Electricity: motive power systems – Plural – diverse or diversely controlled electric motors – Slipping and/or racing control for electric motors
Reexamination Certificate
2000-10-02
2002-01-15
Dang, Khanh (Department: 2837)
Electricity: motive power systems
Plural, diverse or diversely controlled electric motors
Slipping and/or racing control for electric motors
C318S139000, C477S015000, C180S065600
Reexamination Certificate
active
06339301
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to AC motor controllers for vehicles having an electrical drive. In particular it relates to the integration of an AC motor into a vehicle having a multi-speed transmission as part of the power train connecting the motor to the wheels in a system that provides for regenerative braking.
BACKGROUND TO THE INVENTION
Existing AC motor systems for electric vehicles use a fixed overall gear reduction ratio generally of about 12:1. The measured rotor speed in such installations is a reliable guide for setting the desired speed of the rotating field in the stator's field windings. For accelerating a small increase is effected in the field windings rotational speed, over the speed of rotation of the rotor. For decelerating (also called regenerative retard), a small decrease in such speed is required. Expensive, specially designed motors which can operate at very high rpms are required to attain a speed of 100 km/h in a typical AC powered vehicle when a fixed gear ratio is employed.
When an electric vehicle is equipped with a typical 4 speed manual transmission overall reduction ratios of 15:1, 8.3:1, 5.6:1 and 3.9:1 are available. At 100 km/h in 4
th
gear the motor only has to turn at 3660 rpm. At this speed the motor is very efficient and standard, inexpensive industrial AC induction motors can be used.
In an electric vehicle having a shifting transmission, prior to shifting the rotor will be turning at a speed that corresponds to the vehicle wheel speed, as modified by the transmission ratio. After shifting, when the rotor is mechanically reconnected to the wheels through the transmission, the rotor will be turning at a new speed, again dictated by the wheel speed and transmission ratio combined.
The shift between 1
st
and 2
nd
transmission ratios is typically a change of 45% which is too large for existing AC controllers to handle.
A mechanical shock will occur when the rotor reconnects to the wheels and readjusts to the new speed dictated by the wheels through the powertrain. But a rotor is relatively light and will conform quickly to the new speed requirement. However, an electrical mismatch can arise when this reconnection occurs. And this electrical mismatch has more serious consequences.
In an AC motor the field coil, to operate effectively, must produce a magnetic field that is rotating at a speed that corresponds to the speed of rotation of the rotor. In a synchronous AC motor, these speeds will be the same. In an asynchronous, induction AC motor, these speeds will differ by the slip that is present. But, with an allowance for slip, the rotational velocity of the stator field will “correspond” to the mechanical rotational speed of the rotor. These conditions apply when the AC motor is operating effectively.
The current waveform fed to the field coil of a variable speed AC motor must be constructed by a wave-form generating motor controller to create a rotating magnetic field. This waveform has a frequency of oscillations that will deliver a rotating magnetic field of appropriate rotational velocity within the stator coils of the motor. It is the function of an AC motor controller to deliver to the stator winding of an AC motor a field coil activating current of appropriate frequency the stator winding of an AC motor that corresponds to the rotational velocity of the rotor.
When an AC motor experiences a transmission shift, if a substantial mismatch occurs between the stator field's rotational velocity and the rotor's rotational speed, then there can be a severe reduction of torque. Further, electrical transients may occur that expose the AC motor control system to voltage or current spikes that require protective features and protocols to be included in such system.
As an example, when upshifting from first to second gear at say 30 km/h the rotor may be initially turning at 4150 rpm. After the shift it may be turning at only 2296 rpm. If the controller were asked to continue driving the field coil at 4150 rpm following the shift, a dangerous stall condition could arise. At best a long delay would occur before the rotor would accelerate to its proper speed. Downshifting is even worse. A downshift from second to first at 30 km/h calls for a typical change in rotor rpm of from 2296 rpm to 4500 rpm. If the field winding of the motor is still powered at the old speed of 2296 rpm following this shift, then a large negative slip condition will arise and pour considerable energy back into the batteries through the motor control circuitry. This can easily raise the instantaneous voltage applied to the electrical circuitry to a breakdown value. For example, the snubber capacitors and IGBT's (internal gate bipolar transistors) components in a controller could fail.
The present invention addresses a means by which an AC motor controller in an electrically powered vehicle may accommodate a transmission shift without exposing the system to prejudicial electrical consequences.
The invention in its general form will first be described, and then its implementation in terms of specific embodiments will be detailed with reference to the drawings following hereafter. These embodiments are intended to demonstrate the principle of the invention, and the manner of its implementation. The invention in its broadest and more specific forms will then be further described, and defined, in each of the individual claims which conclude this Specification.
SUMMARY OF THE INVENTION
To ensure that a mismatch of the stator field rotational velocity for an AC motor does not arise during a gear change, it is a feature of the invention to provide a means for suspending the flow of current to the field coils of the motor when a transmission ratio shift is in progress. According to one variant of the invention the provision of current to the field coil is suspended when the rotor-to-motor speed ratio has departed from a value that is consistent with there being a mechanical connection between the rotor and the wheels. This can be established by detecting a change in such ratio that can only arise when a transmission shift is in progress.
According to another variant of the invention motor speed and wheel speed are monitored on a continuous basis. A controller then proceeds to divide the smaller wheel rpm into the larger motor rpm (or vice versa). If this value is not substantially within the range of known transmission ratios, the motor may be considered to be disengaged from the wheels. In any of these conditions, the controller should, in an abundantly cautious system, immediately suspend activation of the field coils. Alternately, combinations of such conditions may be required.
Reactivation of the field coil is only permitted to occur when the rotor has stabilized at a new rotational speed that corresponds to re-engagement of the rotor to the vehicle wheels. The excitation of the field coils is then reactivated at a rotational velocity that is within a few percent of the rotor's measured velocity, according to the slip condition that is required, if slip is to be present. Whether positive or negative torque is to be generated within the motor is then established by input from the operator.
Reactivation thus occurs proceeding from an inactivated or unpowered state only once the rotor speed has stabilized. This avoids an undesirable mismatch between the field coil excitation velocity and the rotor speed.
The re-powering of the motor may be made subject to analogous tests to those described above. If the resultant ratio value for wheel speed vs rotor speed is close to one of the expected ratios for the transmission, and preferably, if the present measured ratio is the same, within an acceptable tolerance, as the most recently measured previous ratio, then the clutch may be considered to be engaged and the motor's rotary speed may be considered to be reliable for control purposes. It is at this stage that the field coil may be re-excited with the appropriate rotational velocity.
An important condition for re-powering of the motor is tha
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