I've been handling motion control orders for six years. In that time, I've personally made—and documented—14 significant mistakes. Roughly $38,000 in wasted budget, two missed production deadlines, and one very awkward phone call where I had to explain that the "precision" linear guide carriages I ordered had so much play you could wiggle the gantry by hand. Now I maintain our team's application checklist. This article is a slice of that checklist.
The number one question I get from customers is "what size VFD for 5hp motor?" It seems like a simple arithmetic problem. It isn't. The right VFD depends on whether that motor is a three-phase motor or a single phase induction motor, what kind of load it's feeding, and what you're trying to accomplish. The same logic applies to bigger choices: when to spec a Bosch Rexroth servo motor and when to stick with a fixed-speed motor, or when to invest in Bosch Rexroth linear motion technology and when a simpler belt drive gets the job done. There's no universal "best" – only "best for your exact situation." Let me walk you through the three main scenarios, and then give you a quick way to decide which one applies.
Scenario 1: The variable-speed request on a single-phase motor
This is where I made my first mistake. In 2017, a customer asked me to add a VFD to a 5hp single phase induction motor on a conveyor. I looked at the motor nameplate, saw "5hp," and picked a 5hp VFD. The drive lasted a day. Standard VFDs are designed for three-phase motors. Feed a single-phase motor through a standard three-phase drive and you'll get overcurrent trips on startup, overheating, or both. The single-phase motor draws significantly more current, especially under load. That first wrong drive cost me $400, plus about $1,100 in lost production and a $90 expedited freight bill. It hurt.
Here's what I should have done: read the full-load amps (FLA) off the motor nameplate, not the horsepower. For a 5hp single-phase motor, you'll often see an FLA of 20-24 amps at 230V, while a comparable three-phase motor might draw 13-15 amps. The VFD has to be selected based on output current, not horsepower. On single-phase input drives, you also need to check the input current rating. Many "economy" drives under-rated for single-phase input will blow their rectifier diodes within a year. If you're sizing a VFD for a 5hp single-phase motor, the conservative answer is a 7.5hp VFD with single-phase input capability, or a three-phase motor swap plus a 5hp VFD. (Yes, I recommend swapping the motor. Usually the cheaper total cost of ownership.)
The contrarian advice here: sometimes you shouldn't use a VFD at all. Many capacitor-run single-phase induction motors are not compatible with VFD frequency control. The torque drops, the motor runs hot, and you shorten its life. If the process only needs two or three speeds, a mechanical gear reducer or a two-speed motor might be a better investment. The VFD is not always the answer—it's just the first thing you think of.
Also, check the motor's insulation and bearing design. According to NEMA MG1, inverter-fed motors need to handle voltage spikes generated by VFDs. An old motor not rated for inverter duty may fail prematurely, especially if the cable between motor and drive is long. If you insist on keeping the single-phase motor, at least use a drive with an output reactor or filter. I lost a motor bearing to that exact problem in 2020. (Note to self: never skip the output filter on a long cable run.)
Scenario 2: New machine, need positioning—servo or stepper? Ball screw or belt?
Here's where people split into two camps. One camp thinks you always need a closed-loop servo and a ground ball screw. The other camp thinks a stepper motor and a toothed belt will handle anything. Both are right—for completely different machines.
In September 2022, I ordered 12 linear guide carriages for a pick-and-place gantry. I specified the standard "RM" type from a catalog without checking the preload class. (That was the mistake—assuming all carriages are the same.) The carriages had zero preload, so the gantry had several thousandths of an inch of lateral play. On a line designed to place components with 0.1mm accuracy, every placement was slightly off. We spent three days troubleshooting the servo loops before someone measured the mechanical backlash. 12 carriages, $4,700, dead wrong for the application. They all went into a bin.
If you're looking at Bosch Rexroth linear motion technology, you'll see they separate linear guide carriages by accuracy class, preload, and load rating. For precision positioning with high acceleration, you want preloaded carriages (usually "C1" or "C0" preload) and a low-backlash drive—a ball screw with angular contact bearings, for example. A Bosch Rexroth servo motor is a strong fit in that world because it can deliver the high acceleration and precise indexing that precision processes need.
