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#1
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Stalling the BaneBot 775 motors
Hi all,
Part of our design calls for holding the BB775 motors at stall at ~20%-40% of the motors maximum stall torque, by controlling the voltage applied to the motor with the Jaguars. reading this thread, I understand that if properly tested, this method may work with the fisher price motors. I would appreciate your comments and insight from experience regarding attempting this with the BB 775. Things we think we have going in our favor:
Again, your thoughts and insight are highly appreciated. -Leav |
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#2
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Re: Stalling the BaneBot 775 motors
No motor is designed to be stalled for any amount of time. Any power supplied to the motor when it is not turning turns directly to heat. All of the BaneBot motors and FP motors are fitted with a cooling impeller inside the motor. Unless the motor is running at a half decent speed it lacks sufficient cooling to allow operation. You may get away with it for a little while but you WILL eventually cook your motors. I would suggest you look into a wormgear drive or ratcheting system to prevent backdriving rather than relying on stalling the motor.
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#3
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Re: Stalling the BaneBot 775 motors
Also, the Jaguars have a "brake mode" which shorts the inputs to the motor when it is not running. If you spin a motor shaft, it acts like a generator. By shorting the leads you essentially load this generator and it resists rotation of the shaft. This may help prevent backdriving the motor if there is a good gear reduction on it.
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#4
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Re: Stalling the BaneBot 775 motors
As an add on to this thread - if the motors are running at 5-10% of stall torque does this problem still really exist?
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#5
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Re: Stalling the BaneBot 775 motors
Well at 20% of stall current (130A) and 12V, you have a power consumption of ~300W. To put it in perspective, thats about the same amount of heat generated by 10 soldering irons. This heat is localized in the armature and brushes of the motor which can melt the laquer that insulates the windings and overheat the brushes. If you insist on stalling them, I would make sure you have a couple muffin fans blasting air into/out of the motor case as much as possible. But again, stalling the motors is a poor engineering practice. I would strongly suggest you modify the design to include a ratchet, one-way bearing, worm or similar gearset to limit backdriving.
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#6
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Re: Stalling the BaneBot 775 motors
The RS 775 motors do not have a stall current upwards of 500 amps...
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#7
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Re: Stalling the BaneBot 775 motors
?
See above ^ 130A 130A*0.2*12V = 312W |
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#8
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Re: Stalling the BaneBot 775 motors
Quote:
If stalled at 20% of this rated torque, the voltage will be 20%*12 and the current will be 20%*86.7, so the input power will be (0.2)(12)(0.2)(86.7)=41.5 watts, not 300 watts. Even if the OP meant 20% of the 18V stall torque, the power would be (0.2)(18)(0.2)(130) = 93.6 watts. Last edited by Ether : 26-01-2011 at 20:05. |
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#9
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Re: Stalling the BaneBot 775 motors
Ah, quite right - I just took the spec off BaneBots and forgot it is listed at 18V. I'm not sure I agree with you on the calculation method. With your method you would be calculating power at 4%.
Max Power = Volts * Amps = 86A * 12V = 1032W 20% of 1032W = 1032 * 0.2 = 206.4W (312W for 130A) By your method, (86 * 0.2) * (12 * 0.2) simplifies to 0.04 * (86 *12), the 0.04 representing 4% rather than 20% My understanding is that the Jaguars control output by pulsing the power from the battery. They don't actually control the voltage or current. They are essentially a high speed switch which opens and closes at a specific frequency. The output of which is the percieved power to the motor and is essentially the RMS power of the frequency that it is switching at. At full power the switch is closed and supplying full power from the source. At 50% the switch is only closed 50% of the time and only 50% of the power is getting through per unit time. Likewise for other power factors. This is why the old Victor 883s had that annoying whine to them. Same with a cordless drill when you run it slowly. That is the mechanical manifestation of the switch power vibrating the motor windings and conductors just like the 60Hz humm you hear in electrical transformers and panels. Then again, my background is Mechanical - I could be wrong but this is my understanding of how they work. |
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#10
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Re: Stalling the BaneBot 775 motors
Quote:
If you only want 20% of that current, you need to reduce the effective voltage to 20% of 12. So (0.2)(12)(0.2)(87) is the power draw at 20% of stall torque. Quote:
Quote:
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Last edited by Ether : 26-01-2011 at 20:42. |
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#11
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Re: Stalling the BaneBot 775 motors
I dunnooo... I know and totally agree with you on Ohm's Law and how it affects current. My thinking however, is that when the rotor is stalled, for each pulse the applied voltage per pulse would still be 12V and the current drawn would still be 87A for each finite pulse. If those pulses occurred only 20% of the time, they would result in 20% of the electrical power consumption. I have no doubt the "percieved" amperage and voltage change but if you look at the microcasm (?) of each pulse, the resistance of the Jaguar does not vary so neither would the supplied voltage/current per unit pulse. I don't mean to argue, i'm just trying to understand it from an operational aspect. If the Jaguar controlled power output like a Variac or a transformer with a rectifier - changing the actual DC voltage then I would completely agree with you but it doesn't. The jaguar doesn't actually change the voltage of the applied power, only the duty cycle it is applied.
I'll suppose I have to read up on speed controllers a bit more. Do the Jaguars have any published data that shows output power vs duty cycle? |
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#12
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Re: Stalling the BaneBot 775 motors
Hmm, seems like there's more to this than either of us are accounting for - seems inductance and chopping frequency play a part in this too
I hate Inductance... and capacitance... I still think that a 20% output command sent to a Jaguar will result in a ~20% power output for that motor and, 20% of power consumption at stall. In any case- Stalled motor = heat buildup = bad |
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#13
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Re: Stalling the BaneBot 775 motors
If I understand correctly, the motor does draw full power during the on period, but since there is an off period as well the effective power would be the time-wise average of these two values.
Thanks everyone for the replies. we'll be treating this with as possible, but with extreme caution. I'm reading the 2011 manual to try and figure out if cooling fans are allowed. |
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#14
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Re: Stalling the BaneBot 775 motors
what about a system where the motor is being quickly oscillated between the tow extremes, thus driving the lift down and lifting it. then it would be moving, and it would move the lift or whatever you are using up and down a few inches.
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#15
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Re: Stalling the BaneBot 775 motors
Quote:
Additionally, i'm speculating that a fast switch from "drive down" to "drive up" would probably load the motor heavily, maybe even more than just stalling it (since you are trying to turn it one way, but the inertia is pushing it another). -Leav |
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