Safe to power 2 hacked knex motors with a 9v battery?

Hello all, I am a newbie at electronics, and the C language (I have experience in other languages).

I have a question about powering the knex motor in Arduino Robot for Lowest Cost : 5 Steps - Instructables this tutorial. Normally, the motor takes 2 AA batteries (3V) to power it. I was wondering if the motor is safe to power with a 9v battery. I powered it for a short period of time with the battery, and it was spinning very fast, but will it overheat with consistant use? I am also planning on using an L293D to power it, so if it cannot handle 9v, will it still work if I sent a smaller PWM pulse rate to ENABLE like this:

#define SPEED_PIN 11
//will this send 1/3 voltage and/or amperage so it won't overheat if it is a problem?
analogWrite(SPEED_PIN, 255/3)

The motor does not have any information on it besides the company name "S. M. C. MOTOR". I did a bit of searching, but I could not find any specs for the motor Im using.

And if nobody knows, how long should I run it on 9v to be sure it doesn't overheat?

Normally, the motor takes 2 AA batteries (3V) to power it. I was wondering if the motor is safe to power with a 9v battery.

So what do you think? What was your result?

I was wondering if anyone else knew the specs to the knex motors before I go out and try to send 9v through it for a long period of time.

The amount of current drawn by the motor depends simply on the amount of torque that the motor is producing, so when it is unloaded and up to speed the current with a 9V supply will be the same as with a 3V supply. However, at the other end of the scale when the motor is stalled the 9V supply will put up to about three times more current and up to about nine times more heat into the motor. Obviously that would have a drastic effect on the life expectancy of the motor. If you kept a small motor stalled and powered up at three times the rated voltage, you could probably burn it out within a few seconds.

Tripling the supply voltage would triple the free-running speed when the motor was unloaded. The effect of the increased RPM would depend on the quality of the motor. A really cheap motor might start shifting and flexing to the extent that went out of balance and was destroyed.

Basically, this isn't something you would want to do to a motor that you wanted to last.

Thank you, I think I will go with the 2 AA batteries it was powered from. The issue is though, that you assumed it was rated at 3v, but I didn't know what it's maximum voltage rating capacity was, I only knew it was powered by 2 AA batteries.

wcb98:
it was rated at 3v, but I didn't know what it's maximum voltage rating capacity was,

That is not an unreasonable speculation but the critical question is what current it can take rather than what voltage.

You must ensure that you don't operate with a voltage that forces too much current through the motor. As @PeterH has stated the stall current will be very much higher with 9v rather than 3v. Safely increasing the voltage above 3v might be possible but increasing by 3 times seems excessive.

Also, if you are using a small 9v PP3 battery it won't be able to supply a lot of current. A bigger 9v battery or a 9v mains adapter might cause damage to the motor where the battery would not.

I believe there is a considerable voltage drop in the L293 and if so a 3v battery won't provide 3v to the motor. You may need a 4.5v power supply.

...R

wcb98:
The issue is though, that you assumed it was rated at 3v, but I didn't know what it's maximum voltage rating capacity was, I only knew it was powered by 2 AA batteries.

If you re-read my post you will see that I didn't make any assumption about what the motor's rated voltage was. I described how much effect the change from 3V to 9V would have on the current and heat and then said that if you exceed the rated voltage by a factor of three this would be very likely to damage the motor. You haven't said what the rated voltage is, so I have no idea whether you're exceeding it or by how much.