Portable power

G8TDU

Member #6
Full Member
Power in remote area's when operating portable is often a problem and not easily solved. Solar panels are too big and their usefulness depends on the weather, a wind generator is a little over the top and a small petrol generator is often too big for what it has to power.

A while back, i was given the engine end of a small 40cc petrol strimmer, very economical with fuel although a little noisy, i wondered if i could use that for something. I had in my junk box a small 3 phase motor of the type used in model planes and multi-rotor aircraft (an abandoned hobby) and wondered it these could be used as an alternator. They run at a pretty high speed, anything up to 30,000 rpm.

I fitted the motor shaft into the end of my Dremel drill chuck, held things down and switched on, definately producing a voltage but what about the current?

As the motor is 3phase, there are 3 leads coming out of it, i just connected a 55w car spotlight bulb across two wires and switched on. Taking the dremel up to full speed, 30,000 rpm blew the bulb! The bulb was rated at 55 watts, the dremel motor 170 watts. a quick calculation tells me that at 12v the current was just under 5 amps.

So what about a load on all three? 2 more spotlight bulbs were connected to the other wires in Delta connection and the dremel turned back on. Unfortunately, the load was too great for the dremel and all i could get with 3 bulbs was 9.8v

This was enough to tell me that i could use one of these brushless motors as an alternator. Creating a 1:6 drive from the strimmer engine i was able to get a little over 16 amps from this small motor. Next thing i will need to alter is the gearing. 16 amps load doesnt even start to slow the engine down and i could run the engine at a lower speed for better fuel economy and quietness. A better silencer would improve the noise problem.

Whats next. Ive plans to add an automatic speed control, one that delivers full power during transmit to save the battery, and then goes back to a much lower speed to trickle charge the battery.
 

M0LMK

Member #2
Full Member
Staff member
A micro generator for /P?

I love the sound of that. I wonder if you could use a model air plane type glow engine?

Can't wait to see some pictures..
 

G8TDU

Member #6
Full Member
You could Matt use a model airplane engine, but here are the downfalls, First the fuel is very expensive for long term running, both the nitro and the diesel versions. You need good cooling with a prop which would sap most of your power. varying the speed isnt as reliable as a proper carb. You would be better off looking for a small strimmer or chainsaw engine as these are self cooled, run on standard fuel and can be controlled easier. Here is a small 43cc engine from a chainsaw i modified for a dual rotor heli, this engine is in fact bigger then the strimmer engine i used for the generator project, but it gives you some idea what can be done, it also boasts electric start for added lazyness.

https://youtu.be/MjR7eayaA0w
 

M0LMK

Member #2
Full Member
Staff member
That looks great.

Some very good points about the smaller engines John. Thanks :D
 

G8TDU

Member #6
Full Member
I have another small engine here, i may have another crack at making one for the new base station.
 

MI0PYN

Member #77
Article Contributor
That looks awesome! I guess silencing it would probably be the most challenging aspect?

The microcontroller code should be reasonably straightforward, it would be interesting to mock up a control system on Arduino.
 

G8TDU

Member #6
Full Member
The code was the hard bit for me, but mind you im no fun of Arduino. I purchase a couple of the Alegro current sensors, the type that give a small voltage out depending on the current drawn. One was in the feed to the radio, the other in the feed to the battery. The battery was a 55Ah SLA and charge current would be 10% or 5.5 amps. I wanted to generator to provide as much current as it could during transmit but limiting the charge current to 10% I nearly got thing working but not quite.

The silencing side i found much later was quite easy on another slightly larger engine. I could reduce the noise down to a level i think would have been acceptable on a camp site.
 

MI0PYN

Member #77
Article Contributor
The code was the hard bit for me, but mind you im no fun of Arduino. I purchase a couple of the Alegro current sensors, the type that give a small voltage out depending on the current drawn. One was in the feed to the radio, the other in the feed to the battery. The battery was a 55Ah SLA and charge current would be 10% or 5.5 amps. I wanted to generator to provide as much current as it could during transmit but limiting the charge current to 10% I nearly got thing working but not quite.

Would you mind sharing the code (I presume from your answer that you did use Arduino?) for me to have a look at? It would be interesting to have a look at the logic you've used, and perhaps develop it a little more in line with some solar ideas I have too? It would be nice to have a display showing charge current and voltages too, which should be easy to add.
 

G8TDU

Member #6
Full Member
The code im afraid is long gone, it was written in basic and ran on a small micro called the IOIO. What i had done was simple enough, the 2 current sensors monitored the current drawn by the transceiver and the current drawn by the battery on charge. The sensors outpu a voltage proportional to the current drawn. When the TX was keyed, the sensor inline to the radio sensed the high current and progressively increased the RPM of the engine at the same time as looking at the charge current to the battery. This charge current would appear as a negative figure compared to the charging state, that is until the engine revs reached a point where the current generated was fully met by the engine, and also the preset charge current of 5.5 amps was met. ie radio draws 40a and battery needs 5.5a total current from engine = 45.5a. it was the 5.5a charge current that limited the engine speed and not the drawn current of the radio. If the engine could meet the demand of both, it just ran flat out until radio demand dropped and charge current could again be monitored. I did have a delay in the engine speed reduction of 30 secs, where the charge current was limited to 15 amps for this duration, to 'boost' the battery.
 

MI0PYN

Member #77
Article Contributor
Hmmm, thanks for that. I might just have to go and bring that up to date and see if between us all we can come up with a set of plans and code!

I feel a kit coming on, and a rash of purchases in second hand lawnmower shops.... :)
 

G8TDU

Member #6
Full Member
I actually got both of mine from Freecycle. I posted a wanted add and both times got a result.

John
 
Top