Selection of Motor AC or DC

Thread Starter

NewCurly

Joined Mar 20, 2016
16
Hi,

I have a bit of knowledge about electronics but learning more while working on this project.

I am working on a project where I am designing a electronic deivce (close to a robot), which can be used for working on agricultural fields. The size of the device is planned to be 30cm width and 30cm of length. The device that I am planning to make is much closer to the image in the following link :

http://www.odt.co.nz/news/farming/280043/mars-rover-inspires-pasture-based-robot

But a bit smaller in size and also the device would have rotating blades in the front of the device which will be designed to scrub the soil to remove weeds. I am using Atmega328P as my Microcontroller, and planning to use 6 motor, where 4 for the wheel and 2 for helping the blades to rotate in the front to scrub the soil.

I need some advice with the selection of the motor that I can go for. I have done a bit of research for motors and I am new to motors too. With my understanding I have the following details.

What I am looking for is Reliability, Low Cost, Less Maintenance, Longevity, Good Torque (for the motor used for blades), Moderate Torque (for motors used as wheels), Moderate Speed, Should also be used in hot and hazard environment where there could also be possibility of little water.



if possible, Good Starting Torque would also be appreciated for the motors used for Blades.



Voltage Range : 12V to 30V (More voltage can also be considered, if the motor is reliable)

after little research I concluded with the following:

1. DC Permanent Magnet Motor

2. AC Induction Motor (e.g. Squirrel Cage)



DC Permanent Magnet Motor:

These motor are expensive, not sure about the longitivity but provides enough torque for voltage supplied (e.g. 30V)



AC Induction Motor (Squirrel Cage):

These motor are less cost, low maintenance, can be used in hazard environment, can be used in hot and rough environments. But I was not able to find any motor that is small enough to fit in my project. Also require a control or starter circuit for this motor.

The above mentioned are my understandings, If anybody have any other suggestions or other alternatives that I can used. Please let know.



Many Thanks
 

#12

Joined Nov 30, 2010
18,167
Google motor torque curves.
I think motors without carbon brushes are more reliable but AC induction motors have low starting torque. You might have to start the motor before you dig the blades into the job.
 

Alec_t

Joined Sep 17, 2013
10,592
The size of the device is planned to be 30cm width and 30cm of length.
Have you seen a typical agricultural field? I think a machine that small would fall down a rabbit hole or be stopped by the first stone it met :). As an absolute minimum I'd suggest 1m x 1m, and if it's to be used in a field where animals are present it will need to be VERY ruggedly built and poo-proof.
 

Thread Starter

NewCurly

Joined Mar 20, 2016
16
Google motor torque curves.
I think motors without carbon brushes are more reliable but AC induction motors have low starting torque. You might have to start the motor before you dig the blades into the job.
Thanks for your input
 

Thread Starter

NewCurly

Joined Mar 20, 2016
16
Have you seen a typical agricultural field? I think a machine that small would fall down a rabbit hole or be stopped by the first stone it met :). As an absolute minimum I'd suggest 1m x 1m, and if it's to be used in a field where animals are present it will need to be VERY ruggedly built and poo-proof.
Yes. I do agree with the issues that can come up while serving in the field. If the device is operated by a human who can identify the stones and avoid contact would help handle things. In terms of power and stability with 30cm, I would have to check whether it can be achieved by different motors with increasing the power source.
 

MaxHeadRoom

Joined Jul 18, 2013
19,400
Why not look for a DC motor used in portable power tools especially outdoor applications.
The problem is you preferably need a TE or TEFC motor (totally enclosed) but hard to come by in DC brushed.
BLDC do not have brushes and can be TE but are expensive.
Also do you need RPM control?
A source of various strength DC motors are automotive wreckers and automobile winches etc.
Induction motors require a higher voltage (AC) and are not rpm controllable.
You need to find the torque required for each motor.
Is this a tether powered or a remote application?
Max.
 

shortbus

Joined Sep 30, 2009
7,230
If you chose an AC motor, how will you prevent the long extension cord from getting tangled up in the crop/plants in the field?
 

Alec_t

Joined Sep 17, 2013
10,592
On a platform only 30cm square there won't be room for much else apart from the large battery which will be needed. Have you measured/calculated how many Watts of power you will require for driving the machine forward and mashing the weeds?
Let's make some assumptions and do the maths.
Assume a small field 100m x 100m.
The machine will have to travel 100*100/0.3 metres = 3.3*10^4 metres to cover the field in 30cm wide strips.
If the machine moves at an assumed 1m per 3 secs it will take 3*3.3*10^4 = 10^5 secs = 27.8 hours to cover the field.
Assume the power required is only 500W.
Total power to process the field = 500*27.8 = 13900 W-hrs.
For a 12V battery, that equates to 13900/12 = 1158 Amp-hrs.
So, assuming you have 60Ah car batteries and are prepared to run them totally flat (don't do that!) you would need 1158/60 = 20 batteries to power processing of the whole field.
 

Thread Starter

NewCurly

Joined Mar 20, 2016
16
Max:
I did also give a thought about using Universal Motors, but I read they are more used for short uses and not for continuous use.

Alec_t:
Thanks for the calculations, I haven't done these calculation yet. This is my first project with electronics which I am trying to approach. I wanted to try and design a small device to check the feasibility and the possibility to create a reliable electronic device that can be used for operating in fields. I am still doing my research on things to see whether I can achieve it.

I was thinking more in terms of a LiPo battery which can help me achieve this, from your calculations I see even if I use a LiPo battery it is difficult to achieve with a 30cm device. Since I have started working on this project, I am going to keep working and implement it to see what I can achieve in terms of covering a certain size field without mashing the weeds with a DC motor, later it can be equipped with better components.
 

recklessrog

Joined May 23, 2013
985
As #12 pointed out, far better to use some sort of internal combustion engine. A small petrol or diesel engine driving a hydraulic pump can then feed several small hydraulic motors, with correctly set overload bypass valves, you would not have to worry so much about burning out electric motors should something get jammed.
 
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