DC generator & programmable load

[mojoe]

I'm going to use a hoverboard motor as a generator on a stationary bike. The motor will be followed by a 3-phase rectifier. I'm wondering about the best way to test this with my Maynuo 300 W load. I know that the motor output voltage will increase with RPM. Which mode on the Maynuo would be best, Constant Voltage, Current or Power?

The motor is rated 24 V, 400 W. I'd like to see how much I can really get out of it as a generator.

I want to charge a 12 V LiFePo battery. If I can get enough voltage from the generator, then I assume that an MPPT controller could be used. If not enough voltage, then I'll have to use a boost controller.

This is mostly a fun and learning project, although it would be nice to actually get enough power to charge the battery without spending all day on the bike. I love riding, but prefer to do it outside when the weather cooperates. Right now, it's too cold and windy.

[mjkuwp]

I would go with constant current but you can learn something from all modes.  Keep an eye on the voltage to make sure you don't go above what your load can support. I suppose your electronic load may be able to protect itself.

Eventually you will need some kind of switching circuit to knock the voltage down to 15V or whatever you want as the nominal input to your charge circuit.

you may want to spin your generator with a drill or any similar tool and measure the voltages that come out to gauge what kind of speed-up you need from your bike.  you did not mention how you plan to couple these things.

[moffy]

You need to know what the V/RPM is so that you can work out the gearing ratio for your bike.

[mojoe]

I'll measure V/RPM later. Right now, I'd like to know the best way to measure things with my DC load.

I'm doing one step at a time. I should have the 3-phase rectifiers in a day or two, so I can setup a test jig.

I just picked up a 36 V hoverboard for $20, so if the 24 V motors won't generate enough voltage, the 36 V motors should.

[mojoe]

I'll try CC, first. I figured that either CC or CP would be best.

I thought I'd go with an inexpensive solar MPPT controller. At first thought, I would assume that a wind controller would be preferable. The ones I looked at divert power to a dump load above a preset voltage, so I'll go with a solar controller.

As for using a drill, that is exactly what I am going to use, once I have my test jig setup.

Although I said exercise bike in my OP, I'm really going to use one of my riding bikes. I'm looking for an inexpensive bike trainer stand that I can adapt. To start with, I'll use a friction drive. Later, I may explore using a belt or chain, but that is more complicated.

[mojoe]

On these hoverboards, does anyone know if the motor power specified is actually per each motor, or is it the usual marketing lies, where the rating is split between motors? IOW, is it really a 300 W motor, or a 150 W motor?

I've tried searching on the long part number marked on each wheel, but nothing useful comes up.

[schmitt trigger]

I would strongly suspect, that it would be the total, not individual, power.

My suspicion is based on my experience with vendor’s chicanery.
I could be wrong, though.

[mojoe]

I just did a search on AliExpress for hoverboard motors. It looks like the wattage rating is per wheel. All the listings for a single wheel show comparable wattage ratings. So, not marketing lies, after all. Who would have guessed?

For the 36 V hoverboard I just got, some websites say it is 400 W, but most say 300 W. I'll go with 300 W. More than enough for what I want, assuming I can get anywhere near that much power output.

Edit: except for the voltage, the rest of the part number on all these wheels looks like gibberish to me. I'm sure it means something to the manufacturer, but I don't find any information.

[mojoe]

An update if anyone is interested. On all four motors, I removed the cover, cut and removed the wires for the Hall Effect sensors (pulled them all the way out). While inside, I wiped clean and re-lubed the bearings. The 24 V motor bearings were very dirty, probably due to hard use (I previously cleaned dirt and hair from the outside shaft). They all do seem to spin a bit easier by hand, now (or it could be wishful thinking).

Next, I tested to make sure all three phases of each motor was working. All good. Spinning the rubber tire by hand, while holding the shaft in the other hand, the 24 V motors gave about 5-6 VAC. The 36 V motors gave about 9-10 VAC. I did expect that the 36 V motors would be higher.

The older 24 V motors are all metal, and rather heavy. The 36 V motors are only metal where absolutely necessary. The rest is plastic. Needless to say, they are noticeably lighter.

So now I have four working hoverboard motors, ready to be used as generators. Once the 3-phase rectifiers arrive, I'll rig up something to drive the motors at high speed, and see what I get on my Maynuo DC load.

I'm still looking for a used bike trainer stand. There was one in a local thrift store two months ago, but I passed on it, not thinking I had a use for it at the time. Of course it was gone when I looked, today. I visited some other thrift stores, but no luck. Maybe Craigslist will turn one up.

$40 for the two hoverboards, and a few hours disassembly and cleaning. So far, lots of fun :-)

[mojoe]

One more test.

