What Size VFD for a 5HP Motor? A TECO Electric & Machinery Field Story

The 6:40 phone call

At 6:40 on a Tuesday in March, my phone rang. It was the plant manager, and his voice had that tight sound that can only mean one of two things: a safety problem or a line down. It was line 3. The labeler had stopped hard, and the gear motor under it had that burnt-electrical smell you never forget.

Line 3 was our bottleneck line. If it stopped, within twenty minutes we had a dozen people standing around. That day, a truck was scheduled to pick up a large order by 2:00 PM. If the line wasn't running by noon, we'd miss the loading window and trigger a $12,000 penalty clause. I've handled a lot of rush orders in nine years of maintenance procurement, but this one had a clock on it.

Why a gear motor suddenly mattered

Repairing a burned gear motor on site is possible, but not when the motor has eight years of grease and a partially stripped input shaft. Replacement was the only realistic path.

A local distributor had a 5 HP TECO gear motor in stock. Searching “teco-electric” on their site pulled up the motor and the drive in one list. TECO Electric & Machinery has been making motors since 1956, and honestly, their gear motors are among the top gear motors I'd put on a packaging line. That part was easy. The problem was the drive.

The old motor had been running across the line at a fixed speed. For months we'd talked about adding a variable frequency drive to handle different bottle sizes and reduce the slam at startup. The motor failure turned that someday conversation into a same-day requirement. We needed a VFD before the distributor's warehouse closed.

The search that morning

I typed “what size vfd for 5hp motor” into my phone while walking to the panel. The search results gave me the usual mix of forum posts, vendor pages, and one very confident answer: buy a 5 HP VFD. It sounds simple. It isn't.

Most buyers focus on horsepower and completely miss full-load amps. The question everyone asks is “Is a 5 HP VFD big enough for a 5 HP motor?” The better question is “Does the drive have enough output current for the motor under real load?”

Horsepower is a starting point, not a sizing answer. The nameplate tells the truth.

The almost-order

We found a 5 HP drive in stock two towns over. It was rated 7.6 continuous amps, and the motor nameplate showed 7.4 amps at 460V. That's a match, technically. I had my finger on the order button.

Then Art, our senior maintenance tech, asked to see the nameplate. He read it out loud: “5 HP, 7.4 amps, service factor 1.15.” He looked at me. “What's the drive rated for continuous current?” I told him 7.6. He didn't say anything, just raised an eyebrow.

Art was right. A labeler doesn't run at nameplate load. It has an indexing mechanism that stops and starts hard, and it jams occasionally. Our plant voltage runs high during off-peak hours, which means the motor can draw more than 7.4 amps. The service factor gives the motor a little overload room, but the drive didn't have any. On a hot afternoon with a jammed label, that 5 HP drive could trip, and we'd be back where we started.

The clock was not our friend. The distributor had a 7.5 HP drive at another warehouse, but it would take two hours to reach us. That was cutting it close. Still, it was the only option that didn't worry Art.

I don't have hard data on how many VFDs get undersized because someone simply matched horsepower numbers. But based on repair tickets I've seen over seven years, my sense is that a good chunk of premature drive failures trace back to sizing that looked right on paper.

The better call

Had about 45 minutes to make the call before the distributor closed. Normally I'd call the rep, pull the spec sheet, maybe ask an application engineer. There was no time. I looked at Art and said, “7.5.” He nodded.

We ordered a 7.5 HP VFD from the same TECO line. According to the published specs, its continuous current rating is about 11 amps, which gave us a comfortable margin over 7.4. It cost about $260 more than the 5 HP drive. In my head, the math was simple: $260 extra versus a $12,000 penalty and a very long week. It was a no-brainer.

I get questions about brushless motor retrofits all the time. Brushless motors are great for positioning and high-speed precision. This labeler didn't need any of that. It needed torque at low speed and the ability to stop without tearing labels. An induction motor with a properly sized VFD was the right tool. A brushless motor would have been overkill, and it would have pushed the timeline past the truck window.

Of course, it didn't go perfectly. The 7.5 HP drive came from another warehouse, and the delivery driver got stuck behind an overturned semi. The drive showed up at 11:15. We had the wiring done by 12:40. The labeler was running by 1:05, and the truck left at 1:50.

So what size VFD for a 5hp motor?

If someone asks me that now, here's what I say: For a basic induction motor on a fan, a pump, or a light conveyor, a 5 HP drive is usually enough. For a constant-torque load, a machine with hard starts, or a motor nameplate that's hard to read, I go one size up. A 7.5 HP drive on a 5 HP motor is not a problem if you configure the motor parameters correctly. Oversizing a VFD by one step is not a sin; undersizing it by one step can be an expensive mistake.

According to NEMA MG-1 tables, a 5 HP, three-phase, 460V motor has a full-load current around 7.6A. The drive's output current needs to at least meet that, and then you add margin for load, voltage, and starting conditions. The NEMA table gives you the average. The nameplate tells you the actual motor in front of you.

That's the part most people miss. They measure the shaft or compare prices, and they never look at the amps. It's the same mistake I almost made at 9 AM on a Tuesday because I was rushing.

The lesson that stuck

After that morning, I made a twelve-point VFD procurement checklist. It starts with a photo of the motor nameplate and ends with a phone call to someone who has installed the drive before. That checklist has saved us an estimated $8,000 in potential rework and rush fees since then. Five minutes of verification beats five days of correction.

The replacement motor and drive are still running fourteen months later. We run the labeler at 70% speed for most SKUs now, which looks better, labels better, and our power meter says it draws less than it did before. We didn't plan that improvement when we ordered the gear motor. It happened because we stopped thinking about the replacement as just a motor swap.

So if you're reading this because you typed “what size vfd for 5hp motor” while a line is down, take a breath. Find the nameplate. Check the full-load amps. Check the drive's continuous current rating. And if you're not sure, go one size up. An extra $260 is a lot cheaper than a missed truck, a broken labeler, and a very painful explanation at the Monday meeting.