Forums

BLDC motor questions

Started by Unknown November 23, 2016
Awright youse motor folks, I need info. I have, over the last couple
years goofed around with BLDC motors made for RC hobbies. I even made
a couple just to learn winding techniques. The motors are rated in
part by the kv of the motor, which is how fast it will spin when
powered at a given voltage. The kv stands for thousand revolutions per
volt. A hand wound motor, or any motor for that matter, can be checked
for its kv by spinning the motor at a known rpm and measuring the
voltage it produces when operated as a generator. Anyway, I now have a
practical use for a BLDC motor. But it needs to fit in a certain space
and spin slowly. So I am going to wind a motor by hand on laminations
that I will mill myself. I have looked online at sites all about these
motors and nobody is trying to make a really slow motor. What I need
to know is if the kv is really that important. Can't the motor be told
to spin at whatever rpm by the motor controller? No matter what the
voltage is (whithin reason)? The motors I would have a pretty high kv
and the wire I use to wind them is pretty fine. I can use  finer wire
but only so fine because it will get too delicate for me to hand wind
and because the current capability will drop too much. The motor I
want needs to spin slow but have pretty good torque for its size.
Higer torque means thicker wires to handle higher current but thicker
wires also means fewer turns of the windings which means higher kv.
Thanks,
Eric
On 11/23/2016 12:34 PM, etpm@whidbey.com wrote:
> Awright youse motor folks, I need info. I have, over the last couple > years goofed around with BLDC motors made for RC hobbies. I even made > a couple just to learn winding techniques. The motors are rated in > part by the kv of the motor, which is how fast it will spin when > powered at a given voltage. The kv stands for thousand revolutions per > volt. A hand wound motor, or any motor for that matter, can be checked > for its kv by spinning the motor at a known rpm and measuring the > voltage it produces when operated as a generator. Anyway, I now have a > practical use for a BLDC motor. But it needs to fit in a certain space > and spin slowly. So I am going to wind a motor by hand on laminations > that I will mill myself. I have looked online at sites all about these > motors and nobody is trying to make a really slow motor. What I need > to know is if the kv is really that important. Can't the motor be told > to spin at whatever rpm by the motor controller? No matter what the > voltage is (whithin reason)? The motors I would have a pretty high kv > and the wire I use to wind them is pretty fine. I can use finer wire > but only so fine because it will get too delicate for me to hand wind > and because the current capability will drop too much. The motor I > want needs to spin slow but have pretty good torque for its size. > Higer torque means thicker wires to handle higher current but thicker > wires also means fewer turns of the windings which means higher kv. > Thanks, > Eric >
You may have to consider a gear box.
etpm@whidbey.com prodded the keyboard with:

> Awright youse motor folks, I need info. I have, over the last couple > years goofed around with BLDC motors made for RC hobbies. I even > made a couple just to learn winding techniques. The motors are rated > in part by the kv of the motor, which is how fast it will spin when > powered at a given voltage. The kv stands for thousand revolutions > per volt. A hand wound motor, or any motor for that matter, can be > checked for its kv by spinning the motor at a known rpm and > measuring the voltage it produces when operated as a generator. > Anyway, I now have a practical use for a BLDC motor. But it needs to > fit in a certain space and spin slowly. So I am going to wind a > motor by hand on laminations that I will mill myself. I have looked > online at sites all about these motors and nobody is trying to make > a really slow motor. What I need to know is if the kv is really that > important. Can't the motor be told to spin at whatever rpm by the > motor controller? No matter what the voltage is (whithin reason)? > The motors I would have a pretty high kv > and the wire I use to wind them is pretty fine. I can use finer > wire but only so fine because it will get too delicate for me to > hand wind and because the current capability will drop too much. The > motor I want needs to spin slow but have pretty good torque for its > size. Higer torque means thicker wires to handle higher current but > thicker wires also means fewer turns of the windings which means > higher kv. Thanks, > Eric
Try a small stepper motor. -- Best Regards: Baron.
On Wed, 23 Nov 2016 09:34:17 -0800, etpm wrote:

