News:


  • March 29, 2024, 03:46:53 AM

Login with username, password and session length

Author Topic: Picking a voltage for the system.....  (Read 6333 times)

Offline REX1945

  • AMA 19945
  • Trade Count: (0)
  • Commander
  • ****
  • Posts: 145
Picking a voltage for the system.....
« on: October 08, 2015, 05:43:01 AM »
Years ago when Mr Tesla invented the polyphase induction motor, we could
convert brake HP into watts and pick out an electric engine that may work if you could get it to start.

Now we have this system of pulse width modulation that seems to somehow defeat the specification of an electric motor in watts.

My friend recently acquired an E-flite 90 system with an 80 amp (Castle Phoenix)  speed controller. I'm thinking I should try a 6s LiPo battery to power it and keep the current within range. He had a 16-10 (APC) prop for it which I think may be a bit too steep.

As I understand it motor KV multiplied by voltage gives the max RPM available when free-wheeling such that the back EMF offsets the forward EMF. Actual performance would be a bit less; if violently different either the motor overheats, the speed controller dies, or the battery goes into either Tesla mode or "Mark Scarborough" mode (slow idle speed).

The fellow at the local hobby store wants us to buy a 120 amp speed controller.

In previous discussions about (IC) props, Brett indicated the pitch x RPM gets you within about 5% of the actual airspeed of an IC system.  This is somewhat a function of the torque curve of the particular engine or engine/pipe combination since you can get usually only IC RPM's while the plane is on the ground.  Dr. Walker probably had a way to measure it when he was flying "back in the day" of IC.

The only other way I can see to "back into the problem is to look up the HP of an OS .95, convert to watts, and get the current. (1198 watts - 54 Amp).

Numbers so far :

Motor 325 KV
ESC  80 AMP
Prop  16x10
Airplane weight : 9 Lbs.
Limiting RPM : 6012
Airspeed limit : 68

Using 10 Oz per watt : 64 AMP

Rex

PS : Can I reverse the engine by swapping (any) two leads ?


Offline Crist Rigotti

  • 23 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 3856
  • Electric - The future of Old Time Stunt
Re: Picking a voltage for the system.....
« Reply #1 on: October 08, 2015, 07:42:06 AM »
Is this for an R/C airplane or a giant scale model?
Crist
AMA 482497
Waxahachie, TX
Electric - The Future of Old Time Stunt

Offline Target

  • C/L Addict
  • 2019 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 1692
Re: Picking a voltage for the system.....
« Reply #2 on: October 08, 2015, 07:59:16 AM »
Yes, swapping any two of the three leads between esc and motor reverses the motor direction.
Regards,
Chris
AMA 5956

Online Howard Rush

  • 22 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 7805
Re: Picking a voltage for the system.....
« Reply #3 on: October 08, 2015, 08:54:29 AM »
You can also select direction in ESC software.
The Jive Combat Team
Making combat and stunt great again

Offline Target

  • C/L Addict
  • 2019 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 1692
Re: Picking a voltage for the system.....
« Reply #4 on: October 08, 2015, 10:04:09 AM »
And doing the ESC software is easier sometimes if the motor wires are very short, and connection ahead of the firewall.
Found that out on my last plane during the fitting.
So if you  want to switch between a tractor and a pusher prop, and the wires are tough to get at, you might keep that in mind and bring the ESC program card with you to the field...

R,
Chris
Regards,
Chris
AMA 5956

Offline Tim Wescott

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 12804
Re: Picking a voltage for the system.....
« Reply #5 on: October 08, 2015, 11:12:58 AM »
Years ago when Mr Tesla invented the polyphase induction motor, we could
convert brake HP into watts and pick out an electric engine that may work if you could get it to start.

Now we have this system of pulse width modulation that seems to somehow defeat the specification of an electric motor in watts.

The only thing that I know of that "defeats the specification of an electric motor in watts" is the fact that our system present the motor with varying voltage, while the nameplace specifications of a typical induction machine would list voltage, line frequency, current, and output power.

If our motors came specified at one voltage only, then they could easily and honestly be rated in watts.

