Physical system overview. Insulation separates the cable conductors from the metal shield and the motor windings from the frame. Those separated conductors form stray capacitances. During switching, changing voltage can drive current through them. Shield bonds and other connections help complete the return path. This is a conceptual illustration, use Figures 1 and 2 for the circuit relationships and the equipment manual for installation details.

When an inverter switches, it can create an unwanted voltage disturbance shared by the motor’s phase conductors relative to the chassis. This shared disturbance is common-mode noise. These voltage changes can drive current through stray capacitance between the phase conductors and nearby metal, such as the cable shield, motor frame, heatsink, or chassis. To reduce the noise, first locate the switching voltage that drives the current. Then trace how that current returns to the inverter. Next, identify where the common-mode current flows after coupling into the motor frame or cable shield. Follow the shield, protective-earth conductor, and chassis connections toward the inverter enclosure. Then identify the capacitances or other electrical connections that complete the path to the inverter circuitry. Parts of this path can cross insulation through capacitive coupling.

Key Takeaways

●    The faster voltage changes across a stray capacitance, more displacement current flows through that capacitance during the change.

●     At high frequencies, current can return through the cable shield and metal enclosure, so Include those parts when tracing the noise-current loop.

●     After changing how fast the inverter transistors switch, measure emissions again. Also check the transistor temperature and confirm that the motor controls still work correctly.

A motor drive may run correctly on the test bench, but produce excessive emissions when its production cable is attached. Switching noise may also disturb the sensor readings used to control the motor. This article will trace the unwanted current, calculate a brief current pulse, and choose checks that show whether a design change helps. The goal is to improve electromagnetic compatibility (EMC) while keeping the power transistors within their temperature limits.

What Drives Common-Mode Current in a Motor Drive Inverter

Begin at the inverter’s three motor terminals, labeled A, B, and C in Figure 1. Each terminal connects to a motor phase winding. During switching, the inverter’s power transistors connect these terminals to the positive or negative DC supply rail. Those connections change the terminal voltages relative to the rails. The goal is to find where these voltage changes drive unwanted current outside the motor’s intended phase-current circuit.

Figure 1. The upper diagram identifies the inverter terminals. The separate common-mode equivalent circuit groups the physical capacitances into a simpler circuit for analysis. It does not show a wire joining A, B, and C. Resistance and inductance are omitted from this illustration.

Measure the voltage at A, B, and C relative to the same reference point. Here, that point is O, the negative DC supply rail. The average of those three voltages is the inverter’s common-mode voltage:

This equation uses the negative DC rail as its reference. To find common-mode voltage relative to the chassis, also include the voltage of that rail relative to the chassis. For any particular stray capacitance, calculate the voltage difference between the two conductors that form it.

Inside a shielded motor cable, insulation separates the phase conductors from the metal shield. In the motor, insulation separates the windings from the metal frame. In each case, the two conductors separated by insulation form a capacitance. When the voltage across that insulation changes, displacement current flows through the capacitance. The insulation still separates the conductors; it has not become a direct wire connection. A basic circuit schematic often leaves out these physical capacitances.

Follow the current from a cable conductor through its capacitance to the shield. Then follow the shield connection to the chassis and the connections or capacitances that complete the loop back to the power circuit. Repeat that tracing exercise for the motor windings and frame. A chassis bond is the electrical connection joining a shield or metal part to the chassis. ABB describes related current paths through motor bearings in Technical Guide No. 5, Bearing Currents in Modern AC Drive Systems. [1]

Estimate Displacement Current During a Voltage Transition

Once you have identified a capacitance, estimate the current that a switching voltage change could drive through it. This calculation estimates the brief current pulse during that voltage change. To predict an emissions test result, you would also need the rest of the circuit and the measurement setup.

In this equation, C is the capacitance in farads. The term dvC/dt tells you how quickly the voltage across that capacitance changes, in volts per second. Assume an illustrative capacitance of 1 nF and the voltage across it rises by 100 V in 10 ns:

With the assumed straight-line voltage rise, the current through that 1 nF capacitance is 10 A during the 10 ns transition. If the capacitance represents a cable conductor and its shield, 10 A is the estimated current between that conductor and shield during the rise. The current must then complete a loop back to the power circuit. During a falling voltage transition, its direction reverses.

Use the voltage difference across the capacitance in the calculation. For a cable conductor and shield, subtract the shield voltage from the conductor voltage. Both voltages may move during switching.

