A typical motor or generator includes multiple design elements which can cause electromagnetic compatibility (EMC) issues. As well as the main inductances of the motor itself, there is also a parasitic capacitance between the coil and the housing, between the coil and the shaft, and between implemented sensors. This parasitic capacitance is responsible for the common mode coupling and capacitive coupling. This whitepaper shows how EMC relevant properties of electrical machines can be extracted by 3D EM simulation.

There exist two coupling paths for common mode noise to the ground potential (chassis).One is between the coil through the housing and another one by the shaft/rotor to attached gear box/chassis. Both cou-pling paths depend on the individual connection to ground potential. The resulting common mode current depends on several parameters:

■ impedance of the housing itself (Z) ■ impedance of the connection to GND potential (Z) ■ geometrical proportions of the E-motor design

(distance, gaps) ■ material properties of conductive and non-conductive

material (εr) ■ manufacturing variations within the e-machine

(tolerances, stray parameter)

The capacitive coupling inside the e-machine causes noise to be distributed to low-voltage (LV) areas, such as sensors and resolver and will lead to a bad high-voltage/low-voltage (HV/LV) attenuation behavior.

To analyze the EMC behavior of an E-machine the p arasitic couplings, as described above, needs to be considered, as well as diff erent scenarios of motor housing designs and ground connections. In addition the coupling path from the coil to the LV area needs to be considered in order to evaluate the HV/LV attenuation.

Electromagnetic simulation using CST STUDIO SUITE® can be used to calculate the EMC characteristics of a motor for multiple confi gurations and investigate the eff ect of diff er-ent design choices on EMC performance.

Figure 1: Full (left) and simplifi ed (right) representations of an electrical machine.

WHITEPAPERELectromAGNetic ComPAtibiLitY ANALYsis oF eLectricAL mAchiNes

CST AG WHITEPAPER EMC ANALYSIS OF ELECTRICAL MACHINES 2

SimuLAtioN set-uPThe fi rst step in analyzing a design is to produce the model. Often, the CAD data from the mechanical department is not ideally suited to EMC analysis, due to fi ne details that are not electrically relevant. CST STUDIO SUITE includes both mod-eling tools and CAD import tools for a wide range of formats, including tools for cleaning and simplifying imported fi les. Imported CAD data can be also modifi ed and parameterized to allow the eff ect of design variations to be found through simulation, and to make the simulation process more effi -cient. Because motors and generators are often symmetrical, simulation performance can be further improved by using these symmetries to simulate just a portion of the model.

The simulation was set up with the frequency domain solv-er using the tetrahedral mesh, which can resolve the very small details of the motor around the air gap effectively (see Figure 2). S-parameter ports were used to calculate the impedance of the motor.

The geometric and material properties of the model can be parameterized within the CST STUDIO SUITE interface, and the built-in parameter sweep tool automatically carries out a series of simulations for diff erent values of these parame-ters. As an example, the permittivity of the isolator material between the coils and the stator was defi ned as variable pa-rameter to show the eff ect of this parameter on the coupling path. The parameter sweep tool then simulates the resulted parasitic capacitance for diff erent material properties across a range defi ned by the user.

ANALYsisOne of the advantages of CST SUITE STUDIO is that the 3D simulation tool can import and modify external CAD data. This makes possible to run analyses which would not be real-izable or measurable using a real motor prototype. For exam-ple, it is not possible to deconstruct a real motor to measure properly all the parasitic capacitances, which are responsible for common mode noise to the ground potential. This can be carried out virtually by defi ning ports or probes across the air gaps, as shown in Figure 2. Typical capacitance and impedance curves are shown in Figure 3. These capacitances have a critical impact on the behavior of the motor. On the one hand, they aff ect the im-pedance between the motor phases and the housing, and on the other hand, the impedance between the motor phases and the rotor.

HV/LV attenuation is another important value for the EMC performance of a motor, and provides a measure of the coupling between the high-voltage and low-voltage sys-tems. OEMs set HV/LV attenuation requirements on all high voltage components – components that do not meet these specifi cations may pose a risk to the integrity of low-voltage syststems. The HV/LV attenuation can be calculated with a post-processing template in CST STUDIO SUITE (Figure 4), using the results from probes or ports in the HV and LV parts of the system as the input and output respectively.

Frequency / MHz

Capacitance between the coils and the housing1000

300

100

30

10

3

1

0.3

0.1

0.03

0.01

0.003

0.0010.15 0.3 0.5 1 2 3 4 5 6 7 8 10 20 30

parasitic_capacitance_coil_stator

1000500

10050

105

10.5

0.10.05

0.010.005

0.0010.0005

0.00010.15 0.3 0.5 1 2 3 4 5 6 7 10 20 30 50 70 120

parasitic_capacitance_stator_rotor

Frequency / MHz

Capacitance between the coils and the rotor

1000

400

200 100

40

2010

42

1

0.40.20.1

0.15 0.3 0.5 1 2 3 4 5 6 7 10 20 30 50 70 120

Z_coil_housing

Frequency / MHz

Impedance between the coils and the housing

Frequency / MHz

1000

400

200 100

40

2010

42

1

0.40.20.1

0.15 0.3 0.5 1 2 3 4 5 6 7 10 20 30 50 70 120

Z_coil_rotor

Impedance between the coils and the rotor

Figure 2: Parasitic capacitances in the electrical

machine. The mesh representation of the motor

is shown. Figure 3: Calculated capacitance and impedance within the electrical machine.

