Siemens 3TF68 Contactor: Removing Surge Protection for VFDs

Siemens 3TF68 and 3TF69 vacuum contactors ship from the factory with an integrated overvoltage damping network wired across the main current paths. The network is sized for clean 50/60 Hz line-side transients. When the same contactor is placed on the load side of a frequency converter, DC chopper, or any speed-variable power-electronic source, the suppressor is exposed to high dV/dt switching edges, reflected wave peaks, and harmonic content it was not designed to absorb. The documented failure mode is a phase-to-phase short circuit inside the contactor housing. Siemens addresses this by offering the same mechanical frame without integrated overvoltage damping, identified by ordering suffix -Z and order code A02, at no additional price.

This technical reference covers the engineering rationale, the specific catalog numbers affected, the correct way to order the suppressorless variant, the failure mechanisms inside the snubber network, the troubleshooting steps for a 3TF6844-0CM7 deployed on a compressor fed by a VFD, and the related coil-burnout question that frequently arises in the same application. The user is operating the 3TF6844-0CM7 (230 V AC 50/60 Hz coil) on a compressor load with the contactor held in except for E-stop. The line supply feeds the VFD input, and the VFD output feeds the motor. The contactor location in this topology determines whether the integrated suppressor must be removed.

Integrated Overvoltage Damping in 3TF68/69 Vacuum Contactors

The 3TF68 and 3TF69 are large-frame vacuum contactors built for AC-3 and AC-4 motor switching at 400 V, 500 V, 690 V, and 1000 V AC. The interrupting medium is a vacuum interrupter with copper-chromium contacts, which delivers a low chopping current (typically 0.5 A to 1.5 A) compared to air-break or molded-case contactors. The low chopping current is what makes vacuum technology attractive for repeated motor-starting duty: arc energy at contact separation is bounded, contact wear is slow, and electrical endurance is high.

Even with a low chopping current, the transient recovery voltage (TRV) appearing across the open contacts at current zero can stress the downstream load and the contact insulation if left undamped. To control the TRV rate of rise, Siemens installs a surge suppression network across each main pole of the standard 3TF68/69 contactor. The network typically combines a resistor-capacitor (RC) snubber and, in higher-voltage variants, a metal-oxide varistor (MOV) stage. For 690 V class, the suppressor is rated for line-side switching transients expected in 50/60 Hz power distribution: lightning-induced surges, capacitor-bank switching, transformer inrush, and similar events with rise times measured in microseconds to milliseconds.

Typical values for the RC stage on a 400 V / 690 V class 3TF68 pole:

The suppressor is wired between the line and load sides of each pole, inside the contactor housing, and is part of the type-tested construction. It is not field-repairable; it is built into the contactor at the factory. Opening the contactor to cut the snubber wires is not authorized by Siemens and voids the type-test certifications (CE, UL, CCC).

Figure 1 — Vacuum Contactor with Integrated Suppressor

VFD Output Waveform: dV/dt, Reflected Wave, and Harmonic Content

When the same 3TF68/69 is fed from the output of a frequency converter, the operating conditions on the contactor's main poles are fundamentally different from utility power. A modern IGBT-based VFD produces a PWM output with the following characteristics relevant to a load-side contactor:

DC bus voltage (VDC). For a 400 V AC three-phase input to the drive's rectifier, the DC bus sits at approximately 1.35 × V_LL = 540 V. For 480 V AC input, VDC reaches 1.35 × 480 = 678 V. For 690 V AC input, VDC reaches 932 V. The peak line-to-line PWM output can briefly equal VDC at the maximum modulation index.

Switching frequency (f_sw). Most IGBT inverters switch at 2 kHz to 16 kHz. Some modern drives operate at carrier frequencies up to 32 kHz in silent mode. The dV/dt at each IGBT transition typically lies between 3 kV/μs and 10 kV/μs. First-generation inverters and drives with longer cable runs measure at the high end; modern drives with active gate drive control measure closer to 3 kV/μs.

