Servo Drive Wiring — Power, Motor, Feedback, STO, and Brake
A servo axis is a VFD with a closed loop bolted on: the same power and shielding rules apply, but the feedback path, Safe Torque Off, and the holding brake are what make or break the install.
Safety. This guide is educational reference material, not a work instruction. Electrical work is performed de-energized and verified by qualified personnel under your site’s LOTO procedures, following the drive and motor manuals and the authority having jurisdiction. Servo drive DC bus capacitors hold a lethal charge after power removal — observe the drive’s marked discharge wait time. Safe Torque Off is not a disconnect: it does not isolate the mains and is not a substitute for LOTO.
Overview
A servo axis is a closed-loop motion system, and that loop is the difference that matters. Where a VFD typically runs an induction motor open-loop on speed, a servo drive continuously reads a feedback device on the motor shaft and regulates position, velocity, and torque with tight dynamics — adding two wiring groups a basic VFD does not have (feedback and, almost always, Safe Torque Off) plus a motor-mounted holding brake on most axes. The terminal groups:
- Mains / DC bus — supply from the branch-circuit OCPD, plus DC-bus terminals for multi-axis bus sharing and the regen/braking resistor; a stored-energy hazard.
- Motor power — the PWM output to the servo motor; the noisy circuit, treated exactly like the VFD load side.
- Feedback — resolver, encoder, or single-cable digital protocol; signal-level, and the circuit most likely to cause faults.
- STO / safety — dual-channel Safe Torque Off inputs into the safety system.
- Holding brake — 24 V to a motor-mounted brake that holds a stopped axis.
- Control / network — enable, references, and the motion fieldbus.
flowchart LR
SUP[Supply] --> DISC[Disconnect + OCPD]
DISC -->|input conductors| DRV[Servo drive]
DRV -->|shielded motor cable| M((Servo motor))
M -.->|feedback cable| DRV
BRK[Holding brake 24V] -.-> M
REGEN[Regen / brake resistor] -.->|DC bus| DRV
STO[Safety controller<br/>dual-channel STO] -.-> DRV
NET[Motion fieldbus + enable] -.-> DRV
This guide covers a single low-voltage servo axis and builds on the VFD guide rather than repeating it — the power-wiring, shielding, and reflected-wave rules are shared, so this page focuses on what is different for a servo: the feedback loop, STO, the holding brake, and the sizing consequences of high peak torque. Commutation, phasing, and autotune are commissioning steps, covered in the servo commissioning workflow; STO PL/SIL determination belongs to the machine’s functional-safety design. Terminal designations, feedback pinouts, torque values, and cable-length limits are vendor-specific — they come from the drive and motor manuals, never from a guide, including this one.
Before You Start
Have on hand before pulling wire:
- The matched set. Drive, motor, and feedback device are normally selected together, usually from one vendor, and commissioned as a unit. The drive firmware must support the motor’s feedback type — a mismatched device will not commutate. Confirm the combination against the manuals before wiring.
- Motor nameplate and data sheet — voltage, continuous current/torque and peak current/torque (servos run several times continuous torque for short intervals), feedback type, and whether a holding brake is fitted (motor fundamentals).
- Cable-system decision. Digital-feedback families offer a single-cable solution (power and feedback in one hybrid cable, to vendor rules) or separate power + feedback cables. Which is permitted — and the maximum motor and feedback lengths — is set by the drive/feedback system, not a universal number; feedback protocols have hard length limits. Consult the manual.
- DC-bus / regen architecture. On a multi-axis machine, decide upstream whether axes share a DC bus (so one axis’s braking energy feeds another) and how excess is dumped — braking resistor or regen unit. Bus-sharing, fusing, and same-family rules are vendor-defined.
- Drawings — the branch-circuit design, the safety circuit driving the STO inputs, and the field-network topology are decided upstream; this guide assumes you are implementing them.
Sizing & Protection
The governing framework is NEC Article 430 (Part X, adjustable-speed drive systems) with NFPA 79 Chapters 6 and 7 for machinery panels — the same backbone as a VFD.
- Mains input protection. Size branch-circuit conductors and OCPD to the
drive’s rated input current (NEC 430.122), not the motor current, and
use the OCPD type/rating in the drive listing — deviating can invalidate the
drive’s SCCR contribution and the panel’s marked SCCR (NFPA 79 Ch. 6;
UL 508A methodology).
cst motor-branchcomputes the branch chain andcst voltage-dropchecks the run; the wire sizing walkthrough works the full chain. - Motor-cable sizing. Size to the servo motor’s continuous current, then verify the cable and terminations tolerate the peak current the drive can command — a servo’s peak can be several times continuous, so a cable picked from continuous current alone may be under-rated for the peaks. The drive manual states the continuous/peak ratings and any min/max cross-section.
