Describing motors using watts?

[munter]

Do you or your race group use watts to describe or categorize motors in 1/32 scale?

I see there is some mention of it in a proxy discussion.( not trying to degrade that discussion but am after feedback generally)

I usually go on RPM and torque.

Any thoughts or a simple yes or no will keep me happy.

TIA

[Rleog]

This should lead to an interesting discussion.

I have been using RPM. Laser tachometers are readily available, inexpensive and easy to operate. Additionally, several years ago Robert Livingston published a list of commonly used motors upon which he measured RPM after a methodical break-in procedure.

Link to the Livingston data:

http://slotcarnews.blogspot.com/2007/02 ... -list.html

Checking the motor RPM against the manufacturer's spec provides information, and comparing post break-in RPM against pre break-in provides more. Comparison to the Livingston data adds further information for evaluating how well your particular motor performs. Livingston's final break-in data was gathered using a test potential of 12 Volts.

Livingston's article also explains how to derive Watts if both RPM and torque are measured.

Thanks,
Bob G

[Audi1]

I am familiar with that discussion.......

As rleog referenced, Watts (power) is the product of one form of the max rpm and max torque specs for the motor, with a conversion factor thrown in so that the resulting power number is not a fractional value.

So why use it? Most people do use max rpm, fewer pay attention to torque and hardly anyone talks about power - right?

The discussion came about in trying to find a way to level the playing field between SC-130 (S-cans) and SS 050 (slimline) motors. Is it a perfect leveler? No, rather far from it. But it is one way to try to skin that cat.

It came about because we started talking about a spec motor for the upcoming SCCS proxy; another way to level the playing field. During that discussion, several members said that they'd prefer to use a slimcan motor because the particular car that the were thinking of was not wide enough to accept an S-can in the configuration that they'd want to run it. For them, a slimcan motor would work. So, two motors now.........

In order to try to give racers a fair choice, we looked for a common motor spec demoninator and we had to go "up" a step to find it; we had to combine two common motor specs; rpm and torque. That's where the power (Watts) discussion came from.

If you want a motor with more grunt, then the S-can is probably the way to go. On the other hand, if you want a motor that spools to a higher level, then the slimline is probably the way to go. For a lighter car; say 50 gm or less, the slimline motor would be expected to be the better performer, because you don't need as much grunt in a lighter car. In a heavier car; say 75 gm or more, the S-can would be expected to be better. In the middle ground, it could be a toss-up.

That's sufficient kindling........let's see where we go from here.............

Allan

[RichD]

For all of our classes except one a spec motor is used, that one class has no motor restrictions. In our case there is no need to categorize motors. Things are going to get complicated if two different types of motor will compete in the same class. Consider that the published specs for a particular motor may not be useful because different makers might use different test methods, in addition motors that are the same make and model can vary greatly in performance.
Before I put a motor in a car I break it in on the bench and check the RPMs. I do not determine the torque. Hopefully motors that are the same make and model will have about the same torque, in any case taking an RPM measurement will at least weed out the dud motors.
I would expect that two motors of the same type that had the same watt rating would have equal performance on the track. Motors of different types with the same watt rating would probably be a different case. Consider two different types of motor, one that has low RPMs and high torque and another that has high RPMs and low torque. If both motors had the same watt rating probably the one with more torque would accelerate faster and the one with higher RPMs would have more top speed. It is likely that a particular type would have an advantage on a certain type of track. Some tracks are flowing, others are more technical and the length of the straights would also be a factor.
I tend to avoid being obsessed with theory, in this case there would be no substitute for some track testing.

[chappy]

My head hurts :angry-banghead:
I just cant get tthis theory stuff , ive tried but my head just hurts more.
There are input watts and output watts, load and no load, final rpm and motor rpm and then the braking factor and acceleration comes from torque and the gcm .
So the watts going in would be under no load, and the watts output would have to be measured through a slave motor set up.
So i have taken several Tylenol extra strength and decided to go with whatever the proxy decides on. Simples.
Bob

[Rleog]

And when you have all your evaluation data, ......

"...... there would be no substitute for some track testing."

[SuperSlab]

[RichD]

With DC motors torque is measured with the motor stalled, so factors like balance, comm trueness and motor brush tension are mostly factored out. You can test motors by connecting the shafts of two motors and applying 12 volts to one of them. If you measured the RPMs you could make note of the value. If you shorted the terminals of the driven motor that would put a load on the powered drive motor and it would slow down. The amount that it slowed down would be an indication of how much power the drive motor was putting out. A variation of that idea would be to power the drive motor to get a certain RPM, say 20K. You could then measure the voltage at the terminals of the driven motor, that would have to be proportional to the amount of power that it would put out.
Moving on here, if a car has sluggish acceleration you can always change the gear ratio, but if the track has long straights you may see a loss of top speed. Having lots of power is not always a blessing, if you can't put the power down your lap times would be likely to go up. Hence the track testing.