Many of them are still fairly honestly rated in watts, if you remember that the power rating is for the motor with the maximum number of cells that are called out in the specification.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Mark Scarborough

  • 2015
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 5918
Re: Picking a voltage for the system.....
« Reply #6 on: October 08, 2015, 07:51:07 PM »


Motor 325 KV
ESC  80 AMP
Prop  16x10
Airplane weight : 9 Lbs.
Limiting RPM : 6012
Airspeed limit : 68

Using 10 Oz per watt : 64 AMP

ballpark,, for 3D type flying or cl stunt,, 150 watts per pound,, for sport flying 100 watts per pound,,

with a KV of 325X 25Volts = max theoretical rpm of 8125 rpm,, thats pretty slow,, you would need some serious pitch to make any kind of flying speed,, so a 10 picth prop isnt out of the realm of reality,,

from eflights web site,,

Recommended Prop Range:   16x8 to 18x8
Voltage:   21.6–31.2V
RPM/Volt (Kv):   325Kv
Resistance (Ri):   .02 ohms
Idle Current (Io):   2.00A @ 10V
Continuous Current:   50A
Maximum Burst Current:   65A (15 sec.)
Cells:   6S–8S Li-Po or 18–26 Ni-MH/Ni-Cd
Speed Control:   85A High Voltage Brushless
so,, a 16x10 is in the lower range of listed specs,, or low mid range more exactly,,

what kind of plane is this,,

this stuff is a LOT simpler than you are making it out to be buddy,,

and for the record,, I am not worthy of having a "battery voltage drop off " named after me,, even though I lost another pack in Salem this way,, lol
For years the rat race had me going around in circles, Now I do it for fun!
EXILED IN PULLMAN WA
AMA 842137

Offline REX1945

  • AMA 19945
  • Trade Count: (0)
  • Commander
  • ****
  • Posts: 145
Re: Picking a voltage for the system.....
« Reply #7 on: October 08, 2015, 09:04:08 PM »
Is this for an R/C airplane or a giant scale model?

Yes, it is a Senior Telemaster, high wing RC. Obviously, someone like Howard Rush could calculate the (maximum) airframe drag coefficient to keep within current range for the voltage chosen. The same would apply to a CL airplane.  Right now, I wouldn't know how to size the prop, voltage, and ESC size for a 45 Oz Strathmore with an E-Flite 25. It probably needs a 4s, but can I get away with a 3s if I keep the pitch down.

When planes are large, it's quite expensive to gear up with undersized components.  However, I believe the LHS is sandbagging it to stick in a 120 amp speed controller.

On the other hand, I don't want to stick in a 6s batter for $130+ just to fry the speed controller if too much power is required.

What voltage would you pick ?


Rex

Offline Mark Scarborough

  • 2015
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 5918
Re: Picking a voltage for the system.....
« Reply #8 on: October 08, 2015, 09:56:39 PM »
well what I recommend, is a few minutes on Google,,
here
http://www.wattflyer.com/forums/showthread.php?t=59731

lots of good info there,, I would read it,, kick the lowest power setup and highest,, the middle ground should be a reasonable start,

I think on a Telemaster, I would think slow speed,, flatter prop,, more voltage,,
I would probably look at something like a 16 x 6 or 17 x 6,, on 8S.... or two 4S packs in series ( 4S packs work well with an Eflight 25 in a 600 inch stunter  so some motivation could be created for two 4S instead of a single 8S pack) A setup like this will "drive " the plane better at the lower speed envelope,, IMHO,, but !! I have not flown something this size on electric,, hence my google search,,
oh,, and heres another possible source for info,,
http://www.wattflyer.com/forums/showthread.php?t=74966

I will help with theoretical knowledge ( sic ) ,, as much as I can Rex,,
For years the rat race had me going around in circles, Now I do it for fun!
EXILED IN PULLMAN WA
AMA 842137

Offline Tim Wescott

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 12804
Re: Picking a voltage for the system.....
« Reply #9 on: October 08, 2015, 10:09:56 PM »
However, I believe the LHS is sandbagging it to stick in a 120 amp speed controller.

