What Size VFD for a 5HP AC Induction Motor? A SEW-EURODRIVE Procurement Checklist

Start with the answer, then check it

For a standard 5HP, 460V, three-phase AC induction motor, a 5HP VFD is usually the right starting point. The full-load current is around 7.6A, and a good 5HP drive is rated to cover that. For a 230V motor, the current is roughly 15.2A, so the drive's output rating needs to be at least that high. But the word "usually" is doing a lot of work in that sentence.

This is a checklist for buying, not an engineering course. I'm a procurement manager, not a drive specialist. Over the past six years, I've tracked about $180,000 in drive and gearmotor purchases for our plant. That's not a huge number, but it's enough to show patterns.

When I first started buying VFDs, I assumed a 5HP motor gets a 5HP VFD and that's it. Then I spent a summer watching a new drive trip on overload because I'd ignored the motor's service factor and the conveyor's starting torque. The drive was the right horsepower. It wasn't the right drive for the load.

The 5-step VFD sizing checklist

From the outside, VFD sizing looks like a lookup table. The reality is that the motor's nameplate current, the gearbox, and your plant's actual conditions decide the purchase.

Step 1: Read the motor nameplate, not the brand

Voltage. Phase. FLA. RPM. Service factor. That's the minimum.

The SEW-EURODRIVE logo on a motor tells you who built it. It doesn't tell you what the motor draws electrically. Find the nameplate and write down those five numbers. If the nameplate is missing, stop and get the motor data from the original supplier before ordering. That sounds basic, but I've seen purchase orders go out with only a model number and a guess at horsepower.

Step 2: Compare the drive's output current, not the horsepower label

What I mean by that is the drive has to supply continuous current to the motor without overheating its own output stage. If you pick a drive whose output amp rating barely matches the motor FLA, you might be fine on paper and overloaded on the plant floor.

For a 5HP motor at 460V, the National Electrical Code full-load current tables put the motor around 7.6A. A 5HP-rated VFD usually has enough output current for that. But your motor's nameplate is the spec that matters. In Q2 2024, I compared three "5HP" drives for a conveyor motor. The continuous output current ratings varied by about 25%. Same HP label. Different headroom.

Step 3: Match the load type: constant torque vs variable torque

If this is a pump or fan, variable torque duty is fine. If it's a conveyor, mixer, or press, you need constant torque capability. SEW-EURODRIVE gearmotors are common on material handling equipment precisely because they're built for continuous load, but the VFD still needs to handle the worst-case torque at low speed.

Everything I'd read about VFD sizing said "match the horsepower." In practice, for SEW-EURODRIVE gearmotors driving loaded conveyors, the output current rating and overload capacity matter more than the HP label.

Step 4: Check the gearbox before you run at variable speed

If you're adding a VFD to an existing SEW-EURODRIVE gearmotor, download the current "SEW-EURODRIVE gearbox oil level chart PDF" before you send the PO. The gearbox has to survive the new speed profile, and oil level is part of that picture. According to SEW-EURODRIVE's documentation portal, lubrication intervals and oil levels change with mounting position and gearbox type. The chart exists because guessing isn't a maintenance plan.

I once used an outdated oil-level chart from a third-party site and filled a unit to the wrong level. We caught it during commissioning, but it cost us a few hours and an oily floor.

Step 5: Audit the wiring and environment

Long motor cable runs, voltage imbalance, heat, and enclosure type all affect VFD reliability. Some AC induction motors need an output reactor or a dV/dt filter when the cable distance is significant. This isn't a luxury add-on. I once skipped a line reactor on a 5HP conveyor drive because the budget was tight. The drive lasted fourteen months. The replacement taught me that hidden costs are still costs.

A quick sourcing note: verify the product

If you're buying a used SEW-EURODRIVE gearmotor or drive, check the actual nameplate. Genuine units carry a full type designation, a serial number, and a clean SEW-EURODRIVE logo. If you haven't looked at a real one recently, pull a reference image for the "sew eurodrive logo". A knockoff often uses a loose font or a blurred label. I learned this after a "discount" unit delayed a line startup by two days.

What about servo motors and drives?

If someone starts the conversation by telling you to forget VFDs and switch to servo motors and drives, ask for the motion profile in writing. Servo motors and drives are excellent for positioning, synchronized motion, and high dynamic response. For a simple conveyor or plant air handler running from an AC induction motor, they're usually more technology than the process needs.

To be fair, there are cases where a servo system is the right answer. But a 5HP fixed-speed gearmotor with a variable-speed request is usually not one of them.

Common mistakes I've seen on 5HP VFD purchases

• Matching horsepower only. The drive and motor both say 5HP, but the drive's output amp rating is lower than the motor FLA. Check the amp rating.
• Ignoring constant torque vs variable torque. A screw conveyor is not a fan. Size for constant torque or go one frame up.
• Forgetting the gearbox. If the gearmotor is SEW-EURODRIVE, look up the gearbox oil level chart and lubrication specifications. A VFD changes the operating range.
• Buying on sticker price before total cost. The cheapest drive on the shelf can cost more once installation, commissioning, and downtime are included. I've made that mistake; I don't recommend it.

So, what size VFD for a 5HP motor? Start with a 5HP drive, then verify with the motor FLA and load type. That's the honest procurement answer. The HP label gets you in the door. The nameplate and application details decide whether you buy that drive once or twice.