[slothead]

Hey Chappy - are you also the guy who said he can't listen to Hendrix? If so, no Hendrix and no theory sounds like a tough life. I credit my music and math appreciation to my dad buying me big bottles of pineapple soda and boxes of Funny Bones as a kid. That and mom's potato salad.

As for watts, I know its a unit for measuring power but I'm trying to relate the formula for calculating watts in electricity (volts x amps) with what it's being used to represent here. Torque is a measure of rotational force. I get it that a motor could spin at high rpms but stall out when a load is applied, meaning it has very little torque. Likewise a high torque motor could get a heavy car moving but not have a lot of acceleration. I just have to pause and wrap my head around how watts can be used as a unit of measure to capture the relationship between rpms and torque as we might measure it for slot car motors. Are we assuming that current draw (amps) will vary with load and one can be a fair representation of the other?

I gotta find my Purple Haze CD and go get some pineapple soda and Funny Bones, then get back to this later.

Slothead

[RichD]

Watts are a measure of power. In one case we are talking about electrical power and in the other case we are talking about mechanical power. The formula used for calculating the mechanical power using torque and RPM figures is buried somewhere on the Slot Car News site. https://slotcarnews.blogspot.com/
The current draw of the motor is not included in the calculation. If a motor is free running the current draw will be nearly constant over a wide range of applied voltages and RPMs. When the motor is under a load the amp draw would be proportional to the load. How much current a motor will draw when it is not turning (stalled) can be calculated if you know the ohm value of the armature windings and the applied voltage. For a common 6 ohm HO armature at 18 volts that would be 18/6 or 3 amps. I'll bet that you did not think that it would be that much. Once the motor starts to turn it becomes a generator and the voltage is the opposite polarity of the applied voltage. That has the effect of the armature increasing in resistance as the RPMs go up, because of that the amp draw will be greatly reduced when the motor is turning. In order to determine how large of a power supply you might need I have taken amp readings as a car circulated around the track. An HO car with a 6 ohm motor would draw 0.5 amps when it was accelerating off of a corner and 0.25 amps when it had reached top speed.
DC motors have their maximum torque at low RPMs, the torque drops off at higher RPMs. There are charts showing that on the Mabuchi site.

[chappy]

"Hey Joe",,, i mean Slot head , i loved Jimi, and my math was like " A smashing of Amps",,, just couldnt figure the watts lol :laughing-rolling:
Bob

[slothead]

Well done Chappy - Hendrix was the Penske-Donohue of guitarists.

Since getting involved in this thread I've watched about an hour of videos and done some math problems on calculating watts, both for electricity and for measuring mechanical work, and still don't have it straight how to compute the latter for a slot car motor. Or, if I knew what the watt rating was for some motors which to choose. If I saw a watt rating on a motor I'd probably interpret it as electrical usage (volts x amps), like a light bulb. In that case if a 60 watt is sufficient using a 90 watt one would be inefficient or wasteful.

One thing I have realized that hadn't occurred to me before is that the gears used to transmit motor rotation to axle rotation not only reduce the rpms at the axle (10 tooth pinion with 30 tooth crown gear means the axle will rotate once for every 3 rotations by the motor), the gears also modify the torque since the size (radii) of the gears are like levers or pulleys. When I've built custom brass chassis I tend to go heavy to keep the total center of gravity low, which means those cars need more torque at the rear wheels to accelerate. More math to play around with on my next build.

And yes, good outcomes can also come about from track testing without any concern for the math or physics responsible. My dad was a tinkerer, who did quite well due to a lifetime of experience despite little formal education. If he was building something like an electric water pump he'd say he needed a pulley for the motor shaft that was 'so' big, making a circle with his hands - and we'd go looking for one. He would have considered 'working it out on paper' a waste of time, something the guys in white shirts did to keep from having to get their hands dirty. But, whenever a race team unloads a new car that's fast right out of the hauler, someone has done their homework.

Slothead

[RichD]

If you feel that the motor in your car is not powerful enough how do you pick something better? There are dozens of motors of each type to choose from. There is more info on how measurements can be done here: http://slotcarnews.blogspot.com/2007/02 ... -list.html
This is the formula for calculating watts: ((Max RPM/2)*(max torque/2))/100,000 = Watts , the torque would be in gram centimeters.
If there are watt ratings available for a motor that you are interested in that would be helpful.
In the past I was involved in doing some motor testing, I was given two sets of motors, each set was made to the same specifications. I broke the motors in on the bench and measured the RPMs at 12 volts. I then put each motor in a car and ran it on the track. The car did not have traction magnets and I ran it at both 10 and 12 volts. I made note of the best lap times, how easy the car was to drive and anything else that stood out about each motor. After the track testing I measured the RPMs again.
One result of my testing was that RPMs alone would not tell you which motor would be best on the track. In one set the motor that gave the best lap times and driveability did not have the best RPMs. I was later informed that other people that had done similar testing had reached the same conclusion.
Having a more powerful motor is not always going to increase your chances of winning races. Before you switch motors you need to determine if lack of power is your real problem. Having a more powerful motor might get you a fast lap every now and again, but if the car is more difficult to drive your average lap times and number of offs might suffer.