Reflected wave peak voltage. A traveling wave is launched into the motor cable on every IGBT transition. When the cable surge impedance (Z_cable, typically 30 Ω to 90 Ω for unshielded, 15 Ω to 40 Ω for shielded VFD cable) does not match the motor high-frequency impedance (Z_motor, typically 500 Ω to 5000 Ω at the frequencies of interest), a reflection returns from the motor terminals. The reflection coefficient is:

Γ = (Z_motor − Z_cable) / (Z_motor + Z_cable)

For typical values, Γ approaches +1, and the peak voltage at the motor terminals is:

V_peak = VDC × (1 + Γ) ≈ 2 × VDC

For a 400 V drive, V_peak ≈ 1080 V at the motor. For 480 V, V_peak ≈ 1356 V. These peaks double the stress on motor insulation and on any contactor between the drive and the motor.

Critical cable length. The reflection reaches its full magnitude only if the cable is long enough for the wave to travel to the motor and back before the IGBT transition completes. The critical length is:

L_c = (v_p × t_r) / 2

where v_p is the propagation velocity of the cable (≈ 150 m/μs for typical XLPE-insulated VFD cable) and t_r is the IGBT rise time (≈ 0.05 μs to 0.2 μs). For t_r = 0.1 μs, L_c ≈ 7.5 m. For t_r = 0.2 μs, L_c ≈ 15 m. Cables longer than the critical length see the full 2× VDC reflection; shorter cables see a partial reflection.

Common-mode voltage. The IGBT bridge produces a common-mode component with a peak equal to half the DC bus, switching at the carrier frequency. Common-mode current returns through motor bearings, stray capacitances to ground, and any parallel paths including cable shields and conduit. The 3TF68/69 suppressor has no provision for common-mode stress; it is a line-to-line device.

Harmonic content. The PWM waveform is rich in low-order and high-order harmonics. Total harmonic distortion (THD) at the VFD output terminals typically exceeds 30%, with significant energy at the carrier frequency and its sidebands. The 3TF68/69 snubber is rated for the fundamental 50/60 Hz and the small transient energy from utility switching, not for continuous dissipation of carrier-frequency harmonics.

The integrated snubber in the 3TF68/69 is engineered to handle the first column. It is not engineered to handle the second column.

Failure Mechanisms Inside the Surge Network

Three failure mechanisms act on the integrated suppressor when the upstream source is a converter. All three converge on the same outcome: a phase-to-phase short circuit inside the contactor.

1. Capacitor overcurrent and thermal runaway. The RC snubber's capacitor sees di/dt proportional to C × dV/dt. At a VFD dV/dt of 5 kV/μs and a typical snubber capacitance of 0.22 μF, the peak current through the snubber on every IGBT transition is:

i_peak = C × dV/dt = 0.22 × 10^(-6) × 5 × 10^(9) = 1.1 A peak

At 8 kHz carrier, that is 8 × 10^3 switching events per second. The RMS current is the peak divided by 2√2 for a sinusoidal approximation, or roughly 0.4 A RMS continuous. The power dissipated in the snubber resistor (assuming 47 Ω) is:

P_R = i²_rms × R = (0.4)² × 47 = 7.5 W continuous

A snubber resistor rated for 10 W single-pulse operation will overheat in continuous VFD service. As the resistor's temperature rises, the capacitor's dielectric losses also rise (polypropylene dissipation factor increases with temperature above 85 °C). The capacitor eventually goes into thermal runaway, venting or shorting internally. A shorted capacitor is a direct phase-to-phase fault through the snubber wiring.

2. Metal-oxide varistor (MOV) fatigue. If the suppressor stage includes a MOV, the high repetition rate of voltage peaks causes cumulative degradation of the MOV's clamping characteristic. The MOV's leakage current rises, its capacitance shifts, and at some point the MOV fails short. A shorted MOV at 400 V class is typically a 1 Ω to 10 Ω resistive path that draws tens of amps from the line. Upstream protection will operate, but the contactor is already damaged.

3. dv/dt-triggered flashover inside the snubber assembly. The original snubber is laid out for a 50/60 Hz environment, with creepage and clearance distances set by that stress. The fast edges of a PWM waveform cause uneven voltage distribution across the snubber's physical layout; the end of the snubber network closest to the switching device sees the highest instantaneous stress. Over time, partial discharge carbonizes the insulation, leading to surface tracking and ultimately a phase-to-phase short across the snubber housing.