I placed my cordless drill against the rubber tire of both types of motor. Measuring just one phase, I get a constant:

24 V motor = 8.0 VAC
36 V motor = 12.8 VAC

The Ryobi drill is spec's for a maximum of 1750 RPM. Calculating the ratio of the two diameters, it looks like I may have been spinning the motor wheel at about 430 RPM, assuming the drill was maxed out.

That works out to roughly:

24 V motor = 54 RPM per Volt
36 V motor = 34 RPM per volt

Allowing for some losses (guessing at this point), I'd need these speeds to charge a 12 V LiFePo4 battery:

24 V motor = 900 RPM or more
36 V motor = 580 RPM or more

Edit: Looking at this later, I realized that the ratio of the motor voltages is 1.5, and the RPMs is the same (allowing for inaccuracies in testing). Considering that both motors are the same physical size, this makes sense.

Using a bike with gearing, I'm not sure what speeds I'm likely to get. With the bike tire being much larger than the motor tire (friction drive for now), I  don't think it will be a problem for the 36 V motor. I'm not sure about the 24 V motor.

More testing when the rectifiers come.

[mojoe]

The 3-phase rectifier showed up. With a slightly more stable setup, but still using the cordless drill to spin the 36 V motor, I can get 17.5 VDC @ 1.6 A = 28 W with no load.

When driven by a much larger bike tire, and using the bike gears, I should be able to generate much more power at a comfortable pedaling cadence. As this project progresses, we'll see what I actually get. I saw a YT video where a guy got over 400 W for a few minutes. Not sustainable, but fun to watch.

At this point, I'm not going to bother with the 24 V motor. Once I get a complete setup, and some measurements from the 36 V motor, I may swap in the 24 V motor just to see if it produces enough power to be usable.

[mojoe]

While waiting for other items, I thought I'd do a little experiment. I hooked the output of the rectifier to a ZK-4KX buck-boost controller, set to output 12.0 VDC.

As expected, with no load on the output, the motor spins easily with the drill pressing against the wheel, and I get a nicely regulated 12 VDC output.

Adding a 16 Ohm power resistor to the output, the motor is a bit harder to spin. I get 12 VDC @ 0.75 A = 9 W. Using an 8 Ohm resistor, I can't spin the wheel fast enough with the drill to get a stable 12 VDC output from the buck-boost controller.

Earlier, with no load on the motor, I was able to get 23.3 W, about 2.6 times more. Part of this is due to the efficiency of the buck-boost controller, but mostly the inability to spin the wheel fast enough with a load attached. I should be able to do much better with a bike and gearing.

[PCB.Wiz]

Earlier, with no load on the motor, I was able to get 23.3 W, about 2.6 times more.

I'm curious how you get watts out, whilst claiming it is without any load ?
Perpetual motion should be the next step...

[mojoe]

To clarify for those who don't get it - Just like with a solar panel, I measured the voltage open circuit, and the current short circuit. Solar panels are rated this way, and don't rely on perpetual motion.

[mojoe]

The bike trainer stand arrived, but it is damaged. I'll need to get a replacement.

I just logged some data using a PZEM-003 module, while hand cranking the 36 V hoverboard motor as fast as I could. Using the 3-phase rectifier, the DC load was a 55 W automobile headlight bulb.

It's not easy spinning this by hand, but the highest values I got were: 5.16 VDC @ 2.84 A = 14.65 W. The bulb did glow a reasonable amount.

Hopefully, I'll get the replacement trainer stand soon, so I can fabricate some type of mount for the motor. Then I'll be able to see how much power I can actually produce.

[Andy Chee]

To clarify for those who don't get it - Just like with a solar panel, I measured the voltage open circuit, and the current short circuit. Solar panels are rated this way, and don't rely on perpetual motion.

Generators cannot be rated in this manner.

[Langdon]

You need to know what the V/RPM is so that you can work out the gearing ratio for your bike.

A higher gear will always work better, it your legs can take it.

[Kleinstein]

To clarify for those who don't get it - Just like with a solar panel, I measured the voltage open circuit, and the current short circuit. Solar panels are rated this way, and don't rely on perpetual motion.

This is not how solar panels are rated - they use the maximum power that the solar panel can deliver with 1000 W/m² of light and usually AM1.5 spectrum.
Usually this is something like 10% less than open circuit voltage times short circuit current.

For the motors one must take into account that the power rating is for use as a motor, so electricity in. This can include quite some losses, even though under normal operation the efficiency can be relatively good. The point to look for is a maximum current the motor can stand. This than translates to a maximum torque. The actual power than depends on the speed you get. The 24 V / 36 V are more nominal voltages - the motor will also go faster / slower with different voltage. For the mechanical power going in the current times unloaded voltage at that speed way is OK. The voltage drop from loading is to a large part from the heat loss in the generator, though there is also magnetic field weakening that reduces the torque.

To simulate a bike ride a simple resistor load may be better than constant current.