It usually arrives as a short email or a call after lunch: “What size VFD for a 5 HP motor?” The question sounds like it should have one number for an answer. In the real world it doesn’t. The correct size depends on what the motor is driving, what kind of motor it is, and what the machine is trying to do at the end of each move.
I’m the person at Thomson Linear who picks up urgent replacement requests. I’ve spent the better part of the last decade helping machine builders and plant engineers sort out motion failures—including more rush orders than I can count (somewhere north of 200 at this point; I’d have to check the CRM to give you an exact number). When somebody is standing next to a stopped machine, they want a definite answer. But the fastest way to make a bad situation worse is to quote a drive before understanding the machine’s situation.
For a 5 HP motor, I see the same three situations over and over:
- A three-phase induction motor that only needs adjustable speed. The axis moves faster or slower, but there is no positioning loop involved.
- A single-phase induction motor, or a shop with only single-phase power, where someone expects a VFD to make it work.
- An axis that actually needs positioning—which is a servo motor drive conversation from the start.
These three get different answers. Let’s go through them.
Read the Motor Nameplate Before You Read Anything Else
One technical point saves more confusion than anything else I can say: a VFD is sized by current, not by horsepower. The horsepower on a motor nameplate is a mechanical output rating. The full-load amperage (FLA) tells you what the drive’s output stage has to deliver electrically.
For reference, a typical 5 HP, three-phase, 460 V induction motor has a full-load current of 7.6 A under NEC Table 430.250 (National Electrical Code, NFPA 70). At 230 V, the same 5 HP motor draws about 15.2 A. If you pick a drive by horsepower alone and ignore voltage and nameplate current, you are essentially guessing.
So, to give the short version of the short answer: the minimum VFD for a 5 HP, three-phase motor is one with a continuous output current rating equal to or greater than the motor’s nameplate FLA. With that said, here are the situations that change the advice.
Scenario 1: Three-Phase Induction Motor, Variable Speed, No Positioning
This is the clean case. Imagine a belt-driven carriage riding on a Thomson linear shaft—the 60 Case LinearRace family, for example—with a five-horsepower three-phase induction motor. The operator wants to change the speed from a screen, and the carriage doesn’t have to stop at a commanded position with high repeatability. This is exactly what a VFD is good at.
Size the drive to the motor’s nameplate FLA. If that 5 HP motor shows 7.6 A at 460 V, choose a drive with at least 7.6 A continuous output. That usually works out to the “5 HP” model from a drive manufacturer, but read the drive’s current rating, not its horsepower label. If the load has heavy acceleration peaks—a screw jack that occasionally shocks the system, a carriage that can jam—move up one frame for overload headroom.
If the motor is connected to a screw-driven actuator, respect the duty cycle. Every actuator in the Thomson linear actuator catalog lists one for a reason. A screw actuator is a constant-torque load: motor current follows thrust. At low speed, a standard motor’s shaft-mounted fan moves less air, so if the profile holds high torque at low speed for long stretches, motor temperature can become the real limit, long before the drive is overloaded.
Oh, and one thing I should add: if the load is vertical, plan a mechanical brake. A VFD’s DC holding function is not a parking brake.
If you are pairing an older general-purpose motor with a new VFD, also check the motor’s insulation rating. Inverter-fed motors meet NEMA MG-1 Part 31 insulation requirements; older motors may not. If the motor isn’t rated for inverter duty and the drive is far from the motor, protect it with an output reactor or a sine-wave filter.
Scenario 2: A Single-Phase Induction Motor (or Single-Phase Power Only)
Now the answer changes. A standard VFD produces three-phase output and is designed to drive a three-phase induction motor. A single-phase induction motor is a different machine. The advice “just put a 5 HP VFD on it” tends to end with a burnt motor and longer downtime than if you had stopped to check the nameplate first.
