I don't think you need to. One normally runs at 100rpm and the other operates at 90rpm, so they are close enough that they both should run at 95rpm without any trouble or tweaking.

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...

Close, but not right
 

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.

Andy B.

I have this to say about that...
 

Andy,

I disagree with you about the typical free speed of the drill motor. If your system has so much drag that the motors are topping out at only 10,000 rpm, I think something must be wrong with your drive.

Seriously, that would mean that wind and friction are driving you to half your free speed. In this case, each drill motor would be drawing half its stall current (60-70 amps) when it is running at top speed. I seriously doubt that this is the case. If it is, again, I think something is seriously wrong.

As to what happens when you put 2 motors with different stall torques and free speeds on the same shaft, there is no magic here. At any give speed, the torques add. If you plot the speed-torque curves with torque along the X-axis (as every right thinking person does ;-) all you need to do to get the aggregate speed-torque add the torque from each motor at each speed.

Since everything is linear, you only need two points to define your line. One point can be the zero speed torque = Stall Torque of motor1 + stall torque of motor2. An easy second point is the free speed of the slower motor. At this point the aggregate torque is the torque of the faster motor at the slower motors free speed.

Notice that at speeds faster than the free speed of the slower motor, the aggregate torque is LOWER than the torque of the faster motor alone. Why? Because some of the torque from the faster motor is going to make the slower motor spin faster than its no-load speed. The slower motor is acting as a brake! It only makes sense.

My two cents worth.

Joe J.

P.S. As to why the F-P motor was hotter than the Drill: There are a number of reasons for this. The simplest is that the cooling of the F-P is just that MUCH LESS efficient. The F-Ps get hotter even though the drill motor is providing more power and doing it less efficiently than the F-P (by this I mean the it requires more electrical power in per unit of mechanical power it gets out -- the difference turns up as heat that must be dissipated).

There are 2 main reasons for this. Number 1 is that the surface area of the F-P much smaller. Smaller surface areas for dissipating a fixed amount of thermal power drives up the temperature. Number 2 is that the cooling fan is much less effective. The tip velocity of the fan is smaller and the fan blade size is smaller. Both of these mean that the temperature of the F-P is going to be higher.

Another reason that the F-P may be getting hot is that when it is acting as a brake, it is generating heat. This heat also can be a contributing to the higher temperature of the F-P motors.

Finally, this is one thing that many folks miss: the "factory" motor mounts allow air to flow through the motor from the shaft side along the axis of the armature through to the electrical side with the fan on it. This is true on both the drill and the F-P. Many "homebrew" designs block this flow. If you are blocking the flow on the F-P but leave the drill motors as is, that is even more reason why the F-P motors would be hotter than the drill motors.

Re: I have this to say about that...
 

Joe,

You misunderstood what the 10,000 rpms represent. The 10,000 rpms I referred to not free speed, but rather the speed of the motors while the robot is at top speed. The friction I mention not only includes losses in the drivetrain, but also losses due to a 130 lb. robot being driven around the carpet. Maybe I should call this "peak loaded speed".

What you mention on the torque adding is similar to what I discuss. It suprises me that many people get hung up on the fact that IF the speed is over 15,000, then the F-P motor acts as a brake. Yes, this is true... so they shouldn't design their ratios so that they are exceeding 15,000rpm. As long as 15k is not met, then the F-P motor is adding a boost.

As for the motor cooling info... that makes sense.

Andy B.