Electricity: motive power systems – Switched reluctance motor commutation control
Reexamination Certificate
1999-12-13
2001-03-27
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Switched reluctance motor commutation control
C318S800000, C318S801000, C318S803000, C318S804000, C318S807000, C318S813000, C363S034000, C363S035000, C363S039000, C363S040000
Reexamination Certificate
active
06208098
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a variable frequency inverter and, more particularly, to a noise attenuation circuit used with a pulse width modulated inverter.
BACKGROUND OF THE INVENTION
A motor drive system, in one known form, comprises an AC source supplying three-phase AC power to a variable frequency inverter (VFI). The VFI includes an AC/DC converter to connected by a DC link to a DC/AC converter. The DC/AC converter may comprise a pulse width modulated inverter using insulated gate bipolar transistors (IGBTs).
In general, the output of the VFI is modulated at a carrier frequency ranging from one kHz to 20 kHz. The higher carrier frequency, particularly in small sized VFIs up to 75 kilowatts, along with the fast rise time of the IGBTs employed results in high dv/dt. This, in turn, yields non-trivial common mode or ground currents. If the distance between the motor and the VFI is long and there exists a mismatch in the cable and motor surge impedance, then there is voltage amplification at the motor terminals. Hence, the dv/dt effects in conjunction with larger cable capacitance enhances the common mode currents.
In practice, the over voltage at the motor terminals, which depends upon the distance between the motor and the VFI as well as the impedance mismatch between the cable and the motor surge impedance, can reach as high as twice the DC bus voltage. In certain cases, due to overlap of the modulating pulses, the peak transient appearing across the motor windings can be as high as three times the DC bus voltage. The high rate of rise of voltage pulses in the range of a few hundreds of nanoseconds give rise to ground currents due to cable capacitance to ground and motor winding capacitance to ground.
The over voltage at the motor terminals due to long lead lengths can cause premature to insulation failure in the motor. There exists a non-trivial parasitic capacitance between the stator and the rotor, which is instrumental in creating a charge path between the stator and the rotor. Due to large dv/dt of the common mode voltage, and the above mentioned parasitic capacitance, the rotor develops a voltage similar to a charge pump. When this voltage exceeds the breakdown voltage of the thin lubricant film between the inner and outer rings of the bearing, there is a miniature flashover. This causes pitting in the bearings and is the main reason for premature bearing failure. If not properly mitigated, high frequency ground currents can also create interference with the power system ground and affect other equipment on the power system. This phenomenon contributes to conducted EMI.
Our pending application No. 09/045,333, filed Mar. 20, 1998, the specification of which is incorporated by reference herein, discloses a passive circuit for canceling common mode current. The passive circuit uses a neutral point provider in the form of a three-phase iron core transformer and a common mode transformer.
A DC output is generated in induction motor drives during braking conditions. In servo applications, this DC output component is responsible for developing holding torque. The three phase iron core transformer that creates the neutral point can saturate.
The present invention is directed to solving one or more of the problems discussed above in a novel and simple manner.
SUMMARY OF THE INVENTION
In accordance with the invention there is provided a noise attenuation circuit operable to cancel main common mode current and prevent saturation.
Broadly, there is disclosed herein a noise attenuation circuit used in a motor drive system. The motor drive system includes a pulse width modulated inverter providing three phase power output on three phase conductors for driving a motor. The noise attenuation circuit includes a common mode transformer including a common mode choke having a core, with first, second and third choke windings on the core, each connected in series with one of the three phase conductors, and a fourth winding on the core. A three phase iron core transformer creates a neutral point representing common mode voltage. The iron core transformer has three primary windings connected in a wye configuration. A capacitor circuit is connected between the iron core transformer and the three phase conductors. The fourth winding is connected between the neutral point and the DC bus to reduce main common mode current.
It is a feature of the invention that the fourth winding is wound in the same sense as the three choke windings. All four windings are on the same core.
It is another feature of the invention that the fourth winding is connected to a midpoint of the DC bus to force a current, dependent on voltage of the neutral point relative to midpoint of the DC bus, in the fourth winding of the common mode transformer in an opposite direction to cancel main common mode current. Two capacitors are series connected across the DC bus, and a junction of the two capacitors defines the midpoint of the DC bus.
It is a further feature of the invention to provide a differentiating circuit connected to the fourth winding. The differentiating circuit comprises an RC network. The differentiating circuit comprises a resistor connected across the fourth winding and a capacitor connected between the fourth winding and the DC bus.
It is another feature of the invention that the capacitor circuit comprises a blocking capacitor connected in series with each of the three primary windings. The blocking capacitors are connected between the primary windings and the three phase conductors. The blocking capacitors have values selected to provide a resonant point formed with the primary windings to be less than a switching frequency of the inverter and higher than a fundamental frequency of the inverter.
In accordance with another aspect of the invention there is disclosed a noise attenuation circuit used in a motor drive system. The motor drive system includes a variable frequency drive having a pulse width modulated inverter providing three phase power output on three phase conductors for driving a motor. The noise attenuation circuit includes a common mode transformer including a common mode choke having a core, with first, second and third choke windings on the core, each connected in series with one of the three phase conductors, and a fourth winding on the core. A three phase iron core transformer creates a neutral point representing common mode voltage. The iron core transformer has three primary windings connected in a wye configuration. A capacitor circuit is connected between the iron core transformer and the three phase conductors. The fourth winding is connected between the neutral point and the DC bus to reduce main common mode current.
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Kume Tsuneo
Swamy Mahesh M.
Leykin Rita
Nappi Robert E.
Wood Phillips VanSanten Clark & Mortimer
Yaskawa Electric America, Inc.
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