FP 2011 (00801-0673) versus 9015 power

[de_]

fyi: We had a 9015 on before (16K no load), and a 0673 on afterwards (19k no load). The extra rpm was surprising visible (actually way too fast full power) and the power required to sustain the higher rpm was clearly higher (wheel hubs have wind resistance, also the AM planetary has resistance presumidly proportion to the speed they turn). We had a clip on amp meter monitoring amp draw and it would go over 30 amps peak when a ball when through until the rpm recovered. And of course they had to test a ton of balls.. So things got hot.


Update Clarification (sorry): We had the motor driving a AM 3.7/1 planetary driving a chain based gear reduction of 1 to 0.7 (ie wheel turns 0.7 rev per rev of output of AM gearbox). Our wheels are 8". Clearly the motors have to overcome planetary loses, chain loses and wheel turning loses so they are not running at no load speed but they are still pretty fast. We added a second motor also with a fan to spread the heat load between the motors (until we can prove one motor can survive). My best guess we could get away with final chain ratio of 2/1 reduction and one motor.

Last I heard, these motors were only 60% efficent so even at 15 amps, 0.4 x 15 amps x 12v = 72 watts (compare to a 60w light bulb) so things heat up. I've never regretted adding a (little victor) fan to blow in and around one of the brush cooling slots !

[Ether]

Originally Posted by de_

Current design has a 0.7/1 speed reduction

That sounds way too fast.

What is your wheel diameter ?

[nitneylion452]

.7:1? That puts the motor at ~21k RPM and a measly 44.6 oz-in of torque (assuming no loss in the gearbox. More practically, you could expect 40.1 oz-in assuming 10% loss). To put that into perspective, the Hitec HS-322 HD servo has 53.3 oz-in of torque.

[de_]

Sorry, updated above post to clarify that we have a AM planetary providing 3.7/1 reduction followed by a sprocket based reduction of 1.42 (1/0.7) (=5.2 net) driving 8" wheels.

[nitneylion452]

We are running 2 RS-550s (slightly more powerful than -9015) for our shooter, each drives 2 8" wheels and we haven't had any heat issues. We run them through 5:1 reduction BB P60 gearboxes and direct drive to the wheels. I would look at your shooter a little more closely if you're having issues with your motor heating up.

[de_]

The heating was very apparent when we upgraded our original single motor 9015 to a single 0673 for hopefully improved recover time. The motor direct drove the shooter via the AM 3.7/1. When a ball was shot with the 0673, the current peaked at 30 amps+ and then ramped back down to normal over the ~ 3-4 seconds recover time. If during testing, you shoot a couple of dozen balls one every 3 seconds, that's a lot of heat the motor has to dissipate. In the competition, I suspect we would be lucky to shoot 6 balls over 2 minutes (at nowhere near full power). My original post was basically to highlight the quite visible effect of the higher KV motor had to our wheel speed (too much, had to slow it down with sprockets) and the increased power consumption (and risk to the brushes/motor life) that followed the extra speed.

[Ether]

Originally Posted by de_

The heating was very apparent when we upgraded our original single motor 9015 to a single 0673 for hopefully improved recover time. The motor direct drove the shooter via the AM 3.7/1.

Ignoring friction for the moment:

20770.....0673 motor free rpm
3.67......gear reduction
5659......wheel free rpm
94.3......wheel free rev/sec
8.........wheel diameter, inches
2.09......wheel circumference, feet

197.6.....wheel tangential free speed, ft/sec

Why did you gear it so fast?

[de_]

Because that was the only remotely appropriate gearbox we had in inventory ! We hoped we could get away with a single FP or 550 size motor to minimize high up weight. We had hoped to reduce chain related losses (and sprocket & chain weights) by direct driving (through the AM). We also had no idea what sort of drop in RPM a largely unloaded motor would incur trying to shoot the ball (drop in RPM means drop in exit velocity of the ball relative to the no-load rpm). Heh I'm not in mechanical But then we found the recover time slower than we would like. Shooting distance was limited to 14 ft which some team members wanted more so we put on a more powerful motor. Unfortunately its KV was higher.

[Ether]

The way to get fast recovery time for a shooter wheel is to use a reasonable gear ratio and control the motor closed-loop at an operating point where it has sufficient headroom to rapidly accelerate the wheel back to that operating point after being disturbed. Some design guidelines were suggested here.

[de_]

Great points you make. We actually do have an encoder and PID on it to set and manage target speeds based on shooting distances. When shooting from any distance that has a reasonable probability of actually scoring (5-12ft), the wheels are not spinning anywhere near full power and the PID code does attempt to aggressively recover the target RPM. So I think we ended up close to what you recommend.

[Ether]

Originally Posted by de_

...When shooting from any distance that has a reasonable probability of actually scoring (5-12ft), the wheels are not spinning anywhere near full power

May I ask:

1) how fast are the wheels spinning under those conditions?

and

2) under those conditions, what voltage is being supplied to the motor (or how many amps is the motor pulling) ?

[de_]

You may ask... It may take me a day or so to get the data. I need to know it for future reference as well. I'm also going to try to get the RPM of the wheels for the 7', 12' and max power off our encoder data, maybe even how much the wheel slows down shooting a ball at the various distances. Did a lot of shooting this afternoon at 7 and 12 feet. Motors completely cool (with fans), recover was 2 seconds or less.