Patent Description:
The typical brushless DC or three phase electric motor driven by a three phase bridge or inverter uses the motor windings to integrate the square wave output drive pulses from the bridge. However, timing and switching of power in an inverter to drive a motor generates significant electromagnetic interference (EMI). When the controller, and thereby the bridge or inverter driver circuit is remotely located from the motor load, a three conductor shielded cable is commonly employed as the interface between the driver and motor. This cable, in conjunction with the reactance of the motor and parasitic winding capacitance, creates high Q factor common mode resonances that result in high frequency damped sinusoidal ringing on the square wave bridge driver output pulse edges. This high frequency ringing increases electromagnetic emissions from the system and interferes with the ability to monitor and control the motor phase currents.

Snubber circuits are typically used to reduce the Q factor of the motor interface at the ringing frequency as a way to control electromagnetic emissions and improve the pulse shape of pulse output by the driver bridge. The snubbers could be grounded to the motor case, but this could increase radiated electromagnetic emission levels due to increased structure currents. Therefore what is needed is a way of reducing the Q factor of the motor interface without increasing structure currents and electromagnetic emissions Electric motor drive systems related to the invention are disclosed in <CIT>, <CIT>, <CIT>, <CIT> and <CIT>.

In general, EMI noise can be divided into two major groups: differential mode (DM) noise and common-mode (CM) noise. DM noises are conducted between phases of the motor or inverter. CM noises are conducted together with all phases through the parasitic capacitance of the motor windings to structure ground. CM noises can be problematic for motor drives because CM noises increase the EMI in the motor drive and can damage the motor bearing and winding insulation. Unfortunately, in certain applications, solutions such as adding CM filters to attenuate CM noises are not viable due to the significant weight penalty of each CM filter.

According to the invention a drive system for an electric motor is provided as defined by claim <NUM>.

In any of the above method embodiments, or in the alternative, the drive system further including a rectifier bridge, the rectifier bridge operably connected to an alternating current and voltage power source and the DC link, the rectifier bridge configured to rectify the alternating current and voltage to DC to supply the DC link.

In any of the above method embodiments, or in the alternative, the drive system further including that the rectifier bridge is an active rectifier bridge.

In any of the above method embodiments, or in the alternative, the drive system further including a controller operably connected to the inverter, the controller configured to generate control signals to cause the inverter to generate a plurality of motor excitation signals.

In any of the above method embodiments, or in the alternative, the drive system further including that each snubber circuit of the plurality of snubber circuits includes an inductor.

In any of the above method embodiments, or in the alternative, the drive system further including that the plurality of snubber circuits are configured to reduce a Q factor of an interface between the motor and the inverter at an oscillation frequency associated with the plurality of excitation signals.

In any of the above method embodiments, or in the alternative, the drive system further including that the plurality of snubber circuits are at least one of disposed closer to the motor than the inverter, disposed within three feet (<NUM>) of the motor, and disposed at the motor. Moreover, in any of the above method embodiments, or in the alternative, the drive system further including that the interface cable includes the transmission line. In addition, or in the alternative, the drive system further including that the transmission line is a single shielded wire.

In any of the above method embodiments, or in the alternative, the method further including rectifying an alternating current and voltage to DC to supply the DC link with a rectifier bridge, the rectifier bridge operably connected to an alternating current and voltage power source and the DC link.

In any of the above method embodiments, or in the alternative, the method further including generating control signals to cause the inverter to generate a plurality of motor excitation signal with a controller operably connected to the inverter.

In any of the above method embodiments, or in the alternative, the method further including that each snubber circuit of the plurality of snubber circuits includes at least two of resistor, a capacitor, and an inductor.

In any of the above method embodiments, or in the alternative, the method further including that each snubber circuit of the plurality of snubber circuits includes a resistor and capacitor in series.

In any of the above method embodiments, or in the alternative, the method further including disposing the plurality of snubber circuits at least one of closer to the motor than the inverter, within three feet (<NUM>) of the motor, and disposed at the motor.

In any of the above method embodiments, or in the alternative, the method further including that the interface cable includes the transmission line.

In any of the above method embodiments, or in the alternative, the method further including that the transmission line is single shielded wire.

Additional features and advantages are realized through the techniques of the present disclosure. Other embodiments and aspects of the disclosure are described in detail herein. For a better understanding of the disclosure with the advantages and the features, refer to the description and to the drawings.

The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:.

