Solarcell powered motor

J

Jacobsen

New Member
Question to PICAXE forum

I'm trying to connect an electrical motor to a solarcell, as seen on the Schematics.

Problem: When the suns intnesity degrades we need to aditionally control a mosfet by PWM to reduce the current draw on the solarcell. (DIP Regulating), and it almost works!

Schenatic:
www.sunwind.dk/projekt/Schematic.jpg

Flowchart:
www.sunwind.dk/projekt/Flowdiagram.pdf

Many of these codes works OK, but I'm having trouble making it work well all the way down through the codes, please help.
When going through the codes it's obvious that I've tried to make them step by step, in order to make it look like small individual 'chapters' working with each voltage setting one by one.
Starting with 20Volt going down to 15Volt accordingly reducing the PWM mode from 100% to 10%.
Perhaps one of you have some experience regarding the possibility to make the flow run even smother, eventually with a code example.

Would it be an advantage to use the comand 'Case' here?

Codes can be viewed on the atached file.

With all best regards to all of you
I hope that one here will help me out.

Monie
 

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C

ciseco

Senior Member
Hi, I really suspect there's so little current at the lower voltages it wont do a great deal anyway, you've given code but not really described the problem, what panel & motor would also help. I've done some research into something slightly similar in the past and you might be better looking at charge pumping a capicitor using dc buck or sepic methods then "dropping" the charge to the motor, this will effectivly jump, jump, jump it, the greater the voltage the more jumps (sounds like PWM dont it) :) but not quite the same thing, in my mind you need to store the energy before putting it out to the motor.

have a look at this,

http://www.voltscommissar.net/minimax/minimax.htm

Cheers

Miles
________
full melt bubble hash
 
Last edited:
G

gengis

New Member
This might not help your quest . . . and I'm not being critical of your efforts, I just want to "grok" it. I'm not good at code just electronics.

Why choose a positive displacement pump (diaphragm example in the "maximizer") ? Centrifugal pumps have low starting torque and automatically suited to solar cells.

Have you looked at something called "beambots?" These folks use a solar panel to charge a cap and when the voltage reaches a predetermined point the cap is dumped into a motor that moves the "robot." Some ideas there if your intent is to get over starting torque to get a pump moving when the light level is high enough to support turning it with solar power.

Tell me more about your project, it sounds interesting.
 
M

Mycroft2152

Senior Member
I may be reading the schematic wrong, but it appears that your voltage divider is putting 12 volts into the PICAXE.

The current for the starting torque that was mentioned can easily be 4 to 5 times the 'average' running current.

Myc

BEAM robots-- where did I hear that before :)
 
M

moxhamj

New Member
I don't understand the circuit - it seems to be throwing away energy. The PWM will allow the volts to change but the current will change too. The aim is to get the maximum number of watts out of the cell, not the max volts or amps. Have a read of Maximum Power Point Tracking (search for MPPT) as these devices are commercially available and are also ideal for running on a picaxe. The basic components are a storage capacitor on both the input and output and a switched inductor to convert the volts. You need to measure amps and volts and work out the watts.

For example, a 36 cell array might have an open circuit voltage of 18V and a current of 1 amp (18W). If you put it across a 12V motor or battery it may give 12V but still only 1 amp (12W). But if you put it through a buck regulator at 90% efficiency you might get 1.35A at 12V = 16.2W. The maxiumum power point changes all the time depending on cloud cover and temperature and the angle of the sun so the circuit needs to continually adjust to find the best PWM to drive the buck circuit.

Hope that makes sense!
 
B

BeanieBots

Moderator
Whilst I agree with Dr_Acula that MPPT is the best way to get the most out of a solar cell, I have found in practice that things can be done in a much simpler way with a little extra hardware.
The problem with MPPT is that the controller (PICAXE) needs to make continuous adjustments, test to see if things improved and then continue or change adjustment direction accordingly.
If you add another much smaller solar panel of the same material as the main panel, you can use it to drive a constant resistive load. The voltage across that load will then be an indication of the main panel power output. It is then just a question of using a lookup table to determine the optimum PWM duty.
If your main load is constant, then just use the calculated PWM value. If it is a variable load, then you will get better results by using the lookup value to determine the optimum voltage for the load to 'control' the main panel to.

The method I've described avoids the need for current sense and significantly simplifies the required code. However, it DOES NOT maintain optimum control if ANYTHING other than the light intensity changes. Re-calibration of the lookup table will be required periodically as the panel degrades and or if the panel is re-sited.
 
J

Jacobsen

New Member
To all

Sorry about my delayed answer, I needed a bit help with the translation to English.

Can anybody help me with code suggestions regarding PWM output.?
(pwmout 1 , period, duty), I would like to be able to regulate the PWM duty cycle between
HighVoltage 100 % to LowVoltage 10%
PWM frequency approximately 20000Hz

With the function at Read the ADC (10Bit) with the command:
Readadc10 0,Speed

Symbol LowVoltage = 307 '1.5V
Symbol HighVoltage = 409 '2.0V

I have used PICAXE Manual 2 BASIC Commands: Mathematics and tried many times with no luck.
I will be very pleased, if anyone will help with ideas how to do the coding.

My coding is attached

Best regards.
Monie
 

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