Creating electrical power on your own is a
pretty marvelous feeling I would say.
That is why I built this hand cranked generator
out of a DC motor during a previous project
video of mine.
By utilizing this 12V 21W light bulb as a
load hooked up to my energy meter and the
generator, we can see that with the help of
the hand crank, I can create around 2W of
power which is certainly enough for emergency
situations but not to power anything useful.
Now of course we could cheat by using an electric
drill to turn the generators shaft faster
and thus create around 15W of power.
But then again the drill is powered by electric
energy so by using it, we would basically
just waste energy.
Thankfully though I got a bicycle, which after
lifting up its back wheel, can also reach
a high rotational speed but this time through
muscular strength.
The only question left was, how can we hook
up this DC generator to the wheel and the
answer is that we would have to build a holder
for it and also come up with a kind of adapter
for the shaft that accepts a bicycle chain
and that sounded like a lot of work.
But luckily I recently got myself this three
phase induction motor, which due to its size
and construction does not need a motor holder
and you can also easily buy a gear wheel for
its shaft which kind of work with bike chains.
So in this video let’s find out how easy
it is to use such an asynchronous induction
motor as a generator!
Let’s get started!
This video is sponsored by JLCPCB!
Feel free to visit their website JLCPCB.com
to not only find out what awesome PCB and
Assembly services they offer but also to easily
upload your Gerber files and thus order affordable
and high quality PCBs quickly.
First off in order to test the mechanical
bike setup, I slid the gear wheel onto the
motors shaft and secured it in place with
two zip ties.
Then I positioned a new bike chain onto the
existing gear system of the bike, closed the
chain up with an adapter and placed it around
the motors gear wheel and that was the moment
the problems started to arrive.
You see the gear wheel of the motor is not
fully compatible for a bike chain which is
why at higher speeds it always feels off.
Now the gear wheel does work perfectly with
a bigger motorcycle chain but then we have
the problem that the bike does not work with
it.
So basically put this whole bike motor idea
was a mess and I decided to abort this mission
but I still wanted to find out whether we
could use such an induction motor as a generator.
Luckily you can also spin its shaft rather
easily with your hand or a drill which makes
testing possible.
So it was time to measure how much voltage
the phases of the motor can output without
any modifications but as you can see on the
oscilloscope, the motor output pretty much
no voltage.
To find out why we can take apart the motor
and have a look at its rotor.
We can see that it comes with a squirrel cage
which basically consists of conductors that
are shorted to one another and are definitely
not magnetic on their own which is the main
problem here.
You see when in motor operation aka the motors
coils are being powered, the induced voltages
into the conductors create a magnetic field
which repels the magnetic field of the coils
and thus a rotational force is created and
the motor spins.
And if you are completely confused right now
then feel free to watch my video about such
induction motors.
The problem is now that when just the rotor
is spinning, there is of course no magnetic
field which could cut through the stator coils
and thus create induced voltages which we
could use to power stuff.
So we need to do add what this rotor of a
BLDC motor already comes with and those are
neodymium magnets.
I actually modified the coil windings of this
particular BLDC motor in a previous video
so that when used as a generator it can output
a high enough voltage to power certain loads
and yes this is also proof that our induction
motor just needs some magnets.
That is why I measured the dimensions of the
squirrel cage and thereby decided on those
30 by 10 by 5mm neodymium magnets.
The next question was how many to position
on the rotor and does it matter whether the
magnetic north or south pole faces outwards.
To answer that question I once again had a
look at the BLDC motor and realized that the
polarity of the magnets has to alternate constantly.
And by doing a bit more research it was also
clear that you need an even number of magnets
like 2,4,6,8 and so on to achieve the best
results.
So my plan was to add 8 magnets to the induction
motor rotor which means it was time to somehow
remove at least 5mm of the squirrel cage material.
But I quickly realized that this was impossible
because the material was very robust and dense
and thus I was forced to remove it completely
which was tedious to accomplish with my mini
power tool but as soon as we used the angle
grinder, it worked out much quicker.
And with that being done we now have space
for a magnet holder on the rotors shaft which
I promptly designed in Fusion 360 with 8 square
holes for the magnets.
For 3D printing this holder, I used proto
pasta filament which is actually a bit ferromagnetic
and thus it will make the magnet assembly
simpler to do later on.
So as soon as the 3D prints were complete,
I mixed up strong two component adhesive which
I firstly used to glue the holder onto the
shaft and afterwards I used it to permanently
secure the magnets into their pockets while
making sure to constantly alter their magnetic
orientation.
And just like that we got a hopefully functional
rotor for our generator which I simply inserted
back into the motor and screwed everything
in back place.
As a first test let’s once again have a
look at the coil voltages while using a drill
to spin the rotor.
And to my own surprise, the result was rather
disappointing with terrible looking sine waves
and peak voltages of only 3.5V which was basically
just enough to light up an LED.
At this point I was not entirely sure whether
my magnet holder construction was the main
culprit or the motor windings which I damaged
a bit during the induction motor video by
hooking them up incorrectly.
So to be on the safe side I got myself a second
induction motor with the same mechanical dimensions
but this time with 2 pole pairs and thus a
reduced rotational speed which should equal
higher induced voltages into the coils in
generator mode.
After removing its rotor, I realized however
that its squirrel cage was quite a bit bigger
than the 1 pole pair induction motor one.
But nevertheless the modified rotor did fit
and basically just featured more empty space
to the windings.
And after closing everything up and turning
the rotor we can see that this time we are
getting much nicer looking sine waves with
peaks of up to 4.8V which was a good start.
So I continued testing by hooking up a rectifier,
energy meter and light bulb to the generator
coils and started turning the rotor once again
but as you can see we are only creating a
power of minor 2mW.
By changing the load to something smaller
like a 0.5W LED, I was able to reach a maximum
output power of 9mW which is a joke compared
to the price and size of this generator.
So in conclusion you can turn an induction
motor into a generator but in order to get
a useful one you either need more powerful
magnets or rewind the stator coils but let
me tell you that both options are not that
easy to implement.
That means this induction motor does not work
mechanically or electrically as my bike generator
which also means I will have to come up with
another idea for it.
But that is a subject for another video.
Until then don’t forget to like, share,
subscribe and hit the notification bell.
Stay creative and I will see you next time!
