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Three wired DC Motor question.

P

PaPaPeng

Jan 1, 1970
0
I am hijacking the RC unit from a toy boat for motorizing another
plastic scale model. I need to know how the motors are wired.

Go to [ http://www.sharperimage.com/ ] and in their search box go to
"boats". They have a minispeedster kit NK001 for $19.95.

Also go to
http://www.mikromodell.de/index_e.html and see the small plastic
models that have been motorized and R/Ced.

I have already cut open the minispeedster to see the insides. The
guts are real teeny. They are ideal for what I want to do. The
minispeedster has two cell phone type motors mounted in external pods.
The R/C Tx can signal both motors to spin forward or in reverse. By
stopping one motor or putting it in reverse while the other motor is
still running forward it will cause the boat to turn. There is no
rudder.

A four AA cell charger tops up a two cell NiCd unit sealed inside the
minispeedster. Its good for a 10-15 minute run time.

Each motor has three hair thin wires and I am reluctant to scrape off
the insulation at this time. I can envisage four wires at the battery
source end feeding the polarity change switch for the motors for
fwd/rev. The motor will of course have only two wire leads for the
brushes.

The question therefore is why does the motor have three leads and how
does that implement the motor's fwd/rev polarity change from the Rx
PCB.

Thanks in advance.
 
you could have two motor windings and one ground..this enables you to
excite each winding separate;y to go forward or reverse...this is
similar to an electric antenna on a car..
 
J

James Sweet

Jan 1, 1970
0
you could have two motor windings and one ground..this enables you to
excite each winding separate;y to go forward or reverse...this is
similar to an electric antenna on a car..


The electric antenna on my car just uses a DPDT relay to reverse the
polarity, I've never seen a DC motor that had more than one winding. More
likely it's a brushless motor.
 
P

PaPaPeng

Jan 1, 1970
0
The electric antenna on my car just uses a DPDT relay to reverse the
polarity, I've never seen a DC motor that had more than one winding. More
likely it's a brushless motor.


But of course. I don't know why I never thought of looking up what a
brushless motor is although I have come across it enough times in the
reading materials.

The URL www.allegromicro.com/techpub2/compumot/a17a19.pdf gives a
very clear explanation of the brushless motor technology involved. It
does require three wire leads and its obvious how the direction of
rotation is derived.

My problem is now how to wire a regular two pole mini motor that had
been clearly labelled as a cell phone vibrator motor to work from the
brushless motor. The general idea is that in a three propellar PT
boat the two side motors will be from the minispeedster set up. My
intent for the middle propellar is to OR the inputs from the two side
props such that so long as one of them is rotating in the forward
direction the middle prop will spin forward. I can steer without
losing forward speed or use a rudder. If both side motors are idle or
in reverse the mid prop will be idle.
 
A

Asimov

Jan 1, 1970
0
"James Sweet" bravely wrote to "All" (13 Sep 04 04:43:19)
--- on the heady topic of "Re: Three wired DC Motor question."

JS> From: "James Sweet" <[email protected]>

JS> said:
you could have two motor windings and one ground..this enables you to
excite each winding separate;y to go forward or reverse...this is
similar to an electric antenna on a car..


JS> The electric antenna on my car just uses a DPDT relay to reverse the
JS> polarity, I've never seen a DC motor that had more than one winding.
JS> More likely it's a brushless motor.

Look at some CD player drive motors they have 4 terminals labled:
+ - A B
The + - are the power and the A B are the speed feedback.

A*s*i*m*o*v

.... No electrons were harmed in the posting of this message.
 
J

jakdedert

Jan 1, 1970
0
Asimov said:
"James Sweet" bravely wrote to "All" (13 Sep 04 04:43:19)
--- on the heady topic of "Re: Three wired DC Motor question."

JS> From: "James Sweet" <[email protected]>




JS> The electric antenna on my car just uses a DPDT relay to reverse the
JS> polarity, I've never seen a DC motor that had more than one winding.
JS> More likely it's a brushless motor.

