RS-550 current draw

[Jon Stratis]

I've got a question for the brilliant minds here at CD... How much current is too much for a 550 when running it at full speed (not stalling) for 2+ minutes? In other words, any idea what the limit is before you burn out the motor?

On our shooter, each 550 is drawing about 7A when the shooter is at full speed (and 8-9A when it's spinning up the flywheel to get to full speed). There is as little binding as possible in the system (we had more binding in our shooter for Lunacy than we do here), we checked. Instinct tells me we should be fine with that, but I figured it wouldn't hurt to come on here and see what other teams have experienced!

PS. I'm going to try to get the team to launch a ball straight up at full speed at some point at North Star, just to see if we can hit the rafters in Mariucci Arena! Our robot is going to have no trouble launching the ball to the other side of the field... now programming just has to slow it down so we can actually score.

[Ether]

Originally Posted by Jon Stratis

I've got a question for the brilliant minds here at CD... How much current is too much for a 550 when running it at full speed (not stalling) for 2+ minutes? In other words, any idea what the limit is before you burn out the motor?

On our shooter, each 550 is drawing about 7A when the shooter is at full speed (and 8-9A when it's spinning up the flywheel to get to full speed).

First, a point of clarification: if it's drawing 7 amps, it's not running at "full speed". I think you meant "full voltage applied". It's running at 93.3% of full speed. That's faster then the max efficiency point and generating less waste heat (watts_in minus watts_out). So at that speed, the motor should be able to run continuously, assuming it is vented properly!

Read this:
http://www.chiefdelphi.com/forums/sh...72&postcount=5

Motor Calculator  build MCALC32ab 1/19/2012 610pm

M5-RS550-12 (-B) @ 12.00 volts:

@ free (no load):
   oz-in      Nm     rpm    rpm%    amps   watts out    watts in    eff%
     0.0   0.000   19300   100.0     1.4         0.0        16.8     0.0

@ stall:
   oz-in      Nm     rpm    rpm%    amps   watts out    watts in    eff%
    70.5   0.498       0     0.0    85.0         0.0      1020.0     0.0

@ max power:
   oz-in      Nm     rpm    rpm%    amps   watts out    watts in    eff%
    35.3   0.249    9650    50.0    43.2       251.5       518.4    48.5

@ max efficiency:
   oz-in      Nm     rpm    rpm%    amps   watts out    watts in    eff%
     8.0   0.057   17105    88.6    10.9       101.4       130.9    77.5

Select input:
1)oz-in  7)watts_in    3)rpm    5)amps   8)eff%
2)Nm     6)watts_out   4)rpm%   9)volts  a)rpm&ozin   5


enter amps: 7

M5-RS550-12 (-B) @ 12.00 volts:
   oz-in      Nm     rpm    rpm%    amps   watts out    watts in    eff%
     4.7   0.033   18007    93.3     7.0        62.9        84.0    74.9

[Jared Russell]

At 7A draw, you are at less than 7% of stall. You are drawing 84W of electrical power, and outputting ~61W of mechanical power (taking a quick look a the speed/torque curve). So there is 23W of heat being generated. However, your motor is spinning at pretty close to its full speed, so it will be cooling itself pretty well.

You ought to be fine. That said, why bother running your shooter for all 2+ minutes? You are likely to be shooting for a fairly small portion of that.

[Jon Stratis]

Thanks for the quick replies! We probably won't be running it the entire match (it only takes a second or two to ramp up to full speed), but we've learned to always plan for the worst case scenario!

And yes Ether, I was referring to full voltage applied on the motor. Generally speaking, we're only really concerned with the actual end speed of the shooter, not the full range available from the motor. So for us, "full speed" means the maximum speed of the shooter, not the maximum speed of the motor.

And there's plenty of room around the vents on the motors - we made sure of that!