11.1v li-po for pololu micrometal motor 30:1 6v?

Robotics and electronics discussion Mechanical design and actuators
1

Hello there,
I am using the motor mentioned above…i was wondering if i could use an 11.1v Li-po battery…(i ordered a the li-po earlier as i thought there were more powerful motors in the 12v range…but they are too bulky for me).
its a 2200 mAh 30c 3s 11.1v li-po…can i give it a go?
Also…could you explain me a bit about the torque given by the motor. My robot weghs around 300 grams. Will the motor do the job ?
Thankyou very much :smiley:
Enig

2

Hello, Enig.

Can you be more specific about the motor you have (e.g. do you have a product number)?

- Ben

3

Hello,
I thought it to be a pololu motor.
However, i am really sorry. It was a solarbotics motor. Could you tell me about it anyways.
solarbotics.com/product/gm18/
I had ordered it via my brother abroad. It turned out to be solarbotics, i had ordered for a pololu one.
Could you also give information about the torque?
thankyou

4

In general, a motor is rated for voltage for two reasons:

  1. Too much current for too long will develop too much heat, which will ruin the motor. Voltage drives current; too high voltage drives too much current.

  2. Too high voltage may exceed the isolation specification of the motor windings. Small motors use thin insulation, which may have less voltage isolation. Typically, though, you’ll need to get to at least hundreds of volts for this to be a problem.

So, assuming 2) is not a problem, you can drive as much voltage into a motor as you want temporarily as long as you don’t exceed the allowable current. You may even be able to exceed the allowable current for short periods, as long as you don’t develop too much heat.

Now, motors are big inductors. Inductors make the actual current go up slowly when voltage is applied, The higher the voltage, the faster the current rises. Some motor controllers are designed specifically for taking advantage of this. They will sense current, and apply tons of voltage to drive the current where it should be; then they will turn off the voltage, wait until the current drops a little bit, turn on the voltage, … This leads to much better start-up torque than a controller that just applies “safe” voltage and waits. Especially, bipolar stepper motors are usually driven this way. The motors may be rated for 1-2 Volts, but you’ll drive them with 24 Volts and a chopping driver, to get them moving as quickly as possible.

OK, what does this mean for your motor and battery?

If you apply the battery full force, your motor will probably melt down, or at a minimum, wear out quickly.

If you apply a chopping driver that knows how to regulate current to whatever the rated current limit is for your motor, then you’ll do fine, and in fact do better with a higher voltage than the rated voltage.

If you don’t have a chopping driver, then you have to fake it, using PWM. Because current is proportional to voltage, and heat averages over time, as long as you keep your PWM duty cycle to 50% or less, you should be fine with the motor rating you’re suggesting. This will not achieve the same high start-up torque as a chopping driver, but it will save the motor, and probably still run better than a non-modulated signal at 6 Volts.

5

Dear jwatte,
thankyou very much for the in depth explanation.
I am using the l293d as the motor driver so I think it supplies around 500mA of current which the motor can take?
I really appreciate all the info!!
enig

6

The question is what the controller does if the motor tries to draw more power. There are three possible behaviors in a motor controller under these circumstances:

  1. Stop driving the motor altogether.
  2. Regulate current to an average of the limit.
  3. Overheat.

I don’t know what l293d does offhand – but I don’t think it’s 2) which is the behavior you want.