Documented Failure Mode: Phase-to-Phase Short Circuit

Siemens explicitly documents the failure mode in the 3TF68/69 catalog documentation. The relevant language is that the surge protection circuit could be damaged by the voltage peaks and harmonics and cause phase-to-phase short circuits. This is not a hypothetical concern; it is the manufacturer's stated failure mode for the standard product when used on a converter-fed circuit.

The phase-to-phase short presents in service as one or more of the following:

• The contactor housing shows signs of internal arcing on inspection: carbon tracking on the snubber assembly, melted insulation at resistor or capacitor leads, or an acrid odor.
• The VFD trips on output phase loss (e.g., Siemens SINAMICS fault F30002 / F30003, Allen-Bradley PowerFlex fault F5 / F12, ABB ACS880 fault 2340), ground fault (F30021 / F313 / 2E81), or overcurrent (F30001 / F4 / 2310), depending on which side of the contactor the fault sits.
• Upstream protection (circuit breaker, fuses) operates if the fault is severe enough to draw sustained current from the line.
• Intermittent operation if the fault is a high-resistance carbon track that becomes conductive only when heated by load current.

A short inside the contactor that occurs while the contactor is closed will be cleared by upstream protection (the VFD's electronic protection, the line circuit breaker, or both). A short that occurs while the contactor is open (or partially open, during the TRV interval) is more dangerous because the open contact gap becomes the only barrier between phases, and the vacuum interrupter is not designed to interrupt a bolted fault from the line side. The upstream protection must clear the fault before the contact gap breaks down.

Affected Catalog Numbers

The -Z A02 ordering option applies to the 3TF68 and 3TF69 vacuum contactor families. The relevant catalog numbers include:

The user's contactor, 3TF6844-0CM7, is directly affected. The -0CM7 suffix decodes approximately as: 0 = main pole configuration code, C = auxiliary contact block style (typically 2 NO + 2 NC), M7 = 230 V AC 50/60 Hz operating coil. Suffix decoding is regional and changes between catalog editions; always confirm against the rating plate of the contactor in hand and the current Siemens catalog.

For authoritative part-number and accessory data, the Siemens Industry Online Support portal (search for "3TF68" or "3TF69") provides the current catalog pages, dimensional drawings, and ordering aids. The original 3TF68/69 catalog includes the explicit statement that the surge-suppression circuit is not required for operation in circuits with DC choppers, frequency converters, or speed-variable operating mechanisms.

Ordering Code -Z A02: Contactors Without Integrated Damping

To order a 3TF68/69 vacuum contactor without the integrated surge suppressor, append the suffix -Z and the order code A02 to the base catalog number.

The price of the -Z A02 variant is the same as the standard contactor. Siemens does not charge extra to remove the suppressor because the suppressor is the more expensive component to manufacture and stock; the suppressorless variant is, in production terms, the simpler build.

When ordering, the suffix and code must both be specified: 3TF6844-0CM7-Z A02. If only the -Z is given without the A02 code, the order will not be processed with the suppressor removed (Siemens will interpret -Z as a request for information, not a specific adder). If only the A02 code is given without the -Z, the same applies. The two are a pair. Siemens' ordering systems expect the format "-Z A02" with a single space between -Z and A02.

Lead time for the -Z A02 variant is typically the same as for the standard product, because the suppressorless variant is built on the same assembly line. Confirm lead time with the regional Siemens distributor at the time of order.

Coil Burnout: A Separate Root Cause Analysis

The integrated overvoltage damping in the 3TF68/69 is wired across the main current paths (line-to-load on each pole), not across the operating coil. The coil is a separate circuit fed through its own terminals (typically A1/A2 on the contactor). The snubber network has no galvanic connection to the coil winding, and the snubber's failure modes (capacitor short, MOV short, creepage flashover) occur on the main-pole circuit. A snubber failure cannot cause overcurrent or overvoltage in the coil circuit.

