Motor Mania Reprise |
| Technical |
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Sometimes I look back on something I've written and wonder where my head was at. Then again I know where it
was, right in the middle of things as I understood them at the time. I like to think of learning as a process and not so
much a destination. This gives me latitude for mistakes...ones I can correct and build on as I go, because brother,
once you think you know it all you just quit learning. I'm sure there will be a few flaw here as well but better.
was, right in the middle of things as I understood them at the time. I like to think of learning as a process and not so
much a destination. This gives me latitude for mistakes...ones I can correct and build on as I go, because brother,
once you think you know it all you just quit learning. I'm sure there will be a few flaw here as well but better.
The above drawing is as simple as it gets. A wire in close proximity to and at right angles to the end of a bar
magnet. As this is a copper wire (non-magnetic) and as there is no current flowing through the wire, absolutely
nothing is going to happen. Doesn't matter how strong the magnets is or isn't. However, as soon as we put a
voltage to this wire it draws current over the resistance of that wire and the wire itself emits a magnetic field all its
own. I've drawn an arrow within the wire to show the direction of current. If this were the case that wire would
come right out of the screen at you as the two opposing fields repel each other. The field of the bar magnet and
the field of the wire. This force on the wire has a value and as were talking about motors will label that value
torque. Zero current = zero torque and some current = some torque. The more current that is driven through the
wire the more torque there is on that wire.
A magnet, as the one below, has lines of magnetic force that run through it connecting N and S poles. Each line
is called a line of flux. Flux is the electrical equivalent of current. A Gauss reading will tell us how many lines of
flux are present per unit area, a square centimeter. It is the measurement of the magnetic Intensity of the field,
More Gauss = a more intense field but tells us little about how strong that magnet is unless we know the number
of square centimeters that intensity is distributed over. Gauss times area is the measure of Total Flux. It is
total flux and NOT magnetic intensity that gives our motors punch!
How many lines of force per unit area, in this case the square centimeter is also the number of Dynes of force.
One dyne of force, pay attention here....is the amount of force required to produce an acceleration of 1 centimeter
per second squared acting on a mass of 1 gram.
The mass you are accelerating is the armature and by extension the resistance force. That is your introduction
to the resistance force. Everything else the Magnetomotive force is bucking.
magnet. As this is a copper wire (non-magnetic) and as there is no current flowing through the wire, absolutely
nothing is going to happen. Doesn't matter how strong the magnets is or isn't. However, as soon as we put a
voltage to this wire it draws current over the resistance of that wire and the wire itself emits a magnetic field all its
own. I've drawn an arrow within the wire to show the direction of current. If this were the case that wire would
come right out of the screen at you as the two opposing fields repel each other. The field of the bar magnet and
the field of the wire. This force on the wire has a value and as were talking about motors will label that value
torque. Zero current = zero torque and some current = some torque. The more current that is driven through the
wire the more torque there is on that wire.
A magnet, as the one below, has lines of magnetic force that run through it connecting N and S poles. Each line
is called a line of flux. Flux is the electrical equivalent of current. A Gauss reading will tell us how many lines of
flux are present per unit area, a square centimeter. It is the measurement of the magnetic Intensity of the field,
More Gauss = a more intense field but tells us little about how strong that magnet is unless we know the number
of square centimeters that intensity is distributed over. Gauss times area is the measure of Total Flux. It is
total flux and NOT magnetic intensity that gives our motors punch!
How many lines of force per unit area, in this case the square centimeter is also the number of Dynes of force.
One dyne of force, pay attention here....is the amount of force required to produce an acceleration of 1 centimeter
per second squared acting on a mass of 1 gram.
The mass you are accelerating is the armature and by extension the resistance force. That is your introduction
to the resistance force. Everything else the Magnetomotive force is bucking.
A wire carrying current has a like field to a permanent magnet. Note the "space" between the wire and
the magnet in figure 1. This is the air gap. This is where the magnetic field of the magnet and the magnetic
flied of the armature do their work. This is the space they repel each other. It is the total flux of both fields in
that gap that do the work. The stronger the magnet and the more current the wire carries the more force there
is on that wire.
