I'm a quality/compliance manager at a fluid power and drive company. I review every motor and drive shipment before it reaches customers—roughly 200 unique items in 2024. I rejected 6% of first deliveries last year due to nameplate, wiring diagram, or documentation errors. That context matters, because this article is going to be about specifications as much as motor types.
The question I hear most often goes like this: what size VFD for a 5HP motor? It usually comes up after someone has been comparing 3 phase AC motors vs an AC servo motor. Both can run on a VFD, but they're not the same machine. I've reviewed HYDAC products in both families, and I can tell you the choice comes down to the load profile, not the motor's frame size.
What This Comparison Is (and Isn't)
Let's be clear about the comparison. This is not a brand ranking. It's a direct look at two motor families commonly used in industrial equipment:
- 3 phase AC motors — induction motors supplied by three-phase power. With a VFD, you can vary the speed over a wide range.
- AC servo motor — a permanent-magnet synchronous motor with built-in feedback. It is designed for precise speed, position, and torque control.
I'll compare them on four dimensions: speed control, torque behavior, feedback, and VFD sizing. Each dimension ends with a practical conclusion.
Speed Control: Constant Speed vs. Dynamic Response
A 3 phase AC motor is a constant-speed machine by nature. At 60 Hz it runs near its rated speed. Change the frequency with a VFD, and you get variable speed—but the response is limited by the motor and drive's open-loop behavior. For pumps, fans, conveyors, and continuous processes, that is usually enough.
An AC servo motor operates differently. It has closed-loop feedback, so the drive knows the shaft position and speed in real time. That means faster acceleration, tighter speed holding, and more controlled deceleration.
Conclusion: If your process needs frequent speed changes or position-based moves, the AC servo motor wins. If you need 'set the speed and leave it,' a 3 phase AC motor with a VFD is often the better investment.
Torque: The Tempting Rule That Fails
It's tempting to think torque is just about horsepower or frame size. But for a 3 phase AC motor, torque is not flat across the speed range—especially on a VFD. The motor's cooling fan is mounted on the shaft, so at low speed, less air moves across the motor. Continuous torque can be dramatically reduced at low RPM.
An AC servo motor is designed to deliver rated torque from standstill up to rated speed, with extra peak torque for short periods. That makes it the obvious choice when you need holding torque at zero speed or quick torque spikes.
I've reviewed sizing sheets where the designer sized a 3 phase AC motor by horsepower alone. The math said it was 'big enough.' The actual low-speed torque said otherwise. That kind of mistake leads to a machine that stalls at the worst possible moment.
Conclusion: If you have constant torque at low speed, a standard 3 phase AC motor with a VFD may need a larger frame or external cooling. The AC servo motor avoids that low-speed derating problem.
Feedback: What the Motor Actually Knows
This is where the two motor families separate most clearly. A 3 phase AC motor is usually open-loop. The VFD sends frequency and voltage to the motor, but unless you add an encoder, it doesn't know the exact shaft position. You can build a closed-loop system with a vector drive and an encoder, but that's additional cost and wiring complexity.
An AC servo motor includes an encoder or resolver as standard equipment. The drive controller knows position, speed, and current at all times. If your application requires indexing, registration, or cut-to-length accuracy, that feedback is not optional.
Conclusion: Need position accuracy? AC servo motor. Just need speed? A 3 phase AC motor with a good VFD is usually enough. This is the cleanest A vs B split of the entire comparison.
What Size VFD for a 5HP Motor?
Now let's answer the practical question. The honest answer is: start with a 5HP-rated VFD, but don't stop at horsepower. Horsepower is output power, not current. The VFD must be sized to the motor's full-load amps (FLA) and to the load's starting torque.
A 5HP, 230V motor might have an FLA around 15–17 A. At 460V, it's roughly 7.5–8.5 A. If you use a 5HP VFD, verify its continuous output current is equal to or greater than the motor FLA. According to NEC Article 430, motor overload protection is based on nameplate current, not rated horsepower. That's a good discipline to follow for any motor and drive pair.
But here is where the misleading rule comes in. The load type changes the VFD size. For a centrifugal pump or fan, a 5HP motor with a 5HP VFD is usually fine. For a constant-torque load—conveyor, mixer, or compressor—the motor may need 150% torque during acceleration. A VFD rated for 110% overload for 60 seconds may trip before the motor reaches speed. In that case, you might need a 7.5HP VFD.
Don't hold me to a universal 'one size up' rule. I have mixed feelings about that advice. On one hand, oversizing a VFD is cheaper than replacing a drive after nuisance trips. On the other hand, it doesn't replace the torque calculation. Use the motor nameplate, the driven equipment curve, and the VFD overload rating.
So glad I double-checked an FLA nameplate before approving a VFD spec last year. The original quote followed a generic chart. The actual motor had a 1.0 service factor and drew 8.9 A at 460V. The matching VFD had just enough current capacity but no margin for overload. One spec change saved us from a startup headache. I should add that long cable runs and poor power factor also affect VFD sizing, so the nameplate is the starting point, not the final answer.
Which One Should You Choose?
Here's where a professional boundary comes in. A supplier who tells you one motor family is always better isn't helping you. I'd rather work with a specialist who knows their limits than a generalist who overpromises. In a machine design, the load determines the motor.
Choose a 3 phase AC motor + VFD if:
- Your load is continuous or variable torque, like pumps and fans.
- Speed regulation within a few percent is acceptable.
- You want lower installed cost and simpler maintenance.
Choose an AC servo motor if:
- You need position control, precision indexing, or rapid dwell moves.
- You need high torque at standstill or throughout the speed range.
- Machine production rate depends on dynamic response.
HYDAC's electric drive range includes both motor families, plus VFDs, gear reducers, and feedback accessories around them. The HYDAC logo on a product is supposed to mean traceability: a defined part number, a serial number, and a test record. I've rejected products that looked like HYDAC products but didn't have the documentation. The logo isn't decoration—it's the start of a paper trail.
For a 5HP motor, the right VFD size is the one whose output current matches or exceeds the motor FLA at the applied voltage—and whose overload rating covers the load's starting torque.
At least, that has been my experience across industrial machinery. No universal answer will replace reading the nameplate and the drive datasheet. But if you compare 3 phase AC motors and AC servo motors with these dimensions in mind, you'll ask the right questions before you buy.