Electric motors having EMI reducing circuits and methods therefor

An electric motor having a circuit for reducing electromagnetic interference (EMI) includes a field stack such as a stator, an armature including a shaft, a commutator mounted on the shaft, and an armature core electrically connected to the commutator, whereby the armature core is rotatably mounted within the field stack. The electric motor includes a brush assembly adapted to deliver electrical power to the commutator. The circuit includes a delta capacitor network and a conductive lead electrically interconnecting the delta capacitor network and the brush assembly. In one embodiment, the circuit includes motor field windings or coils, and a power source electrically interconnectable with the delta capacitor network and the motor field windings. In one embodiment, the motor is a universal motor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is generally related to electronic devices, and is more specifically related to circuits for reducing electromagnetic interference (EMI) generated by electric motors.

2. Description of the Related Art

Electromagnetic interference (EMI) is a disturbance that may interrupt, obstruct, degrade or limit the effective performance of an electronic circuit or electronic device. An EMI disturbance may be due to either electromagnetic conduction or electromagnetic radiation emitted from a source, such as an electric motor. Electromagnetic conduction and electromagnetic radiation are differentiated by the way the electromagnetic field propagates. Conducted EMI is caused by physical contact between conductors, and radiated EMI is caused by induction, without physical contact between conductors.

An EMI disturbance may result in adverse consequences including the uneven distribution of an electromagnetic field around a conductor, skin effects, proximity effects, hysteresis losses, transients, voltage drops, electromagnetic disturbances, EMP/HEMP, eddy current losses, harmonic distortion, and reduction in the permeability of a material.

One solution for minimizing a device's susceptibility to EMI is electromagnetic shielding. EMI shielding, however, is expensive and has negative consequences. Another method to reduce EMI involves twisting wires, however, many facilities have tens of thousands of feet of wire, so this solution is not always practical.

Another solution for minimizing EMI emissions includes providing an electronic device with an EMI filter. For example, U.S. Pat. No. 6,400,058 to Liau discloses a universal motor having reduced EMI emission characteristics. The universal motor has a stator, a rotor, and brushes. The EMI filter includes a filter circuit provided on a printed circuit board (PCB). The PCB including the filter circuit is mounted over the brushes on the motor. When electrical arcing occurs between the commutator and the brushes, the PCB filter acts as a shield that absorbs a portion of the radiation emitted by the arcing so as to reduce the EMI characteristics of the universal motor.

Most countries have legal requirements that mandate the electromagnetic compatibility of devices that produce electromagnetic fields. These legal requirements mandate that equipment manufacturers produce electronic devices that work properly when subjected to certain levels of EMI, and that do not emit EMI at levels that will interfere with other equipment.

In 1982, the United States enacted Public Law 97-259, which granted the Federal Communications Commission (FCC) the authority to regulate consumer electronic equipment for EMI. After the law was promulgated, the FCC worked with equipment manufacturers to develop acceptable EMI standards for electronic hardware.

Under the FCC compliance program, electronic devices must be tested to insure that they meet acceptable EMI standards. There are generally three types of EMI compliance tests: emission testing, immunity testing, and safety testing. Emission testing insures that a product will not emit harmful interference by electromagnetic radiation. In one emission test, one or more antennas are used to measure the amplitude of the electromagnetic waves emitted by a device. The amplitude of the emitted waves must be under a set limit, with the limit depending upon the classification of the device. Immunity testing insures that a product is immune to common electrical signals and EMI disturbances that will be found in its operating environment, such as electromagnetic radiation from a local radio station or interference from nearby products. Safety testing insures that a product will not create a safety risk from situations such as a failed or shorted power supply, and power line voltage spikes and dips.

One type of EMI testing device is sold under the trademark R&S® ESU by Rohde & Schwarz GmbH & Co. The R&S® ESU device combines a testing receiver and a spectrum analyzer in one component. The R&S® ESU device makes a wide variety of measurements related to EMI testing including peak, AV, RMS, CISPR-AV and quasi-peak measurements.

In spite of the above advances, there remains a need for improved, reliable, and economical EMI reducing circuits for electronic devices that minimize the likelihood of EMI disturbances and that meet or exceed FCC standards.

SUMMARY OF THE INVENTION

In one embodiment, an electric motor, such as a universal motor, preferably includes an electromagnetic interference (EMI) reducing circuit. The motor preferably has a field including a field lamination stack and coils, an armature including an armature lamination stack, a shaft, a commutator mounted on the shaft, a winding coil electrically connected to the commutator, whereby the armature is rotatably mounted within the field stack, and a brush assembly adapted to deliver electrical power to the commutator. The brush assembly may include brushes and a brush box.

