*Originally posted by Ken Leung *
**Under no load, the drill motor itself runs at 20,000 rpm, while the fisher price run at 15,000 rpm…
The 100 rpm is only when the motors have their own gearbox reduction attached. But in this case, the two motor is directly attached at their output shaft, so they would want to match the two motor’s speed or else one motor will be dragging the other along…
So in this case, the drill motor will need to reduce it’s free speed to 15,000 rpm… Is this right? that’s like 3/4 voltage, which is 9v… **
Ken, you are close. Your above statements are all correct, until you get down to thinking that the drill motor’s free speed will need to be reduced.
Look at it this way: no motor under an applicable load will reach it’s free speed.
I’ll try to explain:
Let’s initially ignore that the F-P motor is on this assembly. The Drill Motor is attached to it’s gearbox, and it is turning a sprocket. This sprocket then drives a chain which drives the wheels. There is much friction, drag, and mechanical loss going on after the torque is transferred to the shaft leaving the Drill Motor gearbox.
The different types of friction losses range from friction in bearings to friction in propelling the robot across the carpet. With all of these losses added up, the Drill Motor’s speed might get to 1/2 its 20,000 rpms… and that is if your drive train is aligned properly.
OK… now we can figure in the Fisher-Price motor. Considering that the Drill Motor is grunting away at 10,000 rpm without the F-P motor, it gets a “boost” when the F-P motor gets added to it’s backside. Ideally, this “boost” can last until both assemblies get up to 15,000 rpm, since that is the F-P motor’s free speed.
Now, above 15,000 rpm, the F-P motor begins to become a drag to the assembly… but we don’t really care at this point. At 15,000 rpm, we already have a 50% speed boost.
I am not sure where our final rpms were peaking on this assembly, but we do have some evidence that it was around 14,000 or 15,000 rpm. Here is our proof:
Our top speed was around 8 or 9 ft./sec. This translates back through all of the reductions to the motors running at 14-15,500 rpm.
Also, we noticed that the F-P motors got hotter than the Drill motors after being ran a long time. This means that the F-P motors are being maxed out while the the Drill motors may have more to give… making us think that we are getting close to the free speed of the F-P motors.
And… this assembly is almost the same as the assembly we used in 2000, except with the F-P motor added. We also used the same ratios (approximately) from the motors to the floor as we did in 2000. In 2001, we saw a 40% speed increase. This would agree with our assumptions that we ran 10,000 rpm in 2000 and 14,000 rpm in 2001.
We have not put this assembly on a dyno, but we plan to over the holidays. Once we get some results, I’ll post them.