I recently bought a 1/4hp motor that i want to recoil differently. The
rotor contains four electro magnet and the stator contains two. The
metal is about twice on the stator than the rotor. I managed to put
550 turns of 29 gauge wire on the rotor, it gives an somewhat
appreciable electromagnetic force. I want to get the same on the
stator but I am not getting the results. I first coil with 550 turns
of 29 gauge, but twice the metal made it a lot weaker then the rotor
and by putting them side by side, they would stick together even if
they had a repulsive polarity. I then used 200 turns of 24 gauge wire
it did give me a good electromagnet but pulls 4 amps of current, even
if i use more turns, it still uses a lot of current. will the rotor
never heats up. They are all hooked up to a 12v battery.
What are the steps of obtaining the same magnetic field on the stator
and the rotor. ? without using too much current on the stator?
I received the motor already stripped from its wires, It had been
passed in an oven to remove all the copper wire, I got it from a shop
where they rebuilt motors.
The mass is the soft metal core, otherwise it would be the coil metal,
just referred to as coil windings.
The mass i do not know cause the stator are all connected together,
The size would be the area of the metal.
If you want I can take a picture and post it on a web site.
Are the copper commutator bars under the tape? If not, what
do you intend to attach the rotor wires to? Does the motor
case include brush holders?
What makes you think that modifying the stator pole pieces
as you describe (cutting off the "long legs" and drilling
holes will improve the operation of the motor?
It's been 20 or 25 years since I worked in the rewind shop, but I'll give my
2-1/2 cents worth.
Looking at the rotor picture, it is divided into four sections and each
has slots for more than just one coil per section. I think it would accept 3
per section. The best picture I can find is here
It shows each slot is shared by two coils.
I hope you plan on dipping and baking the rotor and stator to secure the
windings, it won't last long if you don't.
I just went back and reread your original post, you said
"I want to recoil differently" so my response may have little relevance.
The original rotor had 2 interwoven coils in each slot using 24gauge
wire , I believe the amount of turns was about 50, I get the real
number later, but it not relevant in my motor Since I am using
different gauge wire and position in my motor, it is totally
different, no comparison possible.
Also , yes there is a slip ring where the electrical tape is, it is
not shown on the picture because i did not think it was useful for
what I am trying to do at this stage. So the two stator coil will be
in series and two coils on the rotor will also be in series.
the reason I cut the stator core metal was to make wight of both
stator and rotor identical. I was hoping to get the same amount of
metal with the same numbers of turns. This would of given me two
identical electromagnet on the rotor and stator repulsing themselves
until the next 1/4 turn. The only different i can see is the waffle
layers are thicker on the stator then the rotor.
I worked on the motor all day today. I made an excel sheet and would
like to know if my calculations are correct.
It seems that no matter how many turns I have, my electromagnet will
have the same force but uses more or less current depending the length
used. Is this correct?
Here is the link of my excel sheet:
Ok but for a ball park figure i guess this is good enough.
So if you look at the power consumed P=V*I,
There is a lot more power used for a 100 turn then for a 1500 turn,
both giving almost the same amp turn.
So you can save energy by increasing the number of turn, using less
power and still having a motor giving the same mechanical output !?
Ah... um... let me be as polite about this as I can be. It is clear from
reading this thread so far that you really don't understand the design of
motors very well.
Cutting the 'ears' off of the stator pole pieces was a bad idea. They were
there to spread the magnetic field from the poles out so that it would cross
as much of the rotor iron as possible to the stator pole piece on the
opposite side. Drilling holes in the stator pole pieces just reduces the
permeability of the pole piece and reduces the field strength. The total
'mass' of the pole pieces is only vaguely related to how strong a magnetic
field it can sustain.
Measuring the strength of the magnetic field when out of the motor frame is
pretty pointless. The maximum 'pull' of an electromagnet is a function of
the MMF (amp-turns) and the total reluctance of the magnetic circuit. Air
is a much poorer 'conductor' of magnetic 'lines-of-flux' then iron. In a
proper motor frame, the frame itself completes have of the magnetic
'circuit'. The 'lines-of-flux' come out the pole face, traverse a thin air
gap to the rotor iron, flow across the rotor iron (assuming you haven't
drilled it full of holes :-( ) across another thin air gap to the opposite
pole piece, through that pole piece around the circumference of the motor
frame back to the back-side of the first pole piece. Putting more air in
this 'circuit' (i.e. drilling holes in the stator pole pieces) will make the
reluctance of this higher and *reduce* the total magnetic flux around the
rotor's conductors. That will reduce the capabilities of the machine.
