CIM Voltage and Motor Suggestions

[jakep]

Hey,

I was looking at the CIM Motor specs, and I was wondering if anyone has tried running them at 24 volts? All the spec sheets I saw say 12 Volts, but I was wondering if that was just because FIRST bots are always on a 12 volt system, so performance at higher voltages just isn't reported.

For this one robotics project, we are looking at using 2 CIMs to power it. However, according to our calculations, we will need to double the Horsepower on these motors if they will ever work for our needs. They are rated at ~.34 HP, and we need something that performs at ~.7 HP. My theory was if we doubled the voltage, this would work. However, I'm not sure if this will burn out the motors.

If this is not possible, and our small team is getting a bit desperate with this summer project of ours, can anyone recommend some relatively cheap motor/gearbox combination that would have around 250 inch-pounds of torque at 40amps? I saw: http://www.npcrobotics.com/products/...at=20&mode=gfx
but those just seem way overpriced.

Any help would be greatly appreciated.

[Kevin Sevcik]

Originally Posted by jakep

Hey,

I was looking at the CIM Motor specs, and I was wondering if anyone has tried running them at 24 volts? All the spec sheets I saw say 12 Volts, but I was wondering if that was just because FIRST bots are always on a 12 volt system, so performance at higher voltages just isn't reported.

For this one robotics project, we are looking at using 2 CIMs to power it. However, according to our calculations, we will need to double the Horsepower on these motors if they will ever work for our needs. They are rated at ~.34 HP, and we need something that performs at ~.7 HP. My theory was if we doubled the voltage, this would work. However, I'm not sure if this will burn out the motors.

If this is not possible, and our small team is getting a bit desperate with this summer project of ours, can anyone recommend some relatively cheap motor/gearbox combination that would have around 250 inch-pounds of torque at 40amps? I saw: http://www.npcrobotics.com/products/...at=20&mode=gfx
but those just seem way overpriced.

Any help would be greatly appreciated.

Is your estimated horsepower need at the 250 in-lb point on the torque-speed curve? Basically, do you need roughly 180 RPM at this torque or can you just gear something really low?

Generally speaking, the maximum power output of a motor goes up as the square of the voltage. If you up the voltage you up the current by the same ratio, so twice the voltage is four times the power. If you wanted twice the peak power, up the voltage by root 2. So figure about 18V, not 24V.

I think you could operate the CIMs at peak power for a reasonable amount of time at 12V, but I'm not sure sure they'd like having to dissipate twice the heat.

[jakep]

Yes, we would need roughly 180RPM at this torque, if not closer to 200 or so.

I see what you mean about the 18 volts, but can the CIMs handle it?

[Cuog]

DC motors can run at as much as double their intended voltage "safely" however 150% is usually the maximum recomended so 18V should be ok, just make sure to stop using it if the motors start getting too hot to avoid damaging the motors.

[sanddrag]

You could try one and if it doesn't work you're out $28. However, how about just using more motors?

[Kevin Sevcik]

Well at max power, the CIM is gonna be drawing around 100A at 18V, so it's going to have to sink about 1200W of heat or so. Which is a good bit. I'm not saying it's going to catch fire, but you're probably going to want some sort of cooling on it. I figure a ballpark estimate would be a rise of atleast 1 deg C every second the motor is running at max power at 18V, probably more.

And don't forget you'll want a pair of Victor 885s to power those.

[Richard Wallace]

Check out the CIM data given by its manufacturer. In particular, read these notes:

1) DUTY CYCLE:INTERMITTENT, CCW 3 MINUTES ON 2 SECOND
CW 3 MINUTES ON 30 MINUTES @64.0 OZ-IN,
2) LIFE:1000 CYCLES MINIMUM

shown near the lower right corner under the heading "Special Features." Admittedly it's a little cryptic, so let me try to parse it. In plainer English it says that, if you use the motor to drive a 64 oz-in (4 lbf-in) load that requires a cycle of motion in which it first turns counterclockwise as fast as it can from a 12V supply for 3 minutes, then rests for two seconds, then turns clockwise as fast as it can from a 12V supply for 3 minutes, and finally rests for 30 minutes before starting the same cycle again, then the motor will last for at least 1000 of these cycles. The load cycle thus described is typical of a winch, which is the application for which the CIM was originally designed. So the total operating time for this load rating (12V, 27A, 4 lbf-in, +/-4320 RPM intermittent reversing duty) is 6000 minutes or 100 hours.

Back to your question: can you get more power out of the CIM by increasing the supply voltage. Kevin correctly pointed out that you can approximately double the peak power if you increase the voltage by a factor of sqrt(2). Now your question is: can the CIM handle that?

The question amounts to asking how to de-rate the CIM for operation at higher voltage and higher loading. Cuog recalls correctly that dc motors can typically be operated at up to 150% of rated loading; however, this increase does not come for free -- the penalty for overloading a motor is that it will wear out faster. You may not care about this if your robot only needs to last for a few operating hours.

Two overload stress factors will tend to shorten the CIM's expected life. One is running at higher speed, which will cause both the bearings and the brushes to wear faster. The other is running at higher currents, which will cause the armature coils to run hotter, thereby degrading the life of the insulation, the commutator, and the bearings, since these are all parts of the overheated rotating assembly.