Generally, the switching of power electronics devices in actively controlled inverters also generates electromagnetic interference (EMI). EMI filters are designed to attenuate EMI noise to satisfy the EMI standards, which are defined for particular applications, but EMI filters add weight and complexity for the motor drive system. Thus, alternative means to reduce EMI are commonly considered. In general, embodiments herein relate to a motor drive that receives DC power from a DC bus supplied by an active or passive rectifier bridge. The motor drive is located remotely from the motor and significant EMI can result. A snubber network and transmission line is employed to address the EMI concerns. In particular, the embodiments herein relate a snubber network and its connection between a motor and the DC bus. Embodiments herein set forth a drive and motor system and/or method for control of motor system driven by a motor drive or inverter to control EMI. In an embodiment, three snubbers designed to reduce the circuit Q factor at the system resonant frequency are connected in a "Y" configuration at the motor. In an embodiment, the common point is interfaced back to the DC power bus at the motor bridge through a single shielded conductor transmission line which is isolated from the three shielded motor drive signals.

It will nevertheless be understood that no limitation of the scope of this disclosure is thereby intended. The following description is merely illustrative in nature and is not intended to limit the present disclosure, its application or uses. As used herein, the term controller refers to processing circuitry that may include an application specific integrated circuit (ASIC), an electronic circuit, an electronic processor (shared, dedicated, or group) and memory that executes one or more software or firmware programs, a combinational logic circuit, and/or other suitable interfaces and components that provide the described functionality.

Additionally, the term "exemplary" is used herein to mean "serving as an example, instance or illustration. " Any embodiment or design described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments or designs. The terms "at least one" and "one or more" are understood to include any integer number greater than or equal to one, i.e. one, two, three, four, etc. The terms "a plurality" are understood to include any integer number greater than or equal to two, i.e. two, three, four, five, etc. The term "connection" can include an indirect "connection" and a direct "connection".

As shown and described herein, various features of the disclosure will be presented. Various embodiments may have the same or similar features and thus the same or similar features may be labeled with the same reference numeral, but preceded by a different first number indicating the figure to which the feature is shown. Thus, for example, element "a" that is shown in Figure X may be labeled "Xa" and a similar feature in Figure Z may be labeled "Za. " Although similar reference numbers may be used in a generic sense, various embodiments will be described and various features may include changes, alterations, modifications, etc. as will be appreciated by those of skill in the art, whether explicitly described or otherwise would be appreciated by those of skill in the art.

In one embodiment, the three-phase rectifier and inverter is utilized in a power system of an aircraft. It will be appreciated that while the embodiments described herein are provided with reference to aircraft systems, other applications are possible. For example the described embodiments may be employed in other motor controls systems where inverter motor controls are employed. For example the described embodiments may also be applicable to building systems such as heating ventilation and air conditioning or refrigeration system (HVAC/R). For example, a building HVAC/R can employ a chiller system driven by a power system including a motor drive with inverter as described herein. The drive may also include a power electronics inverter (e.g., as a variable speed alternating current (AC) motor drive) to improve the performance of the chiller system. Similarly, the described embodiments may be utilized in an electric motor system of an elevator system. The elevator system also includes a hoistway having a plurality of lanes or shafts. In each shaft, one or more elevator car travels to deliver passengers to a desired floor of a building. The electric motor system utilizes the power electronics inverter (e.g., as variable speed alternating drive (AC) motor drive) to improve the performance of maneuvering the elevator cars. Other applications and embodiments of the three-phase passive front-end rectifier include powers systems for trains, boats, planes, etc..

<FIG> schematically illustrates a motor drive <NUM> including an active or passive rectifier <NUM> and a motor drive inverter <NUM> as may be employed to implement the described embodiments. A three phase power source <NUM> provides electrical power to the motor drive <NUM>. The current from the three phase power source <NUM> is passed through the input EMI filter <NUM> to the rectifier <NUM>. In an embodiment, the active rectifier <NUM> includes a conventional six-switch voltage source pulse width modulation (PWM) converter. In one example, the active rectifier <NUM> converts a three-phase AC input power into <NUM> volts DC output power at a DC link <NUM>. In another embodiment the DC link <NUM> is supplied from an aircraft battery and a 28VDC bus. In another embodiment, the rectifier <NUM> is comprised of a conventional rectifier bridge. In an embodiment the DC link includes a positive terminal and a negative terminal. The DC link <NUM> may also include two or more DC link mid-point capacitors <NUM> connecting the terminals of the DC link <NUM>. In addition, one or more bulk capacitor(s) <NUM> may be arranged in parallel to the two DC link mid-point capacitors <NUM> across the DC link <NUM>.