Good time to bring this up. Note the ends of the field in figure 2. See how the further away you move from the
poles face the more spread out the lines of force become? There is a finite area to the end of the magnet
and a finite area to the crown of the armature pole. ONLY the total flux density acting over the smaller of the
two areas counts. The rest is pushing against air and doing no work. That has it's own lessons attached for
another time.
Thing is that a single wire as in figure 1 will not produce much force even for a large wire carrying a large
current.
the magnet in figure 1. This is the air gap. This is where the magnetic field of the magnet and the magnetic
flied of the armature do their work. This is the space they repel each other. It is the total flux of both fields in
that gap that do the work. The stronger the magnet and the more current the wire carries the more force there
is on that wire.
Good time to bring this up. Note the ends of the field in figure 2. See how the further away you move from the
poles face the more spread out the lines of force become? There is a finite area to the end of the magnet
and a finite area to the crown of the armature pole. ONLY the total flux density acting over the smaller of the
two areas counts. The rest is pushing against air and doing no work. That has it's own lessons attached for
another time.
Thing is that a single wire as in figure 1 will not produce much force even for a large wire carrying a large
current.
Figure 1
Figure 2
So how to amplify the armature field is the question. Figure 3 is one pole of an armature and one
magnet. By wrapping wire in a coil around a magnetically soft material we can amplify the field strength. This
is the same as figure 1 but turned 90 degrees around the vertical axis. The X to the left of the coil is now the
direction of current into the screen and the torque on the armature will be in the direction of the arrow
assuming the permanent magnets N pole is the lower face and the S pole the upper face. Crude drawing I
know but simple enough so show the relationships.
If we know the number of turns and the number of amps we know the Ampere-Turns of the coil. It is a unit
of Magnetomotive Force (MMF) also called a Gilbert. It is the force the gives cause to magnetic flux in the
armature pole. Sort of like voltage is to the electrical circuit. Having said that there is not a linear relationship
between MMF and Flux. Part of this is due to way the individual coils fields interact with each other,
Inductance, and partly because of the magnetic Permeability of the material it is wound around. And lastly
the geometry of the iron within the coil.
Inductance is a ratio of ampere-turns to the flux it creates. Part of inductance is, as mentioned earlier, the
way the coils reinforce each other. To do so they need to be in close proximity to each other. Thus a coil with
more layering, fatter, shorter coils have a higher inductance. The number of coils doing the reinforcement
and finally, the permeability of the material it is wound around.
Inductance also affects the rate of time rise of the coil. How quickly the coil charges and discharges. Rise
time can be found by dividing inductance by resistance. I'm not going to spend allot of time on this and for
these two reasons. Inductance is frequency dependant in an AC circuit, read rpm in our case. Yes, your
armature, by virtue of commutation is an AC circuit. Rise time is, as your can see from the formula, is the
ratio of inductance to resistance so to know if you are doing yourself any good you have to compare the two
not just choose the lower inductance armature.
Permeability is how easy it is to magnetize a material. More literal Permeability is the flux density divided
by the magnetizing force. Permeability, in some magnetic materials is also frequency dependant.
Lastly the geometry of the pole, which by the way is called an Inductor. This is because the coil induces a
flux in the pole. Okay, back to geometry. Just like a wire that carries a current a larger, fatter, shorter wire
having a lower resistance than a thin long one, the inductor will have a low Reluctance. At least it will if it too
is short and fat. The magnetic circuit equivalent to resistance. Oddly there is no unit for reluctance, only a
value. Reluctance = Length / area x permeability.
The three pieces to the puzzle then are Flux (Current), Magnetomotive Force (Voltage) and Reluctance
(Resistance) and they have the same relationship in the magnetic circuit they have in the electrical circuit.
Current = Voltage / Resistance and then Flux = Magnetomotive Force / Reluctance.
