This invention relates to electromagnetic interference filter designs and, in particular, to an improved method and filter design employed with inverter controlled dynamoelectric machines. While the invention is described with particular emphasis with respect to its use with brushless permanent magnet motors, those skilled in the art will recognize the wider applicability of the inventive principles disclosed hereinafter.
Inverter controlled dynamoelectric machines of various types are well known in the art. Inverter controls can be employed with conventional induction motors, brushless permanent magnet motors, and switch reluctance motors, for example. These motors have in common a stator assembly including a core and stator windings carried by the core. A rotor is mounted for rotation with respect to the stator. Primarily, the motors differ in the design of their rotors. An induction motor, for example, employs a squirrel cage rotor, while the permanent magnet motor has permanent magnets associated with the rotor structure. A switch reluctance motor uses a preformed, solid core rotor design that does not include either the squirrel cage windings of the induction motor or the permanent magnets of the permanent magnet motor. The motor types also differ in that rotor position information is required for controlling operation of the brushless permanent magnet motors and switch reluctance motors. Such information is not required for a controlled induction motor.
It long has been recognized that brushless permanent magnet motors and switch reluctance motors offer higher operating efficiency in most applications as compared to their induction motor counterparts. The increase in operating efficiency, however, is weighed against the cost of the electronic control required to operate the motor properly for all operating conditions. As will be appreciated by those skilled in the art, induction motors are ubiquitous in application. If one is willing to bear the cost differential for motor controls, either the brushless permanent magnet motor or the switch reluctance motor can be substituted for the induction motor in any particular application requirement.
Although both brushless permanent magnet motors and switch reluctance motors have for many years been promoted in high efficiency applications, their widespread substitution for induction motors has lagged because of the cost associated with the motor controls. One problem associated with any inverter operated dynamoelectric machine is the generation of electromagnetic interference (EMI). For example, a domestic appliance, such as a washing machine or a furnace blower motor, can cause considerable interference on the power lines which consumers find objectionable. The United States government, in fact, has regulations controlling the electromagnetic interference which products must meet before they are sold commercially.
The common way to eliminate EMI problems is to utilize some form of filter in conjunction with the motor control.
In the past, filters designed for use in a motor control were and are expensive because they employ high cost precision components.
I am aware of low cost filter designs, for example, as described in the Handbook of Filter Synthesis by Anatoli Zverev, published by John Wylie and Sons, Inc., 1967. These designs, prior to my invention, were unsuitable for application to inverter controlled dynamoelectric machines. I have found that a highly efficient, relatively low cost filter for use in inverter controlled dynamoelectric machines can be designed when a very lossy balun choke is inserted between the power supply link and the inverter control used to control the dynamoelectric machine. The lossy choke provides a real impedance that can be used to determine other reactive components that provide good attenuation while maintaining an almost ideal impulse response. The addition of the lossy balun wound choke allows EMI filter design to be accomplished using standard linear lossless ladder networks. While the filter is highly effective, the components employed with the filter, including the lossy choke, are substantially reduced in cost as compared to previous EMI filter designs employed in inverter controlled dynamoelectric machines with which I am familiar.
One of the objects of this invention is to provide an efficient low cost EMI filter for an inverter controlled dynamoelectric machine.
Another object of this invention is to provide a method of designing an EMI filter for an inverter controlled dynamoelectric machine.
Still another object of this invention is to provide an EMI filter for a dynamoelectric machine which employs conventional components which provide good attenuation for EMI generated by the inverter/motor combination.
Another object of this invention is to provide a filter which maintains an almost ideal impulse response.
Other objects of this invention will be apparent to those skilled in the art in light of the following description and accompanying drawings.