From the specs that Mark is quoting the maximum burst current is 65A -- that's the current that you can sustain for a few seconds, but would fry the motor if you kept it up.  So 120A to the motor is "fry it right now!".

I'm not sure why they're calling out an 85A speed controller for a 65A motor -- probably because motors are more robust to overloads than transistors, so you want the margin to go to the ESC.

It could be that the only high-voltage ESC the hobby shop had in stock was the 120A unit, or the guy might not know his stuff (I've been behind a counter -- trust me, it does not make you a genius), or he was, indeed, just sandbagging.  On the bright side, a 120V ESC probably won't hurt anything.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Fred Underwood

  • Trade Count: (0)
  • Captain
  • *****
  • Posts: 807
Re: Picking a voltage for the system.....
« Reply #10 on: October 08, 2015, 10:15:45 PM »
An example here



more discussion here
http://www.wattflyer.com/forums/showthread.php?t=59731

and much more on searching electric conversion of Senior Telemaster.
Fred
352575

Offline REX1945

  • AMA 19945
  • Trade Count: (0)
  • Commander
  • ****
  • Posts: 145
Re: Picking a voltage for the system.....
« Reply #11 on: October 09, 2015, 10:11:25 AM »
well what I recommend, is a few minutes on Google,,
here
http://www.wattflyer.com/forums/showthread.php?t=59731

lots of good info there,, I would read it,, kick the lowest power setup and highest,, the middle ground should be a reasonable start,

I think on a Telemaster, I would think slow speed,, flatter prop,, more voltage,,
I would probably look at something like a 16 x 6 or 17 x 6,, on 8S.... or two 4S packs in series ( 4S packs work well with an Eflight 25 in a 600 inch stunter  so some motivation could be created for two 4S instead of a single 8S pack) A setup like this will "drive " the plane better at the lower speed envelope,, IMHO,, but !! I have not flown something this size on electric,, hence my google search,,
oh,, and heres another possible source for info,,
http://www.wattflyer.com/forums/showthread.php?t=74966

I will help with theoretical knowledge ( sic ) ,, as much as I can Rex,,


Thanks for all the help Mark. You fly so well and your planes are so pretty that about the only way I can beat you is if you run out of juice.

The idea of using 2 4s in series in order to share (batteries) with other planes is certainly one that would have otherwise passed me by.
And, going with a flatter pitch and higher RPM's helps most everything except a 4s slimer....

I guess I'll have to break down and get a computer interface for the ESC, since changing stuff around is probably a good reason to learn to program the ESC. Somehow, at my age, I am a bit reluctant because of all the meaningless "computer techniques" that I learned that were only valuable for the duration of a couple of projects. On the other hand, the skill of sanding balsa wood and sniffing glue has never lost it's value. Ooops...I'm probably the last of the glue sniffers now that SIG no longer sells silkspan.......

Thanks again for the help.

PS : Possibly Dane has the new record for running out of juice.


Offline Tim Wescott

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 12804
Re: Picking a voltage for the system.....
« Reply #12 on: October 09, 2015, 11:06:22 AM »
I guess I'll have to break down and get a computer interface for the ESC, since changing stuff around is probably a good reason to learn to program the ESC. Somehow, at my age, I am a bit reluctant because of all the meaningless "computer techniques" that I learned that were only valuable for the duration of a couple of projects.

So, ESC in one hand, punched card in the other, confused look on the face in between?
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Mark Scarborough

  • 2015
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 5918
Re: Picking a voltage for the system.....
« Reply #13 on: October 09, 2015, 02:33:42 PM »
So, ESC in one hand, punched card in the other, confused look on the face in between?
that right there is funny,, I dont care who you are LOL

ok well maybe you have to understand early programming ,, but still,, funny,

programming is easy,, its like everything,, learn the language,, ( everything is a language inherant to the endevour,, sewing,, flying,, glow motors,, ),, then the programiing will be easy
For years the rat race had me going around in circles, Now I do it for fun!
EXILED IN PULLMAN WA
AMA 842137

Offline REX1945

  • AMA 19945
  • Trade Count: (0)
  • Commander
  • ****
  • Posts: 145
Re: Picking a voltage for the system.....
« Reply #14 on: October 09, 2015, 08:33:19 PM »
So, ESC in one hand, punched card in the other, confused look on the face in between?