But if your machine is a high-speed transfer that just needs to drop a part into a bin within ±0.5mm, a belt-driven actuator with standard non-preloaded carriages will be lighter, faster, cheaper, and easier to maintain. The precision-grade stuff would actually hurt performance by adding inertia and cost. I've seen designers put a 20mm-pitch ground ball screw on a fast labeling machine and end up with a bottleneck. The labels still came out slightly crooked—because the actual error wasn't in the drive. It was in the unsupported carriage. So the lesson: precision doesn't mean "best". It means "matched to the process."
The preconception that "ball screws are always more accurate" comes from an era when ball screws were exotic and cheap stepper drives didn't exist. Today, a well-supported belt drive with an inline encoder can match the accuracy of an unloaded ball screw for a fraction of the cost. You just have to know your real tolerance.
Scenario 3: Slapping a cheap VFD on an existing motor to save energy
This is the "budget upgrade" mistake. A lot of plants think a VFD will instantly make a motor efficient and pay for itself. It can, but only if sized right and matched to the load. Let me put some numbers on it.
We tested a $149 "economy" VFD from an online retailer on a 5hp three-phase motor driving a centrifuge. It worked for two weeks. Then it started tripping on overcurrent every startup. We replaced it with another $149 unit. That one died in a month, and it took a motor bearing with it. The motor repair: $380. The labor: $540. Downtime: 6 hours at $750/hour. Total damage: over $5,000 for a $149 piece of equipment. A proper industrial VFD, like the ones Bosch Rexroth offers, would have cost us $700 at retail and would still be running today. The lesson isn't "buy expensive." The lesson is "calculate the total cost of failure before you buy cheap."
And the cheap-drive issues aren't just about failures. Cheap VFDs often lack the software features you need—like field-oriented control, dynamic braking, or even a simple RS-485 port for the PLC. You end up buying extra hardware or a replacement drive. That's the hidden cost of saving $500.
For the specific question "what size VFD for 5hp motor", here's the practical answer:
- For a 3-phase motor, a 5hp VFD is usually correct, provided the drive's rated current is at least the motor's FLA. If the motor is at the end of a long cable (>50m), go one size up.
- For a single phase induction motor, you need a VFD rated for single-phase input and at least 1.5-2x the motor's FLA, which often means a 7.5hp frame.
- For a high starting torque load (conveyor, compressor, pump), go one size up regardless, to avoid nuisance trips during acceleration.
Don't oversize beyond one frame unless your load has huge inertia. A VFD that's too big can cause overvoltage trips during deceleration, especially if the load back-drives the motor. I've tripped that breaker too. (Not my proudest moment.)
"The cheapest VFD you can find will cost you three times its price in downtime and repairs." — my plant manager, after the third failure.
How to tell which scenario you're in
I don't want to just give you three scenarios and let you guess. Here's the 3-question check I run with every new project:
- What exactly does the motor need to do? If it just needs on/off at a fixed speed, you don't need a VFD. If it needs adjustable speed, write down the speed range. If it needs precise position control, you're in Scenario 2.
- What is the positional tolerance? If you need repeatable positioning under ±0.1mm, go with servo + preloaded carriages + ball screw. If ±0.5mm is fine, a belt drive with standard linear guide carriages will save you money and time.
- What's your duty cycle? High start/stop and high acceleration (>20 cycles/min) demand servos and preloaded bearings. Low start/stop duty can tolerate simpler, cheaper components.
If you're still unsure, get the motor nameplate data and the machine's torque-speed curve. Send it to a supplier's applications engineer. The cost of a 30-minute phone call is nothing compared to a mis-specified component.
I still kick myself for not asking those basic questions early. One of my biggest regrets: not documenting the load inertia on a gantry before picking a servo motor. The motor was half the size it needed to be, and we spent $2,200 on a replacement plus express shipping. That lesson alone paid for the checklist I now use.
So when you search for "bosch rexroth servo motors" or "linear guide carriages," don't just compare spec sheets. Compare your application against the scenarios above. The right answer is the one that matches your machine's real demands—not the one that looks best on a brochure. If you take one thing from this article, let it be this: measure the load, question the assumptions, and check the preload. Your future self (and your budget) will thank you. And if you're browsing the bosch-rexroth website for technical data, remember: the part number tells you the preload class—believe it.