.m.=V0, i.e. the point retaining the linear movement speed is in the centre of mass (Fig.2b). Differential design of steering mechanism meets such requirement. Such steering mechanisms are installed on such tanks as M60A1 Abrams, Leopard, etc. Also apply the steering mechanisms for tracked platforms in which the point that retains linear movement speed at turning is behind the leading track, therefore, not only the speed of the lagging track is reduced when turning but also the speed of the leading one, that is V0> V2> V1 (Fig. 2c).
In cases when the steering mechanism ensures transfer of all power from the lagging track to the leading one at any turning radius, it can be called a perfect steering mechanism. In any case, there is a complete power recuperation at any turning radius. Such mechanism can be created using electromechanical transmission. In this case, the recuperation power received from the lagging side of the tracked platform varies constantly, and unlike the recuperation power received on similar tracked platforms with mechanical transmission, is independent of the movement speed.
Strict requirements for tractive effort of a tracked platform, especially in turning mode with minimal braking of the lagging side involves complication of the design of electric drive system and is crucial for choosing the type of electromechanical transmission used. Currently there are several main trends in the development of electromechanical transmissions for combat tracked platforms. Let us consider some basic variants of electromechanical transmissions that can be used for combat tracked platforms:
- mixed-design electromechanical transmission;
- consecutive-design electromechanical transmission;
- consecutive-design hybrid electric drive.
Mixed-design (consecutive-parallel) electromechanical transmission combines a kind of medium variant between mechanical and electromechanical transmission and is a central transmission with the second power flow from internal combustion engine (ICE) [5,6]. Such trend stipulates development of parallel power flows on the basis of electric machines transmitting only a part of the ICE power. The major part of the engine power is transmitted via the main mechanical branch of the transmission, at the same time the transmission remains automatic. The main disadvantage is that there remains kinematic connection between the engine and track sprocket-wheels, lack of layout flexibility and raise of certain difficulties when installing additional equipment (MG, TEM, power converter, etc.) in the existing combat tracked platform.
Classical approach to using consecutive electromechanical transmission in combat tracked platforms accepts that motor-generator is driven from ICE, and two traction electric motors, placed at the sides, drive sprocket wheels of the left and right side tracks via final drives. The disadvantage of such transmission is the lack of source of additional power ensuring high mobility capability of the combat tracked platform and the use of more powerful MG. Therefore, it is necessary to achieve full correspondence of the power level generated by MG for the needs of TEM of the left and right sides, and at the same time to provide power supply for operation of other power users of the combat tracked platform mains.
Currently the hybrid electric drive is regarded as the most promising option to be used on combat tracked platforms [7,8]. As a rule it includes the following elements: ICE (diesel engine), motor-generator, two traction electric motors on the left and right side, energy storage buffer (ESB), power electronics unit. For the use on a combat tracked platform, the most appropriate variant is hybrid electric drive with consecutively connected elements (no rigid kinematic connection) and ESB on board. The main advantage of ESB is to enable compensation for the difference between mean and maximum power of the electric drive system, is required for movement and acceleration of the combat tracked platform accordingly.
Physical requirements for the power of MG, TEM and their cooling system, as well as ESB are mainly connected with the need to provide minimum requirements of operational and physical characteristics of the combat tracked platform, first of all, generation of the tractive effort required for movement, steering and manoeuvring at high speed.
Note the main advantages of consecutive hybrid electric drive used on a combat tracked platform:
- possibility to rapidly develop high torque on TEM, when acceleration is required, due to simultaneous operation of MG and ESB;
- capability to accumulate energy in the storage buffer generated when the tracked platform is braking in order to use it for the further acceleration, turning, hill climbing, high mobility and stealth actions;
- 20-30% reduction of actual power of the used power plant (ICE) with equal tractive parameters compared with the similar vehicles with automatic transmission;
- 10-15% reduction of fuel consumption during movement;
- capability to move a combat tracked platform over short distances in stealth mode with decrease of visibility in infra-red light using the energy of the storage buffer with the main engine shut down;
- ensures the possibility to use a combat tracked platform as an independent power source with around 800-900kW capacity;
- ensures the possibility to accumulate considerable amount of power for future weapon types;
- reduction of maintenance labour and its cost.
The main disadvantages are:
- relatively big size of converters, ESB, that occupy considerable part of the hull volume of the combat tracked platform;
- high voltage on-board requires introduction of additional safety measures for the crew, and the vehicle hull has to be air tight;
- requires extensive cooling system for electrical power units (MG, TEM, power converter).
The electric motors applied for combat tracked platforms have to be capable to work for a long time at rather high torque at low movement speed of the tracked vehicle. Moreover, they have to ensure the movement of the tracked platform at maximum possible speed providing the required tractive effort, and provide additional (reserve) power required for manoeuvres and steering.