Tim,

No, that's the IBM folks with their EBCDIC code.  My stuff was the pure EIA RS 244 or 348 (ASCII) with or without a
parity punch in column nine.....

Now, getting back to model airplanes and Mr Tesla :

Are these 3 phase motors AC or DC ?    I assume they are pulsed DC (Pulsed Width Modulation) to achieve the speed control, sort of
like stepping motors.

If they are AC induction motors, I wouldn't know what all the rare earth magnets are for.

Rex

PS : Sad to hear about Randy's plane.  Did the engine (motor) survive with only a bent shaft ?

Offline Tim Wescott

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 12804
Re: Picking a voltage for the system.....
« Reply #15 on: October 09, 2015, 10:58:58 PM »
Are these 3 phase motors AC or DC ?    I assume they are pulsed DC (Pulsed Width Modulation) to achieve the speed control, sort of
like stepping motors.

If they are AC induction motors, I wouldn't know what all the rare earth magnets are for.

Our brushless motors are not induction motors -- as you've noted, there's magnets, not a squirrel cage.

As to whether they're AC or DC, there's some terminology collision, so it kind of depends on how you hold your mouth.

If you were going to buy something like one of our motors from an industrial supplier, they'd be called a "brushless DC" motor.  That doesn't mean that you just feed them DC -- that means that the coils are wound so that the motor back-EMF on each coil isn't a nice sine wave -- instead, it's a so-called trapezoid wave that's flat on top and bottom.  That makes driving it from a DC source easier (that's where the "DC" in "brushless DC" comes from).  There's a kind of brushless motor called "brushless AC", which has a sinusoidal back-EMF, and are basically just permanent-magnet, fixed-field synchronous motors.  We don't use them, but they're out there in industrial applications.

After all that, just to confuse things -- the voltage across the coils of our motors alternates, as does the current.  So from that point of view they're strictly AC machines.

(In case anyone's wondering how this relates to DC brushed motors, of the sort where you just hook a motor up to a battery and watch it go -- the brushes in a DC brushed motor switch the input voltage to the motor coils in a manner pretty close to what an ESC does.  You can think of the brushes as a mechanical ESC, or you can think of an ESC as a set of "electronic brushes", and either way you'd be pretty close to correct.)

Our brushless motors are only sorta-kinda like steppers.  In a sense they're like bipolar stepper motors, but they're optimized for entirely different things than steppers are optimized for, and they can put out hugely more power for their size than steppers can.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Mark Scarborough

  • 2015
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 5918
Re: Picking a voltage for the system.....
« Reply #16 on: October 09, 2015, 11:52:41 PM »
Our brushless motors are not induction motors -- as you've noted, there's magnets, not a squirrel cage.

As to whether they're AC or DC, there's some terminology collision, so it kind of depends on how you hold your mouth.

If you were going to buy something like one of our motors from an industrial supplier, they'd be called a "brushless DC" motor.  That doesn't mean that you just feed them DC -- that means that the coils are wound so that the motor back-EMF on each coil isn't a nice sine wave -- instead, it's a so-called trapezoid wave that's flat on top and bottom.  That makes driving it from a DC source easier (that's where the "DC" in "brushless DC" comes from).  There's a kind of brushless motor called "brushless AC", which has a sinusoidal back-EMF, and are basically just permanent-magnet, fixed-field synchronous motors.  We don't use them, but they're out there in industrial applications.

After all that, just to confuse things -- the voltage across the coils of our motors alternates, as does the current.  So from that point of view they're strictly AC machines.

(In case anyone's wondering how this relates to DC brushed motors, of the sort where you just hook a motor up to a battery and watch it go -- the brushes in a DC brushed motor switch the input voltage to the motor coils in a manner pretty close to what an ESC does.  You can think of the brushes as a mechanical ESC, or you can think of an ESC as a set of "electronic brushes", and either way you'd be pretty close to correct.)