Look at some CD player drive motors they have 4 terminals labled:
+ - A B
The + - are the power and the A B are the speed feedback.
Most CPU fans these days have three wires...one, I guess would be common,
and the other two: + and feedback.

jak
 
P

PaPaPeng

Jan 1, 1970
0
Most CPU fans these days have three wires...one, I guess would be common,
and the other two: + and feedback.


Now that you mentioned it CPU fans are brushless. I did wonder how it
managed to rotate without the brushes you would find in any ordinary
motor. OK the 60Hz AC supply was a quick if uninteresting answer. But
because its such a cheap item and there is little else one could use
the motor for - too weak, too slow and the fan is molded into the
rotator unit - I never cared to find out how it worked not that I knew
brushless motors was a new technology anyway (circa the 1980s.) But
then again I did wonder why a simple motor fan would cost several tens
of bucks back then and still cost more than brushed motors even now.
With the hair thin magins in PCs wouldn't they cut costs especially
for a low tech item like a fan?
The URL www.allegromicro.com/techpub2/compumot/a17a19.pdf gives a
very clear explanation of the brushless motor technology involved. It
does require three wire leads and its obvious how the direction of
rotation is derived.

Brushless motors can function only with three leads to handle AC
polarity pattern generated by an electronics circuit necessary to
drive the motor. There is no + - and certainly no feedback. Why
would a simple cooling fan need to provide feedback anyway? The
beauty of a circuit generated AC is that it can be minituarized and
the cct can be programmed for speed control as well as for rotational
direction. It also means that the brushless motor have a controller
PCB. You can't just take the two leads off a battery and apply it to
the brushless motor.

In the URL http://www.mikromodell.de/index_e.html mini circuits and
motors have been used to RC scale plastic models, a project I am doing
now. I was puzzled when one of the contributors said he wrote a
control program for his model. I was think WOW these guys must be
rocket scientists who knew the innermost secrets of electronics
miniaturization, programming, etc. Surely they must have custom built
their toys because no manufacturer would cater to such a small niche
market. Well things are beginning to look a lot more achievable now
although I cannot program anything.

Do hop over to Radio Shack and take a look at their latest miniature
RC toys the one that comes complete with a little docking bay battery
charger. Among the minicars they have aftermarket motors with gear
sets that you can use to exchange with the setup of a standard model.
Those motors are exquisite with a diameter of perhaps 3 mm and a
length of 10 mm. How anyone can manufacture such a small precision
motor is amazing. This size would have been impossible with regular
brush motors.
 
J

jakdedert

Jan 1, 1970
0
PaPaPeng said:
Now that you mentioned it CPU fans are brushless. I did wonder how it
managed to rotate without the brushes you would find in any ordinary
motor. OK the 60Hz AC supply was a quick if uninteresting answer. But
because its such a cheap item and there is little else one could use
the motor for - too weak, too slow and the fan is molded into the
rotator unit - I never cared to find out how it worked not that I knew
brushless motors was a new technology anyway (circa the 1980s.) But
then again I did wonder why a simple motor fan would cost several tens
of bucks back then and still cost more than brushed motors even now.
With the hair thin magins in PCs wouldn't they cut costs especially
for a low tech item like a fan?


Brushless motors can function only with three leads to handle AC
polarity pattern generated by an electronics circuit necessary to
drive the motor. There is no + - and certainly no feedback. Why
would a simple cooling fan need to provide feedback anyway?

The feedback--more aptly termed 'tachometer,' I guess--lead provides speed
info to the majority of modern motherboards which display RPM values in BIOS
and in utility programs supplied by the manufacturer. Up until such systems
were common, most CPU fans were only two-wire.

