There's a moment in every motion-control design where you ask yourself the brake resistor question. Mine came while speccing a small parts lift: a NEMA17 stepper, CAN-controlled, hauling 10 kg up and down on a 10 cm pulley. Line-powered. Big inertia. Four stops a minute.
Do I need a brake resistor, or will a fat bus capacitor eat the braking energy?
If you've sized one of these before, you know the reflex: braking energy is kinetic energy. Compute ½Jω², compare it to what the caps can swallow, done. Every forum thread does it this way. I started doing it this way.
The reflex calculation
500 rpm is 52.36 rad/s. The 10 kg load on a 10 cm radius reflects back as J = mr² = 0.10 kg·m². So:
E_kinetic = ½ × 0.10 × 52.36² ≈ 137 J
137 joules per stop. Substantial — already suspicious territory for a capacitor-only answer. But I was designing in a chat session with whatic, working the numbers line by line through its calculator (it refuses to do arithmetic "in its head," which I've come to appreciate), and the next line it set up wasn't one I'd asked for:
h_avg = v × t/2 = 5.24 m/s × 0.5 s = 2.62 m
E_grav = m × g × h = 10 × 9.81 × 2.62 ≈ 257 J
The part my intuition skipped
This is a lift. During that one-second deceleration, the load doesn't politely freeze — it's still descending, 2.6 meters on average, and every one of those meters hands its potential energy to the motor, which hands it to the drive, which has to put it somewhere.
Gravity contributes 257 J. Inertia contributes 137 J. The term I almost didn't compute is nearly twice the term everyone computes. Total: ~394 J per stop — around 26 W of average braking power, with ~394 W peaks during each one-second event. That is not a bus capacitor's job. That's a brake resistor, sized properly, with the driver's overvoltage clamp threshold and the resistor's pulse rating checked against their datasheets — which we did next, in the same conversation, citations and all.
No datasheet would have saved me here — and that's the point
Here's what makes this one interesting: the 394 J figure isn't printed in any component document. It falls out of the mechanics. But every decision it forces — the driver's overvoltage threshold, the bus cap's surge rating, the resistor's wattage and pulse capability — is a number buried deep in some PDF, and getting them wrong means the smoke arrives at the worst possible demo.
What I actually needed was a design partner that could hold the mechanics and the datasheets in the same conversation: run the energy balance honestly, then immediately pull the cited limits of the parts that have to absorb it. That's the whole reason whatic exists — it's not an EDA suite, it's the colleague who's read every datasheet and shows you the page number.
If your axis moves vertically, run the gravity term before the kinetic one. And if you want a second pair of eyes that cites its sources: whatic.io