Our brushless motors are only sorta-kinda like steppers.  In a sense they're like bipolar stepper motors, but they're optimized for entirely different things than steppers are optimized for, and they can put out hugely more power for their size than steppers can.

Tim,, did you get kicked out of the tv room or something,, man I bet you have hours in these responses,, LOL ( which are great by the way ),, but seriously,, go build something that will fly as good as you can now ,, sheesh
Keep on keepin on man,,
hoping to make a trip to Estecada in a month or so,, maybe do some flying?

For years the rat race had me going around in circles, Now I do it for fun!
EXILED IN PULLMAN WA
AMA 842137

Offline Mark Scarborough

  • 2015
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 5918
Re: Picking a voltage for the system.....
« Reply #17 on: October 09, 2015, 11:53:47 PM »
by the way,, informative posts Tim,,
For years the rat race had me going around in circles, Now I do it for fun!
EXILED IN PULLMAN WA
AMA 842137

Offline REX1945

  • AMA 19945
  • Trade Count: (0)
  • Commander
  • ****
  • Posts: 145
Re: Picking a voltage for the system.....
« Reply #18 on: October 10, 2015, 09:03:12 AM »
......In a sense they're like bipolar stepper motors, but they're optimized for entirely different things than steppers are optimized for, and they can put out hugely more power for their size than steppers can.....

And here all this time I thought bipolar was a mental condition......

Only an EE would be able to invent a hardware version of it...

Thanks for the insight.  As you are probably aware, Mr Tesla invented the poly phase induction motor which has no magnets or brushes in it. It is all
done by magnetic induction from field windings to the armature. In order for it to work, George Westinghouse had to change all his AC systems to 60 Hz from 33 Hz which was too slow to run induction motor-generators....

As a result, they are constant speed.

Rex

Rex

Offline dave siegler

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 1128
  • sport flier
    • Circlemasters Flying club
Re: Picking a voltage for the system.....
« Reply #19 on: October 18, 2015, 05:50:31 AM »
Brush-less DC is the worst terminology.  I avoid using it.  It started when suppliers replaced DC motors with permanent magnet synchronous motors AND speed controllers.

So there is no brushless DC motor alone.  You replaced a DC motor with a controller + a AC PM motor. 

The system would look like a DC motor - controller to the control system, and have none of the pesky communicator brush maintenance of DC machines    


As far as over-sizing the ESC.  Electrical components have a lot longer life when not run at their maximum rating.  Especially power transistors and power circuits in general.  In industry we call that derating the device.  At my day job, we  may choose to run a transistor at 1/2 of its load to increase its life.  

While the designer SHOULD have some margin in his design, everyone wants to be inexpensive, and there is a watts/$$ point that one tries to hit.

If you fly when it is hot, don't have good airflow etc, you may not be able to run at the max rating and get a long life out of it.

So over-sizing the ESC is a pretty common thing to do.  Also as others have said watch the burst rating  vs continuous rating.  
Dave Siegler
NE9N extra class
AMA 720731
EAA 1231299 UAS Certificate Number FA39HY9ML7  Member of the Milwaukee Circlemasters. A Gold Leader Club for over 25 years!  http://www.circlemasters.com/

Offline Tim Wescott

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 12804
Re: Picking a voltage for the system.....
« Reply #20 on: October 18, 2015, 11:29:29 AM »
Brush-less DC is the worst terminology.  I avoid using it.  It started when suppliers replaced DC motors with permanent magnet synchronous motors AND speed controllers.

So there is no brushless DC motor alone.  You replaced a DC motor with a controller + a AC PM motor. 

The system would look like a DC motor - controller to the control system, and have none of the pesky communicator brush maintenance of DC machines

Yet there's a distinction between "Brushless DC" motors and "Brushless AC" motors.  It's a geeky technical one (it has to do with the shape of the back-EMF waveform: ask if you must know), but the motor driver for one is not appropriate as the motor driver for the other.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline dave siegler

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 1128
  • sport flier
    • Circlemasters Flying club
Re: Picking a voltage for the system.....
« Reply #21 on: October 18, 2015, 05:17:56 PM »
Yet there's a distinction between "Brushless DC" motors and "Brushless AC" motors.  It's a geeky technical one (it has to do with the shape of the back-EMF waveform: ask if you must know), but the motor driver for one is not appropriate as the motor driver for the other.


