Behavioural models for common mode EMI filters

EMC is defined as the “ability of an equipment or system to function satisfactorily in its electromagnetic environment without producing intolerable electromagnetic disturbances to anything in that environment”. EMC means that equipment shall be designed and manufactured, in such way that:
- The electromagnetic disturbance generated does not exceed the level above which radio and telecommunication equipment or other equipment cannot operate as intended.
- It has a level of immunity to the electromagnetic disturbance to be expected in its environment which allow it to operate without unacceptable degradation of its indented use.
Electric motors convert electrical power into mechanical power within a motor‐driven system. Electric motors and the systems they drive are the single largest electrical energy consumer, more than twice as much as lighting, the next largest energy consumer and represent close to 70% of the electrical energy consumption in the industrial sector worldwide [1]. One of the major problems faced by motor drives is Electromagnetic Interference (EMI) which produces unwanted effects on surrounding electronic devices. A traditional way to control EMI in motor drives is the introduction of EMI filters. However a successful and first-time-right design is given only to a few experienced designers. The filter is though often not optimized in terms of weight, cost and EMI reduction abilities. Enhancing these performance.
criteria and supporting a first-time-design at the earlier stage of development, are the objectives of the work presented in this thesis.
Behavioural models have been chosen as modelling technique because they relates the designable parameters of the EMI filter to its final performance placed in the motor drive and respect the underlying physics involved.

The thesis starts with a description of the generation mechanisms of EMI in motor drives when no precautionary action is taken to reduce them. A focus is made on common mode current as they are the main cause of conducted and radiated electromagnetic emission.
Behavioural models of the common mode EMI filter involve the complete motor drive. It has been modelled step by step, starting from a component point of view towards a global approach. It combines functional parameters together with the EMI ones.
The constitutive components of the filter are the Y-capacitors and the Common Mode Choke (CMC). A new equivalent circuit of this last component has been developed. The choice of the proper material is a trade off between cost, size and performance. Their properties have therefore also been described and compared.
The common mode choke is first placed alone at the output of the motor drive and a first level of an in-situ behavioural model is developed. Initial common mode circuit flowing in the drive are used along with the common mode impedance of the rest of the circuit. Deviation and sensitivities studies are also used to link designable parameters with the final performances of the component placed in the drive also. The overall concept of a behavioural model for a CMC representing its actual performance is new.
All common mode EMI filter topologies have been considered afterwards. Each structure is investigated and a new equivalent circuit is proposed and validated for each of them. Traditional design rules are described and complemented with their corresponding behavioural model. The proper description of the common mode current flowing un the cable towards the motor is essential for the prediction of level of radiated emission around the motor drive system. This current is the main contributor to the level of radiated electromagnetic emission. A quick evaluation method is proposed to evaluate the worst case scenario in term of radiated emission. It has been extracted from existing models of the common mode input impedance of thin linear antennas. The link between the lowest values of the common mode input impedance, highest values of current and the highest levels of electromagnetic field is validated using measurements performed on a complete motor drive.