The DC link <NUM> is connected to an input of the motor drive inverter <NUM>. The motor drive inverter <NUM>, in turn, converts the received DC input power into a three phase AC output power on lines <NUM> to power the motor <NUM>. The motor drive inverter <NUM> includes a conventional six-switch voltage source PWM inverter. The motor drive inverter <NUM> receives control signals <NUM> from controller <NUM> to generate a set of motor excitation signals <NUM>. The inverter <NUM> generates high frequency (HF) voltage components that cause HF leakage currents and conducted electromagnetic interference (EMI) noise which flows within power-feeding paths, and between the drive system <NUM> and the ground. Due to the low-duty cycle utilization of the motor drive system for engine start applications, concerns related to the bearing currents and shaft voltage are reduced, while the common-mode noise associated with the high-frequency leakage currents due to motor windings capacitive coupling to the ground are addressed by incrementally increasing size of the input EMI filter.

The control signals <NUM> generated by the controller <NUM> may be pulse width modulation (PWM) signals, commonly used in n-level drives and many inverter control applications. In conventional PWM the duty cycle of the control signals <NUM> is varied as required based on the output current requirements of the load (in this instance motor <NUM>). For example, if more torque is required in by the motor <NUM>, the pulse width of the control signals <NUM> is increased, thereby the switching devices of the inverter <NUM> remain on for a commensurate duration and directing more current to the motor <NUM>. Likewise, if a reduction in the output current from the drive <NUM> is needed, the duty cycle of the control signals <NUM> is decreased by the controller <NUM>.

In some embodiments, where employed, a switched-mode active rectifier <NUM> will also generate high frequency (HF) voltage components that cause HF leakage currents and conducted electromagnetic interference (EMI) noise. In some instances, the switching frequency of the active rectifier <NUM> and the motor drive inverter <NUM> may be the same, and therefore introduce the same frequency of common mode noise into the system. In alternate examples, the switching frequencies are different (i.e., the inverter <NUM> switching frequency may be lower in comparison to the active rectifier <NUM> switching frequency in order to reduce inverter switching losses when operating the inverter <NUM> at higher output current levels), and introduce different frequencies of common mode noise into the system <NUM>.

<FIG> illustrates a more detailed diagram of a portion of the motor drive <NUM> including the snubber circuits <NUM> of an embodiment. In the figure an optional AC power source <NUM>, input EMI filter <NUM> and rectifier <NUM> are not depicted for simplicity. In an embodiment, the motor <NUM> may be part of an actuator, machine, and compressor and the like. In an embodiment the motor <NUM> is located some distance from the motor drive <NUM> and in particular the inverter or bridge <NUM>. To facilitate operation a wiring harness or interface cable <NUM> carries a set of excitation signals <NUM> to the motor <NUM>. Unfortunately, the bridge or inverter <NUM> generating the motor excitation signals <NUM> is remotely located from the motor <NUM> the cable <NUM> in conjunction with the reactance of the motor <NUM> and parasitic winding capacitance in the motor <NUM> creates high Q factor common mode resonances that result in high frequency damped sinusoidal ringing on the square wave edges of the motor excitation signals <NUM> This high frequency ringing increases electromagnetic emissions from the system <NUM> and interferes with the ability to monitor and control the motor phase currents.

To address this ringing, in an embodiment, a snubber circuit <NUM> is connected to each excitation phase of the motor <NUM> closer to the motor <NUM> than the inverter <NUM>. In another embodiment, the snubber circuit <NUM> is placed within a few feet (<NUM> foot = <NUM>) of the motor <NUM>, for example, within three feet (<NUM>). Finally, in yet another embodiment, the snubber circuit is placed at or very near the motor <NUM>, i.e., within one foot (<NUM>). In particular, in an embodiment, three snubber circuits <NUM> with a first terminal <NUM> are connected in a "Y" configuration at the motor <NUM>. In an embodiment the snubber circuits each comprise a resistor and capacitor in series. In another embodiment the snubber circuits <NUM> may comprise a resistor and an inductor. In yet another embodiment the snubber circuit <NUM> may comprise at least two of resistor, an inductor, and a capacitor. It should be appreciated that the snubber circuit <NUM> has been described as a resistor capacitor circuit for the purpose of illustration. Many other circuit configurations and topologies are possible, including, but not limited to more complex resistor capacitor networks, inductive,-resistive networks, inductive capacitive networks, and resistive inductive, capacitive networks. The only requirement is that the snubber circuit <NUM> operates to reduce the Q factor of the motor interface. The snubber circuit <NUM> reduces the Q factor of the motor <NUM> interface at the ringing frequency as way to control electromagnetic emissions and improve the pulse shape or the excitation signals <NUM> from the from the driver bridge or inverter <NUM>. The snubber circuit <NUM> second terminal <NUM> could be commonly connected or grounded locally with the case of the motor <NUM>. However, such an approach could increase radiated emission levels due to increased structure currents. To address this concern, in an embodiment, the snubber circuit <NUM> second terminals <NUM> are connected together at a common point shown as reference number <NUM> that is terminated back to the drive bridge or inverter DC link <NUM>. In an embodiment, a fourth conductor <NUM> shown as a single shielded conductor transmission line <NUM> which is isolated from the three shielded motor drive signals <NUM> in the interface cable <NUM> between the drive bridge or inverter <NUM> and the motor <NUM>. Advantageously, this termination scheme for the snubber circuits <NUM> does not increase high frequency common mode structure currents.