- DC-bus and regen protection. Shared-bus links and braking-resistor circuits are high-energy; protect and thermally monitor them per the manual, and wire the resistor thermal switch into the control/safety chain.
- STO is not overcurrent protection. Safe Torque Off removes torque capability; it does not protect conductors and is no substitute for branch-circuit or motor protection — a distinction worth keeping clear.
- Disconnecting means — a disconnect within sight of the drive is required (NEC Art. 430 Parts IX/X; NFPA 79 Ch. 5), and it is what you LOTO for maintenance — not the STO inputs.
Power Wiring
The servo motor circuit follows the VFD load-side rules; see the VFD power-wiring section for the full reasoning, summarized here with the servo differences.
- Shielded servo motor cable, 360-degree termination at both ends. A full-circumference gland or clamp at the drive and motor end — the same both-ends rule as the VFD motor cable, and for the same reason: a pigtail is a significant impedance at PWM frequencies and largely defeats the shield. Verify against the drive’s EMC installation instructions.
- Keep motor power away from feedback. In a two-cable system, route motor power and feedback apart. In a single-cable/hybrid system the segregation is engineered inside the cable to vendor rules — follow those rules, do not improvise a substitute hybrid from separate cables in one conduit.
- Reflected-wave / lead length applies as on a VFD: long leads overshoot the terminal voltage and stress winding insulation. Threshold and remedy are vendor- and voltage-class-specific — from the manual, not a universal number.
- DC-bus link wiring (multi-axis): vendor bus bars or cables, short runs, correct polarity and fusing. Regen / brake-resistor wiring is a high-energy switched circuit — short, shielded/twisted, routed away from control and feedback wiring, thermal contact wired into the safety chain.
- Torque discipline — terminal torque values and wire ranges come from the drive manual; record the values used.
Control / Signal Wiring
This is the core of a servo install. Three circuits — feedback, STO, and the holding brake — have no real counterpart on a basic VFD.
Feedback
- Use the specified feedback cable. Resolver, incremental/absolute encoder, or a single-cable digital protocol — the cable, connector, pinout, and grounding are defined by the feedback system. Consult the manual; do not assume across vendors or motor families. Ground and terminate the feedback shield per the manual — no pigtails, any more than on the motor cable.
- Never route feedback with motor power beyond the manufacturer’s single-cable rules. Motor-cable coupling into the feedback path is the classic servo fault — it shows up as position error, following error, or commutation faults only while the axis moves.
Safe Torque Off (STO)
- Dual-channel, into the safety system. STO is typically wired as two independent channels to a safety relay or safety controller, as part of a rated safety function per ISO 13849-1 and IEC 62061. The wiring, diagnostics, and PL/SIL determination belong to the machine’s safety design and the planned safety-circuit guide.
- STO removes torque — it is not a brake. On a vertical or loaded axis the load can still fall when torque is removed; STO alone does not hold it. That is what the holding brake and the safety design are for.
- STO is not galvanic isolation and not a maintenance disconnect. It stops the drive from producing torque but leaves the mains and DC bus live. For maintenance you still LOTO the upstream disconnect (IEC 60204-1 treats STO as a stop function, not an isolation function).
Holding brake
- The brake holds; it does not stop. A motor-mounted holding brake is typically 24 V, fail-safe (spring-applied, electrically released), and rated to hold a stopped axis — not to decelerate a moving load repeatedly, and not as a safety or e-stop stopping function.
- Sequence release with enable. The drive must hold the load before the brake releases, and the brake must re-apply before enable drops; get the order wrong and the axis drops or jerks. Sequencing is configured in the drive — verify it against the manual.
- Suppress the coil and size for the current. Fit the freewheel diode or varistor the manual specifies across the brake coil, and account for brake current, which can be significant relative to a signal-level 24 V load.
Grounding, Shielding & EMC
Servo systems are aggressive EMC sources — fast switching and high dynamics. Device-specifics here; the deep treatment is owned by the noise & EMC mitigation guide.
- PE first, sized on the NFPA 79 Table 8.2.2.3 basis (largest upstream OCPD) — procedure per the table, values not reproduced here. See panel grounding & bonding.