Coil burnouts in 230 V AC contactors are caused by one or more of the following mechanisms:

1. Supply overvoltage at the coil terminals. A nominal 230 V AC coil is rated for operation at 0.85 to 1.1 × Uc per IEC 60947-1. Sustained operation at 250 V or above will overheat the coil. In industrial plants, voltage can sit at 240 V to 250 V continuously, which shortens coil life by 50% or more compared to nominal operation. The user's plant should be checked with a true-RMS voltmeter logging for 24 hours to characterize the actual supply.
2. Mechanical wear of the magnet assembly. As the magnet faces wear, the air gap increases, the inrush current stays high for a longer portion of the cycle, and the coil overheats. Vacuum contactors in compressor service that cycle frequently (more than 100 operations per hour) are candidates for this failure mode. The 3TF68/69 magnet assembly is serviceable; the magnet face can be inspected and the contactor refurbished rather than scrapped.
3. Failed economizer. Some 3TF68/69 variants include an electronic economizer that drops coil current after pull-in. If the economizer fails, the coil sees continuous inrush current and burns out in minutes to hours. The economizer is a separate add-on module; check the part number list for the installation.
4. Voltage sag and re-energization. When the supply dips below the holding voltage but not below the drop-out voltage, the contactor chatters, and the coil sees inrush current repeatedly without ever fully pulling in. This is a common cause of coil failure in plants with weak power systems, large motor starts, or weak transformers.
5. Foreign material on the magnet faces. Dust, oil mist, or scale on the magnet faces prevents proper sealing, increases the air gap, and causes the same heating as mechanical wear. Compressor rooms in particular have oil mist in the atmosphere; sealing the contactor enclosure is a good practice.

The user's contactor is normally energized (held in except for E-stop), and the E-stop cuts the line supply. This is a critical point: in a normally-energized contactor, the coil is under continuous stress. Any of the five causes above will burn out the coil, and the surge suppressor is not one of them.

Recommended coil-life mitigations for the user's installation:

• Feed the coil through a control transformer. A 230 V control transformer with adequate VA rating (typically 75 VA to 150 VA for a 3TF68 coil) provides isolation from line disturbances and stabilizes the supply voltage to the coil. The transformer should be sized for the inrush, not just the sealed VA; control transformer inrush is 10× to 20× the sealed VA for the first 100 ms.
• Add an RC snubber across the coil terminals (A1 to A2). For a 230 V AC coil, typical values are 0.1 μF (X2-class, 275 V AC rated) in series with 100 Ω, 2 W. This suppresses the inductive kick at drop-out (which can reach 1500 V on an unsuppressed coil) and extends contact life on the control relay. The RC across the coil is unrelated to the integrated overvoltage damping across the main poles; they are two different suppressors for two different transients.
• Add a metal-oxide varistor (MOV) across the coil as a secondary suppressor, sized to 275 V clamp for a 230 V AC coil. A typical choice is a 14 mm radial-leaded MOV with 275 Vrms continuous rating, 430 V clamping at 5 A test current, and 25 J energy rating (such as the Littelfuse V275LA20A or equivalent).
• Add a coil voltage monitor relay (undervoltage and overvoltage) to trip the upstream control breaker if the supply deviates from the 195 V to 253 V AC range.
• Consider an electronic coil economizer (Siemens 3RT19 accessory or equivalent) to reduce coil power consumption by 70% to 90% after pull-in, dramatically extending coil life. The economizer is particularly effective for normally-energized contactors.

System Topology: Line, Drive-Side, and Bypass Contactors

The 3TF68/69 in this application is positioned either between the line and the VFD, between the VFD and the compressor motor, or as a bypass contactor routing the motor around the VFD. The function of the contactor — and therefore the surge suppressor exposure — depends on which position it occupies.

Figure 2 — VFD System Topology with Line, Drive, and Bypass Contactors

Line-side contactor (K1 in Figure 2): The contactor is upstream of the VFD, between the utility supply and the drive's rectifier. The contactor sees 50/60 Hz line voltage with normal utility transients. The integrated overvoltage damping is appropriate, and the standard catalog number is correct. No -Z A02 required. Mechanical and electrical endurance are determined by the number of starts, not by VFD output stress.

Drive-side contactor (K2 in Figure 2): The contactor is downstream of the VFD, between the drive's inverter output and the motor. The contactor sees the PWM output described in Section 2. This is where the integrated suppressor is exposed to converter-induced stress, and the -Z A02 order code is required if the contactor is to be placed on the motor side of the VFD.

