Three Magnet Rotor One: Lessons from a Pulse Motor That Ripped Apart

Hook

I finally got both circuits running together on my pulse motor and the torque is stupid for two little 5-oz coils. Then I remembered the time a three magnet rotor one went nuclear at almost 3000 RPM and bent a quarter-inch steel shaft.

Watch this experiment

I clip the meter to each coil in turn, spin the rotor by hand, and show how little push it needs once both circuits are live. You can hear it rip and see the shaft shake when it gets up to speed.

What you'll learn

How two separate batteries and coils add torque, why an off-center toroidal magnet causes vibration, and the exact failure that happened when I once built a thin rotor with too much magnet clearance.

From the bench

Both circuits are hooked up, each with its own battery and its own coil. One battery sits at 12.78 V. I move the multimeter between the coils and catch fast spikes. The 20-gauge and 30-gauge coils are paralleled on the drive side while the triggers are linked in series. Spinning by hand barely gets it going now; it used to take a much harder flick.

How it works / what we changed

I removed the second motor completely and started from scratch. The toroidal magnet in the center of the current rotor was glued a little off middle then zip-tied. At 2000 RPM that tiny error shows up as shaft shake. I figure above some speed it might level out but I’m not risking it after what I saw before.

Doing made rotor

Back when I was first learning CAD I made a rotor that was a little thin. I left too much space around the magnets. Once it hit almost 3000 RPM one magnet moved a tiny bit, the extra clearance let it pop free, and the whole thing came apart. That doing made rotor taught me to respect balance and glue placement the hard way.

Three-magnet rotor. one

The rotor that failed was a three-magnet rotor. one. All three magnets were glued in and the trigger was linked across them in series. It ran nicely in that configuration until the speed got high enough for the mechanical forces to rip it apart.

Rotor. one magnet

In the failure story it was a rotor. one magnet that first moved in its pocket. That single magnet had just enough room to shift, which let it pop out and destroy the rest of the assembly. One magnet path was all it took to start the chain reaction.

One magnet path

The one magnet path had been given too much clearance in that early thin rotor. Once the magnet started to move the extra space let it gain momentum until the magnet path popped completely out of the PLA, bending the shaft 60 degrees on the way out.

Magnet path popped

The magnet path popped literally broke the PLA and sent the magnet flying. The rotor the shaft which bent at a 60° angle. It was only a quarter-inch shaft but the force was enough to kink solid steel. That memory still gives me a little PTSD when I hear the current rotor get loud.

Magnet here awesome

With the current eight-magnet rotor I can see the interaction clearly. Even with only four magnets placed I had a one two three setup and the magnet here awesome interaction worked better in series than a simple one two. The triggers hit in a double-hit pattern that feels strong.

Doing made rotor

That same doing made rotor mistake changed how I build everything now. I never let an accident stop me but it definitely made me measure glue location and magnet clearance twice. The current rotor is much closer to balanced even if the center toroidal still looks a hair off.

Builder checklist

• Charge each circuit on its own battery before hooking up
• Glue magnets dead center then add zip ties while checking alignment
• Measure rotor thickness and magnet pocket clearance before first spin
• Link three triggers in series when using a short 100-turn coil to fire eight drives
• Start spins by hand and listen for shake before letting it rip
• Keep multimeter on one coil at a time to watch spike behavior

Troubleshooting

Shaft shakes at speed? Check the toroidal magnet in the middle for off-center glue. Rotor won’t fire all coils? Single short trigger coil can’t overcome eight drives in series; link three triggers instead. Meter shows nothing? Confirm battery is at 12.78 V and clips are solid.

Safety

These things can get out of hand real fast. I’ve seen a magnet path popped destroy a rotor and bend steel. Always wear eye protection, keep hands clear at high RPM, and never stand in line with the rotor plane when first testing a new build.

FAQ

Why does the rotor need almost no push now? Both circuits on separate batteries and coils give way more torque than I expected from two 5-oz coils.

What caused the old rotor to explode? Too-thin material plus extra magnet space let one magnet move, then the magnet path popped and the shaft bent 60 degrees.

Why link three triggers? The 100-turn 16-gauge coil is too short to trigger all eight series drives alone; three in series makes the trigger pulse long enough.

Is the toroidal magnet a problem? It’s glued a little off center so at 2000 RPM the shaft shakes. I’m watching it closely.

Related on this site

• How to Build a Pulse Motor: Parts, Coil Gap, First Spin
• Pulse Motor Rotor Design: Magnets, Materials and Balance
• Pulse Motor vs Bedini Motor: What's the Difference?
• Best Neodymium Magnets for Pulse Motor Rotors

Watch this experiment on YouTube

• ▶ Watch full on YouTube