Motor generator

Part II of "Generator for LEDs"

I grabbed out some stepper motors and a couple of magnet DC motors and
hooked them to the scope.

With the stepper motors, I read a flat line until I hooked up an LED. I
don't understand why.

Otherwise I observed what I thought was a pretty nice looking sine
wave. I presume a bridge rectifier would flip the bottom waves to the
top, is this correct?


I don't know which choice of motor makes more sense. If I turned the
stepper motor pretty fast by hand, I saw it range +/- 40v. The other
style motor went to +12 or so by hand and about +30v if turned by a high
speed drill (engaged by a large attachment, so the motor spun much
faster than the drill)

I don't know which type to use.

With voltage being subject to such variation, I don't see how I can
reliably limit the proper current to LEDs. Is there another means
available beyond a resistor/LED pair? What do you do if voltage is
not a static variable?


Thank you.

 

All done here,

http://www.freelights.co.uk/


No battery with LED's.DIY kits as well.

 

With voltage being subject to such variation, I don't see how I can

The number of LEDs is still negotiable, but I imagine I want anywhere
from 12 to 32, if the power supply can handle it.


As for the type, I have only a few assorted LEDs right now so I will
probably have to order some. I'd like some of the really bright ones
like flashlights use, but if those are expensive, I think some medium
size bright red ones show up the best.


As David kindly posted a link below, that is nearly what I want to do.

In addition I want to explore those 1 farad backup memory capacitors to
keep providing power when I'm stopped. I want to make it very very
cheaply.

I'm open to either a motor (which seems easier) or a magnet on the wheel
(if it has an advantage).


I will have to learn about current regulators. If I make one, do I
still have to put a resistor with each LED or does one regulator suffice
for all?

 

---
You've got to make up your mind what you want to do first. That is,
if you want a certain number of LEDs, then the power supply _has_ to
be designed to handle that number. On the other hand, if your power
supply decision is fixed, then the number of LEDs you can have will
also be fixed. For example, if we look at a super-bright red LED
with a forward voltage (Vf) of 1.7V at a forward current (If) of
20mA, then each of those LEDs will consume:


P = If * Vf = 0.02A * 1.7V = 0.034W = 34 milliwatts


and the array will consume 36 times that, or about 1.22 watts.

That's not an awful lot of power, and probably either of your motors
could generate that.

The nice thing about the LED array is that it can be arranged to
"fit" whatever your motor can put out by wiring the LEDs in series,
in parallel, or in series-parallel.

That is, if you wired all 36 LEDs in series their Vf's would add,
but their If's would stay the same, so the power supply would have
to be able to put out 36 * 1.7V = 61.2V at 20mA.

On the other hand, if you wired all the LEDs in parallel their Vf's
would add, but their Vf's would stay the same (well... kinda. More
on that later.) so the supply would have to be able to supply
36 * 20ma = 720mA at something greater than 1.7V. Greater, because
the Vf's of the LED's won't all be the same, so the current into
each LED will have to be limited by a resistor or a current
regulator of some sort in order to keep the LEDs with low Vf's from
being current hogs and committing suicide.

Let's take a look at a preliminary design using your stepper motor
(alternator) with an output of +/- 40V, assuming that that's what
you'll be able to get under "normal" cycling conditions, whatever
that means.

Let's also make the assumption that it's a 1° stepper and that at
+/- 40V out the output frequency is about 720 Hz.

Next, we'll full-wave rectify it using some fairly fast diodes in a
bridge, smooth it, and we'll wind up with:


ACIN>---+--------+
| |
[1N5817] [1N5817]
|A |K
| +--------+-->39VDC
| | |+
| | [BFC]
| | |
+--------|--------+-->0VDC
| |
|A |K
[1N5817] [1N5817]
| |
ACIN>---+--------+


Since 36 LEDs will drop about 61.2V (let's say 60V) 18 will drop
30V, so if we make two series strings and connect them in parallel,
we'll have a load that'll draw 40 mA and drop 30V. Since we have
39V available and, presumably, can draw more than 40mA from the
alternator, we have more than enough to run through a current
regulator and even some left over to charge the BFC, and the circuit
will look like this:


ACIN>---+--------+
| |
[1N5817] [1N5817]
|A |K
| +--------+---------+---------+
| | |+ | |
| | [BFC] [LED1] [LED19]
| | | | |
+--------|--------+ . .
| | | . .
|A |K | . .
[1N5817] [1N5817] | [LED18] [LED36]
| | | | |
ACIN>---+--------+ | [IREG1] [IREG2]
| | |
+---------+---------+


The problem, of course, is going to be that at some speed the
alternator's voltage is going to drop to the point where it will no
longer be able to run the LEDs. If you're pedalling along
'normally' and all of a sudden have to come to a stop, then the BFC
will have to supply all the current and it'll be able to do that
for:

C dV 1F * (39V - 30V)
t = ------ = ----------------- = 225s = 3min 45s
I 0.04A

Not bad.

Of course, the BFC will start charging again once you get under way,
and the amount of time it takes it to get to 39V is going to depend
on how much current your alternator can put out. But... as long as
the voltage across the cap rises to what the current regulator needs
for headroom, the LEDs will light. Which brings up another thing.
If the LEDs are going to flash, adjusting their duty cycle will
lengthen the time the charge will last. For example, 100ms on and
900ms off will increase the discharge time to about a half hour
(assuming a perfect BFC)

Anyway, I think the next step would be for you to either get or
build a full-wave bridge and measure what kinds of voltages and
currents you can get out of the alternators and motors (generators)
at various RPMs.