To calculate the shape of the actual current pulse, you must include the other parts of the loop. Cable and motor capacitances provide paths between insulated conductors and metal structures. Copper traces, wires, and chassis bonds add resistance and inductance. These parts change how the current rises, rings, and decays. The 10 A calculation applies to the single capacitance we assumed. Calculating motor-bearing current would require the path through the bearings as well. [1]

 Differential-Mode Current vs. Common-Mode Current

There are two noise-current patterns to look for at a two-wire DC input. First, consider unwanted current traveling toward the drive on DC+ and returning on DC−. This opposite-direction pattern is differential-mode noise. The normal current delivering power also uses DC+ and DC−. This section examines the unwanted disturbance added to that power current. [3]

Now consider an unwanted current traveling in the same direction on both DC+ and DC−. This same-direction pattern is common-mode noise. Its return path for this current must lie outside those two wires. For example, current can pass through stray capacitance to the chassis and return through the power source’s connection to the chassis. Figure 2 separates the two patterns so you can follow each current loop. [3]

Figure 2. Differential-mode and common-mode noise currents at a two-wire DC input. The arrows show current direction at one instant. The dashed lines represent a return through structures outside the two supply wires; they are not direct wire connections.

Before choosing a filter, trace the current causing the problem. A differential-mode current returns through the other supply wire. A common-mode current returns outside the pair. Because those loops contain different components and connections, they oppose current differently at each frequency. One filter may therefore reduce the two types of noise by different amounts.

Next, check whether the unwanted current actually passes through the filter. Stray capacitance can provide a route around it. If most of the unwanted current takes that route, adding the filter may produce little improvement. [2, 3]

Check the cable shield’s connection to the chassis as well. During a fast voltage transition, the inductance of that connection can oppose the noise current. A connection that works adequately at low frequency may therefore perform poorly during switching.

First determine whether you are investigating differential-mode or common-mode noise. Then trace where that noise current goes and how it returns. This tells you which physical parts to investigate. Possible changes include rerouting a conductor, reducing capacitive coupling between conductors and nearby metal, or changing a shield’s bond to the chassis. Choose a change based on the current path you found, and then measure whether it reduces the noise. [3]

How to Reduce Switching Noise Without Overheating Devices

Suppose emissions increase after you make the power transistors switch faster. During each switching transition, a transistor briefly carries current while also supporting voltage. Shortening this overlap of voltage and current often reduces the energy converted into heat. But faster switching also changes the unwanted currents and voltages around the transistor. A faster voltage change drives more current through nearby capacitances. A faster current change produces a larger voltage across the loop inductance, which can increase voltage overshoot.

You could slow the switching transition to try to reduce emissions, but the longer overlap of transistor voltage and current may then generate more heat and raise junction temperature. Figure 3 illustrates the comparison during turn-off. Changing the gate resistor is one way to adjust switching speed. The gate driver, transistor capacitances, and PCB connections also influence the result. [4]

Figure 3. Illustrative transistor voltage and current during turn-off. These curves are conceptual, they are not measured or simulated results. The slower transition shows less overshoot and a longer interval with voltage and current present together. Integrating their product over that interval gives the switching energy.

After you slow the switching, you may still see a peak at a particular frequency in the emissions measurement. The cable and PCB both have inductance and capacitance. Those elements can exchange stored energy and produce a resonance, so some frequencies respond more strongly than others. The opposition to current in the return connections also changes with frequency. Compare emissions before and after the adjustment across the measured frequency range. Check which peaks fell, which stayed, and whether any increased. [1, 3]

Change one variable to identify what improved the design

To find a useful switching speed, you need to know which adjustment reduced noise and how it affected transistor temperature. Start with one change, such as the gate resistor value. Keep the motor load, DC bus voltage, cable arrangement, and measurement setup unchanged. If you also reroute the cable and change its shield connection, it will be hard to tell which change caused the measured improvement.

Record the transistor voltage and current during switching before making the change. Also record the emissions spectrum and transistor temperature. Repeat those measurements after the change. You can then check whether the new gate resistor slowed the transition, reduced an emissions peak, or increased heating.

Once you understand the effect of each change, test them together. Repeat the tests over the drive’s intended operating range. A combination of cable routing and switching speed may behave differently from either change alone.

Pre-Compliance Testing with the Production Setup

To catch problems before final EMC testing, use the parts and connections intended for the finished product. Connect the intended motor and cable length. Fit the enclosure and heatsink, and use the intended cable shield terminations and external interfaces. These physical parts change the capacitances and connections that carry unwanted current. Testing them together can reveal a problem that an open enclosure or short bench cable would miss. [1, 3]

During those tests, check the signals used to control the motor. Switching noise may enter the current-sensing circuit or position-feedback wiring. If the controller receives an incorrect measurement, it may respond with an unwanted motor command. Compare sensor disturbances with the timing of inverter switching. [5]

Choose the EMC tests that apply to the product. IEC 61800-3:2022 covers adjustable-speed power drive systems and machine tools. Its scope excludes traction applications and electric vehicles. IEC also lists a 2025 corrigendum, meaning a published correction to the standard. Ask the EMC team to confirm the applicable standard and its correction. Agree on the operating environment, test limits, and physical test setup before testing. [6, 7]

Common EMC failures and What to Check

When a test fails, start with the physical change that accompanied the failure. For example, an emissions increase after fitting a longer motor cable gives you a reason to investigate that cable and its shield connections. The table below links several observations to possible causes. Use the next check to investigate each cause before concluding that it explains the failure.

