Three decisions, not one

"Spec a drive" sounds like a single task, but for a machine builder it's really three separate decisions layered on top of each other: what category of drive the axis actually needs, how to size that drive correctly against the motor and load, and which brand and model you can source on the timeline the build calls for. Getting any one of those wrong tends to surface late — during commissioning, or worse, after the machine has shipped.

This is a working guide to machine builder VFD selection and servo sizing for OEM builds, covering all three decisions in order.

The first fork in the road

A VFD is the right call when the axis just needs to run at a commanded speed — a conveyor, a fan, a mixer — with no requirement to hold a precise position. It's simpler, less expensive, and easier to commission.

A servo drive is the right call when the axis needs to hit a specific position, hold it under load, or move through a coordinated motion profile with other axes — a pick-and-place, an indexing table, a multi-axis transfer. Servo systems add closed-loop feedback and tighter tuning, and cost more to spec and commission, but they're the only option once positioning accuracy actually matters to the process.

Getting this decision wrong in either direction shows up fast: a VFD asked to hold position will hunt and never settle; a servo spec'd for a simple speed-control axis is unnecessary cost and complexity.

What actually goes into the calculation

Once the drive category is settled, sizing comes down to matching the drive to the motor, the load, and the environment it will run in.

Sizing by nameplate current alone is the most common shortcut that causes trouble later — duty cycle and overload requirements are what actually separate a drive that runs comfortably from one that trips or overheats in the field.

Making sure the drive can actually talk to the rest of the machine

For any servo axis, the feedback device and the drive have to agree on signal type and resolution — an incremental encoder, an absolute encoder, or a resolver each require different drive-side configuration, and mismatching resolution against the application's positioning tolerance either wastes precision or falls short of it.

Just as important is confirming the drive's communication card matches the machine's control architecture before it's ordered — a drive that supports the wrong motion bus means either a card swap or a redesign once it arrives. If you're not sure a given drive/feedback/protocol combination will work together, IAC's sizing tool and engineering team can cross-reference the combination against your control architecture before you commit to a part number.

The right spec still has to be available on your timeline

Once the technical spec is locked, brand choice often comes down to lead time, cost, and what the end customer's plant already standardizes on.

IAC stocks across brands so a tight timeline doesn't force a compromise

IAC carries Invertek, ABB, and Yaskawa drives alongside Fagor encoders, so if your first-choice brand is backordered, there's usually a compatible alternative in stock rather than a redesign. IAC's sizing tool and engineering team can also cross-reference a drive/motor/feedback combination before you commit to a part number.

Warranty and verification

Every component IAC ships carries a 2-year in-service warranty — twice the industry standard for refurbished industrial parts. Components are functionally tested before they leave the warehouse.

Same-day shipping

In-stock parts ordered before 4:00 PM Eastern ship same day. For urgent needs, call (877) 727-8757 during business hours — quote turnaround is typically under five minutes. You can also submit a part number via the quote form ↗ or email sales@iac.us.com.