If you say so, but my machines professor was very clear, and he wrote the book.   Brushless DC includes the controller. 

You could say the motor used in a brushless system isn the same as an industrial motor.  But is dose not meet definition of DC, in any way. 

The only difference between the 2 is one is designed for sinusoidal extrication and one isn't. 
It has 3 terminals, the input is polyphase time varying. 
And the rotor speed is proportional to the excitation frequency. 

nothing DC about it. 


But this isn't about the question that asked. 
This conversation has run way down a rat hole. 



get the speed controller one sized larger.  It will last longer. 
Dave Siegler
NE9N extra class
AMA 720731
EAA 1231299 UAS Certificate Number FA39HY9ML7  Member of the Milwaukee Circlemasters. A Gold Leader Club for over 25 years!  http://www.circlemasters.com/

Offline Igor Burger

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 2165
Re: Picking a voltage for the system.....
« Reply #22 on: October 19, 2015, 01:52:19 AM »


And the rotor speed is proportional to the excitation frequency. 

nothing DC about it. 


I do not see it so straight, while RPM of AC motor is proportional to frequency (because of phase shift at load - that makes the current and torque to keep it), the BLDC motor (and yes I agree including the ESC) keeps RPM proportional to VOLTAGE, so there IS something about DC ... that is reason of different construction and also mode of work. You canot use BLDC motor connected to 3 phase sinusoidal waves and expect usefull run and also you cannot use classic AC motor with BLDC ESC and expect usefull efficiency.

That also leads to different RPM controll, BLDC has simple step down module feeding commutation with voltage proportional to expected RPM, while AC needs proper sinus waves for wanted RPM.

Offline Igor Burger

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 2165
Re: Picking a voltage for the system.....
« Reply #23 on: October 19, 2015, 02:05:02 AM »

get the speed controller one sized larger.  It will last longer. 


That is also true, however it will work well in industry, be we are about airplanes, we must save the weight also :- ))) My setup will take 60A on ground and full power and it is so udersized that it will burn in few seconds if I let it run there.  VD~

Offline dave siegler

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 1128
  • sport flier
    • Circlemasters Flying club
Re: Picking a voltage for the system.....
« Reply #24 on: October 19, 2015, 04:57:54 AM »
I do not see it so straight, while RPM of AC motor is proportional to frequency (because of phase shift at load - that makes the current and torque to keep it), the BLDC motor (and yes I agree including the ESC) keeps RPM proportional to VOLTAGE, so there IS something about DC ... that is reason of different construction and also mode of work. You canot use BLDC motor connected to 3 phase sinusoidal waves and expect usefull run and also you cannot use classic AC motor with BLDC ESC and expect usefull efficiency.

That also leads to different RPM controll, BLDC has simple step down module feeding commutation with voltage proportional to expected RPM, while AC needs proper sinus waves for wanted RPM.

Right a motor wound to expect a trapezoidal waveform runs pretty poorly on a sine wave and vise versa. 

yes in a regulated system (vector control) even induction motors are sort of like that.  The regulator raises phase voltage to deal with load.

But doesn't a BLDC motor have a Vlt/ hz curve?  I think we agree.  Vlts / RPM is the same thing.  The hobby doesn't want to deal with motor poles and supply frequency and just gives you the slope of the line. 

Sure the control can't just increase fundamental frequency it will stall.  But it can't just increase voltage either.  It will just heat up.  In the open loop case the is a simple relationship between voltage speed.  RPM and frequency only differ  by the number of poles in the motor.



Dave Siegler
NE9N extra class
AMA 720731
EAA 1231299 UAS Certificate Number FA39HY9ML7  Member of the Milwaukee Circlemasters. A Gold Leader Club for over 25 years!  http://www.circlemasters.com/

Offline Igor Burger

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 2165
Re: Picking a voltage for the system.....
« Reply #25 on: October 19, 2015, 05:08:36 AM »
But doesn't a BLDC motor have a Vlt/ hz curve?  