The numerous slots in the rotor were designed to carry many small coils as
you have deduced from its original construction. This maximizes the number
of current-carrying conductors that will be within the magnetic field of the
stator at any given moment, thus maximizing the amount of torque. Also
having coils in each slot means the torque developed will be much smoother.
Your 'four-pole' rotor design will have severe 'cogging' as the rotor
rotates around with some positions producing zero torque and others
In your sketch you show the rotor and stator pole pieces directly facing
each other. While it is true that the repulsive force will be maximum at
that point, you will get *zero* torque at that point. The repulsive forces
will push directly toward the rotor's axle and not develop any torque at
all. To develop torque you need a force and a moment arm. In the position
of your sketch, you have no moment arm.
Using many turns of fine wire for the stator (i.e. shunt field winding)
*does* reduce the I^2*R losses in the stator. As you surmised, using large
wire means more current but fewer turns, while fine wire means less current
but more turns. Either way you get about the same number of amp-turns to
generate the stator's magnetic field.
But the amount of current flowing through the rotor coils is more
complicated. Not only is there the resistance of the wire, but the fact
that the wire is moving through a magnetic field (at least hopefully). So
there is a voltage induced in this wire (ala Faraday) that will complicate
the current calculation. In fact, in most DC motors this electromotive
force is the predominant term and the resistance is a couple of orders of
magnitude less significant (i.e. about 100 times less). Without going into
all the math, the result is simply TANSTAAFL (There Ain't No Such Thing As A
Free Lunch). To get more mechanical power out, you have to put more
electrical power in. And electrical power flow in will *always* be higher
than mechanical power flow out.
In commercial motors, the many coils on the rotor are usually connected in
series or parallel or some combination (google for 'lap-winding' and
'wave-winding' to get an idea how they would be connected). These coils are
connected such that the coils carry current almost all the time. Being in a
wide magnetic field (owing to those pole tips you've cut off), they have
forces acting on them almost all the time and hence produce a nice steady
torque. The one instant that a particular coil does *not* have current
flowing is when the current is about to be reversed by virtue of the
commutator. Curiously, this happens when the coil is located exactly where
your sketch shows. At that moment, the amount of electromotive-force
induced in the coil is nearly zero and the current can be reversed without
excessive arcing as the commutator segments connected to this coil are
momentarily shorted out by one of the brushes.
When you mention 'slip rings', you probably mean 'commutator'. DC motors
use a segmented ring where each segment is insulated from the others and
connected to opposite ends of two coil windings. Thus the brushes can
alternately connect to different coils and at different ends of the coils as
the rotor rotates so as to keep the current through the coils in the proper
direction to maintain smooth torque generation. 'Slip rings' is a term used
for a continuous ring that is insulated from the shaft. It would connect
one coil connection to one brush all the time and not provide this
electro-mechanical switching. You *could* use solid-state electronics to
control the current flow through the coils, but you would also need some
sort of shaft encoder/ sensor so that your electronics could detect the
shaft position and know exactly when to reverse the current.
Better go study some DC motor theory for a while. Or just continue to
experiment on your own, that's fine if you want. But chances are if you do
your own experiments, you'll start to realize that the 'normal' design of DC
motors is the-way-it-is because it works better then all the other designs.
Over 100 years of 'experimentation' is how the current design came to be
what you see today.
thanks for the theory.
I was told that the more metal used the less powerful the magnet. is
this true ?
My goal is to make a very simple motor. Once this is turning, I will
fill in the void, by that I mean when the motor has no impulse and is
just spinning on momentum. I will add mot coil.
Tell me something, I put up an excel spreadsheet on an early link. On
this, my calculation showed me that even with more turn of a fine (30
gauge wire) I can never make an electro-magnet as power as when I used
an 18 gauge wire. is this true?
Most DC motor i have seen so far have a permanent magnet. The motor I
am using was initially an AC motor, that I am rewinding it in a DC
fashion motor. I know it is not going to be as efficient as an overlap
winding motor but I want to learn about the winding, gauge wire and
metal aspect. I have not found a book speaking about the theory of
this. OOoh lots of book on calculations, efficiency and how a motor
generally works, but they do not go on details on gauge wires or such
things. Any books you recommend on this subject?