Advantageously, the isolated conductor <NUM> provides improved damping of the ringing on the drive output pulses by eliminating the common mode coupling between the drive outputs and the return signal with a <NUM> conductor interface cable. The shielded conductor transmission line is employed because the snubber common point return signal is out of phase with the motor drive transients on the motor drive signals <NUM>. This occurs because the common mode signals on the motor excitation signals carried on the motor interface cable <NUM> are <NUM> degrees out of phase with the signal from the common point of the snubber circuits <NUM>, so any shield leakage will reduce the snubber circuit <NUM> effectiveness in reducing the circuit Q. The circuit Q reduction is greatest when the differential transient voltage across the snubber circuit <NUM> is maximized. The differential voltage loaded by the resistors of snubber circuits <NUM> provide a cancelling effect for any radiated emissions.

Turning now to <FIG> for a depiction of the method <NUM> of reducing the resonant effects of reactive loads in an electric motor system <NUM> in accordance with an embodiment with a DC link <NUM>. The DC link <NUM> has a positive terminal and a ground terminal. The system also includes an inverter <NUM> operably connected to the DC link <NUM>. The method <NUM> initiates with an optional step of rectifying an alternating current and voltage to DC to supply the DC link <NUM> with a rectifier bridge <NUM> as depicted at process step <NUM>. The rectifier bridge <NUM> is also operably connected to an alternating current and voltage power source <NUM> and outputs to the DC link <NUM>. At process step <NUM> the method <NUM> continues with generating a plurality of motor excitation signals <NUM> with the inverter <NUM>. The inverter <NUM> is connected with an interface cable <NUM> to the motor <NUM> where the motor <NUM> is remote from the inverter <NUM> at process step <NUM>. The motor <NUM> is configured to be responsive to the plurality of motor excitation signals <NUM>. Finally at process step <NUM> the method continues with reducing a Q factor of an interface of the motor <NUM> at an oscillation frequency associated with the plurality of excitation signals <NUM> with a plurality of snubber circuits <NUM>, each of the snubber circuits <NUM> of the plurality of snubber circuits having a first terminal <NUM> operably connected to a winding of the motor <NUM>, and a second terminal <NUM> operably connected to a first end of a transmission line <NUM>, wherein a second end of the transmission line <NUM> is operably connected to the positive terminal of the DC link <NUM>.

In view of the above, the technical effects and benefits of embodiments of a drive system <NUM> include achieving reduced CM-voltage and EMI that enables control of a remotely placed motor <NUM>. Eliminating common-mode voltage and ringing for the inverter output results in significant reductions of CM and radiated EMI, and facilitates eliminating a need for CM EMI filters, along with a reduction of an input current ripple, DC side (e.g., DC capacitor) current ripple, and a conducted EMI.

Claim 1:
A drive system for an electric motor, comprising:
a DC link (<NUM>) having a positive terminal and a ground terminal;
an inverter (<NUM>) operably connected to the DC link, the inverter configured to provide a plurality of motor excitation signals;
an interface cable (<NUM>), the interface cable operably connected to the inverter, and configured to transmit the plurality of motor excitation signals;
a motor (<NUM>) remote from and operably connected to the inverter via the interface cable, the motor responsive to the plurality of motor excitation signals (<NUM>);
a transmission line (<NUM>) having a first end and a second end, wherein the transmission line is radiation shielded, characterized by
a plurality of snubber circuits (<NUM>), each of the snubber circuits of the plurality of snubber circuits having a first terminal (<NUM>) operably connected to a winding of the motor, and a second terminal operably connected to the first end of a transmission line, wherein the plurality of snubber circuit are connected in a Y configuration at the motor; each of the snubber circuits further comprising a series connection of a capacitor and a resistor, and wherein the common point of the Y-connected snubber circuits is connected to the motor end of a conductor (<NUM>) of the transmission line (<NUM>); and
wherein the second end of the transmission line (<NUM>) is operably connected to the positive terminal (<NUM>) of the DC link