In one embodiment, the circuit preferably includes a delta capacitor network and a conductive lead electrically interconnecting the delta capacitor network and the brush assembly, either directly to the brush or indirectly to brush through the brush box if the brush box is made of electrically conductive material (e.g. brass or steel). The delta capacitor network preferably includes three capacitors. In one embodiment, the circuit may include field windings or field coils, and a power source electrically interconnectable with the delta capacitor network and the field coils.

In one embodiment, the conductive lead electrically interconnecting the delta capacitor network and the brush assembly desirably includes an electrical component adapted to reduce EMI emissions. The electrical component may be a resistor, an LC combination or a choke.

In one embodiment, a circuit for reducing electromagnetic interference (EMI) emitted by an electric motor preferably includes a field, an armature, a brush assembly adapted to deliver electrical power between the field and the armature, a delta capacitor network, and a conductive lead electrically interconnecting the delta capacitor network and the brush assembly. In one embodiment, the circuit may also include a power source electrically interconnectable with the delta capacitor network and the field coils. The delta capacitor network desirably includes three capacitors, and the brush assembly may have brushes and a brush box.

In one embodiment, a method for reducing the electromagnetic interference (EMI) emissions of an electric motor desirably includes providing a motor having a field including a field lamination stack and coils, an armature including an armature lamination stack, a shaft, a commutator mounted on the shaft, and a winding coil electrically connected to the commutator, whereby the armature is rotatably mounted within the field stack, and a brush assembly adapted to deliver electrical power to the commutator. The method preferably includes forming a circuit having a field stack, a brush assembly adapted to deliver electrical power to the field stack, motor field windings, and a delta capacitor network. The method desirably includes electrically interconnecting the delta capacitor network and the brush assembly.

The electrical interconnection may be formed using a conductive lead. In one embodiment, the electrically interconnecting step preferably includes using an electrical component adapted to reduce EMI emissions for forming at least a portion of the electrical interconnection between the delta capacitor network and the brush assembly. The electrical component for reducing EMI may be a resistor, an LC combination or a choke.

These and other preferred embodiments of the present invention will be described in more detail below.

DETAILED DESCRIPTION

FIG. 1shows a schematic diagram of a prior art circuit2for a universal electric motor. For simplicity, only some of the components of the motor circuit are illustrated. The circuit2includes a power source4connected with a two lead capacitor network6, commonly referred to as an X capacitor network. The circuit includes motor field windings8, brushes and a brush box10, and a field stack12. The power source4may be an alternating current (AC) or a direct current (DC) power source.

FIG. 2shows a schematic diagram of a second prior art circuit22for a universal electric motor. For simplicity, only some of the components of the motor circuit are illustrated. The circuit22includes a power source24, a capacitor network16, motor field windings28, brushes and a brush box30, and a field stack32. The circuit22also includes a lead wire34that interconnects one of the leads of the capacitor network26with the field stack22. In theFIG. 2embodiment, the capacitor network26is a Delta capacitor network.

Referring toFIG. 3, in one embodiment, an EMI reducing circuit100for an electric motor, such as a universal motor, preferably includes a power source104connected with a Delta capacitor network106having three leads107A,107B, and107C. For simplicity, only some of the components of the preferred motor circuit are illustrated. The EMI reducing circuit100preferably includes motor field windings108, brushes and a brush box110, and a field stack112. The EMI reducing circuit100desirably includes a lead wire114that interconnects the third capacitor lead107C of the Delta capacitor network106with the brush and/or the brush box110.

Referring toFIG. 4, in one embodiment, an EMI reducing circuit200for an electric motor preferably includes a power source204coupled with a Delta capacitor network206having a first lead207A, a second lead207B and a third lead207C. For simplicity, only some of the components of the preferred motor circuit are illustrated. The EMI reducing circuit200preferably includes motor field windings208, brushes and a brush box210, and a field stack212. The EMI reducing circuit200desirably includes a lead wire214coupling the third capacitor lead207C of the Delta capacitor network206with the brush and/or the brush box210. The lead wire214preferably includes an additional EMI reducing component216such as a resistor, an LC, or a choke. Thus, in one embodiment, the additional EMI reducing component216is desirably disposed between and coupled with one of the leads of the Delta capacitor network206and the brush and/or the brush box210.

