INTRODUCTION:
Electric motors are a tremendous source of electromagnetic emissions, noise, or interference (EMI). It is to be appreciated that EMI may refer to either radiated or conducted emissions, noise, or interference, as understood by those skilled in the art. Most automobiles contain a number of electric motors to control the various engine functions, such as pumping hydraulic fluid for braking and steering systems, and for performing other vehicle functions such as operating windshield wipers, electric windows, electric adjustable mirrors, retractable antennas and a whole host of other functions. In addition, electric motors are used extensively in a number of home appliances such as washing machines, dryers, dishwashers, blenders, hair dryers. Due to the prevalence of electric motors and increased electromagnetic emissions standards there is a need for an assembly having differential and common mode filtering.
EMI can be radiated and conducted through electrical lines from such sources as the motor, the control units, and the like. Other sources of interference are generated from equipment coupled to the electrical lines, such as control circuits, computers, switching power supplies, and a variety of other systems, which may generate significant interference which is desired to be eliminated to meet international emission and/or susceptibility standard requirements. Examples of such standards include automotive component EMC specification GMW - 3097/3100.
Several other sources of EMI include cross talk and ground bounce. Cross talk in most connectors or carriers is usually the result of mutual inductance between two adjacent lines rather than from parasitic capacitance and occurs when signal currents follow the path of least inductance, especially at high frequencies, and return or couple onto nearby conductors such as conductive tracks positioned parallel with or underneath the signal current track. Ground bounce is caused by shifts in the internal ground reference voltage due to output switching of a component. Ground bounce causes false signals in logic inputs when a device output switches from one state to another.
Differential and common mode currents are typically generated in cables and on circuit board tracks. Controlling these conducted/radiated emissions is necessary to prevent interference with other circuitry or other parts of the circuit generating or sensitive to the unwanted noise. Based upon the known phenomenon of EMI, a variety of filter and surge suppression circuit configurations have been designed as is evident from the prior art. Most electric motor assemblies include multiple inductors and multiple capacitors within a housing of the motor, as well as exterior to the motor, to suppress EMI. However, as packaging space becomes limited, it is more difficult to package these components within the motor housing.
SUPPRESSING EMI/RFI WITH FERRITES
Ferrites of their unique magnetic characteristics have been extensively used to suppress unwanted signals in electronic circuits. The ferrites used for suppression are Ni-Zn material which are high resistive component available in different sizes and shapes like rod, bead, toroids, balun cores etc.
The most economical and universally applied EMI attenuators are ferrite shield beads. These are usually small ferrite cylinders which are simply slipped over component leads or circuit wiring. The beads introduce a small inductance at low frequency current, but as frequency is increased a sharp reduction in magnetic permeability occurs. Simultaneously, the magnetic losses increase so that the unit now behaves like a resistor in the circuit. Effectively the device is converting the high frequency signal energy to heat through magnetic losses, while at dc and low frequency there is little or no effect.
In case of FERRITE RODS used in open magnetic circuit the ferrite material only occupies part of the path of the magnetic lines generated by the current flowing in the winding. Because of the nature of the magnetic circuit, rod permeability is always much lower than the intrinsic permeability of the material, and the difference between these permeabilities increases as the length to diameter ratio decreases. Rod Inductance parameters will be low compared to bead ferrites due to their nature of open magnetic circuits. On the other hand core saturation will be better for rod ferrite compared to any other core geometry of closed magnetic circuits.
BALUN CORES
Are designed for balun transformers, matching balanced to unbalanced circuits in the frequency spectrum. These multi apertures cores are designed as suppression components which are compact in size and provide high resistive impedance over a wide frequency band. Impedance matching are perfect meaning windings on two aperture will have the same inductance value for a given number of turns.
Disclosed is a balun line filter with coils wound through various winding processes such that an inductance characteristic for a certain frequency can be improved through changes in current flowing direction and/or area, whereby the balun line filter can compensate for a poor EMI characteristic at the certain frequency without adding extra components such as an inductor or a capacitor. The balun line filter comprises a Bi annular core and coils wound around the annular core. The coils are wound in such a way that first winding is wound over an area of the one annular core and second winding around another annular core. GAP is introduced on two opposite side of the two annular region of the Balun core.
Similarly a new concept using COUPLED INDUCTOR having two inductors assembled in single unit, used as suppressor for inward and out ward filter for DC motors in automobile. The footprint and profile of coupled Inductor matches the demand required in automobile industries.
DIMENSION(mm) Inductance@10KHz, 6A DC Bias CISPR 25, CLASS 3
10 X 10 (Coupled Inductor) 4uH±10%(each inductor) PASSED
12 X 12(Coupled Inductor) 5.5uH±10%(each inductor) PASSED
Visit the MagDev Ltd website for more information on EMI/RFI Filters