Characteristics and manoeuvrability of the currently developed types of electric motors differ greatly . Electric motors comprising permanent magnets cannot generate electric power (generator mode) to be transferred to the leading side and to charge ESB at braking, in addition, they are expensive.
Currently, the most preferable variant to be used in electric drive system (MG, TEM) of combat tracked platforms is an AC induction motor with squirrel-cage rotor, which has no moving contacts (no brushes and slip rings). A significant advantage of the induction drive compared with the other types of electric drives is that the power limitation is provided by restriction of power supply voltage of the induction motor due to respective weakening of the magnetic field, which requires less actual power of the power converters, and as a result, the whole drive system becomes cheaper. The absence of moving contacts ensures higher reliability and reduces maintenance requirements. Also the induction drive is characterized by the best price - performance ratio. The use of modern power converters, the maximum output frequency of which can be adjusted in the range of up to 500Hz, provides the possibility to reduce the weight of traction electric motors and motor-generators without significant reduction of their efficiency.
Let us formulate the basic requirements for the electric drive system for tracked combat platforms weighing up to 50 tonnes:
- power plant - diesel engine with estimated capacity of 800-900kW;
- consecutive-design hybrid electric drive with no rigid kinematic connection between ICE and sprocket wheels. The torque developed by diesel engine should be comparable with the torque of MG which should be formed on the working area of the diesel engine;
- motor-generator is an AC induction motor (motors) with squirrel-cage rotor, which has no moving contacts with total capacity of 800-900kW with liquid cooling system, connected to the output shaft of the power plant;
- two traction electric motors are AC induction motors with squirrel-cage rotor, capable to operate in generator mode to provide ESB charging in braking mode of the combat tracked platform with estimated capacity of 400-450kW each, with liquid cooling system, connected to the sprocket wheels via final drive;
- energy storage buffer is developed on the basis of consecutively connected Lithium-ion batteries (LiFePO4)with estimated capacity of 120kW at 600V voltage, which makes it possible to drive the combat tracked platform in stealth mode without starting the main engine for 4-5 km;
- power converter is executed using IGBT-transistors, that allow to change direction of power transfer - MG control in the engine mode when starting the powerpack as well and control of generator mode of TEM when the tracked platform is braking or turning;
- power converter - (600/28)V DC for power supply of low voltage equipment of the vehicular mains;
- vehicular steering and control system with display of current parameters of the main elements of electric drive system (rpm, temperature, voltage, current, etc.) on driver's panel.
Software of modern CPUs (if equipped with corresponding sensors) makes it possible to implement steering algorithms to avoid skidding and track slip when moving in various road conditions in order to improve characteristics of combat tracked platform acceleration, braking and turning.
Fig. 3. Functional diagram of hybrid traction electrical equipment of a tracked platform.
Development of hybrid electric drive for 50 tonnes combat tracked platform, probably using the following components that are currently in mass production.
1. A regular diesel engine 6TD-2 with a capacity of 882kW (1200h.p.) can be used as the power plant. 2. Traction motors HDS200 with a capacity of 200kW and HDS300 with a capacity of 230kW by BAE Systems company with water cooling can be used as the motor-generators. AC induction generators are installed coaxially with the output shaft of the power plant and can operate both in generator and motor modes, providing start of the power plant (diesel engine). Total capacity of the motor-generators, excluding the capacity of the regular starter-generator of the power plant is 860kW. 3. As traction motors there can be used two induction traction motors with liquid cooling, with a capacity of 450kW each, connected to the sprocket wheels of the tracks on left and right sides via final drive. 4. Energy storage buffer can be made on the basis of consecutively connected Lithium-ion (LiFePO4) batteries (3.2V, 200A·h each) with total nominal voltage of 600V and 120kW·h capacity of the storage buffer. The storage buffer can be placed on the right above-track plate instead of a part of the fuel tanks and regular power unit that is not needed when the hybrid drive is applied. 5. Power control of the traction equipment can be placed symmetrically on the above-track plate of the left side. Energy of the storage buffer goes to the DC voltage converter (600/28V) and then it can be transferred for supply of low-voltage equipment of the power users of the combat tracked platform mains.
Summary. 1. It is possible to use consecutive-design hybrid electric drive with no rigid kinematic connection between ICE and sprocket wheels and with the energy buffer on-board for the combat tracked platforms with up to 50 tonnes weight without considerable weight increase of the vehicle with respect to the similar vehicles with mechanical transmission. 2. As components of electric drive (MG and TEM) it is invited to use AC induction motor (motors) with squirrel-cage rotor, that has no moving contacts and can work both in motor and generator modes, and ESB is made on the basis of consecutively connected Lithium-ion (LiFePO4) batteries.
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