> Awright youse motor folks, I need info. I have, over the last couple > years goofed around with BLDC motors made for RC hobbies. I even made a > couple just to learn winding techniques. The motors are rated in part by > the kv of the motor, which is how fast it will spin when powered at a > given voltage. The kv stands for thousand revolutions per volt. A hand > wound motor, or any motor for that matter, can be checked for its kv by > spinning the motor at a known rpm and measuring the voltage it produces > when operated as a generator. Anyway, I now have a practical use for a > BLDC motor. But it needs to fit in a certain space and spin slowly. So I > am going to wind a motor by hand on laminations that I will mill myself. > I have looked online at sites all about these motors and nobody is > trying to make a really slow motor. What I need to know is if the kv is > really that important. Can't the motor be told to spin at whatever rpm > by the motor controller? No matter what the voltage is (whithin reason)? > The motors I would have a pretty high kv and the wire I use to wind them > is pretty fine. I can use finer wire but only so fine because it will > get too delicate for me to hand wind and because the current capability > will drop too much. The motor I want needs to spin slow but have pretty > good torque for its size. Higer torque means thicker wires to handle > higher current but thicker wires also means fewer turns of the windings > which means higher kv. Thanks, > Eric
No matter what wire size you use to wind with, as long as you're filling about the same amount of space with copper the amount of torque that you can generate per watt of I^2-R losses in a permanent-field motor is roughly constant. That's because as the turns go up the wire gets smaller in area and longer in length roughly proportionally with the number of turns, so the resistance goes as turns squared. The torque per current is proportional to the number of turns. So for an N-turn motor with good packing, resistance is roughly proportional to N^2, and the current needed to generate the same torque is proportional to 1/N. (1/ N)^2 * N^2 = 1. What that means is that the torque that you can develop is a function of the motor's mechanical and magnetic design -- try to get more torque out of it and you'll just burn up the motor. Winding the motor differently is just a convenience to match whatever power rail you're running off of, and your motor controller's ability to deal with a mismatch between the motor's back EMF and the power rail. You can sort of test this theory by getting onto the Pittmon or Maxon or Faulhauber motor sites and looking at data sheets -- they'll have lines of motors that are identical mechanical assemblies with different windings, and a bit of math will show you that the torque (and power dissipated) at maximum-rated continuous current is roughly constant across the middle of the voltage range, and trailing off at the ends (because at the low end it's hard to wind a few turns to fill a space, and at the high end you start ending up with a lower ratio of copper to insulation). So, I think you need a gearbox. The only alternative is if your space allows for a very nontraditional diameter (as in -- BIG) and a custom motor. A gearbox is probably better. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com I'm looking for work -- see my website!
etpm@whidbey.com wrote:
> Awright youse motor folks, I need info. I have, over the last couple > years goofed around with BLDC motors made for RC hobbies. I even made > a couple just to learn winding techniques. The motors are rated in > part by the kv of the motor, which is how fast it will spin when > powered at a given voltage. The kv stands for thousand revolutions per > volt. A hand wound motor, or any motor for that matter, can be checked > for its kv by spinning the motor at a known rpm and measuring the > voltage it produces when operated as a generator. Anyway, I now have a > practical use for a BLDC motor. But it needs to fit in a certain space > and spin slowly. So I am going to wind a motor by hand on laminations > that I will mill myself. I have looked online at sites all about these > motors and nobody is trying to make a really slow motor. What I need > to know is if the kv is really that important. Can't the motor be told > to spin at whatever rpm by the motor controller? No matter what the
You can spin a motor slow, but you also lose output power rating if the motor wasn't made to run slow. Harping on speed constants of a motor won't solve this problem. It's only part of the issue. You need to identify the speed and torque rating you need. Everthing else comes after that.
On Wed, 23 Nov 2016 22:10:23 +0000 (UTC), Cydrome Leader
<presence@MUNGEpanix.com> wrote:

>etpm@whidbey.com wrote: >> Awright youse motor folks, I need info. I have, over the last couple >> years goofed around with BLDC motors made for RC hobbies. I even made >> a couple just to learn winding techniques. The motors are rated in >> part by the kv of the motor, which is how fast it will spin when >> powered at a given voltage. The kv stands for thousand revolutions per >> volt. A hand wound motor, or any motor for that matter, can be checked >> for its kv by spinning the motor at a known rpm and measuring the >> voltage it produces when operated as a generator. Anyway, I now have a >> practical use for a BLDC motor. But it needs to fit in a certain space >> and spin slowly. So I am going to wind a motor by hand on laminations >> that I will mill myself. I have looked online at sites all about these >> motors and nobody is trying to make a really slow motor. What I need >> to know is if the kv is really that important. Can't the motor be told >> to spin at whatever rpm by the motor controller? No matter what the > >You can spin a motor slow, but you also lose output power rating if the >motor wasn't made to run slow. Harping on speed constants of a motor won't >solve this problem. It's only part of the issue. > >You need to identify the speed and torque rating you need. Everthing else >comes after that.
My speed range is from 30 rpm to 220 rpm. I dunno how much torque yet, but pretty low. I'll be measuring the torque wirh an inch pound torque wrench pretty soon. As others have said a gearbox is probably in order. I don't have a lot of room and so can't stuff a lot of gears in the space. I guess I need to gigure how much torque is required. Eric
On 2016-11-23, etpm@whidbey.com <etpm@whidbey.com> wrote:
> Anyway, I now have a > practical use for a BLDC motor. But it needs to fit in a certain space > and spin slowly.
> The motor I > want needs to spin slow but have pretty good torque for its size.
Use stronger magnets, consider having several poles.
> Higer torque means thicker wires to handle higher current but thicker > wires also means fewer turns of the windings which means higher kv.
Thicker or thinner does not effect max torque, only cross-section does. ten wires carrying 0.1A or one wire carrying 1A, the field is the same only the voltage is different (assuming 10 wires is 10 turns of 1 wire). but maybe you really need a gearmotor? -- This email has not been checked by half-arsed antivirus software
On Wed, 23 Nov 2016 13:42:44 -0600, Tim Wescott
<seemywebsite@myfooter.really> wrote:

>On Wed, 23 Nov 2016 09:34:17 -0800, etpm wrote: > >> Awright youse motor folks, I need info. I have, over the last couple >> years goofed around with BLDC motors made for RC hobbies. I even made a >> couple just to learn winding techniques. The motors are rated in part by >> the kv of the motor, which is how fast it will spin when powered at a >> given voltage. The kv stands for thousand revolutions per volt. A hand >> wound motor, or any motor for that matter, can be checked for its kv by >> spinning the motor at a known rpm and measuring the voltage it produces >> when operated as a generator. Anyway, I now have a practical use for a >> BLDC motor. But it needs to fit in a certain space and spin slowly. So I >> am going to wind a motor by hand on laminations that I will mill myself. >> I have looked online at sites all about these motors and nobody is >> trying to make a really slow motor. What I need to know is if the kv is >> really that important. Can't the motor be told to spin at whatever rpm >> by the motor controller? No matter what the voltage is (whithin reason)? >> The motors I would have a pretty high kv and the wire I use to wind them >> is pretty fine. I can use finer wire but only so fine because it will >> get too delicate for me to hand wind and because the current capability >> will drop too much. The motor I want needs to spin slow but have pretty >> good torque for its size. Higer torque means thicker wires to handle >> higher current but thicker wires also means fewer turns of the windings >> which means higher kv. Thanks, >> Eric > >No matter what wire size you use to wind with, as long as you're filling >about the same amount of space with copper the amount of torque that you >can generate per watt of I^2-R losses in a permanent-field motor is >roughly constant. That's because as the turns go up the wire gets >smaller in area and longer in length roughly proportionally with the >number of turns, so the resistance goes as turns squared. The torque per >current is proportional to the number of turns. So for an N-turn motor >with good packing, resistance is roughly proportional to N^2, and the >current needed to generate the same torque is proportional to 1/N. (1/ >N)^2 * N^2 = 1. > >What that means is that the torque that you can develop is a function of >the motor's mechanical and magnetic design -- try to get more torque out >of it and you'll just burn up the motor. Winding the motor differently >is just a convenience to match whatever power rail you're running off of, >and your motor controller's ability to deal with a mismatch between the >motor's back EMF and the power rail. > >You can sort of test this theory by getting onto the Pittmon or Maxon or >Faulhauber motor sites and looking at data sheets -- they'll have lines >of motors that are identical mechanical assemblies with different >windings, and a bit of math will show you that the torque (and power >dissipated) at maximum-rated continuous current is roughly constant >across the middle of the voltage range, and trailing off at the ends >(because at the low end it's hard to wind a few turns to fill a space, >and at the high end you start ending up with a lower ratio of copper to >insulation). > >So, I think you need a gearbox. The only alternative is if your space >allows for a very nontraditional diameter (as in -- BIG) and a custom >motor. A gearbox is probably better.
Is Pittmon the Jamaican site Pittman? Can the back EMF be a problem because it might harm the controller or because of speed regulation. The controllers I have now probably won't spin a motor slow enough. But I have found chips for the purpose that use hall effect sensors for speed control.
On Wed, 23 Nov 2016 19:21:44 +0000, Baron <baron@linuxmaniac.net>
wrote:

