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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
im trying to formulate a specific concise question here. but i might have to rewrite this top post a few times...
that being said, in my quest to maximize the flight time of a 1.5kg bicopter drone, im stuck with certain limitations. first the competition rules wont allow me to vary the props much, im pretty much stuck with a tractor and pusher tri-prop of 10 in dia, and 7 inches in pitch. therefore, i must optimize the battery, ESC, and motor. and really at this point its only the battery i need help understanding. the battery masses also seem to vary, +- 7 grams or so of the 250 gram nominal mass sizes, even within the same type and lot numbers.
The usual setup: 250 gram lipo battery to a 50 amp ESC and a RCtimer motor with 10x7 prop. however the batteries could be 4s, 2200mAH, 248 gram lipo, -or- 3s, 3000mAH, 242 gram lipo...
Calculations: lipo batteries of my type often equal 500kJ per kg, in specific energy. both the 3S and 4S whiegh 250ish grams...
The question: Is a 3 phase electronic speed control for brushless motors, effectively a step down SMPS? And by this i mean, will the 6 transistor h-bridge narrow the power pulses for the 14.8v lipo battey, and lengthen the pulses for the 11.1v battery? (assuming im trying to levitate the same 1.5kg in both cases.)
The simulated possibilities: Not ready yet...
the 3-cell battery, and the simulation from eCalc website.
the 4-cell battery, and the simulation from eCalc website.
keep in mind i dont trust these simulations, but changing one variable at a time, and then looking at the result over the course of several hours, can still be useful in spotting trends.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Carbon_Rod wrote ...
The calculator I referenced previously contains a list of common brands:
Intro to picking a prop:
On most smaller ESCs, the back emf is actually used to estimate motor rotation position without sensors.
im kinda stuck with the Master Airscrew 10x7 tri-prop, and I realize that the prop is the "transmission" which couples the motor to the air. but when he said "compounding the air around" I laughed, that's what ive been trying to explain to others... let me continue to clarify a specific point, with more facts and evidence... I kinda muddled that first post up a bit...
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
ESC's usually run at a fixed PWM duty cycle (depending on the input control value from the receiver), but since I assume you will have the props locked in a feedback loop (if you are hovering, the output power is fixed...) the duty cycle will increase with lower battery voltage to keep the same output power. This should translate to almost exactly constant power draw from the battery (regardless of voltage), save for slight differences in motor/esc efficiency with different duty cycles (in general efficiency will be lower for lower input voltage since there will be higher resistive losses)
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Steve connor made an excellent point, as usuall , on a related thread. He reminds us that the EMF and waveform matter, and that CEMF or poorly sent power can oppose rotation and generate heat. SO im willing to bet my castle ESCs are better than the cheap Chinese ones for this task.
tired...need sleep, ill think about these statements more and reply in the morning...
EDIT: everything is blurry due to lack of sleep, but the graphs indicate a great difference in RPM, maybe this explains the 4s 5.9 minute hover vs the 3s 6.4 minute hover? (14k vs 11k in the "optimum hover" text box) perhaps this is the explanation for a greater efficiency, same thrust generated for less power used? but the propeller is the same in both cases...
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
You're knee-deep in variables here. Once you get into a few percentage points efficiency then almost everything is up for grabs. Splitting the system apart into its components should help, as long as you don't take it too far.
If you are stuck with a particular geometry of prop, that is a *good thing*, because it means several fewer variables.
For a given weight of aircraft, ie downforce to generate, you should require a specific RPM, which will require a specific torque and hence power to generate.
This should mean you can split the system apart at the motor shaft, and optimise the motor and ESC to produce that shaft power at the lowest input power. I think you'll find that measuring the motor shaft power into a brake will be quicker and more diagnostic than measuring hover time of a bicopter. One specific test that will easier to do like that is to vary the ESC input voltage, measure the current, and being able to control the load precisely, get a direct plot of power efficiency versus supply voltage for your motor and power point. No 'was the air warmer' or 'was the prop soiled' questions for uncertainties about the hover time. What can make quite a good brake is a motor of a similar or larger size, run as a generator. Even though you might be a bit vague on the current output / shaft torque conversion, at least for constant current output at constant speed, you can reckon that shaft torque is constant. As you are trying to optimise hover time, not power consumption, it may help to further split the system at the battery terminals and see whether a 3 stack or a 4 stack provides more shaft Joules before the end point, when run through the motor efficiency curve.
