Getting a motor to run as fast as possible..?

[Botanic]

Hello everyone!

Me and three other people are going to construct a robot as a school project. The mission of the robot is to get from a point to four different points as fast as possible. The robot is going to have a sensor to detect the four different points. There's some other requirements for the robot also, which I will probably ask for help with later on.

What we have so far is only the chassis with two motors that we have to use for the robot. We have no documentation for the motors.
The motors are directly connected to the rear wheels.

How do I get the motors to run as fast as possible without overheating them with too much current? Any practical information is appreciated.

The only thing that's on the motors is this: "RA250077-50C01  DC 6V  S.Y. TAIWAN".
I have googled some for the specifications but can't find anything.

Please help.

[Admin]

This should help:
http://www.societyofrobots.com/actuators_dcmotors.shtml

Your motor says 6V, which means your motors run most efficient (therefore minimum heat output vs torque/speed) at 6V. Depending on your load (heavy robot? fast acceleration?) you can probably get away with 7 or 8V without melting your motors. Higher voltage of course meaning higher torque and speed. You can also heat sink your motor too.

The documention, if there is any, wont tell you about over driving the motor.

This may or may not be useful for you to optimize your robot speed and acceleration:
http://www.societyofrobots.com/mechanics_dynamics.shtml

[Botanic]

Thanks! I had already read the first article. I thought that the documentation would give me some information about how much current the motor can take before meltdown...

I guess that we just have to test with different current to see what it can take before going too hot. Then get the biggest wheels we can fit it with without slowing it down.
We did some tests, and noticed that the motors can take 9 Volts atleast for a minute or so.

I found the manufacturer of the motors after some more googling. The company is called "Shayang Ye Industrial". I emailed them and asked for some documentation, just for kicks. I am quite sure that no one else of the groups in my school has any documentation. So it would be nice to boast that we have.

Any tips on how to test how much current the motors can take?

Any tips on a cheap and effective dc/dc converter that converts 6V to 6-9V for the motors? We are going to have a PIC to control the motors.

[Admin]

Well, what you can do is this:

attach a wheel to the motor
apply your desired voltage
with your hand slow the wheel down
measure current and temperature

You will then notice the current go up (use a multimeter to measure current). Try and keep the current steady, and let your motor heat up. Best if you measure temperature and wait until the temperature becomes stable. Apply more and more torque until the motor gets like 200F (I think?). Slowly increase torque so that the motor temperature reaches steady state (doesnt get higher).

The reason you want to do this slowly is so that heat has a chance to travel from the coils to the outer casing of your motor. If you do it fast, you have a problem of instananeous melting, meaning that you have a high concentration of heat in a tiny area, causing melting in just that tiny area before the heat has a time to dissapate.

Once you feel that the motor shouldnt get any hotter, record the current being used. Thats your maximum current - meaning not even a current spike should go above that value.

In the end, it really all depends on the melting characteristics and heat dissapation of the components in the motor you have.

[Afroman]

"Any tips on a cheap and effective dc/dc converter that converts 6V to 6-9V for the motors? "
Unless this is a motor that draws under an amp, it is almost certainly not worth the time/money involved in stepping up from 6 to 9. If you really want the higher voltage, a 7.2V NiCd pack sounds ideal for your situation.

[Botanic]

Well. We have to use a dc/dc converter to step up the voltage. It's a part of the mission. The robot would be too slow with only 6V.
We also have to use 8 AA batteries. 4 for the electronics and 4 for the motors.
Any suggestions are welcome.

[Botanic]

The robot has to travel to all those four points within 150 seconds. There is a prize for the fastest robot.
The robot would not make it under 150 seconds if the engines would be running with 6 V.
To power the two motors we have 4 AA batteries, 1,2 V Ni-MH. Which gives 4,8 V.

I did some tests on the motors today. I got a little surprised of how little heat there was. I ran the motor with 20 V without problems.
With 20 Volts, running for 5 minutes:
The motor was not too hot to hold with my fingers. According to my multimeter the temperature was 55 Celsius. Running with a tire without putting any pressure on the tire, it was 0,7 A. When I slowed down the speed as much as I could with some pliers directly on the motor, it was about 0,5 A.

With 6-8 Volts I was able to brake the motor to stand still, the current went up to 0,79 A in that case.

How does the servo affect the motor? I didn't notice before I did my tests that the motors had servo. Does the servo make the motor create as much heat as it would without it?

What should the maximum output of the dc/dc convertor be? The motors are quite strong, so there is no chance that the robot would ever need more current than 0,5 A. 0,25 A would probably be enough.

Upto how many volts would it make sense to drive these motors with, with these four AA's? How may Watts should the DC/DC converter be able to cope with atleast?

Sorry for all these questions, but it turned out to be more complicated that I had planned with 

[Admin]

If you are only allowed to use 4 1.2V batteries, how would you get more than 4.8V?!? The best you can do is keep your batteries fully charged, as a fully charged battery has a somewhat higher voltage. Perhaps 1.4V/battery, depending.

Is it a typo when you say .7A when the motor is spinning freely but .5A when under load? Do you mean the reverse? The maximum current you found before overheating should be your maximum allowed current spike in normal operation. Current spikes are usually twice that of stall current (when going from full forward to full reverse), so you would now want to use a voltage that is half of what you found. So if 15 volts gave you an overheating stall current, use 7.5V on your motors.

What servo? Are you running basic DC motors or servos (little black boxes)?

If your motor has a lot of torque but not enough speed, use big wheels with large diameters. Larger diameters mean more speed, assuming torque isnt an issue.

[Botanic]

We are allowed to use only four AA batteries. According to what I read on the page that you ponted at before, Ni-MH are good to use if I am not mistaken. 4 * 1,2V = 4,8V. I am supposed to have a DC/DC converter to convert that 4,8V to something bigger. My big problem right now is to find a suitable DC/DC converter that will have the correct output without costing way too much.

Sorry, it was a typo. When the motor runs freely at 6V the current is 0,12 A. When the motor runs freely at 19V the current is 0,17 A.

I fed the motor with 20 V without overheating it. I could have gone even further, probably 25 V without damaging the motor it seemed.

The motor is two parts put together. One basic motor, just like the ones I am used to from my battery driven cars I had when I was little. Directly attached to that motor there is another box, that looks like the motor. From that box the axle comes out where the tire is mounted. My guess is that it is some kind of gearbox. I had the impression that it is called a servo, but I guess I was wrong. Sorry for that 

We are going to fit bigger wheels to the robot. But just changing the tires is not enough for the speed.

I hope that what I write makes sense 

[Admin]

If you want high current, use NiCad not NiMH. NiMH can can hold more amp hours, but it cant pump out current as fast as NiCad.