Referring toFIGS. 5A-5E, in one embodiment, an electric motor preferably includes a field stack212having field coils or field windings208. The motor also desirably includes a brush assembly210including a brush box and a brush. The motor desirably includes an armature240having a commutator242. In one embodiment, the electric motor desirably includes a field case244having a central opening246adapted to receive the field stack212and the armature240. The electric motor desirably includes an end plate248securable to an end of the field case244. The end plate248preferably includes a central opening250adapted to receive a shaft252on the armature240.FIG. 5Bshows the field stack212including a field lamination254and the field coils208.FIG. 5Cshows the armature rotatably mounted within the field stack212. The brush assembly210is accessible outside the field stack212.FIG. 5Dshows a Delta capacitor network206as described above in the circuit shown inFIG. 4. The Delta capacitor network206desirably includes a first lead207A, a second lead207B, and a third lead207C.FIG. 5Eshows the electric motor assembled inside the field case244with the end plate248covering an end of the field case244. The shaft252of the armature240extends through the central opening250in the end plate248.

As noted above, pursuant to Public Law 97-259, the FCC has enabled equipment manufacturers to implement a voluntary compliance program for minimizing EMI disturbances. The circuits disclosed inFIGS. 3 and 4are preferred embodiments of the present invention that insure that authorized EMI levels are not exceeded. More particularly, the preferred circuits ofFIGS. 3 and 4desirably minimize emitted noise levels in electronic components so as to maintain a buffer or safety zone of at least 3-5 dB from the acceptable standard without requiring additional expensive EMI reducing components found in prior art systems. Although the present invention is not limited by any particular theory of operation, it is believed that providing a circuit with an electrical connection between one of the capacitor leads of a Delta capacitor network and the brush or brush box of an electric motor reduce the EMI noise without requiring additional expensive components. Set forth below are graphs and tables that show the EMI test results for two different electrical devices when using the prior art circuits ofFIGS. 1 and 2and the preferred circuit ofFIG. 4. As will be discussed in more detail below, the circuit ofFIG. 4exceeds the EMI standards by more than 5 dB, however, the circuits ofFIGS. 1 and 2fail to meet EMI standards at all of the measured frequencies.

In order to confirm the EMI reducing benefits provided by the preferred circuits disclosed in the present invention, the circuits were placed in electronic devices and EMI testing was conducted. The first electronic device tested for EMI compliance was a Black & Decker power tool having model number LAG D28492. The specifications of the electric motor used with the LAG D28492 power tool are well-known to those of ordinary skill in the art.FIGS. 6A-6Band7A-7B show the respective conducted band and radiated band test results for the prior art circuit ofFIG. 1;FIGS. 8A-8Band9A-9B show the respective conducted band and radiated band test results for the prior art circuit ofFIG. 2; andFIGS. 10A-10Band11A-11B show the respective conducted band and radiated band test results for the preferred circuit ofFIG. 4.

More specifically,FIG. 6Ais a graph illustrating EMI test results for the prior art circuit ofFIG. 1incorporated into the Black and Decker power tool identified by model number LAG D28492. The desired noise level safety zone is preferably 3-5 dB. The graph inFIG. 6Aillustrates the conducted band test results achieved using the prior art circuit ofFIG. 1. The table shown inFIG. 6Billustrates the conducted band test results plotted in the graph ofFIG. 6A. As shown in the table ofFIG. 6B, the first prior art circuit shown inFIG. 1does not satisfy the quasi-peak (QP) Delta values for the frequencies at 0.54 MHz and 0.6 MHz. At 0.54 MHz, the QP Delta is 0.8 dB and at 0.6 dB the QP Delta is 0.36 dB. At a frequency of 0.525 MHz, the average value (AV) Delta reading is −1.47 dB, and at 0.645 MHz, the AV Delta is −1.81 dB. These conducted band test results indicate that the prior art circuit ofFIG. 1does not meet the desired EMI “safety” zone of at least 3-5 dB.

FIGS. 7A and 7Bshow the radiated band test results for the LAG D28492 power tool when using the prior art circuit ofFIG. 1. As shown in the table ofFIG. 7B, the circuit ofFIG. 1satisfactorily passes the EMI test because the QP Delta and AV Delta values for all of the measured frequencies are above the 3-5 dB safety zone. Although the circuit ofFIG. 1satisfactorily passed the radiated band test, the circuit ofFIG. 1is not acceptable because it did not meet the standards for the conducted band test described above inFIGS. 6A and 6B.

FIGS. 8A and 8Bshow a graph and a table illustrating the EMI test results for the prior art circuit ofFIG. 2when the circuit is incorporated into the Black & Decker power tool LAG D28492. As shown in the conducted band test results table ofFIG. 8B, the prior art circuit ofFIG. 2does not satisfy the 3-5 dB buffer standard, having a QP Delta of 4.85 dB at 0.39 MHZ, an AV Delta of 1.56 dB at 0.435 MHZ, and an AV Delta of 4.88 dB at 0.57 MHz.