Another useful item to know is a feature of the inductor called Remanence. We induce a flux in to the
inductor when we energize the coil. When the coils voltage is removed the flux in a soft material decays but
not all of it leaves, what remains is a small amount of flux, a remnant if you will. A really good armature pole
will have both a low reluctance and a low remanence.
If we know the ampere-turns we can convert this to gilberts with a bit of multiplication. If this were then
divided by the reluctance we would know the flux, gauss. Point is that, it is a magnetic field we create in the
armature pole that opposes the magnetic field of the magnet. How strongly those two fields push against
each other creates a force and that force acts on the radius of the armature to produce torque. The radius is
the length of the lever the force acts upon.
An aside: That sets up some interesting contradictions. One for instance, an armature of larger diameter
and with a longer leg gives not only an inductor with a higher reluctance but a coil of lower inductance hence
a lower total flux for the same amp turns. However the lesser field is also acting on a longer lever. Just a
thought to show how such information is useful. Of course it also gives more room for larger wire, less
resistance for the same number of turns. I'm just saying.....
Let's start knitting some of this together in a practical way. There are things about motor construction you
just can't do much about other that make informed choices. That friend is why I make these post. There are
other items you have allot more influence over. So a quick recap of the above.
The object of the game it to construct two large magnetic forces and place them in a geometrical location
that gives the greatest net "leverage", and I might add, although not discussed above, over the smallest
inertia moment, touched on in the first paragraph, remember?
So let's separate the two and have a look. The field is the catalyst. It is comprised of the magnets and the
can which completes the magnetic circuit. As can be seen in figure 2 the magnetic lines of force, once
outside the magnets body stray or become unfocused. It is the cans job to concentrate or focus the lines of
force and bend them back in to a more productive area. Some cans are better at this than others. The lines
of force will want to follow the can if the can is also magnetic and thick enough and permeable enough to
contain the field. Any force not contained is called leakage. It is waisted into the surrounding area to have no
effect on the armature field. At a flux density of around 1.5 Tesla (15,000 gauss) magnetic materials become
Saturated. That is to say they can contain no more flux. Like a sponge full with water. So how thick does the
can need to be? Depends upon how permeable it is and that is a number we will not know. Not a lost cause
though. Measuring the field intensity on the cans exterior is a good guide. The lower the gauss reading the
better. Or the interior then the higher the better. Use the same magnet set however to do you
measurements. A stable point of reference is required.
magnet. By wrapping wire in a coil around a magnetically soft material we can amplify the field strength. This
is the same as figure 1 but turned 90 degrees around the vertical axis. The X to the left of the coil is now the
direction of current into the screen and the torque on the armature will be in the direction of the arrow
assuming the permanent magnets N pole is the lower face and the S pole the upper face. Crude drawing I
know but simple enough so show the relationships.
If we know the number of turns and the number of amps we know the Ampere-Turns of the coil. It is a unit
of Magnetomotive Force (MMF) also called a Gilbert. It is the force the gives cause to magnetic flux in the
armature pole. Sort of like voltage is to the electrical circuit. Having said that there is not a linear relationship
between MMF and Flux. Part of this is due to way the individual coils fields interact with each other,
Inductance, and partly because of the magnetic Permeability of the material it is wound around. And lastly
the geometry of the iron within the coil.
Inductance is a ratio of ampere-turns to the flux it creates. Part of inductance is, as mentioned earlier, the
way the coils reinforce each other. To do so they need to be in close proximity to each other. Thus a coil with
more layering, fatter, shorter coils have a higher inductance. The number of coils doing the reinforcement
and finally, the permeability of the material it is wound around.
Inductance also affects the rate of time rise of the coil. How quickly the coil charges and discharges. Rise
time can be found by dividing inductance by resistance. I'm not going to spend allot of time on this and for
these two reasons. Inductance is frequency dependant in an AC circuit, read rpm in our case. Yes, your
armature, by virtue of commutation is an AC circuit. Rise time is, as your can see from the formula, is the
ratio of inductance to resistance so to know if you are doing yourself any good you have to compare the two
not just choose the lower inductance armature.