Maybe yes, but only at certain load. If you load it, V/Hz changes as the current goes up, resistance will make effective voltage lower, so at certain voltage, rpm (and thus also frquecy) dprops. That is not what happens at AC motor. That is why we do not deal with frequency on BLDC. ESC commutates regrading rotor position, it does not give some fixed frequency. There is only one reason where we have to take care with frequency, every ESC has some limit of commutation speed, so we cannot use motor with lot of poles in hi-rev applications, but otherwise we not need to know frequency.

Offline dave siegler

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 1128
  • sport flier
    • Circlemasters Flying club
Re: Picking a voltage for the system.....
« Reply #26 on: October 19, 2015, 10:28:23 AM »
Maybe yes, but only at certain load. If you load it, V/Hz changes as the current goes up, resistance will make effective voltage lower, so at certain voltage, rpm (and thus also frquecy) dprops. That is not what happens at AC motor. That is why we do not deal with frequency on BLDC. ESC commutates regrading rotor position, it does not give some fixed frequency. There is only one reason where we have to take care with frequency, every ESC has some limit of commutation speed, so we cannot use motor with lot of poles in hi-rev applications, but otherwise we not need to know frequency.

I agree with everything you said.
I get it now that voltage may be the more fundamental control variable in a BLDC system, and that the hall effects determine the frequency.  That is the difference between the a PM AC and BLDC with hall effects.
Dave Siegler
NE9N extra class
AMA 720731
EAA 1231299 UAS Certificate Number FA39HY9ML7  Member of the Milwaukee Circlemasters. A Gold Leader Club for over 25 years!  http://www.circlemasters.com/

Offline Igor Burger

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 2165
Re: Picking a voltage for the system.....
« Reply #27 on: October 19, 2015, 10:49:52 AM »
Yes, however we do not use hall sensors with our small motors, it is typical only with larger and more expensive types.

Offline Tim Wescott

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 12804
Re: Picking a voltage for the system.....
« Reply #28 on: October 19, 2015, 10:58:06 AM »
Yes, however we do not use hall sensors with our small motors, it is typical only with larger and more expensive types.

Not so much larger and more expensive as for motors that need good torque around zero speed -- RC car motors are brushless with hall sensors, and they have ESCs to match.  The can take off instantly from zero speed, in either direction, and if anything their maximum torque demand is when they start.  Conversely, there are some large (to us RC types) blower motors in cars that use BLDC systems with no hall sensors because they're doing the same thing we are (I know a guy who designed the electronics for one.  He said that one of the interesting challenges was to start the motor properly if it was already blowing backward in the wind-stream).
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Igor Burger

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 2165
Re: Picking a voltage for the system.....
« Reply #29 on: October 19, 2015, 11:05:30 AM »
I mean our airplanes, they use also sensors, but only larger types.

I know about cars, they need it especially because of acceleration, because sensorless ESC has slow (blind) spin-up mode with reduced power - it is not usefull on cars.

Offline Tim Wescott

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 12804
Re: Picking a voltage for the system.....
« Reply #30 on: October 19, 2015, 11:18:39 AM »
I mean our airplanes, they use also sensors, but only larger types.

Eh -- I hadn't seen that.  I've looked at fist-sized motors in the hobby shop for big airplanes, but mostly to raise my eyebrows at and then move on to what interests me.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Igor Burger

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 2165
Re: Picking a voltage for the system.....
« Reply #31 on: October 19, 2015, 11:22:44 AM »
Friend of mine has such motor in large R/C model, it is such flat (short) type with large diameter. He had problems with ESC and synchronization so he simply gave up and used the same type with sensors and problem solved. I can ask him what type it was.

Offline Igor Burger

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 2165
Re: Picking a voltage for the system.....
« Reply #32 on: October 19, 2015, 11:33:41 AM »
so here it is ... Gigatex 3500W

Offline Tim Wescott

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 12804
Re: Picking a voltage for the system.....
« Reply #33 on: October 19, 2015, 11:42:39 AM »
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.


Advertise Here
Tags:
 


Advertise Here