I remember a call from a plant that replaced a failed three-phase motor with a 5 HP single-phase induction motor because it was cheaper and, in their words, “horsepower is horsepower.” By the time they asked us for help, that motor had already failed. The money they saved on the purchase disappeared quickly into reinstallation labor and lost production. (I’m mixing up the exact amount—it was either $400 or $600, but the lesson was identical.)
A reference number helps. NEC Table 430.248 lists the full-load current of a 5 HP, 230 V single-phase induction motor at 28 A—almost double the 15.2 A of its three-phase counterpart at the same voltage. That difference tells you the input side is not a small detail.
So what actually works in this scenario?
- If three-phase power is available at the motor: replace the single-phase motor with a three-phase, inverter-duty induction motor, and size the VFD as described in Scenario 1.
- If only single-phase power is available: still replace the motor with a three-phase, inverter-duty motor, and feed the drive from a VFD that is specifically rated for single-phase input. Follow the manufacturer’s derating table. At 5 HP, you will likely need a larger drive chassis than the “5 HP” label suggests. That is not a sales trick; the input current is simply much higher. Do not guess at this.
- If positioning is part of the requirement: skip ahead to Scenario 3.
Scenario 3: What You Really Need Is a Servo Motor Drive
Here is where the conversation shifts from “which VFD?” to “do you need a VFD at all?” If the machine describes its motion as “move to 250.0 mm and hold there,” or if it needs fast, controlled stops and starts every few seconds, you are not in VFD country. You’re in servo territory.
A VFD driving an induction motor is a speed-control tool. It does that well. But an induction motor has slip, and slip makes it a poor positioning device under a varying load. You can add an encoder and closed-loop vector control, but by the time you’ve done all that, you have built something with servo-like complexity and less-than-servo performance. I do not mean this as a criticism of VFDs; I mean it as a boundary. If positioning is the requirement, spec a servo motor drive.
For a screw-driven linear actuator, the servo motor and drive are selected by peak torque and RMS torque over the cycle, not by the horsepower of the motor being replaced. A 5 HP induction motor might give way to a servo motor with a smaller frame and much higher dynamic performance—or the converse, if the inertia of the screw and carriage is high. That is why I cannot honestly give you a “servo size” in this article. The data needed is different: load profile, gear ratio, screw lead, inertia, and required cycle time.
This is also where the Thomson linear actuator catalog becomes useful in a different way. If you are building a machine around a catalog actuator, the question is not how to bolt a servo motor onto something you already have; it’s which actuator package can be configured with the right motor, gearbox, and servo motor drive. That is a normal part of our work. But the first step is still defining what the axis must actually do.
How to Tell Which Scenario You’re In
If all you want is a defensible answer before you order something, answer these three questions:
- What does the motor nameplate say? Single-phase or three-phase? What voltage and FLA? If you’re not looking at the nameplate, stop and take a photo.
- What is the endpoint of the move? Is the machine asking for a speed (e.g., “run the conveyor at 80%”) or a position (“move to 250 mm, hold, repeat”)? Speed → Scenario 1. Position → Scenario 3.
- What is between the motor and the load? A belt, a gearbox, a ball screw, a coupling, an actuator? Screw-driven and belt-driven axes behave differently, and vertical loads add holding requirements.
If the answers put you in Scenario 1, the VFD sizing rule is the nameplate FLA plus the load profile. If they put you in Scenario 2, plan to change the motor before you buy a drive. If they put you in Scenario 3, start the servo motor drive conversation and bring duty-cycle data.
When a machine is down, I know it’s tempting to ask for a part number before asking for an application review. But in the last few years, I’ve watched more than one rush order turn into a second rush order because someone skipped the nameplate step. A photo of the motor nameplate, a 30-second video of the mechanism, and the cycle time will get you a better answer than a guess. That’s not me being difficult—that’s me trying to get you the right part the first time.
This article is general engineering guidance, not a substitute for the motor or drive manufacturer’s instructions. Always verify nameplate data and follow applicable electrical codes.