Is a ESC a buck SMPS? Yes, at the fundamental level. Does that mean you can further split the system down into an ESC working into three loads? Almost certainly not. An ESC reacts to the motor position, and the timing of drive pulses will affect motor efficiency. Sensor ESCs read the motor position from an encoder, back-EMF ESCs infer it from the back-EMF. Either could be implemented well, or badly, or be a good match for your particular motor. There will be a difference between Castle and Chinese, there will be a difference between sensor and back-EMF. You either get chest-deep into ESC control algorithms, take lots of advice for which one on specialist forums, or try one of each brand into a motor+brake at your hover shaft power point (can you try before you buy?).
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yes, an ESC is basically three synchronous buck converters. If you increase the battery voltage, it will draw less battery current for a given motor power.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
I'm not sure how much use this is for your application, Patrick, but here is JohnF's schematic for a three phase controller for a turbo-molecular vacuum pump. There is more info regarding it in the 'vacuum technology' forum on Fusor.net, if you want to search for it.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Steve Conner wrote ...
Yes, an ESC is basically three synchronous buck converters. If you increase the battery voltage, it will draw less battery current for a given motor power.
I was trying to wrap my mind around the principle of operation, and I kept coming back to the Buck SMPS as the only way to really operate this system...
Dr. Slack wrote ...
You're knee-deep in variables here. Once you get into a few percentage points efficiency then almost everything is up for grabs. Splitting the system apart into its components should help, as long as you don't take it too far.
If you are stuck with a particular geometry of prop, that is a *good thing*, because it means several fewer variables.
For a given weight of aircraft, ie downforce to generate, you should require a specific RPM, which will require a specific torque and hence power to generate.
yep, about 2 months ago I gave up trying to find some "magic" prop, with ideal properties. there are combinations that aren't made, or are only made in tractor types... so I settled for 10x7 MA tri-blade as a good all-around prop.
Dr. Slack wrote ...
...Is a ESC a buck SMPS? Yes, at the fundamental level. Does that mean you can further split the system down into an ESC working into three loads? Almost certainly not. An ESC reacts to the motor position, and the timing of drive pulses will affect motor efficiency. Sensor ESCs read the motor position from an encoder, back-EMF ESCs infer it from the back-EMF. Either could be implemented well, or badly, or be a good match for your particular motor. There will be a difference between Castle and Chinese, there will be a difference between sensor and back-EMF...
I try and I try to explain this analog to digital and EMF issue to users of the cheap (13$) ESCs,but they just refuse to listen, needing 4,6, or 8 ESC for a machine is probly why they don't want to pay 70$ for one really good ESC type. these people would rather use a large battery inefficiently, rather than a lighter battery efficiently for the same or greater flight time...
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
I try and I try to explain this analog to digital and EMF issue to users of the cheap (13$) ESCs,but they just refuse to listen, needing 4,6, or 8 ESC for a machine is probly why they don't want to pay 70$ for one really good ESC type. these people would rather use a large battery inefficiently, rather than a lighter battery efficiently for the same or greater flight time...
... and if you set up a motor/brake combination to absorb X watts at Y RPM, does the $70 ESC draw less power than the $13 one? If so, how much? Without numbers, everybody is just shooting the breeze.
Now $13 and and $70 *are* solid numbers. Until you come up with some like-for-like power consumption figures, I have every sympathy with their cheapskate point of view.
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Is a 3 phase electronic speed control for brushless motors, effectively a step down SMPS?
, on a related thread. He reminds us that the EMF and waveform matter, and that CEMF or poorly sent power can oppose rotation and generate heat. SO im willing to bet my castle ESCs are better than the cheap Chinese ones for this task.