 

Neither do I. I have observed the voltage output of a moving,
unloaded step motor lots of times.

Yes. If each winding has just two leads (a 4 wire motor) then you can
use two bridge rectifiers to combine their outputs into a single DC
output. If they have 6 leads (each winding center tapped), you can
also use a pair of diodes for each phase to combine the outputs into a
single DC supply.

The best choice may get down to which one is a more efficient
generator or which one is easiest to mount, though the stepper has the
advantage of no brushes to wear out. The rectifiers on the stepper
will also prevent energy going back into the motor when you slow down,
but a single diode on the brush DC motor will provide the same effect
for it.

The most effective solution may be a switching regulator (that
converts power in to power out with little loss) that accepts a wide
range of input voltage and regulates an output current that feeds all
LEDs in series or a regulated output voltage that feeds all the LEDs
in parallel (or small seriesed groups in parallel), with a small
ballast resistor in series with each one (or group).

But I doubt you will be ready to design such a regulator till you have
quite a bit more experience.

 

Ryan wrote:
(snip)

If the current is regulated, and has enough voltage behind it to drive
all LEDs in series, then no additional resistor is needed. The only
purpose for the resistor is to stabilize the current and the current
regulator does that.

 

neither do I, I can only succest operator error.

use current regulation.

typically this means starting with DC at a voltage above what is needed
and limiting the current by means of an electronic circuit that measures the
current and takes action to correct it.


Bye.
Jasen

 

there's a vast range of brilliance some LEDs are 1000 times more
luminous but only cost ten times as much snd want less than three times the
power if their dimmer cousins.

You'll do much better buying a few high intensity LEDs than buying many
cheap ones.

they aren't very cheap.

you'll need one regulator for each series group of LEDs, if you have
multiple groups you may be able to use some resistors to balance the
current and still only use one regulator.

 

John Fields said:

Something I was wondering about with a similar problem (using dimmable
luxeon LEDs for room lighting) is whether you can get away with using just
a single current regulator circuit for several strings if you put a small
resistor in series with each string.

+---------+------------+
| | |
| | |
| [LED] [LED]
+-----------+----+ | |
| Current source | [LED] [LED]
+-----------+----+ | |
| [...] [...] more leds.
| | |
| [R] [R]
| | |
+---------+------------+

Intuitively, I would think that the resistors would help balance out any
manufacturing variations in the LEDS so that the current would go more
equally between the two strings, but I don't know if this is right - e.g.
is there a runaway burn out scenario here where one of the strings gets
hotter and then starts hogging the current more and gets even hotter?

 

This is one of the choices I described, and is certainly practical
with an appropriate value of resistor. They can be much lower
resistance than when they must limit the current from an unregulated
voltage. 10 ohms per LED in the string work. At 20 mA, that wastes
0.2 volts per LED in the string. Something like 11% of the power.

 

John Popelish said:

That's what I was thinking - thanks.

 

Because the voltage is going to vary over a wide range as you speed up and
slow down etc, I would be thinking of using a NiCd or such to power the
leds (handy when youve stopped too) and using the steppers thru a regulated
charging circuit to recharge the NiCd(s)

David - just playing with ideas

 

You've got to make up your mind what you want to do first. That is,

Ok. The particular stepper motor of interest is PM42L-048. I found a
PDF that says it is a 7.5 deg/turn 24v motor and as I understand is
rated for 600mA. Is this information meaningful to me as a generator?

I can't find good data sheets on the other motors I got.

I think I have a bridge rectifier that I pulled from a dead power supply
or something. Any reason this won't suffice?

My other challenge is how to attach to the shaft of my motor. It has a
gear on the shaft and I can't fathom a way to make it have a 2 to 2.5"
wheel on it to engage the bike tire, or else interface with the chain
like a sprocket.

My initial experiments with this method were not encouraging.


Thank You.

-Ryan

 

the resistors have to be greater than the negative-resistance effects of
overheating.

Bye.
Jasen

 

yes, it means don't try to get more than 600mA out of it
also voltages far exceeding 24V aren't a good idea. (I'd draw the line at
100V)

7.5 degrees means 48 poles in the stator, these motors are normally two
phase, so 24 poles in the rotor, - 12 cycles of AC per revolution to each
phase.

(most bicycle generators have a 4 pole rotor)

so this motor should produce usable output and around 1/12 the speed
so you could use a bigger wheel on it to reduce drag....

I can't see the gear, can you put a tyre on it?
can it be removed and replaced with something else?

i'd be worried about buckled rims...

Bye.
Jasen

 

the stepper motor has two outputs, for best results you should use two
bridge rectifiers.

a toothed pulley? did this motor come out of an inkjet?

make one using epoxy putty

Bye.
Jasen

 

---
No, that's the rated current for the bipolar chopper version of the
motor to put out the torque it's rated for. The only way you're
going to find out if the motor you have will work for you is to try
it. Determine how fast it'll be spinning when it's on your bike,
spin it up through the range with an electric drill, and load it
down electrically to to find out how much power you can get out of
it.
---

---
Doesn't matter. Your approach has to be experimental with any of
the motors, so the procedure is the same: Try it!
---