Hand Crank Generator: Intro

I ran into a coworker who was giving away a bunch of electronics, and when I took a look, I found a BLDC motor! Let’s build a hand crank generator!

Motor or Generator

Normally, when you give a motor power, it spins a shaft. Interestingly, if you turn the shaft of a motor, most motor types actually generate power, just like a generator! The difference between a motor and a generator is what they’re optimized for; they both work on the same principle.

A BLDC motor has two components: the rotor and stator. The stator is a bunch of coils of wires, and pumping electricity through them causes them to generate a magnetic field, just like a solenoid. The rotor is the rotating shaft, and it has a bunch of magnets on it. When the motor is in operation, the coils of wire generate a magnetic field and force the magnets on the rotor to move to align with that field. The energized coil is then turned off, and then the coil next to it is turned on. The rotor will then rotate again to align itself with the new magnetic field. This process repeats over and over again, resulting in the rotor rotating.

If the motor isn’t powered, then turning the shaft (the rotor) will generate power. When the shaft is rotated, the magnets on the rotor move towards and then away from the coils of wires on the stator. This causes EMF according to Lenz’s law, which means the coils of wire will have a voltage difference between its two ends. With enough preparation, you can use that voltage difference to power something!

Why a BLDC?

There are several motor types available which could be used to generate electricity. For this project, I was looking for DC motors, since they’re smaller and easier to work with. The three main types of DC motors are brushed DC motors, steppers, and brushless DC (BLDC) motors. Even before I got the motor from my coworker, I was planning on using a BLDC motor. Why?

Brushed DC motors are great because of their simplicity for this project. You turn the shaft, and you get a rectified sinusoid out on its two terminals. Put a capacitor across it, and you’re done, right? The downside is that they, as their name suggests, have brushes. Inside the motor is a commutator, which wears out over time. To be fair, I probably won’t be using this generator all that often, but it’s still best practice to avoid brushed motors for that reason. Additionally, brushed motors would output single phase sinusoid, but I would prefer two or three phase power.

Stepper motors don’t use brushes, so no need to worry about the commutator failing. Even better, most stepper motors have two phases, which means spinning the stepper motor would produce two phase power (two sinusoids, 90° apart). Perfect, right? Well, two problems. One, stepper motors aren’t very comfortable to turn. By design, the shaft rotates in steps. That means when you try to turn the shaft, instead of one smooth motion, it’s a stop-and-go motion, which is pretty awkward and uncomfortable. That would also make it hard to turn the shaft quickly. Two, two sinusoids in quadrature is better than just having one phase, but surely there’s something better?

Enter the BLDC motor. It has none of the drawbacks I’ve mentioned previously. It doesn’t have brushes, so you don’t have to worry about wear. Its shaft turns smoothly, so you won’t have the jerky movement problem as steppers did. Best of all, almost all BLDC motors use three phases, which means it will produce three phase power! This means that you’ll have three sinusoid, all 120° apart:

The only problem with a BLDC is that it generates AC power, as opposed to DC, which is what almost all electronics use. We can fix that pretty easily using a three phase rectifier (which I call hex bridges) and a bulk capacitor:

Specifications

I managed to find the specs for the motor my coworker gave me. The most important spec is the one that tells me how much voltage I’ll get out of the motor for a given rotation rate of the shaft. In other words, how fast do I have to turn the shaft to generate (say) 5 V? The number we’re looking for is the back EMF coefficient:

According to this, I need to rotate the shaft at 1000 RPM to generate 6.64 Vp. Seems impossible, but let’s do the math. The back EMF constant, when put in revolutions per second, is 0.3984 Vp/RPS. Since the hex bridge uses peak to peak voltage, not peak voltage, the back EMF is doubled to 0.7968 Vpp/RPS. In order to generate 5 volts, we’d need to rotate the shaft about 6 times per second. I tried doing that, and that’s pretty hard to do. The fastest I can turn the handle of anything is about… 1 revolution per second. Does that mean this project is impossible?

Gear Train

Fortunately not! Through the use of gears, rotation speed can be increased substantially. When a gear rotates and is connected to another gear who’s diameter is twice as big, the larger gear rotates at half the rotation rate of the smaller one. Reversing this, rotating the large gear makes the small gear rotate at twice the speed!

I’ve decided to use gears to increase the rotation rate by a factor of 12. This means that rotating a crank once will cause the motor shaft to rotate twelve times. Using the back EMF coefficient and gear ratio, turning the crank at 1 revolution per second will generate 9.5616 volts across the bulk capacitor. That’s definitely usable if you put it through a buck-boost regulator!

Only problem is that using two gears to achieve a 12:1 gear ratio is impractical. This would mean the larger gear needs a diameter 12 times that of the smaller one, which means the larger gear takes up 144 (12^2) times more area than the smaller gear. Not very practical. Instead, I’ll make a gear train that achieves the desired gear ratio in several steps: a gear ratio of 2, then 2 again, then 3. This gives an overall ratio of 2 x 2 x 3. Let’s take a look at the gear train next time!