Permeability is how easy it is to magnetize a material. More literal Permeability is the flux density divided
by the magnetizing force. Permeability, in some magnetic materials is also frequency dependant.
Lastly the geometry of the pole, which by the way is called an Inductor. This is because the coil induces a
flux in the pole. Okay, back to geometry. Just like a wire that carries a current a larger, fatter, shorter wire
having a lower resistance than a thin long one, the inductor will have a low Reluctance. At least it will if it too
is short and fat. The magnetic circuit equivalent to resistance. Oddly there is no unit for reluctance, only a
value. Reluctance = Length / area x permeability.
The three pieces to the puzzle then are Flux (Current), Magnetomotive Force (Voltage) and Reluctance
(Resistance) and they have the same relationship in the magnetic circuit they have in the electrical circuit.
Current = Voltage / Resistance and then Flux = Magnetomotive Force / Reluctance.
Another useful item to know is a feature of the inductor called Remanence. We induce a flux in to the
inductor when we energize the coil. When the coils voltage is removed the flux in a soft material decays but
not all of it leaves, what remains is a small amount of flux, a remnant if you will. A really good armature pole
will have both a low reluctance and a low remanence.
If we know the ampere-turns we can convert this to gilberts with a bit of multiplication. If this were then
divided by the reluctance we would know the flux, gauss. Point is that, it is a magnetic field we create in the
armature pole that opposes the magnetic field of the magnet. How strongly those two fields push against
each other creates a force and that force acts on the radius of the armature to produce torque. The radius is
the length of the lever the force acts upon.
An aside: That sets up some interesting contradictions. One for instance, an armature of larger diameter
and with a longer leg gives not only an inductor with a higher reluctance but a coil of lower inductance hence
a lower total flux for the same amp turns. However the lesser field is also acting on a longer lever. Just a
thought to show how such information is useful. Of course it also gives more room for larger wire, less
resistance for the same number of turns. I'm just saying.....
Let's start knitting some of this together in a practical way. There are things about motor construction you
just can't do much about other that make informed choices. That friend is why I make these post. There are
other items you have allot more influence over. So a quick recap of the above.
The object of the game it to construct two large magnetic forces and place them in a geometrical location
that gives the greatest net "leverage", and I might add, although not discussed above, over the smallest
inertia moment, touched on in the first paragraph, remember?
So let's separate the two and have a look. The field is the catalyst. It is comprised of the magnets and the
can which completes the magnetic circuit. As can be seen in figure 2 the magnetic lines of force, once
outside the magnets body stray or become unfocused. It is the cans job to concentrate or focus the lines of
force and bend them back in to a more productive area. Some cans are better at this than others. The lines
of force will want to follow the can if the can is also magnetic and thick enough and permeable enough to
contain the field. Any force not contained is called leakage. It is waisted into the surrounding area to have no
effect on the armature field. At a flux density of around 1.5 Tesla (15,000 gauss) magnetic materials become
Saturated. That is to say they can contain no more flux. Like a sponge full with water. So how thick does the
can need to be? Depends upon how permeable it is and that is a number we will not know. Not a lost cause
though. Measuring the field intensity on the cans exterior is a good guide. The lower the gauss reading the
better. Or the interior then the higher the better. Use the same magnet set however to do you
measurements. A stable point of reference is required.
