Motor drive with damper

A motor controller having a damper that dampens resonance in a motor drive of a common mode filter to prevent it from becoming over excited by operation of the motor drive at a resonant frequency of the common mode filter. The damper is connected to an output feeder line of a motor drive in the motor controller and provides a common mode path to ground for the common mode filter. Thus, even if the motor drive is operating at a resonant frequency of the common mode filter, the resulting resonance in the common mode filter is attenuated by rather than amplified by the common mode filter, thereby preventing the common mode filter from excessive excitation.

TECHNICAL FIELD

The present invention relates to motor controllers, and more particularly to a motor control circuit for filtering a motor drive output.

BACKGROUND OF THE INVENTION

Motor controllers are used with motors to provide variable and controllable speed for various applications. The same motor controller may be used for different applications in the same system; for example, a motor controller in an aircraft may be used to start a main engine and to drive an environmental control system.

To minimize the size and weight of the motor used in a given application, it is common to drive the motor at high rotational speeds. This in turn requires the motor controller to operate at relatively high frequencies. To do this, the motor controller may include a motor drive having switches, such as switching transistors, with switching frequencies that can handle the rotational speed of the motor. However, it is also necessary to prevent excessive radiated electromagnetic interference (EMI) emissions due to the high frequency operation of the motor drive. Although shielding of input and/or output lines of the motor drive is often used to control EMI emissions, there are weight-sensitive applications, such as aircraft applications, the extra weight and heat retention caused by the shields would be unacceptable.

Common mode filtering is therefore considered a more desirable way to control radiation emissions from the motor drive. Common mode filtering controls the common mode voltage at the input and/or output feeders of the motor drive. Common mode voltages normally occur at the output lines of the motor drive. As is known in the art, inverter switches, such as insulated gate bipolar transistors (IGBTs), operate at a chop frequency typically ranging in the thousands of kHz and at amplitudes of hundreds of volts. The common mode elements of the chop frequency and its harmonics must be attenuated to a tiny fraction of the original chop voltage (e.g., on the order of millivolts) to reduce EMI emissions to required levels.

Typically, the input feeders of the motor drive are grounded to a system ground (e.g., an airframe) and the output feeders (i.e., the lines going to the motor) float and stay electrically isolated from the system ground. Common mode filtering is typically realized in such a system by adding a common mode filter, such as a low pass filter, to the output feeders of the motor drive.

The load for the motor drive is typically an ungrounded motor, making it easy to design the common mode filter to attenuate the common mode voltages so that they meet emission requirements. That is, none of the motor windings in the motor are connected to ground. However, for motor drive operating frequencies below the emission requirement limits, it may be difficult to keep the common mode voltage under control because the common mode filter itself has a resonant frequency. If the common mode filter is excited at its resonant frequency, it could generate extremely high common mode voltages on the motor and its associated feeder lines because there is no common mode path to ground. Because the motor drive operates over a wide frequency range, there are opportunities for the motor drive frequency to pass through the common mode filter resonant frequency and excite the common mode filter, thereby creating a risk of motor damage due to the resulting high voltage response.

Harmonics of the voltage output of the motor drive may also create undesirable levels of common mode voltage. Although a harmonic (e.g., a third harmonic) of the fundamental motor drive output voltage may be added to the fundamental motor drive output voltage itself to deliver more of the fundamental voltage to the motor (and therefore make more power available to the motor), the common mode filter will react to the harmonic. This reaction must therefore be addressed in the motor controller design to prevent the common mode filter from generating the high voltage response to the harmonic.

There is a desire for a system that provides tighter control over the common mode voltage in a motor controller.

SUMMARY OF THE INVENTION

The present invention is directed to a motor controller having a damper that dampens resonance in a motor drive and prevents a common mode filter from generating a high common mode voltage in response to the motor drive operating at the resonant frequency of the common mode filter. The damper is connected to an output feeder line of a motor drive in the motor controller and provides a common mode path to ground for the common mode filter. Thus, even if the motor drive is operating at a resonant frequency of the common mode filter, the resulting resonance in the common mode filter is attenuated by rather than amplified by the common mode filter.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1is a representative block diagram of an output portion of a motor controller100according to one embodiment of the invention. The motor controller100includes a motor drive102having one or more switches104and a plurality of output feeder lines106that connect to a motor108. In the illustrated example, the motor108is a three-phase motor and each output feeder line106corresponds to one of the phases of the motor108.

An optional differential mode filter110may be connected to each output feeder line106to protect the motor108from voltage and current spikes. In the example shown inFIG. 2, the differential mode filters110comprise inductors and capacitors, but any other components may be used as the differential mode filters110. Note that the differential mode filters110shown in this example are merely for illustrative purposes only and may be omitted from the motor controller100without departing from the scope of the invention.

A common mode filter112is connected to the output feeder lines106to attenuate the common mode elements of the chop frequency and its associated harmonics, as described above, to levels low enough to meet typical EMI emission limit requirements. The common mode filter112itself may be any low pass filter. In the example shown inFIG. 2, each output feeder line106has its own associated common mode filter112comprising a common mode inductor150and a common mode capacitor152coupled together as a low-pass filter. The common mode capacitors152all are connected to the system ground. Of course, other filter configurations may be used as the common mode filters112without departing from the scope of the invention.

To prevent the common mode filter112from being excited when the common mode voltage crosses the resonant frequency of the common mode filter112, a damper160is also coupled to the output feeder lines106. The example shown inFIG. 2includes a damper160connected to all of the output feeder lines106. The damper160is designed to damp a given single line LC low pass filter (i.e., the common mode filter112) by including a damping resistor162in parallel or series with the common mode inductor150or in parallel or series with the common mode capacitor152. The connection of the components in the damper160with respect to the components in the common mode filter112will influence the amount of attenuation in the common mode filter112; thus, it is desirable to connect the damper160in a manner that maintains or even improves the attenuation performance of the common mode filter112.

The embodiment shown inFIG. 2has the damping resistor162connected in parallel with the common mode capacitors152. This enhances the attenuation performance of the common mode filter112, but may cause power dissipation as well, particularly when the damping resistor162is selected to be small enough to provide good damping characteristics. To prevent this power dissipation while still maintaining acceptable damping, the damper160may connect the damping resistor162in parallel with the common mode capacitors152through an electromagnetic device, such as a neutral forming transformer164, as shown inFIG. 2. The neutral forming transformer164is a three-phase inductor connected to all three output feeder lines106and having a high differential mode (magnetizing) inductance and a low common mode (leakage) inductance.

The neutral forming transformer164blocks all the differential mode voltage from the damping resistor162and allows only common mode voltage to reach the damping resistor162. As a result, the damper160is able to provide the correct amount of common mode damping while minimizing any possible power dissipation in the damping resistor162.

By incorporating a damper at the output feeder lines of a motor drive, the invention prevents the common mode filter from being excited when the motor drive operates at the resonant frequency of the common mode filter, thereby keeping the common mode voltage under tight control. This ensures that the common mode voltage is kept low enough to prevent any damage to a motor and its associated feeder lines.

It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. It is intended that the following claims define the scope of the invention and that the method and apparatus within the scope of these claims and their equivalents be covered thereby.