Electricity: motive power systems – Braking – Dynamic braking
Reexamination Certificate
1999-05-12
2001-05-29
Ro, Bentsu (Department: 2834)
Electricity: motive power systems
Braking
Dynamic braking
C318S500000
Reexamination Certificate
active
06239566
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to drive systems for electric motors and, more particularly, to a drive system for a permanently excited electric motor.
2. Description of the Related Art
Permanently excited electric motors are used, for example, as vehicle drive motors which, in the case of hybrid drive systems, receive electrical power from a generator driven by an internal combustion engine or, in the case of completely electrically operated vehicles, receive electrical power from a so-called traction battery. In the case of permanently excited electric motors, and due to their construction, there is, in principle, the problem that, on account of the relative movement which occurs during operation between the armature windings and the permanent magnets, a back e.m.f., known as the field e.m.f., is induced in the armature windings. This induced voltage rises as the speed increases, until it ultimately lies in the range of the supply voltage provided by the voltage source for the electric motor. A further increase in the motor speed can then be obtained only by means of so-called field weakening, in which a phase-winding shift in the current introduced into the windings is produced. If faults occur in such drive systems with permanently excited electric motors, such as the failure of the drive unit performing the field weakening, this may lead to serious problems. As can be taken from characteristic curve A in
FIG. 3
, which reproduces the braking torque for a permanently excited electric motor in the case of passive regeneration by the rotating electric motor, a failure of the field weakening, in particular at high speeds, leads to a considerable braking torque, which is produced when electrical energy is fed back from the motor into the voltage source, for example the traction battery. Due to the voltage overshoot caused by the internal resistance of the battery, there is the risk that damage will occur in the area of the traction battery, which is generally designed to output an operating voltage of about 200 to 300 V. In addition, the occurrence of such large braking torques during driving operation is undesirable, since this may result in endangering a person seated in a vehicle.
If no voltage source is connected to the intermediate circuit, or if, for example, the connection between the voltage source and the intermediate circuit is broken, the field e.m.f. may be present on the intermediate circuit. This can lead to damage in the area of the components present in the intermediate circuit or the power electronics for the electric motor and/or possibly in a generator, in particular semiconductor components.
In order to avoid these problems, the various components of the converter, for example the capacitors and power semiconductors, have been designed such that even the rectified field e.m.f. does not exceed their rated voltage, so that damage to these components is not expected. The consequence of this is that the drive systems could not be designed such that they are able to produce the greatest possible output in the volume available to them and a certain oversizing of the components always had to be provided. In addition, the various components in the intermediate circuit or the power electronics had to be designed for the maximum voltages to be expected, so that higher costs arose in the area of these components.
SUMMARY OF THE INVENTION
It is therefore the object of the present invention to provide a drive system for a permanently excited electric motor having at least one phase winding which is suitable to take suitable measures for avoiding damage to components or for avoiding undesired consequences in the widest possible range of operating states, and in particular fault states.
According to the invention, this and other objects are achieved by a drive system for a permanently excited electric motor having at least one phase winding, comprising a half-bridge arrangement for each phase winding of the motor, an intermediate circuit connecting each half-bridge arrangement to a motor supply voltage source, a drive arrangement for driving each half-bridge arrangement, where, in accordance with the driving by means of the drive arrangement, a voltage or a potential with a predetermined polarity is applied or can be applied for a predetermined duration by each half-bridge arrangement to that phase winding of the electric motor which is assigned to this half-bridge arrangement, and an operating state registering arrangement at least for registering an operating state of the drive system and/or of the electric motor.
In the drive system according to the invention, the drive arrangement is designed such that, when the presence of at least one predetermined operating state in the drive system and/or in the electric motor is registered by the operating state registering arrangement, said drive arrangement generates a command to produce a short circuit in each phase winding of the motor.
For this purpose, the drive system according to the invention is able to take into account a wide variety of operating states and, by means of the specific production of a short circuit in the electric motor, to avoid the occurrence of potential hazards or potentially undesired consequential states. This can be illustrated, in particular, using characteristic curve B in
FIG. 3
, which shows the short-circuit braking torque as a function of the machine speed. It can be seen that, above all in the range of high speeds, the braking torque produced at short circuit is negligible, but is in every case considerably lower than the braking torque which is present when the electric motor is not short-circuited. This means that if, for example, a spontaneous failure of the field weakening occurs, then by means of the specific short-circuiting of the electric motor, on the one hand, the occurrence of an undesired braking torque can be considerably reduced, and on the other hand it is possible to prevent a voltage which may possibly be fed into the intermediate circuit by the electric motor leading to damage to components or to the voltage source, for example the traction battery.
The drive system according to the invention preferably comprises a primary drive arrangement supply voltage source for providing an operating voltage for the drive arrangement, preferably in the region of 12 V.
The above-mentioned predetermined operating state may be, for example, a fault state, which comprises at least one of the following faults:
a) a drop in, or failure of, the motor supply voltage,
b) faults in the area of a motor position sensor,
c) faults in the area of a current sensor for registering the current flowing in at least one phase winding,
d) short-circuit faults, in particular in the area of the motor, and
e) an intermediate-circuit voltage rising above a predetermined limiting value.
However, it should be pointed out here that any other fault which, in particular in the field-weakening range, leads to it being impossible to maintain the specific current flow through the motor, can be considered as a fault defining the predetermined operating state.
In the drive system according to the invention, each half-bridge arrangement comprises at least one first switch element for the selective connection of the associated phase winding of the electric motor to a motor supply voltage or, respectively, to a potential with a first polarity, as well as at least one second switch element for the selective connection of the associated phase winding of the electric motor to a motor supply voltage or, respectively, to a potential with a second polarity. In this embodiment, the command for producing the short circuit causes the at least one first switch element of each half-bridge arrangement to be switched on and/or the at least one second switch element of each half-bridge arrangement to be switched on.
There is, in principle, the problem that faults can also occur in the area of the half-bridge arrangements. For example, one of the switch elements may be defective and
Karg Erich
Ruthlein Alfred
Tareilus Alfred
Cohen & Pontani, Lieberman & Pavane
Mannesmann Sachs AG
Ro Bentsu
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