Introduction: Brushless DC Motor
Let's make an electric motor that spins using neodymium magnets and wire. This shows how an electric current is converted into motion.
We’re building a primitive brushless DC motor. It’s not going to win any efficiency or design awards, but we like to think a simple example makes it easier to see what's going on.
-(2) neodymium magnets
-Rotor (we used a 608ZZ bearing)
-Electronics - Reed switch, transistor, flyback diode, 20ohm resistor, LED, 6V DC power supply. We used 4AA batteries in a battery pack
Step 1: DIY Rotor
The spinning part of an electric motor is called the rotor. Most brushless motors have permanent magnets on the rotor.
Our rotor spins thanks to a 608ZZ bearing stuck on a pencil. This bearing is commonly used in things like skateboard wheels and fidget spinners.
We stuck two 1/4" x 1/4" x 1/8" B442 neodymium magnets on the outer edge of the bearing, 180 degrees apart from one another. Both are oriented with their north poles facing out. This is different than most BLDC motors which have alternating poles facing out. This simplification made our electronic circuitry a bit easier.
Step 2: Get Moving!
How do we get this thing spinning? We could just flick it with our finger, but we're looking for a magnetic push. Bring another magnet near one of the rotor magnets, with it's north pole facing the north pole of the rotor magnet. This will cause the magnets to repel, or push, setting the rotor spinning.
If we push on the magnet hard enough to spin the rotor halfway around, we can do it again to the next magnet. If we were fast enough, we could keep putting the magnet close and taking it away, spinning the rotor continuously.
This is where the electronics come in. We need to create an electromagnet that turns off an on, pushing the rotor magnets.
Step 3: Electromagnet
A simple electromagnet consists of a coil of magnet wire wrapped around a steel core. We used 24 gauge, single strand copper magnet wire with a thin, enamel insulation. A bolt became the steel core.
When we apply a voltage to it, it becomes a magnet. With the electromagnet positioned just right, it should push the rotor's magnet away. Now all we have to do is turn it on and off at just the right moment.
We want to turn the electromagnet on just after one of the rotor magnet passes the bolt, to push it away. After a little bit of travel, say 30 degrees or so, it should turn back off. How can we do this switching electronically?
Step 4: Magnetic Sensor
We chose a reed switch to tell us when the magnets are in the right position. A reed switch is a glass-encased sensor, where two ferromagnetic leads are almost touching one another. Apply a magnetic field to the sensor with just the right magnetic strength and direction, and it causes these two leads to touch one another, making electrical contact and completing the circuit.
With the reed switch positioned as shown, it makes contact only during the correct portion of the rotor's rotation.
Step 5: Final Circuit - Improved
While the simple reed switch setup worked briefly, we quickly ran into problems. We were running a lot of current through that reed switch and it welded the two contacts together. This is because we were essentially shorting out the batteries.
To fix this problem, we added a transistor. Instead of having all of the electromagnet's current go through the reed switch, we used the reed switch to trip the transistor on and off, so the current goes through the transistor instead. A transistor is basically an on-off switch that can handle a bit more current.
The final setup also includes a diode to prevent backflow from the electromagnet. This is called a "Flyback Diode", which prevents the current from frying the transistor when it turns off.
Step 6: Watch It Run!
With the electromagnet switching on only through a small portion of the rotation, the rotor spins continuously! Check it out in the video.
We added an LED that lights up when the electromagnet is activated to help visualize what’s going on.
In the chart, you can see the measured voltage across the coil, turning on and off!
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