Motor

[shrewdPanther46]

Hello guys,

Has anyone started thinking about what motor to use? I am not exactly sure what type of motor and what specs would benefit the given scenario.
One should be aware of the voltage they are planning on running through the motor as well as Kv, turns, etc.
Any advice?

[retired1]

Batteries are limited to a max of 8 AA batteries = 12 V. if You plan on using alkaline batteries, then go for a 12V motor. I would look at Servo City who have 3 levels of motors. Next I would look at Pololu 25D or for a heavier motor their 37D. All have a no load rpm gearing choice.
Somewhere in my early searches, I saw a 9V motor. This is of interest if you use rechargeable batteries (1.2 V) , so 8 would be 9.6 V. This tiny bit of over voltage will be OK.
A fully charged NiMH will have closer to 1.3 V each for a rather short period of time.
My prototype will use a 6 v motor.I will use 6 NiMH batteries which will shorten the life of the motor. This would be bad if the motor were going to run for more than the 5 seconds or less that most will use.
Look at what the rpm (no load) and assume that it will be less under a load. The heavier the load, the lower the rpm will be. Very few motors will have torque/voltage curves available, So I looked at stall torque as a reference.

Remember, that you can not use an electronic speed control, so I think that pretty much eliminates brushless motors.

Watch out for shaft diameter as finding compatible light weight gears is not easy. It has to mate up with the axle gear.

Finally, you will want to consider the weight of the motor.

[retired1]

"Somewhere in my early searches, I saw a 9V motor. This is of interest if you use rechargeable batteries (1.2 V) , so 8 would be 9.6 V. This tiny bit of over voltage will be OK.
A fully charged NiMH will have closer to 1.3 V each for a rather short period of time.
My prototype will use a 6 v motor.I will use 6 NiMH batteries which will shorten the life of the motor. This would be bad if the motor were going to run for more than the 5 seconds or less that most will use."

Problem solved. I found on line that 5 AA cell battery compartments are available.

[TheWingsOfFire]

While I'm trying to find a motor, I'm wondering if the motor should have a fast rpm or a slower rpm. I heard that a faster rpm makes the car not as accelerate as fast, and I have heard that a fast rpm also loses torque, which is better for acceleration.
I was wondering which type of motor to get and whether speed is more important or torque is.

[shrewdPanther46]

While I'm trying to find a motor, I'm wondering if the motor should have a fast rpm or a slower rpm. I heard that a faster rpm makes the car not as accelerate as fast, and I have heard that a fast rpm also loses torque, which is better for acceleration.
I was wondering which type of motor to get and whether speed is more important or torque is.

good question... correct me if I am wrong anywhere in my responses
First, that choice depends greatly on the final mass of your vehicle. If your car is relatively heavy, more torque would be needed in order to pull the vehicle along, whereas in a lighter vehicle, high RPM would be beneficial. Keep in mind that 9-12 meters is relatively long as a distance, and chances are that the vehicle would eventually hit its top speed before braking. Thus, it is important to have a high rpm motor in your vehicle. Since 6V motors aren't even that expensive, I would recommend simply trying out a few different motors and seeing what first best in your car (that is what I did).

But MOST importantly, I believe that the motor you use does not matter that much, in comparison to how it is geared. So don't worry too much about the motor, and try to calculate a gear ratio and go with trial and error.

[TheWingsOfFire]

Thanks for the super quick reply. I'm assuming that with gears that having a higher gear ratio would make it slower, resulting in more torque? Or is it the opposite? Also, what would be considered a good gear ratio?

[shrewdPanther46]

your intuition is correct. Look at the other thread for finding a good ratio(we talked about deriving gear ratios and stuff). Depending on the motor, I would probably anticipate around a 1:5 reduction...

[cheese]

just curious, where did you guys get your gears from? I got some traxxas gears from amazon - a 72 tooth and 16 tooth i think. They work fine with my 9v 150 g/cm torque powered on 5-10 volts.

[shrewdPanther46]

We decided to avoid the traxxas gears since they require the transmission thing which is unnecessary weight, although maybe it runs smoother idk

We just got bare gears from some local guy

[cheese]

We decided to avoid the traxxas gears since they require the transmission thing which is unnecessary weight, although maybe it runs smoother idk

We just got bare gears from some local guy

Yeah the transmission thing is a lot of weight so we kinda just smashed it between a bunch of nuts on our threaded rod and it words perfect.