>etpm@whidbey.com prodded the keyboard with: > >> Awright youse motor folks, I need info. I have, over the last couple >> years goofed around with BLDC motors made for RC hobbies. I even >> made a couple just to learn winding techniques. The motors are rated >> in part by the kv of the motor, which is how fast it will spin when >> powered at a given voltage. The kv stands for thousand revolutions >> per volt. A hand wound motor, or any motor for that matter, can be >> checked for its kv by spinning the motor at a known rpm and >> measuring the voltage it produces when operated as a generator. >> Anyway, I now have a practical use for a BLDC motor. But it needs to >> fit in a certain space and spin slowly. So I am going to wind a >> motor by hand on laminations that I will mill myself. I have looked >> online at sites all about these motors and nobody is trying to make >> a really slow motor. What I need to know is if the kv is really that >> important. Can't the motor be told to spin at whatever rpm by the >> motor controller? No matter what the voltage is (whithin reason)? >> The motors I would have a pretty high kv >> and the wire I use to wind them is pretty fine. I can use finer >> wire but only so fine because it will get too delicate for me to >> hand wind and because the current capability will drop too much. The >> motor I want needs to spin slow but have pretty good torque for its >> size. Higer torque means thicker wires to handle higher current but >> thicker wires also means fewer turns of the windings which means >> higher kv. Thanks, >> Eric > >Try a small stepper motor.
I considered steppers when I first thought about this but all the small ones I found had 15 degree steps which might mean kind of jerky motion at 30 rpm. I need a motor no more than about 1.125" diameter. Maybe I need to look at steppers again. Eric
On Wed, 23 Nov 2016 15:10:46 -0800, etpm wrote:

> On Wed, 23 Nov 2016 13:42:44 -0600, Tim Wescott > <seemywebsite@myfooter.really> wrote: > >>On Wed, 23 Nov 2016 09:34:17 -0800, etpm wrote: >> >>> Awright youse motor folks, I need info. I have, over the last couple >>> years goofed around with BLDC motors made for RC hobbies. I even made >>> a couple just to learn winding techniques. The motors are rated in >>> part by the kv of the motor, which is how fast it will spin when >>> powered at a given voltage. The kv stands for thousand revolutions per >>> volt. A hand wound motor, or any motor for that matter, can be checked >>> for its kv by spinning the motor at a known rpm and measuring the >>> voltage it produces when operated as a generator. Anyway, I now have a >>> practical use for a BLDC motor. But it needs to fit in a certain space >>> and spin slowly. So I am going to wind a motor by hand on laminations >>> that I will mill myself. >>> I have looked online at sites all about these motors and nobody is >>> trying to make a really slow motor. What I need to know is if the kv >>> is really that important. Can't the motor be told to spin at whatever >>> rpm by the motor controller? No matter what the voltage is (whithin >>> reason)? >>> The motors I would have a pretty high kv and the wire I use to wind >>> them is pretty fine. I can use finer wire but only so fine because it >>> will get too delicate for me to hand wind and because the current >>> capability will drop too much. The motor I want needs to spin slow but >>> have pretty good torque for its size. Higer torque means thicker wires >>> to handle higher current but thicker wires also means fewer turns of >>> the windings which means higher kv. Thanks, >>> Eric >> >>No matter what wire size you use to wind with, as long as you're filling >>about the same amount of space with copper the amount of torque that you >>can generate per watt of I^2-R losses in a permanent-field motor is >>roughly constant. That's because as the turns go up the wire gets >>smaller in area and longer in length roughly proportionally with the >>number of turns, so the resistance goes as turns squared. The torque >>per current is proportional to the number of turns. So for an N-turn >>motor with good packing, resistance is roughly proportional to N^2, and >>the current needed to generate the same torque is proportional to 1/N. >>(1/ N)^2 * N^2 = 1. >> >>What that means is that the torque that you can develop is a function of >>the motor's mechanical and magnetic design -- try to get more torque out >>of it and you'll just burn up the motor. Winding the motor differently >>is just a convenience to match whatever power rail you're running off >>of, >>and your motor controller's ability to deal with a mismatch between the >>motor's back EMF and the power rail. >> >>You can sort of test this theory by getting onto the Pittmon or Maxon or >>Faulhauber motor sites and looking at data sheets -- they'll have lines >>of motors that are identical mechanical assemblies with different >>windings, and a bit of math will show you that the torque (and power >>dissipated) at maximum-rated continuous current is roughly constant >>across the middle of the voltage range, and trailing off at the ends >>(because at the low end it's hard to wind a few turns to fill a space, >>and at the high end you start ending up with a lower ratio of copper to >>insulation). >> >>So, I think you need a gearbox. The only alternative is if your space >>allows for a very nontraditional diameter (as in -- BIG) and a custom >>motor. A gearbox is probably better. > Is Pittmon the Jamaican site Pittman? Can the back EMF be a problem > because it might harm the controller or because of speed regulation. > The controllers I have now probably won't spin a motor slow enough. But > I have found chips for the purpose that use hall effect sensors for > speed control.
Yes, but since you're in the US you probably want to get Pittman -- mon. Back EMF isn't a problem -- it's the ratio between the motor controller's power rail voltage and the voltage that the motor needs to be driven at your desired speed. How much torque do you need? If it's more than the motor can deliver, you're screwed. If not, you may want to use a brushed servo motor. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com I'm looking for work -- see my website!