Bypass contactor (K3 in Figure 2): In bypass configurations, a contactor routes the motor either through the VFD or directly to the line. The bypass contactor sees line voltage when the motor is in the bypass position (VFD disconnected from the motor, motor fed from the line through K3 and a parallel branch around the VFD). The drive-side branch sees PWM voltage. Each contactor must be ordered for the position it occupies: the drive-side branch takes the -Z A02 suffix, the bypass branch uses the standard catalog number. Note that when the VFD is in normal operation, K3 is open; when in bypass, K3 is closed and the VFD's K2 is open. The contactors are mechanically and electrically interlocked to prevent both from being closed simultaneously, which would parallel the VFD output with the line.

Verification Procedure After Order Code Change

When a 3TF6844-0CM7-Z A02 is received and installed, perform the following verification before energizing the load.

1. Nameplate confirmation. Confirm the rating plate carries the full order number 3TF6844-0CM7-Z A02, not just the base 3TF6844-0CM7. The -Z and A02 must be visible on the rating plate. If the rating plate shows only the base catalog number, the suppressorless variant was not built; do not energize.
2. Visual snubber check. With the arc chambers removed (or by reference to the catalog drawing), confirm that no RC network is wired across L1-T1, L2-T2, L3-T3. The standard product has visible snubber leads running parallel to the vacuum interrupters; the -Z A02 variant does not. The auxiliary contact block and the coil wiring should be present and untouched.
3. Insulation resistance test. Measure phase-to-phase and phase-to-ground with a 1000 V megohmmeter. Expect > 100 MΩ on a new contactor. A reading below 50 MΩ suggests contamination or damage in shipping.
4. Mechanical operation check. Energize the coil 10 times, confirm the contactor picks up and drops out cleanly. Listen for any mechanical binding; vacuum contactors should be nearly silent in operation. Measure the pickup voltage (must close at or below 0.85 × Uc = 195 V for a 230 V coil) and the dropout voltage (must open at or below 0.75 × Uc = 173 V and above 0.10 × Uc = 23 V).
5. Control circuit voltage check. Verify the supply at A1-A2. For a 230 V AC coil, confirm the supply is within 0.85 to 1.1 × Uc (195 V to 253 V AC) at the coil terminals, not at the panel bus. Voltage drop in the control wiring can be 5 V to 15 V on long runs; check at the coil, not the source. Log the supply voltage over 24 hours to characterize sag and overvoltage events.
6. No-load electrical operation. With the VFD output disabled, close the contactor. Confirm the contactor carries input current at 50/60 Hz without abnormal heating or noise. Use a clamp-on ammeter to verify the line current is within 10% of the motor's nameplate full-load current at the running speed. This validates the line-side wiring.
7. Loaded electrical operation. Enable the VFD output at 0 Hz and ramp slowly to rated frequency over 30 seconds. Monitor contactor terminal temperature with an infrared thermometer or RTD at 10 minutes, 30 minutes, and 1 hour of operation. Steady-state temperature should not exceed 80 K rise over ambient on the main poles, or 65 K rise on the coil, per IEC 60947-4-1 Class B limits. Hot spots on the snubber area (if present) indicate the wrong variant was installed.
8. VFD fault check. Verify no VFD faults are logged over a 4-hour loaded run. Common faults to watch for on a system with a contactor issue: F30002 / F30003 (output phase loss on Siemens SINAMICS), F4 / F5 / F12 on Allen-Bradley PowerFlex, 2340 / 2310 on ABB ACS880. Any of these faults within the first hour of loaded operation indicates the contactor or its wiring is faulty.
9. Bearing current check (optional, recommended for VFD service). With a true-RMS current clamp on the motor shaft (using a shaft grounding brush or an isolated measurement), verify bearing current is below 200 mA RMS. Excessive bearing current indicates inadequate common-mode filtering or improper grounding; this is a VFD output issue, not a contactor issue, but it is good practice to check it during commissioning.

Standards Reference

Contactor selection and application in VFD-fed motor circuits is governed by several standards. The full text should be consulted for binding requirements; the summaries below show where this topic intersects each standard.

Standards are documents to verify against, not guarantees. Always confirm the specific clause text against the latest edition in force in the jurisdiction of the installation. The 2023 edition of NFPA 70 (NEC) and the IEC standards in their current editions apply; older installations may be governed by the standards in force at the time of commissioning.