Someone else will have to explain that. I'm too busy right now. Think
'DC'....

jak

The
beauty of a circuit generated AC is that it can be minituarized and
the cct can be programmed for speed control as well as for rotational
direction. It also means that the brushless motor have a controller
PCB. You can't just take the two leads off a battery and apply it to
the brushless motor.
<snip>
 
J

James Sweet

Jan 1, 1970
0
Asimov said:
"James Sweet" bravely wrote to "All" (13 Sep 04 04:43:19)
--- on the heady topic of "Re: Three wired DC Motor question."

JS> From: "James Sweet" <[email protected]>




JS> The electric antenna on my car just uses a DPDT relay to reverse the
JS> polarity, I've never seen a DC motor that had more than one winding.
JS> More likely it's a brushless motor.

Look at some CD player drive motors they have 4 terminals labled:
+ - A B
The + - are the power and the A B are the speed feedback.


Do they go to brushes? I've seen plenty of motors with tach coils on them,
but all that I've seen have been separate from the armature in the motor and
electrically isolated. Either way I don't think this is what he has here.
 
A

Asimov

Jan 1, 1970
0
"PaPaPeng" bravely wrote to "All" (14 Sep 04 20:38:15)
--- on the heady topic of "Re: Three wired DC Motor question."

Pa> From: PaPaPeng <[email protected]>

Pa> On Tue, 14 Sep 2004 11:37:08 -0500, "jakdedert"
Pa> said:
Most CPU fans these days have three wires...one, I guess would be common,
and the other two: + and feedback.


Pa> Now that you mentioned it CPU fans are brushless. I did wonder how it
Pa> managed to rotate without the brushes you would find in any ordinary
Pa> motor. OK the 60Hz AC supply was a quick if uninteresting answer. But
Pa> because its such a cheap item and there is little else one could use
Pa> the motor for - too weak, too slow and the fan is molded into the
Pa> rotator unit - I never cared to find out how it worked not that I knew
Pa> brushless motors was a new technology anyway (circa the 1980s.) But
Pa> then again I did wonder why a simple motor fan would cost several tens
Pa> of bucks back then and still cost more than brushed motors even now.

Brushless DC motors are a mystery until weu take one apart to find one
or more Hall Effect Switches. Indeed these sense the magnetic field
polarity reversal as the doughnut magnet in the rotor fan rotates. The
logic signal they output is applied to control the commutating
transistors which drive the opposing phase windings. It is a very simple
and rugged circuit but rather low in torque. I think the low torque
problem is because there is still a lot of energy left in the opposite
winding. I guess the size could be scaled up though.

A*s*i*m*o*v

.... That was a fascinating period of time for electronics
 
J

James Sweet

Jan 1, 1970
0
jakdedert said:
Asimov said:
"James Sweet" bravely wrote to "All" (13 Sep 04 04:43:19)
--- on the heady topic of "Re: Three wired DC Motor question."

JS> From: "James Sweet" <[email protected]>

JS> said:
Most CPU fans these days have three wires...one, I guess would be common,
and the other two: + and feedback.

2 wires power the motor, the 3rd wire is a tach signal, it's just a square
wave, probably comes from the hall sensor used in the motor to trigger the
windings.
 
J

James Sweet

Jan 1, 1970
0
The feedback--more aptly termed 'tachometer,' I guess--lead provides speed
info to the majority of modern motherboards which display RPM values in BIOS
and in utility programs supplied by the manufacturer. Up until such systems
were common, most CPU fans were only two-wire.


The tach signal is also used in some systems to shut the machine down if the
fan fails.
 
P

PaPaPeng

Jan 1, 1970
0
I am hijacking the RC unit from a toy boat for motorizing another
plastic scale model. I need to know how the motors are wired.


What do you know. The latest issue Oct 204 of RCM, an aeromodelling
magazine, has an article on how to salvage a CD ROM drive motor and
remanufacture it as a small powerful battery operated motor for
electric flight models. Go to
www.groups.yahoo.com/group/lrk-torquemax for on-going discussions on
this subject.
 
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