I wish I could give you a rule of thumb for de-rating the CIM -- something like "subtract half the operating life for every 10 Celsius degrees beyond the rated insulation temperature." But I don't have one, and I don't know who does.

[Kevin Sevcik]

I'll also point out that if you power just these two motors from even a 20Ah 18V battery, you're likely to get about 3-4 minutes out of a fully charged battery. Assuming the battery doesn't end up in a puddle on the floor. A constant 200A draw is a huge load.

[Kevin Sevcik]

Okay, I've got a sort of lead for you, but it'll probably require some leg work. I ran across this list of battlebots motor specs. two motors in there look like a good idea for you. The Dewalt 24V hammerdrill motor with gearbox, and the NPC 64038, which you'd have to gear up to twice the speed.

It says the dewalt costs $85, but I have no idea what the part number for all the stuff is.

EDIT: So I was bored and did the legwork for you. The drill model is DW006K-2. Get an account on www.dewaltservicenet.com and you can gaze confusedly upon the exploded view of the thing. It looks to me like the parts you'll need will run you from $100-$150 per motor. Depending on how much case you want around it.

[John Gutmann]

Isn't there something you can do to the brushes on the CIMs to give it higher RPMs? Also I recall seeing a DeWalt impact wrench that had 300 ft-lbs of torque on it. That should work.

[jakep]

Hey Kevin, WOW thanks for that awesome spreadsheet! And it's got those numbers and specifications we all love!

For those Dewalt motors/gearboxes:

What do you mean by "the parts you'll need will run you from $100-$150 per motor. Depending on how much case you want around it." ?

I was looking at the ServiceNet link you gave me, the "motor pack" part number is 389010-00 and the transmission part number is 388974-00. Buying one of each costs around $90. For our project, we have a protective aluminum shield around the underside of our robot. (Where the motors, electronics, batteries, etc. are mounted) So, I don't think we need much case around the motors/transmission. Would we be set if we just bought the motor pack and transmission?

[KenWittlief]

One of the limiting factors of electric motor rpm is the centrifugal force on the wires, armature, commutator...

With no load you can keep increasing the voltage, and the motor will spin faster, until you reach the point where the motor rips itself apart.

I think that is the real issue you will face.

Heat dissapation is the second. You can push the HP rating of the motor as long as you back off on the duty cycle, to give it time to cool between application of full power. This would more of less be a matter of trail and error. The failure mode for a motor that overheats is usually the wires melting/unsoldering themselves from the commutator, which is not easy to do since most motors are soldered with silver solder, and the wire is hooked or twisted to give it extra mechanical strength.

right in the same range of abuse, the varnish on the wires will start to cook off from the heat, making a wonderfull pungent scent that all FIRST teams have experienced at some point. If the varnish burns through and wire shorts to wire, then the motor turns into a space heater, and you get those wonderfull clouds of white smoke spewing like Mt Saint Helens.

[Kevin Sevcik]

Originally Posted by jakep

Hey Kevin, WOW thanks for that awesome spreadsheet! And it's got those numbers and specifications we all love!

For those Dewalt motors/gearboxes:

What do you mean by "the parts you'll need will run you from $100-$150 per motor. Depending on how much case you want around it." ?

I was looking at the ServiceNet link you gave me, the "motor pack" part number is 389010-00 and the transmission part number is 388974-00. Buying one of each costs around $90. For our project, we have a protective aluminum shield around the underside of our robot. (Where the motors, electronics, batteries, etc. are mounted) So, I don't think we need much case around the motors/transmission. Would we be set if we just bought the motor pack and transmission?

Have you looked at the Nothing But Dewalts white paper? You can see a bit of how complicated these Dewalt trannies can be. I suspect you need more than just the motor and tranny to make that work. Specifically, I think you need some of the plastic case parts to screw things into and other parts to hook up a shaft to.

So, I think you also need parts 17, 18, 19, 38, and maybe 24. 18, the gear case, would hold everything together. 38 is a nice round-ish spindle to use for a shaft. 17 and 19 almost certainly engage the spindle with what's probably an odd looking asymmetrical output on the tranny itself. 24 is a spring that probably helps the clutch work, but you can probably simply weld/glue and 19 together instead.

EDIT: oh. and 29. A nice left-hand screw to go into the spindle. And 22,to hold the spindle in. So all that works out to $125. I'd personally want the case as well to give it more support and some of the screws for it. That works out to $154 or so.

[jakep]

Thanks for the quick reply! ,

I looked through that whitepaper briefly, but it looks as if it's meant more for CIMs and FIRST motors hooked up to dewalt gearboxes. If we are using everything dewalt, won't it be easier? Also, would there be much machining to do? We don't exactly have easy access to a machine shop

"I'd personally want the case as well to give it more support and some of the screws for it. That works out to $154 or so." - The case is part number 1 right, but what about the other screws?

EDIT: Nevermind, I think the screws are part 6, correct?

[sanddrag]

Is there a specific reason you just can't use 4 CIMs (at 12V) to power this project?