Electricity: motive power systems – Limitation of motor load – current – torque or force
Reexamination Certificate
1999-10-26
2001-09-18
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Limitation of motor load, current, torque or force
C180S065100
Reexamination Certificate
active
06291953
ABSTRACT:
The present invention relates to a drive system and in particular to an electrical drive system.
The invention has been developed primarily for use within hybrid electric vehicles (HEVs) or electrically powered vehicles and will be described hereinafter with reference to that application. However, it will be appreciated that the invention is not limited to that particular field of use and is also applicable to other electrical drive applications having a variable load characteristic.
It is a conventional approach to provide a vehicle such as a car, bus or truck with an internal combustion (IC) engine. Due to a variety of concerns with this form of locomotion, particularly that of pollution, the development of alternative power sources has been encouraged. One of these alternatives is a HEV which includes a combination of an electrical machine and an IC engine. Such a combination promises numerous advantages such as:
1. recovering some of the energy used for acceleration when braking;
2. turning the IC engine off in areas particularly susceptible or prone to pollution or when stationary as the electrical system is still available to propel the vehicle (although this is contingent upon the vehicle having sufficient on-board energy storage);
3. allowing the use of a smaller IC engine; and
4. allowing the IC engine to be run more efficiently.
Other possible HEV configurations include: replacing the IC engine with a high-speed turbine or a fuel cell or combining an engine/turbine/fuel cell and a steam turbine which recovers some of the tail pipe energy.
The power required to propel a vehicle varies greatly over short time periods. Typical analysis of power requirements show that peak power is only required under extreme conditions such as: when accelerating hard; when the vehicle is traveling at its maximum velocity; or when climbing a steep gradient. Accordingly, when use is made of an IC engine, that engine is oversized for normal running, such as freeway cruising or an urban cycle, to cater for far more time limited events such as climbing steep hills and/or accelerating. The larger the engine the more fuel it burns for a given power because of increased friction. Hence, from an economy view point reducing the engine size is preferred. This is effectively achieved in a HEV by obtaining the extra peak power required for acceleration from the associated electrical system. The IC engine is only required to provide the average power.
Additionally, an IC engine has a relatively peaky efficiency characteristic compared to an electrical machine and is most effectively run at its most efficient operating point. In this type of scheme the IC engine is:
1. only turned on when required;
2. run always at its most efficient operating point (that is, it is only run at a fixed power and hence fixed speed and fixed torque); and
3. any excess energy generated above that required by the car at that time is collected and stored.
A compromise approach is to allow the IC engine to run at different powers, but for a given power run at its optimum speed and torque for this power. This reduces the amount of storage required and is done with conventional cars fitted continuously variable gearboxes.
A second compromise approach is run the engine at a speed dictated by the vehicle speed and the gear that the vehicle is in, but to operate it at its most efficient torque. The excess power is stored for later use.
Other alternatives are to replace the internal combustion engine with a high-speed turbine. Such a turbine is subject to considerably less frictional losses and is much more efficient than an IC engine. It also generally operates at higher temperatures, which again improve its efficiency. Moreover, turbines are smaller and lighter than the equivalent IC engine. Small turbines producing up to about 20 kW are common in aircraft where they provide auxiliary power to the aircraft when stationary on the tarmac. This power is used both to start the main engines and to power internal systems on the aircraft. Turbines specifically designed for hybrid vehicles are also available.
The most radical approach to HEVs is to replace any form of mechanical conversion of fuel to energy with a fuel cell. Much attention, particularly in the USA, has been given to fuel cells for cars. However, there are no vehicles powered by fuel cells commercially available as many specific difficulties associated with this technology are yet to be sufficiently addressed.
In another alternative form HEV the waste heat from the engines, whether that be an IC engine, turbine or fuel cell, is used to heat water and turn it into steam. This steam drives a turbine, which, in turn, drives a generator that charges an on-board energy store.
Most HEVs rely upon an electrical machine of some kind, although in combination with one or more alternative power sources. Accordingly, sophisticated electronic control circuitry is required to ensure an effective interface between the electrical machine and the other power source. This leads to an inherent compromise involved in the choice of the electrical machine and control electronics. More particularly, the cost of an electrical machine or generator is approximately proportional to its torque, whereas the cost of the control electronics is approximately proportional to the VA required to drive the electrical machine.
It is an object of the present invention, at least in the preferred embodiments, to overcome or substantially ameliorate one or more of the disadvantages of the prior art, or at least to provide a useful alternative.
According to a first aspect of the invention there is provided an electrical drive system for a load, the system including:
an electrical machine for rotating a drive shaft at a first speed;
an output shaft for connecting to the load and which is responsive to the drive shaft for rotation at a second speed; and
a coupling between the drive shaft and the output shaft which selectively operates in either a locked or and unlocked configuration, whereby in the locked configuration the first speed equals the second speed and in the unlocked configuration the first speed is greater than the second speed.
Preferably, the electrical machine applies to a first torque to the drive shaft and the coupling applies a second torque to the output shaft wherein, in the unlocked configuration, the second torque is greater than the first torque. More preferably, in the locked configuration the first torque is equal to the second torque.
Preferably also, the coupling is a lockup torque converter. More preferably the coupling is a viscous converter.
In a preferred form, the coupling is responsive to an operator's input for varying the second torque. More preferably, during that variation of the second torque the first torque remains substantially constant. Even more preferably, the magnitude of the second torque is required to fall within the range of between 0 and a maximum load torque and the magnitude of the first torque is required to fall within a range between 0 and a maximum drive torque, wherein the maximum load torque is greater than the maximum drive torque.
Preferably, the system includes a control means, which is responsive to an operator's input for controlling both the electrical machine and the coupling such that the first torque is substantially constant.
Preferably also, the system is mounted to a vehicle having at least one driven wheel and the load is that one or more driven wheel. More preferably, the operator provides various inputs to drive the vehicle and the controller is responsive to at least one of those inputs for accelerating the vehicle, whereby acceleration up to a threshold is achieved by a corresponding increase in both the first and the second torques, while acceleration above that threshold is achieved by a larger increase in the second torque. Even more preferably, following an input being provided to accelerate the vehicle above the threshold, and is absence of subsequent like inputs, the controller progressively reduces the first speed to equal
Dunlop John
Lovatt Howard C.
Commonwealth Scientific and Industrial Research Organization
Duda Rina I.
Kenyon & Kenyon
Nappi Robert E.
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