I Didn't Choose Wrong. I Chose Blind.

In my first year as a procurement engineer—2017, to be exact—I made a classic mistake. I was sourcing a motor for a new robotics arm prototype. The requirements seemed straightforward: high torque at low RPM, and it needed to fit within a NEMA 23 stepper motor frame size. I pulled up a maxon-motor catalog PDF, found a motor that matched the voltage and torque specs on paper, and placed an order for 50 units.

The result? A $2,300 mistake. Every single unit had to be re-ordered with a different winding. The ‘perfect fit’ on the datasheet turned out to be completely wrong for the application.

“When I compared my first motor choice side-by-side with the actual application requirements, I finally understood why torque and speed are not the only numbers that matter.”

Here’s what I learned from comparing a standard maxon dc motor datasheet against what I actually needed—and what you should look for when you're choosing between two similar-looking specs.

Dimension 1: The Torque-Speed Curve vs. The Single Data Point

I used to think that if a motor’s datasheet said “Rated Torque: 0.2 Nm,” that was all I needed. That was my first mistake.

When I compared the maxon brushless dc motor I initially chose (based on the single rated torque point) against a second motor that came with a full torque-speed curve, the difference was night and day.

The first motor (the mistake): The datasheet listed a stall torque of 0.5 Nm and a rated torque of 0.2 Nm. Perfect, I thought. My application required 0.15 Nm continuous.

The second motor (the correct choice): The same rated torque of 0.2 Nm, but the torque-speed curve showed that at my required speed of 3,000 RPM, the available torque dropped to 0.12 Nm—below my requirement.

I only believed in the importance of a torque-speed curve after ignoring it once and eating a $2,300 mistake. Now, I won't approve a motor selection without seeing that full curve.

Dimension 2: Speed Capability—How Fast Can a Stepper Motor Turn?

This is where things got interesting. The second dimension of my comparison was speed. My application needed a motor that could run at a low, constant speed of 200 RPM. I initially chose a NEMA 23 stepper motor because I thought “stepper = good at low speed.”

I went back and forth between the stepper motor and a maxon electric servo actuator for a full week. The stepper motor offered lower cost, but the servo actuator had better closed-loop control. Ultimately, I chose the stepper motor for cost reasons.

That was my second mistake. I had never asked myself: “How fast can a stepper motor turn while maintaining full torque?” The answer, in my case, was that it started losing torque at 400 RPM (much slower than I expected). The application required it to maintain torque at 500 RPM for short bursts. The stepper motor couldn't do it without a resonance issue.

“Seeing my rush order for stepper motors vs. the actual speed profile made me realize I had spent 40% more than necessary on artificial emergencies.”
If I had compared the speed-torque curves side-by-side from the start, I would have caught this before placing the order.

Dimension 3: Distributor Support & Documentation

Here's a dimension that’s rarely on the spec sheet but matters more than any number: distributor support.

Option A: I ordered the first batch of motors from a general industrial distributor. They had the part in stock, priced well, and shipped fast. But when the problem arose, their technical support could only say, “Check the datasheet.”

Option B (the recovery): I contacted an authorized maxon motor distributor for the replacement order. Not only did they have a different winding option I hadn't seen in my initial search of the maxon dc motor catalog pdf, but their application engineer asked me three simple questions: “What is your required continuous torque at your operational speed? What's your duty cycle? Do you need a holding brake?”

Those three questions would have saved me $2,300. The distributor didn't just sell me a part—they helped me choose the right one.

I have mixed feelings about this. On one hand, I'm grateful a good distributor exists. On the other, I'm frustrated that I didn't find them first. The lesson? A good distributor is worth more than a 5% discount on the motor, and a bad one can cost you 200% of the order value in rework.

Final Comparison: The Choice Framework

After this experience—and a few more that were smaller but still painful—I created my own pre-purchase checklist for any motion component. Here's the framework I now use:

When to choose a stepper motor (like NEMA 23):
- Your speed is below 300 RPM most of the time
- You don't need ultra-high positioning accuracy
- Cost is the primary constraint
- You can handle minor resonance issues

When to choose an electric servo actuator (or a maxon BLDC with encoder):
- Your speed requirement varies or crosses the 300-500 RPM threshold
- You need closed-loop control for position accuracy
- The torque-speed curve of a stepper can't cover your full operating range
- A failure at high-torque, low-speed would be expensive (like my $2,300 mistake)

My personal policy for working with distributors:
- If the order value is over $500, I call the distributor before placing the order
- I ask them: “Based on my application file, what would you change?”
- I verify the torque-speed curve at my specific RPM
- I ask for a written confirmation that the motor will work at my specific duty cycle

To be fair, I know engineers who have used the same maxon-motor part number for ten years without issues. They know their application, they know the motor, and they have the curve memorized. But if you're new to a project or the application is different from last time—don't trust the one-number summary on a spec sheet. Trust the curve. Trust the distributor. Trust the comparison.

Take it from someone who paid $2,300 for a lesson in reading datasheets: the comparison is always worth your time.