BLDC motor questions

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

 

[Cydrome Leader]

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.

 

[Tom Biasi]

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.

 

[Baron]

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.

 

[Tim Wescott]

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!

 

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

 

[Jasen Betts]

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

 

[Tim Wescott]

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!

 

[Ralph Mowery]

In article <8k6c3ctnbdi2br75q6i3oi5c6d7gpk95qg@4ax.com>,
etpm@whidbey.com says...

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

I take it this is a very small motor so it may not come into a factor
like large motors. The larger motors can have their own cooling fan or
an external fan. This is to keep down the heat. If the motor is
turning too slow it will not cool. Place I worked burnt up several 50
HP motors and wondered why. I looked at them and they were being driven
by a varitable speed drive. The nominal 1800 rpm was only running about
200 rpm. Not enough air flow for it to cool. Put a big fan on it to
cool it and it was fine.

 

[Jasen Betts]

On 2016-11-23, etpm@whidbey.com <etpm@whidbey.com> wrote:

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

You could do microstepping perhaps, still that could be a challenge.

--
This email has not been checked by half-arsed antivirus software

 

[Baron]

etpm@whidbey.com prodded the keyboard with:

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

Did you really mean fifteen degree steps ? A couple of the ones I
have are 1.2 and 1.8 degrees per step.


--
Best Regards:
Baron.

 

[ehsjr]

On 11/23/2016 6:22 PM, etpm@whidbey.com wrote:

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

Here's a small stepper that provides really small steps:
http://www.ebay.com/itm/Stepper-Motor-28BYJ-48-With-Drive-Test-Module-Board-ULN2003-5-Line-4-Phase-5V-/121861312264?hash=item1c5f800f08:g:VKQAAOSwwPhWkb58

Cost is $3.15 and includes a ULN2003 driver. You need to
provide pulses to the ULN2003 circuit from an external
circuit.

See the specs here:
http://robocraft.ru/files/datasheet/28BYJ-48.pdf

You won't get 30 rpm out of it - more like 6-7 RPM.
It has pretty good torque so you can gear up the speed
if you need to.

Ed

 

On Thu, 24 Nov 2016 13:51:48 +0000, Baron <baron@linuxmaniac.net>
wrote:

etpm@whidbey.com prodded the keyboard with:

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

Did you really mean fifteen degree steps ? A couple of the ones I
have are 1.2 and 1.8 degrees per step.

Yeah, I meant 15 degrees. I'm talking about steppers that are about 1
inch diameter, which is the diameter constraint for the motor. I have
many larger steppers with 1.8 degree steps.
Eric

 

[Tim Wescott]

On Wed, 23 Nov 2016 14:45:43 -0800, etpm wrote:

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

Yes, figure out how much torque you need. I'd recommend a motor that's
geared down by between 12:1 and 25:1, so the motor is turning at
2700-5500 (assuming I'm getting my math right). That should be a good
speed for bearing and gearbox life and for keeping things quiet.

I know you said what sized space you needed to fit, but I'm too lazy to
dig it out. Small motors with gearboxes generally get pretty long for
the diameter, though, and at least on the gearboxes that I've worked with
you can't trust the bearings with any significant side or axial loads.
That means that if you use and off-the-shelf gear box you'll need to
either use a coupling going into a bearing block (meaning even more
length), or you'll need to make a bearing block right at the motor
(meaning fiddly machining to get it right).

Most bare motors can't take much in the way of side or axial loads,
either, though, so that's not as big of a deal as it might be. I
remember one famous mechanism that I worked with, designed by a contract
mechanical engineer who was really full of himself. It was supposed to
be a fast mechanism for switching a lens array in and out; because he
depended on the motor to support the axial load on a worm during rapid
deceleration, it turned out to be that _and_ a motor-guts-puller-outer.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

I'm looking for work -- see my website!

 

[Tim Wescott]

On Wed, 23 Nov 2016 19:21:44 +0000, Baron 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.

IME a stepper motor has less torque for the size and mass than a DC
brushed motor, much less DC brushless. Certainly less torque than a
geared DC motor.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

I'm looking for work -- see my website!

 

[Tim Wescott]

On Wed, 23 Nov 2016 18:26:57 -0500, Ralph Mowery wrote:

In article <8k6c3ctnbdi2br75q6i3oi5c6d7gpk95qg@4ax.com>,
etpm@whidbey.com says...

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

I take it this is a very small motor so it may not come into a factor
like large motors. The larger motors can have their own cooling fan or
an external fan. This is to keep down the heat. If the motor is
turning too slow it will not cool. Place I worked burnt up several 50
HP motors and wondered why. I looked at them and they were being driven
by a varitable speed drive. The nominal 1800 rpm was only running about
200 rpm. Not enough air flow for it to cool. Put a big fan on it to
cool it and it was fine.

That can, indeed, affect smaller motors. Life's complicated.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

I'm looking for work -- see my website!

 

[Cydrome Leader]

etpm@whidbey.com wrote:

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

The suggestion somebody had of a stepper may be worth looking into. 30RPM
is real slow- many motors can't even run at such a low speed in the first
place. There's too much drag from the magnetics to overcome, so unless
you're holding them back with a big load, it's not a realistic speed for a
small motor. Big motors with dozens of windings have no problem with
stable low speeds. Examples would be a really old elevator motor.

 

[Ron M.]

On Wednesday, November 23, 2016 at 11:31:22 AM UTC-6, et...@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

Servo City. They have gear motors galore. Even fraction of RPM ranges. ENJOY!!