FIGS. 9A and 9Bshow the radiated band results for the circuit ofFIG. 2when incorporated into the Black & Decker power tool model number LAG D28492. The circuit ofFIG. 2meets the EMI standards at all of the tested frequencies, however, the circuit ofFIG. 2remains unacceptable because it did not satisfy the EMI standards for the conducted band test described above inFIGS. 8A and 8B.

FIGS. 10A and 10Bshow a graph and a table, respectively, illustrating the conducted band test results when incorporating the preferred circuit ofFIG. 4into the Black & Decker power tool model number LAG D28492. As shown therein, the circuit ofFIG. 4exceeds the 3-5 dB safety zone at all of the measured frequencies.

FIGS. 11A and 11Bshow a graph and a table, respectively, illustrating the radiated band test results when incorporating the preferred circuit ofFIG. 4into the Black & Decker power tool model number LAG D28492. As shown inFIGS. 11A and 11B, the circuit ofFIG. 4exceeds the 3-5 dB buffer zone at all of the tested frequencies. Thus, the circuit ofFIG. 4exceeds the 3-5 dBb safety zone specification so that it may be reliably incorporated into the power tool LAG D28492 for minimizing EMI.

FIGS. 12A-12Band13A-13B show the EMI test results for the prior art circuit ofFIG. 1incorporated into the Black & Decker polisher model number DW849, having electric motor specifications well-known to those of ordinary skill in the art.FIG. 12Ashows a plotting of the conducted band test results. As shown in the table ofFIG. 12B, the prior art circuit ofFIG. 1falls below the 3-5 dB safety zone standard, having a QP Delta of 2.85 dB at 0.285 MHz and 1.11 dB at 0.375 MHz.

FIGS. 13A-13Bshow radiated band test results for the prior art circuit ofFIG. 1incorporated into the Black & Decker polisher model number DW849. As shown in the table ofFIG. 13B, the prior art circuit ofFIG. 1exceeds the 3-5 dB standard at all frequencies, however, the circuit does not satisfy EMI standards because it did not pass the conducted band test as described above forFIGS. 12A and 12B.

FIGS. 14A and 14Bshow a graph and a table, respectively, illustrating the EMI test results for the prior art circuit ofFIG. 2incorporated into the Black & Decker polisher DW849. The graph and the table illustrate the conducted band results. The circuit ofFIG. 2is not acceptable for meeting the 3-5 dB safety standard because the circuit has a QP Delta of 0.96 dB at a frequency of 0.285 MHz, and 0.81 dB at a frequency of 0.3 MHz.

FIGS. 15A and 15Bshow the radiated band test results for the prior art circuit ofFIG. 2when incorporated into the power tool DW849. As shown in the table ofFIG. 15B, the circuit exceeds 3-5 dB for the QP Delta and AV Delta results at all frequencies, however, the circuit is not acceptable because it did not pass the conducted band test results shown inFIGS. 14A and 14B.

FIGS. 16A and 16Bshow a graph and a table, respectively, illustrating EMI test results for the preferred circuit ofFIG. 4incorporated into the Black & Decker polisher DW849.FIGS. 16A and 16Billustrate the conducted band results. As shown in the table ofFIG. 16B, the circuit ofFIG. 4exceeds the 3-5 dB specification standard at all measured frequencies.

FIGS. 17A and 17Bshow a graph and a table, respectively, illustrating EMI test results for the preferred circuit ofFIG. 4incorporated into the Black & Decker polisher DW849.FIGS. 17A and 17Billustrate the radiated band results. As shown in the table ofFIG. 17B, the circuit ofFIG. 4exceeds the 3-5 dB specification standard at all frequencies.

Although the present invention is not limited by any particular theory of operation, it is believed that the EMI reducing circuits shown inFIGS. 3 and 4provide efficient, cost-effective structures for minimizing EMI disturbances without requiring additional, expensive EMI reducing components. As shown in the test results herein, the circuits ofFIGS. 3 and 4provide distinct advantages over the prior art circuits shown inFIGS. 1 and 2by exceeding the 3-5 dB safety zone standard at all measured frequencies for both conducted band and radiated band tests.

While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, which is only limited by the scope of the claims that follow. For example, the present invention contemplates that any of the features shown in any of the embodiments described herein, or incorporated by reference herein, may be incorporated with any of the features shown in any of the other embodiments described herein, or incorporated by reference herein, and still fall within the scope of the present invention.