VFD and Soft Starter Fundamentals for Electricians: What Changes in Wiring, Protection, and Testing

VFDs and Soft Starters: What They Change (and What They Don’t)

Variable Frequency Drives (VFDs) and soft starters both reduce mechanical stress and electrical stress during starting, but they do it in different ways that affect wiring, protection, and testing.

• Soft starter: Controls voltage during start (and sometimes stop) using SCRs/thyristors. After ramp-up, many units use an internal bypass contactor so the motor runs on full line voltage. Frequency stays at line frequency.
• VFD: Controls frequency and voltage to control speed/torque continuously. The motor is fed by a PWM (pulse-width modulated) waveform, not a sine wave.

From an electrician’s perspective, the biggest changes are: (1) the motor is no longer on a clean sine wave with a VFD, (2) the cable/grounding practices matter more (EMI and bearing currents), (3) some protective devices must be chosen with drive behavior in mind, and (4) testing procedures must avoid damaging electronics.

1) Basic VFD Blocks and What They Imply in the Field

Block diagram you should visualize

Motor heating at low speed: what changes with a VFD

At low speed, many motors cool themselves less effectively because the shaft-mounted fan moves less air. Meanwhile, the VFD can still command torque (and therefore current). Result: a motor can overheat even when it is turning slowly and “sounds fine.”

• Constant torque loads (conveyors, positive displacement pumps, screw compressors) are most likely to overheat the motor at low speed if run there continuously.
• Constant horsepower loads (some fans/blowers in certain regions of operation) behave differently, but low-speed cooling can still be a limit.

Field implications:

• Verify whether the motor is rated for inverter duty if it must run slowly for long periods.
• Check if an external blower kit is required for continuous low-speed operation.
• Don’t assume “lower Hz = cooler motor.” Check actual motor current and temperature.

Soft starter heating note (different issue)

A soft starter mainly affects starting. If it has a bypass contactor, the SCRs are out of the circuit during run and heating is minimal. If it is non-bypass (or used for soft stopping frequently), the SCRs dissipate heat and enclosure ventilation becomes important.

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2) Shielded Cable, Grounding, and Separation to Reduce EMI

VFD output switching creates fast voltage edges that can radiate or conduct electrical noise (EMI/RFI). This can cause nuisance trips, encoder faults, PLC I/O glitches, radio interference, and premature bearing issues. Good installation practices prevent most of these problems.

Cable selection: what to use and why

• Use VFD-rated motor cable when possible: symmetrical grounds and insulation designed for PWM stress.
• Shielded cable (braid or foil with drain) reduces radiated noise and provides a controlled return path for high-frequency currents.
• Keep motor leads short when practical; longer leads increase reflected wave voltage stress and EMI.

Shield termination: practical step-by-step

1. Terminate the shield 360° (full circumference) to metal at the drive end using an EMC gland/clamp. Avoid “pigtails” when possible; they add inductance and reduce effectiveness at high frequency.
2. Bond to the drive’s PE/ground bar and to the enclosure backplate that is bonded to PE.
3. At the motor end, bond the shield to the motor frame/ground using a proper gland or clamp. (Some manufacturers specify one-end termination for certain situations; follow the drive/motor manual, but two-end bonding is common for VFD motor leads to control high-frequency currents.)
4. Verify continuity of the equipment grounding conductor (EGC) and bonding jumpers; do not rely on conduit alone unless permitted and verified.

Grounding and bonding practices that matter

• Short, wide ground connections are better than long, thin ones for high-frequency noise (lower impedance).
• Bond the motor frame, driven equipment, and nearby metalwork to minimize potential differences that drive bearing currents.
• Use a dedicated PE conductor in the same cable or raceway as the motor leads; keep the return path close to the outgoing path.

Separation and routing: practical rules of thumb

• Separate VFD output cables from control/communication wiring. If they must cross, cross at 90°.
• Do not run analog signals (0–10 V, 4–20 mA) in the same conduit/tray as VFD output conductors.
• Keep encoder/feedback cables shielded and physically separated; terminate shields per manufacturer instructions (often at the drive end with 360° clamp).
• Use metal conduit or continuous metallic tray where appropriate as an additional shield, but still provide a proper PE conductor.

3) When to Use dv/dt Filters or Reactors

VFD PWM switching can create high dv/dt (fast voltage rise) and reflected wave overvoltage at the motor terminals, especially with long motor leads. This stresses motor insulation and can increase bearing currents and audible noise. Reactors and filters are tools to manage these effects.

Output reactor vs dv/dt filter vs sine filter

Practical decision triggers (field-focused)

• Long motor lead length: If the motor is far from the drive, consider at least an output reactor; for very long runs, a dv/dt or sine filter may be required. Use the drive manufacturer’s cable-length limits as the deciding reference.
• Older or non-inverter-duty motor: More likely to need dv/dt mitigation to protect insulation.
• Nuisance overcurrent/ground fault trips: Output reactor can reduce peak currents and capacitive charging effects on long cables.
• Audible motor whine or overheating not explained by load: Filter/reactor may help, but also check carrier frequency settings and motor cooling.

