We smoked an RS-550 motor last night, and were not near the stall point, as far as I can tell.
This is somewhat puzzling. We used the RS550s a couple years ago with no issues, so I thought I would throw this out here to get some opinions.
Here’s what we have:
Banebots RS-550 with a 25:1 gearbox, driving a 3" diameter timing belt pulley which raises our elevator.
The design load is supposed to be around 30 lb, but we have some binding, as I will show below.
When connected to 25:1 gearbox
772 RPM Free speed RPM
1763 in-oz Stall torque
85A Stall current
We were running the elevator up at roughly 8 feet in 2 seconds, using a 3" diameter pulley with a 1:1 timing belt between the pulley shaft at gearbox
Belt speed = 4 ft/sec
Pulley/gearbox RPM = 306 RPM (4 ft/sec/(pi * 3" diameter)*60 sec/min * 12 in/ft)
This is 40% of the free speed RPM.
From the motor curves:
The motor current should be roughly 51A (60% of stall current)
The motor torque should be roughly 1057 in-oz (60% of stall torque)
With a 3" diameter pulley, and ignoring gearbox efficiencies, this should create a lifting force of 66 lb (1057 in-oz/1.5" radius/16 oz/lb).
(Clearly we have some binding to get rid of.)
Even so, this is just over the normal, max power operating point of the motor.
Any ideas why this motor smoked?
You know how infants have “baby teeth”? Banebot motors have “baby smoke”.
They tend to vent a little smoke before running happily ever after (with no major discernible loss of performance). Did you actually destroy the motor, or just vent some baby smoke?
Most of the banebot motors (and the old FP motors) seem to work better if you run them in a little bit. I have never seen the baby smoke but I have certainly seen them change performance after running unloaded for 10 minutes or so…
Actually this is probably a good policy for all motors… a run in period.
At first pass, and without re-running your numbers to verify their accuracy, I would suggest you re-calculate and include the gearbox efficiency(s). I believe you will find you are a lot further along on the power curve than you think.
Secondly, how are you holding the lift stationary once it has reached it’s desired height? If you are simply having the motor hold, that is stalling the motor, although not at it’s max torque. But a stalled 550 with even a 20% load can burn it out. When the motor is stalled, it has no air flow. No air flow is not a good thing!
So that means that Bane Bot’s recommended configuration is to run the motor at 17000RPM, right? (And correct me if I’m wrong).
Well, if I was thinking straight (which I never do), then you were running the motor at 306RPM * 25:1 gear ratio = 7650RPM, well below their peak efficiency.
What I don’t get is how it isn’t dissipating enough heat at 7650RPM. That little fan is going pretty well at that speed and should be able to dissipate the heat.
I’m going to go with the reason being unknown, and go with what John said and call it baby smoke.
Maybe you can add one of those little muffin fans to the motor to force more air on it (like from the back).
Without a little more information, I can’t really complete the full puzzle. Like did it smoke while it was running the elevator up, or after it stopped at it’s set point?
Yesterday we fired up a brand new Tetrix motor and it very quickly started smoking. Upon close examination we noticed the smoke was coming from the factory attached transmission and not actually the motor.
Wow, great responses. I can’t wait to tell my students about baby smoke.
The motor is still working, but I have no tools to tell how much it has degraded. There was a very noticable burnt motor smell, and subsequent runs made a little smoke each time. I will check tomorrow, after a cool down.
I agree that with real-life efficiencies, we are probably running much further down the power curve than we want. The fact that we are getting 300 RPM out of it says that it can’t be too far down the curve.
Hopefully fixing the binding problem will move us to the left side of peak power, and stop us from pumping 100W into the little guy. Still, I’d think you should be able to run a motor at 40% free speed RPM/60% stall torque all day.
With regard to peak efficiency, we were not even trying to operate at that point.
Also, we have a separate brake system, so at no time were we holding torque with no motion.
The timeline was
Time 0: apply 12V (direct from battery), elevator starts to ris
Time 1 sec: smoke starts, elevator about 1/2 way up
Time 2 sec: remove battery power, still smoking
Time >2 sec, smoke tapers off, smell expands through the shop and software team comes over to find out what is going on.
The guidelines I use say to target motor operation at peak power (or a little to the left on the RPM/torque curve, to give wiggle room for gear efficiency and wire impedance loss). This is at roughly 50% stall torque and 50% free speed.
I know we are above the 50% torque point, but not by much. I still think this would be a normal operating point for a motor.
Am I off anywhere? Do any of you use different guidelines for gearbox design?
Note, we are actually targeting to operate at <30% of stall torque, assuming we get rid of the racking/binding. I’m just curious as to what other people use.
J,
Is this a simple arm? Is your required torque calculated with the arm straight out or at some other angle? Are the holes in the motor body blocked in your design?
Where did you get these guidelines? Most motors are not designed for continuous operation at peak power. Continuous operation is usually closer to 80% free speed.
Please note that I am not saying you can’t run them at peak power for short periods. I’m just questioning the “run them all day long” statement.
This is an forklift/elevator belt running off a 3" diameter timing belt pulley, so there is near continuous load, except for a small increase when the 2nd section of the elevator starts to lift.
The motor is out in the open with no obstruction to the air-in/smoke-out holes.
Ether - you are right. The lecture materials I remembered had alot of discussion about finding the peak power point, but also noted the manufacturers typically recommend a normal operating speed at some high percentage of the free speed, such as 80%.
The good news: we should be closer to that 80% before we are done. The bad news, operating at 40% may be too much for the RS-550s to handle, even for 2 seconds at a time.
OK, I crunched some numbers based on what you gave us + a couple assumptions on stuff you didn’t give us.
SR550 with 25:1 reduction should yield 9.185Lb/ft of torque. Assuming you used a 25:1 2 stage planetary trans for reduction, your actual usable torque should be:
9.185*.81= 7.44Lb/ft of torque.
Spinning a 3" dia. pulley yields 29.76 Lb/ft of maximum torque.
I verified that you are running at about 60% torque capacity.
So the load on the motor/trans/pulley is roughly 17.86 Lb.
Now, from all I have read, optimal design is to keep the load at 50% or less of max. So, if you can reduce the load via. weight reduction, alignment, lubrication etc. to less than 15 Lbs, you lift should be in good shape.
Based on all these calculations, I too would say you had a bad case of “Baby Smoke”.
BTW, If you are pulling 51 amps (85*.6), then you are dissipating as much as 612 watts. No wonder it’s smoking! It won’t do this very long, so make sure to reduce the load as much as possible.
Ok Jeff,
I think you are on the right track. One of the issues we have noted in the past, is the efficiency of the internal fan at different speeds and it’s effect on internal heat. Slow speeds just don’t get the job done. So it’s a combo problem with high heat generated and poor fan efficiency occurring at the near the same RPM. The holes in the end plate opposite the motor leads form the inlet for cooling air. If those are blocked, very little air moves.
If the RS550 is operating at 40% rated speed, then it’s pulling ~51.6 amps @ 12 volts or ~619 watts from the power supply. But not all that power is being dissipated as heat. 237 of those watts are output power at the motor shaft. So “only” 382 watts are being dissipated as heat in the motor. Still too much, I know.