- HF grounding of drive and motor. The motor-cable shield/ground is the dedicated high-frequency return for PWM common-mode current back to the drive PE terminal; the building ground alone is not sufficient. Ground the feedback shield per the manual.
- Feedback corruption from the motor cable is the signature servo fault. Coupling into the feedback path produces intermittent position errors — shield integrity and separation on the feedback path are decisive. See the encoder wiring guide for feedback-cable detail.
- Bearing currents — common-mode voltage drives current through motor bearings; mitigation (insulated bearings, shaft-grounding rings) is generally accepted practice for larger machines. The servo motor cable is also a worst-offender for parallel network runs; separation classes and distances are in the EMC guide.
Common Mistakes
- Feedback cable routed with motor power. PWM common-mode current couples into the feedback path; the axis throws position/following/commutation faults only while it moves, and the drive gets blamed for a wiring problem. Keep feedback off the motor cable except within a vendor single-cable system.
- Treating STO as a maintenance disconnect. STO removes torque but leaves the mains and DC bus live — it is not isolation. You still LOTO the upstream disconnect before working on the axis. Assuming otherwise is a shock and unexpected-restart hazard.
- Using the holding brake as a safety or e-stop stop. The brake is rated to hold a stopped axis, not to stop a moving load on demand; relying on it as a stopping function wears it out and does not meet the safety function. Use STO and the safety design for stopping.
- Shield pigtails on the feedback cable. A few centimeters of pigtail is a real impedance at these frequencies — the shield reads continuous on a meter but its EMC performance is gone, and feedback noise returns under load.
- Mismatched drive / motor / feedback set. A motor or feedback device that the drive firmware does not support will not commutate or will fault at enable — confirm the matched set against the manuals before wiring, not at power-up.
- Wrong brake/enable sequence. Releasing the brake before the drive holds torque drops the axis; dropping enable before the brake re-applies lets it fall or jerk. Verify the release/apply sequence against the drive manual.
- Sizing the motor cable from continuous current only. A servo’s peak current runs several times continuous; a cable picked on continuous current alone runs hot on the peaks. Size continuous, verify peak.
Verification Checks
Before and during first energization (evidence-retaining checklists in templates):
- Drive, motor, and feedback confirmed as a supported matched set against the manuals
- Feedback signal reads position/velocity cleanly in drive diagnostics before torque is enabled
- Feedback shield/ground and motor-cable 360-degree glands complete at both ends; no pigtails
- Motor cable sized for continuous current and verified for peak; terminal torques per the manual, recorded
- STO functional test — both channels remove torque, and a channel mismatch is detected and faults — per the safety validation plan
- Brake sequencing test — axis holds through the release/apply cycle with no drop or jerk
- Insulation-resistance test the motor and cable with the drive output disconnected — never megger through a connected drive
- Discharge wait time observed before any terminal re-work (NFPA 79 Ch. 7)
- Hand off to the servo commissioning workflow for commutation/phasing, autotune, and functional checks
Standards References
- NEC (NFPA 70), 2023 — Art. 430 Part X (adjustable-speed drive systems, incl. 430.122 input conductors); Parts IX/X (disconnecting means); 430.6 with Tables 430.247–430.250 (FLC vs FLA discipline for the motor circuit)
- NFPA 79:2024 — Ch. 5 (supply and disconnect), Ch. 6 (protection, SCCR), Ch. 7 (stored-energy discharge), Ch. 8 (grounding and bonding, Table 8.2.2.3 basis)
- IEC 60204-1 — stop categories (STO as a Category-0 style stop) and equipotential-bonding/wiring practices; stop functions are not isolation
- ISO 13849-1 / IEC 62061 — govern the STO safety function (PL/SIL determination, dual-channel architecture); the wiring detail is outside this guide
- UL 508A — SCCR methodology for the drive/OCPD combination in panels
Related Pages
- How to wire a VFD — shared power, shielding, and reflected-wave rules
- Encoder wiring — feedback-cable detail
- Noise & EMC mitigation
- Panel grounding & bonding
- Servo commissioning workflow
- ISO 13849-1 overview
- Motor fundamentals
This site is an independent educational reference for industrial automation standards. It paraphrases and organizes guidance in original language; it is not a substitute for the official published standards, professional engineering judgment, or legal review, and content has not been independently verified against current published editions.
Items marked Review pending await technical review against the identified edition. Items marked Planned have no reviewed source material behind them. Do not rely on this site for compliance determinations, safety-critical design decisions, or legal interpretation.