Motor-drive EMC Evaluation Checklist

A lower emissions reading is only part of the result. The transistors may now run hotter, or the sensor signals may still contain switching disturbances. A different cable arrangement may also bring the noise back. Use the checklist below to record the test setup and check each of those outcomes.

●        Locate the inverter voltage transition that drives the unwanted current. Trace the capacitance and physical connections that let the current return to the power circuit.

●        Determine whether the unwanted current is differential-mode, common-mode, or a mixture. Trace the relevant current loop before selecting or moving a filter.

●        Record the motor, cable length and route, enclosure, and shield-to-chassis connections. Another engineer should be able to rebuild the same test setup from that record.

●        After changing switching speed, repeat the emissions and temperature measurements. Compare switching energy and sensor behavior at the same motor load and supply voltage.

●        Test the drive’s immunity to the specified electromagnetic disturbances. Check whether its sensing, motor control, and protection circuits continue to operate correctly during those tests.

●        Confirm the emissions and immunity tests with the EMC team. Perform the required checks with the hardware and connections intended for production.

Modeling Motor-Drive EMC from PCB to System

Use Allegro X to route the PCB and apply the physical design constraints. For interconnect analysis, Cadence documents a quasi-static extraction workflow in Clarity 3D Layout and Clarity 3D Workbench. That workflow produces resistance, inductance, capacitance, and conductance values, abbreviated RLCG, for circuit models. [8]

A PCB model covers only the structures included in that model. Add the relevant motor cable, motor, enclosure, and return connections when analyzing current outside the board. Use the resulting simulation to investigate current paths and compare design changes. Establishing EMC compliance still requires assessment and testing of the specified complete system.

Frequently Asked Questions About Motor-Drive EMC

Should the motor-cable shield also be terminated at the motor?

For a shielded cable between an industrial variable-frequency drive (VFD) and motor, follow the drive manufacturer’s installation instructions. ABB’s guidance calls for a continuous shield with 360° termination at both ends. That means making electrical contact around the shield’s circumference, using the specified clamp or gland. Eaton likewise describes connecting the motor-cable shield at the drive and the grounded motor. These connections provide a controlled path for high-frequency noise current.

Keep the required protective-earth connections. A shield connection selected for EMC does not by itself establish that the protective-earth requirements are satisfied. Check the specified cable, shield termination, grounding conductors, and bonding arrangement together. Instructions for a low-level sensor cable may differ from those for the motor power cable.

Can a common-mode choke smooth an inverter’s PWM output into a sine wave?

A common-mode choke is designed primarily to oppose current that travels in the same direction through its windings. In a correctly connected two-winding choke, the magnetic flux from equal, opposite-direction load currents largely cancels. That load current therefore sees much less inductance than common-mode current sees.

A real choke has leakage inductance, so it can also affect differential-mode noise. But its common-mode inductance rating does not establish that it will smooth the voltage between the inverter’s output conductors. If an output filter is needed, design it for the intended differential voltage and current, load, switching frequency, and acceptable resonance. Check the choke’s actual frequency response and ratings separately.

How should I design around the resonance in a ferrite bead/LC low-pass filter?

Start by checking the filter’s response over the frequencies where noise is a problem. Include the source, load, capacitors, and the bead’s or inductor’s actual impedance. A bead can behave mainly like an inductor over part of its frequency range. Combined with a low-resistance capacitor, it can produce a resonant peak that increases noise at some frequencies.

Check the response under the intended current and load conditions. DC bias can change a ferrite bead’s impedance. If the filter has excessive peaking, evaluate a suitable damping network, such as a resistor and capacitor connected in series across the filtered supply. Choose its values for the actual circuit, then check attenuation, component heating, and the complete drive’s behavior. Analog Devices AN-1368 shows several damping approaches and their tradeoffs.

Technical references

[1] ABB, Technical Guide No. 5: Bearing Currents in Modern AC Drive Systems

[2] Texas Instruments, TI Precision Labs: Motor Driver EMI Design Tips

[3] Texas Instruments, Best Practices for Board Layout of Motor Drivers, SLVA959B, Sections 1.3.2 and 1.4

[4] Texas Instruments, Fundamentals of MOSFET and IGBT Gate Driver Circuits, SLUA618A

[5] Texas Instruments, Low-Drift, Precision, In-Line Motor Current Measurements With PWM Rejection, SBOA160D

[6] IEC 61800-3:2022: official scope

[7] IEC 61800-3:2022/COR1:2025: published correction

[8] Cadence, Sigrity and Systems Analysis 2023.1 HF2: quasi-static RLCG extraction

[9] Coilcraft, Common Mode Filter Inductor Analysis. Source

[10] Analog Devices, AN-1368: Ferrite Bead Demystified. Source

[11] Eaton, VFD Wiring Best Practices, Application Note AP040076EN, July 2014. Source

[12] Motor Controller Design: A System Level Guide From Requirements to Hardware Verification, v13. Project source 06. Sections 9, 11 and 12 cover switching, PCB constraints and EMC.