Daestrom has given you good advice. The advice you have re use of more
metal- is questionable and not necessarily true(i.e wrapped in "if this,
then that" which is meaningless without the "this" and "that"). In a
typical motor, in the normal operating region, the air gap is far more
The field windings are designed to produce a given flux density at the air
gap which depends on the ampere turns. You can use any combination of turns
and current to get the desired level (which is limited by iron saturation
but mainly determined by the air gap. However, reducing the current and
increasing the number of turns runs into problems in that, due to space and
economic considerations, the wire size would have to be reduced- so with
more turns and smaller wires, the resistance of the field winding increases.
It is not a matter of turns or of current but it is a matter of net ampere
First figure out the required amp-turns needed to produce the desired flux
density in the air gap for the desired pole area. then the problem is to
determine the best balance of turns and current for that situation.
Whatever the field I^2R loss is- it is only a small part of the total power
balance-say 2-5%. so this is a secondary concern.
Now, in a typical DC motor, the field pole is quite wide so that it
"covers" more of the actual winding. The ideal would be to have half the
rotor winding under each pole and these two halves in parallel. Physically
this can't quite be done. Curtailing the pole span can reduce field losses
but will certainly have an adverse effect on the motor operation which is
far more important. Why take a 50% hit on performance to reduce the field
loss from 3% to 2.5% ?
Many small motors do have a permanent magnet. However, once you get beyond
"small" you will have electromagnets.
Another puzzler is that you are rewinding an AC motor as a DC motor.
Somehow this is doomed to failure as a typical AC motor doesn't have a
commutator. If the motor you are winding has a commutator -chances are that
it is a "universal" or series motor which is essentially a DC motor that can
run on AC (actually works better on DC)-so what you are attempting to
achieve is a puzzle.
Daestrom suggested that you look at the theory of DC motors. DO THAT. Look
for texts on motor design. What these will tell you is what has been found
from over a century of practice and theory, is to map out "where there be
dragons" and how to avoid them.
Don Kelly email@example.com
remove the X to answer
Your post doesn't make a lot of sense. Yeah, you may know what you
are doing, but anyone reading the post won't
We could make a lot of assumptions from what you post - like
presumably this is a DC motor or AC/DC? You don't say.
You shouldn't expect the highly subjective idea of magnetic pull to
translate into real world results. It may . . .but don't bet on it.
All engineering is compromise. You need to look at the "ampere turns"
of the original, and translate that for the lower (?) voltage.
Voltage goes down, wire size and current goes up for the same
horsepower output. Don't worry about "pull" or mass of iron involved.
Someone more knowledgeable than you, already made those compromises.
Try to match the original for ampere turns if this is DC. Ampere
turns translates into magnetic force - directly.
Always post exactly what you have (in generic terms) and what you want
And my hats off to you - since you're actually tinkering with it - but
the rules do work, so learn them and apply them.
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Would we assume that at max torque, the gauss of the field is equal to
the gauss of the armature? Lets say 10,000 gauss in each coil (20,000
total) gives force X. If one coil has 5000 and the other 15000 would
the force be 5000? 15000? 20000? Thanks!
The flux of concern is the air gap flux due to the field. The armature
current will produce a flux which is ideally perpendicular to the field flux
and is lower because the air gap is larger between poles. This can have a
secondary and unwanted effect called armature reaction.
Voltage in the armature is due to the flux produced by the field.
Get a text on the operation of DC motors.
Don Kelly firstname.lastname@example.org
remove the X to answer
==================================================I ask about force, you babble about flux, and tell me to get a book on
oeration of dc motors. ("Hook the red wire to the + terminal, the
black wire to the - terminal"). I'd give you an F for reponding to the
question. Let me ask it in a different way. A permanent magnet with
gauss X can pull Y lbs Z inches from a steel plate. Do 2 of these
magmets pull 2*Y lbs at the same distance? If the magnets have
different strength, does the force still arithmetically sum? (I
suspect that it is 'bad design' to have either the field or the
armature a lot stronger than the other. Agree?)
I am not sure about this, but I think they will cancel.
Imagine two people pushing each other, One can push with 200newton and
the other can push with only 100newton, the end result would be the
same as one person is pushing with 100newton and the other one not
pushing at all.
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