Generator Add-on for Kickr Core Smart Trainer

Using my wahoo kickr core smart indoor trainer for quite a while now I often thought about converting all those watts that I put into my training for something useful. At least something more useful than just producing a noticeable amount of heat. The sad thing about it is, that the electro magnetic brake on this device even needs energy to waste my energy, on top I have two fans running, a laptop with Zwift and my phone. So the whole thing is a enormous waste of energy and the situation is completely unsatisfying. On the other hand it's an easy calculation to see that even in my better training weeks I do not produce a single kilowatt-hour of electricity not to mention the amount of food I have to eat for that. But maybe it would be able to produce at least the amount of electric energy that I need to drive the whole setup. Even an off-grid application is at least thinkable. So I started this little project. There are several attempts flying around which show very good solutions for this problem like this zwift capable pedal generator or this modified spinning bike which proves that a bike generator can produce some serious amount of electric watts. Since I have not much space left in my "pain cave" I wanted a small add-on to my kickr core without the need of modifying it in any permanent way, so here is what I did:

• Brushless DC-Motor used as a generator. Try to get one with a very low KV-Value. The KV Value tells you how much rpm you need to produce a certain voltage, so lower is better. I got one with 270KV which works in my case. It is limited by the price you are willing to pay.
• 3phase rectifier using 6 Schottky diodes 30SQ50. These diodes have a very little voltage drop of just a few hundred mV which makes them a much better choice than the normal silicon diodes even though they are a bit pricier.
• stepdown DC/DC converter that produces a stabilized output voltage. In my case I use a dps5015 which has a serial interface that can be used to control the output power by zwift using an arduino bluetooth adapter. It also can be used to charge a battery directly due to it's programmable voltage and current cutoffs. Of course you can choose a much simpler device to get the job done.
• A simple metal plate from your local hardware store with some holes to attach the motor to my bike
• A toothed belt pulley which fits the axis of your motor (8mm in my case) with a reasonable diameter (40mm in my case) and a belt with a length of about 850mm.
• some rubber band to put around the flywheel of the kickr core

Attach the motor to the mounting plate (I did it with a single screw) and then clamp the plate to jour bike axis using the quick release and the big hole in the metal plate. Put the toothed belt pulley on the motor axis and fix it. Put some rubber bands around the flywheel of the kickr to prevent belt to damage the paint or stickers and prevent the belt to slip. Last but not least put the belt around the flywheel and the belt pulley and put a nice tension on the belt using the quick release. Now turning the pedals should let the motor spin very fast.

So in case you want to pedal without the generator you just have to untighten the belt so you can remove it and there you go. Done in a second.

Solder the 6 diodes to the three cables coming from the motor in a way that it makes a three phase bridge rectifier. Add a cable that is capable to carry some amps to the dc/dc converter

Since the belt adds an 1:5 gear ratio between flywheel and motor it's easy to produce some 18 Volts DC even while pedaling on the small chainring. The oscilloscope shows my cadence converted into voltage oscillations (btw: this gives you an idea how smart trainers retrieve your cadence without a cadence sensor). It's then converted to constant 12 Volt. I found a nice 12 Volt to 65W USB-C converter in on of my junk boxes which is a nice way to connect my laptop, phone and charge some USB-Powerbanks while pedaling. I also will find a 230V inverter in another of my junk boxes to connect the fans and the kickr core itself.

When I pedal without any electrical load I need about 30 watts on the pedals due to losses in the drive train, in the motor and the AC to constant 12V conversion. This seems to be relatively independent of the total power output. If I have some devices connected to the USB-C ports that pull 69.4 watts from the dc/dc converter I have to pedal at 97 watts, measured by my powermeter. So efficiency is about 70% at that power level. When drawing more power it will be better due to the constant part of the power losses. So if I pedal at about 200 watts for longer time I can expect to produce about 150 Watt of electric power. Not too bad.

As I've written before the dps5015 is capable to be controlled directly by Zwift. So if I had a bigger (and not fully charged) battery which can eat up a few hundred watts I could let the kickr be switched off and let the dps5015 control the resistance or output power. In a setup with limited power sink like in my case I do something different: I have a base resistant produced driving my setup through the dc motor. The power at the pedals is measured by my powermeter. Zwift then controls the kickr to meet the desired watts or resistance by adding the electromagnetic brake. Of course changes in resistance are not as smooth and quick as you might be used to but it works.

Noise: the belt has teeth which is actually not really necessary. I found this belt in my junk box, so another belt might be a better choice since this thing produces a nice and annoyingly loud noise. The motor rotates fast enough when I pedal on the small chainring and you definitely don't want to switch to the big chainring because of the noise. That sucks.

Buffer battery: even if you don't want a big battery a small buffer would be nice because when you stop pedaling or you pedal to slow all the lights go off and you can literally feel it in your legs when a power consumer is switched on or off.