Installation notes electricians should not miss

• Mount filters/reactors per manufacturer spacing and ventilation requirements; they run hot.
• Keep wiring between VFD and filter/reactor short to reduce radiated noise.
• Bond enclosures and use proper shield terminations even when filters are installed; filters reduce stress but do not replace good EMC practices.

4) Basic Parameter Checks for Troubleshooting (No Deep Programming)

Many “drive problems” are actually setup mismatches between the VFD/soft starter and the motor/load. The goal here is to verify a small set of parameters that directly affect protection and behavior.

VFD: essential checks tied to real symptoms

VFD parameter check: step-by-step workflow

1. Lock out and verify safe state before opening panels. Confirm DC bus is discharged per drive indicator/manual.
2. Confirm motor wiring configuration matches the drive output voltage (e.g., 230 V class drive to a motor connected for low voltage). Do not rely on “it used to run on line power.”
3. Read motor nameplate and compare to the drive’s motor data parameters: volts, FLA, base frequency, RPM (or poles). Correct any mismatch.
4. Check accel/decel: If the drive trips on accel (overcurrent), increase accel time or verify load isn’t jammed. If it trips on decel (overvoltage), increase decel time or verify braking resistor/chopper setup if used.
5. Check current limit/torque limit: If the motor won’t pull load, confirm limits aren’t set too low.
6. Run and measure correctly: Use the drive’s display for output current and frequency; use a true-RMS meter rated for VFD use if measuring, and understand that PWM can distort readings.

Soft starter: essential checks

• Motor FLA setting: Drives overload protection model (if integrated) and current limit behavior.
• Ramp time and initial torque/voltage: Too low causes stalling; too high defeats the purpose and can still cause mechanical shock.
• Current limit setting: If set too low, motor may never accelerate to speed.
• Bypass operation (if equipped): Verify bypass contactor pulls in after ramp; if it doesn’t, the soft starter may overheat and voltage drop may persist.

5) Safe Testing Guidance: Why Procedures Differ with Connected Electronics

Traditional motor testing habits (especially insulation resistance testing) can damage VFDs/soft starters or produce misleading results if performed with electronics still connected. The key rule is: test the motor and cable as a motor-and-cable system, not through the drive, unless the manufacturer explicitly allows it.

Megger/insulation resistance testing: do’s and don’ts

• Do not megger through a VFD output (U/V/W to ground) with the drive connected. The DC bus capacitors, IGBTs, MOVs, and filters can be damaged by test voltage.
• Do not megger through a soft starter unless the manual states a safe method; SCRs and surge components can be affected.
• Do isolate the motor leads from the drive/soft starter before insulation testing.

Safe insulation test procedure (step-by-step)

1. LOTO the supply feeding the drive/soft starter and verify absence of voltage.
2. Wait for DC bus discharge on VFDs. Confirm with the drive’s indicator and/or measure per manufacturer guidance.
3. Disconnect motor leads from the drive output terminals (U/V/W) or from the soft starter output. Label conductors for correct reconnection.
4. Test motor + cable: Perform insulation resistance tests from each phase conductor to ground and phase-to-phase as required by your procedure/spec. Use the test voltage appropriate for the motor/cable rating and site standards.
5. Discharge the motor windings after meggering (many testers do this automatically; verify). This prevents a stored charge from surprising you or stressing components when reconnecting.
6. Reconnect and torque terminals to specification; loose terminations on VFD outputs can increase heating and noise.

Continuity and grounding checks that are especially important

• Verify PE/EGC continuity from drive enclosure to motor frame. High-frequency currents will find a path; you want it controlled and low impedance.
• Check shield bonding at both ends (as specified). A shield that is only “sort of connected” can worsen EMI.
• Inspect gland plates and paint: paint under bonding points can insulate and defeat EMC clamps; use proper bonding hardware.

Voltage measurements: why standard meter habits can mislead

On a VFD output, the waveform is PWM. Many meters read “something,” but it may not represent true fundamental voltage. For troubleshooting:

• Prefer drive diagnostics (output frequency, output current, DC bus voltage, fault history).
• Use appropriate instruments if measuring output (true-RMS meter specified for VFD use, or a scope with proper differential probes and safety category ratings).
• Do not use a standard phase rotation meter on VFD output unless it is rated for VFD PWM; use the drive’s direction command and verify rotation mechanically.

Capacitors and stored energy: practical safety note

VFDs contain DC bus capacitors that can hold charge after power is removed. Always follow the manufacturer’s specified wait time and verification method before touching internal components or terminals, even if the motor is stopped.