CST AG WHITEPAPER EMC ANALYSIS OF ELECTRICAL MACHINES 3

PArAmeteriZAtioNA parameter sweep is especially useful for fi nding a good compromise value between opposing design requirements, and for calculating the eff ect of material variation and geo-metrical tolerances in the manufacturing process.For example, an isolator with lower permittivity results in smaller parasitic capacitance and reduces the coupling be-tween the coils and the housing, as well as between the coils and the rotor. The infl uence of the permittivity on the im-pedances can be calculated with a parameter sweep across diff erent values of permittivity (Figure 5).

The impedance behavior illustrates the eff ect of the coupling path (Figure 6). These impedances depend on the ground connection of the housing to the chassis. The lower the im-pedance of the ground connection the better – this reduces the EMC risk posed by the coupling paths, by conducting the majority of the common-mode noise to the chassis.

CoNcLusioNThe EMC behavior of an E-machine can be analyzed consid-ering diff erent material and geometric parameters of the isolator. This paper has demonstrated two examples of the analysis of an electrical machine: the analysis of the isolator between the coils and the stator, and the ground connection between the motor and the chassis. The two coupling paths are between the coil through the housing and another one by the shaft/rotor to attached gear box/chassis. They are re-sponsible for common mode noise to the ground potential and depend on the parasitic capacitance. The smaller the capacitance the lower is the eff ect of the coupling path. Also they depend on the ground connection, which must be very good (lower impedance) to reduce the eff ect of the coupling path by conducting the common mode noise to the chassis.

Additional the HV/LV attenuation between the coils and the resolver was analyzed. Depending on the applicable EMC norm several counter measures should be taken in account to fulfill the requirements. These may involve changes to the structure and materials, or the inclusion of additional connections and components such as ferrites. EM simu-lation is a powerful tool for the analysis of potential EMC mitigation strategies.

Figure 4: HV/LV attenuation curve between the coil

(HV) and the resolver (LV).

Figure 5: Results of a parameter sweep showing

how the impedance curve varies with the permit-

tivity of the isolators.

Figure 6: Results of a parameter sweep showing how

the impedance curve varies with the impedance of the

ground connection of the housing to the chassis.

1000

400

200

100

40

2010

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1

0.4

0.20.1

0.15 0.3 0.5 1 2 3 4 5 6 7 10 20 30 50 70 120

eps_r=1.5eps_r=3eps_r=6

Frequency / MHz

Impedance between the coils and the rotor

140

120

100

80

60

40

20

0

-20HV/LV attenuation

0.15 0.3 0.5 1 2 3 4 5 6 7 8 10 20 30Frequency / MHz

HV/LV attenuation

1000

400

200

100

4020

10

4

2

1

0.40.20.1

0.15 0.3 0.5 1 2 3 4 5 6 7 10 20 30 40 60 120

Z_Housing_chassis=150nZ_Housing_chassis=15nZ_Housing_chassis=500n

Frequency / MHz

Impedance between the coils and the rotor

1000

400

200

100

40

2010

42

1

0.4

0.20.1

0.15 0.3 0.5 1 2 3 4 5 6 7 10 20 30 50 70 120

eps_r=1.5eps_r=3eps_r=6

Frequency / MHz

Impedance between the coils and the housing1000

400

200

100

4020

10

4

2

1

0.40.20.1

0.15 0.3 0.5 1 2 3 4 5 6 7 10 20 30 40 60 120

Z_Housing_chassis=150nZ_Housing_chassis=15nZ_Housing_chassis=500n

Frequency / MHz

Impedance between the coils and the housing

CST AG WHITEPAPER EMC ANALYSIS OF ELECTRICAL MACHINES 4

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the European Union, and other countries. Other brands and their products are trademarks or registered trademarks of their respective holders and should be noted as such.

CST – Computer Simulation Technology AG, Bad Nauheimer Str. 19, 64289 Darmstadt, Germany

About CST

Founded in 1992, CST offers the market’s widest range of 3D electromagnetic field simulation tools through a glob-al network of sales and support staff and representatives. CST develops CST STUDIO SUITE, a package of high-per-formance software for the simulation of electromagnet-ic fields in all frequency bands, and also sells and sup-ports complementary third-party products. Its success is based on a combination of leading edge technology, a user-friendly interface and knowledgeable support staff. CST’s customers are market leaders in industries as diverse as telecommunications, defense, automotive, electronics and healthcare. Today, the company enjoys a leading position in the high-frequency 3D EM simulation market and employs 250 sales, development, and sup-port personnel around the world.

CST  STUDIO SUITE is the culmination of many years of research and development into the most accurate and efficient computational solutions for electromagnetic designs. From static to optical, and from the nanoscale to the electrically large, CST STUDIO SUITE includes tools for the design, simulation and optimization of a wide range of devices. Analysis is not limited to pure EM, but can also include thermal and mechanical effects and circuit simu-lation. CST STUDIO SUITE can offer considerable product to market advantages such as shorter development cycles, virtual prototyping before physical trials, and optimiza-tion instead of experimentation.

Further information about CST is available on the web at https://www.cst.com

Authors

Reindl Hartwig, EMC Director, AVL Trimerics

About AVL Trimerics GmbH

The AVL Trimerics GmbH – located at Stuttgart and Regensburg – is part of the world-wide acting AVL Group. As a development partner we offer engineering expertise in the fields of electrical machine (EM) and electromagnetic com-patibility (EMC).

We will support you from concept phase up to series for E-motors, generators & drives; electromagnetic valves; electro-magnetic actuators; electromagnetic sensors; ESD and pulse; signal integrity; virtual measurements and EMC in all systems.

Our company know-how is based on many years of ex-perience of our employees and their profound and broad knowledge in the field of electromagnetism and electro-magnetic compatibility.

For development and optimization of our designs with re-gard to electric, magnetic, thermal, mechanical and EMC aspects we use state of art analytic and numeric simulation methods.

For more information: http://www.avl.com/emc