Field Notes and VFD Output Filtering

Three field notes from installations of this contactor family in VFD service:

1. Mis-ordered contactors are the most common cause of failures in this category. A 3TF6844-0CM7 ordered without the -Z A02 suffix arrives with the suppressor installed. If the installer is unaware of the issue, the suppressor is left in place and the failure mode is set. The fix is to verify the order code on receipt, before commissioning. Add a note to the purchase order: "Siemens 3TF6844-0CM7-Z A02 — vacuum contactor without integrated overvoltage damping, for VFD output service".
2. The -Z A02 variant does not need an external surge suppressor on the main poles. The 3TF68/69 is a vacuum contactor; the vacuum interrupter handles line-side switching transients at 50/60 Hz, and the IGBTs in the VFD handle the PWM edge themselves. Adding an external snubber across the main poles is unnecessary and can introduce additional failure modes. The exception is high-voltage (1000 V class) or high-current installations, where a separate snubber can be added to suppress TRV peaks from long cable runs; consult the VFD manufacturer for guidance.
3. The -Z A02 variant is still a vacuum contactor. It is suitable for use at 50/60 Hz on the line side of a VFD or in any position where the contactor sees utility power. The -Z A02 option is "without integrated overvoltage damping" — it is not "for VFD use only". A line-side 3TF68/69 with -Z A02 is acceptable and equivalent to a standard unit without the snubber, in any utility-frequency application. There is no functional penalty for ordering the -Z A02 variant on the line side; the only difference is the absence of the snubber, which is benign for the line-side duty.

Although the -Z A02 order code resolves the contactor's internal suppressor problem, the underlying VFD output stress on the motor, the cables, and any other equipment on the load side remains. For installations with long motor cables (over 30 m for 400 V systems, over 15 m for 690 V systems), or with motors that have old windings, the following should be considered in addition to the contactor change:

• dV/dt filter at the VFD output. Limits the edge rate to 500 V/μs or less, reducing the stress on motor insulation and on any load-side contactor. Typical dV/dt filters use an L-C or R-L-C topology with a series inductance of 5% to 10% and a shunt capacitance sized to limit the edge. Cable-side peaking must be checked with the specific cable length.
• Sine wave filter at the VFD output. Converts the PWM waveform back to a near-sinusoidal shape, eliminating reflected wave peaks. Required for older motors or for motors with unspecified insulation class. Sine wave filters introduce a small voltage drop (typically 5% to 8%) and a small additional derating of the VFD's output current capability (5% to 10%).
• Common-mode choke at the VFD output. Reduces bearing currents and the common-mode voltage stress on the motor. A common-mode choke is a single-phase three-winding reactor (one per phase, wound on a common core) that presents high impedance to common-mode currents and low impedance to differential currents.
• Symmetric VFD-rated motor cable with three phase conductors, three symmetric ground conductors, and a continuous corrugated aluminum armor bonded at both ends (such as VFD cable with a Type MC-HL construction). Reduces the common-mode current path and provides a low-impedance return path for high-frequency components.
• Shaft grounding ring on the motor. Diverts bearing current to ground before it arcs through the bearing race, extending bearing life in VFD service.

These are load-side mitigations and do not change the contactor selection. The contactor must still be ordered as -Z A02 if it sits on the load side of the VFD.

Cross-Reference: VFD Fault Codes Associated with Output Contactor Failures

When a 3TF68/69 suppressor fails on the load side of a VFD, the drive's electronic protection typically logs one of the following fault codes. The list below covers the major VFD platforms; consult the drive manual for the exact code on a specific installation.

Fault codes on a VFD-driven compressor system should be interpreted in the context of recent work on the contactor, recent cable changes, and recent motor changes. A ground fault that appears immediately after a contactor replacement points to the contactor; an overcurrent that appears at high speed with a long cable points to a reflected wave issue; an output phase loss that appears under load but not at no-load points to a high-resistance contact or a failing snubber.

Troubleshooting Matrix for a Failed 3TF6844 on a VFD-Driven Compressor

Use the matrix below to diagnose a failed 3TF6844 in a compressor application fed by a VFD. Score each row; multiple hits in the suppressor-related rows indicate a -Z A02 ordering issue.

Frequently Asked Questions