Figure 3
Figure 4
Refer to figure 3 for a moment and picture the parallel field on the left of figure 4 running up and down. The
armature field in figure 3 runs the direction of the wires, right to left so that the armature in this position is
presenting it's field at perfect right angles to the magnet field. This is the only position that provides the
maximum amount of force in the direction of rotation. Rotate that armature a bit right or left and an angle is
created between the armature field and the magnet field. This is called the Slip Angle. The greater this angle
the less torque is being produced on that armature. The net torque is a function of the root square mean
average of all the positions the armature occupies while energized. It also says that the armature coil, to deliver
the maximum torque has to be fully energized right at that specific point. IF the motor were operating like most
motors on this planet this is little problem as it concerns armature timing. Just position the brushes so that the
coil is full at precisely this point and at that current load. There is the other side of that coin as well. Depending
upon reluctance and speed of the motor at any given time the time the coil is turned off gives that coil a certain
amount of time to discharge as far as it can which drives a voltage into the coils, the BEMF. If that voltage is
sufficiently high by the time it is time to turn the juice back on we get a big fat spark at the commutator which
erodes it. Bottom line, there is no perfect timing that can be set. There is a timing that will enhance various
portions of the rpm scale. This is a constantly moving target as frequency affected inductance an reluctance
and rpm driven "time" is all moving about. We can't solve all of the issues but we can help by using a field
magnets with radial orientation or the next best thing, segmented magnets. The more orientation we have the
fatter (area under the curve) we have, the longer the com last is a nice side benefit as well.
There is also the issue of armature Reactance associated with motor commutation timing. It is distortion in
the magnetic field(s). Sort of an "S" wave in the total field that can be partially corrected with timing. If fact in
stationary motors it is the prime goal. Again, there are issues as the amount as it is driven by field strength and
degree of slip angle which in our application is a constantly moving target. What I'm saying in these last few
paragraphs is that it is a compromise at best.
Again refer to figure 3. Note that in the drawing the crown of the pole is the same width as the arc of the
magnet? Yea, that isn't normal at all in C and D can motors. Strap motors with short, height wise, magnets may
actually have a crown wider than the magnet face. Part of the reason you don't see these arms with the degree
of timing advance we normally use.
For the forces to oppose each other at right angles, which is the goal, they have to do so with the maximum
amount of current AND the maximum amount of area. This means keeping the pole under the magnet while
the pole is at full MMF. As we are shooting at a moving target we can further say the maximum average area
during the power on cycle. Here's why;
Kt (per amp drawn) = 1352.4 / Kv
This is the relationship between the voltage generated, BEMF, and the torque produced called the
Generator Constant. The torque, in once inches per amp drawn is equal the voltage generated in millivolts. It is
inversely proportional. The lower the BEMF the higher the motor spins but the less torque it makes and
visa-versa. It is in equal proportion. That is to say if you increase the timing to reflect a 10% increase in rpm and
you've reduced the torque by a like percentage. This is true up to the point where the pole face comes out from
under the magnet face. At this point the torque drops like a stone in a well with little increase in rpm. It should
be evident then that the narrower the crown as a percentage of the magnet face the more timing the motor will
tolerate before falling into that well. One word for ya..Low dwell armatures.
So you may be asking yourself about now why run high timing? Field strength is that answer. It is the total
field that broadly defines the torque a motor will make. Say we have a motor that will develop 800 gr/cm of
torque at stall and has a no load speed of 70,000 rpm. If we increased the magnet field so that number is now
1,000 gr/cm the rpm would be reduced to 56,000 rpm. If we then advance the timing so that the rpm is again
70,000 rpm then the torque is then reduced back to 800 gr/cm. Looks like a wash, right? Maybe it is, maybe it
isn't. You have to answer two questions first. Did the amount of timing added to restore the rpm keep the pole
under the magnet? IF not then it becomes a loss as the pole pushes against air and not flux. Does this
combination provide the gearing that is inside the envelop of possibilities?
Another point to consider is time and time is equivalent in our motors to degrees of timing. Both inductance
and reluctance are frequency dependant. Higher is worse. Remember I2R? That is the static case. Our motor
is dynamic. In the dynamic state it is (I2 + H2) / R where H = Henries inductance. The faster is spins for a given
static inductance the higher the dynamic inductance the higher the BEMF the lower the forward voltage across
the winding the fewer amps it can draw.
A tricky one next. An armature is a series winding from pole to pole. Commutation places the three coils in
this state. Two in series with the third in parallel with the first two. This means that all three poles are in some
state of charge always...except...during overlap where both brushes are contacting all three leaf segments at
the same time which of course results in a dead short. The pole standing alone is the primary pole we've been
concerning ourselves with. One of the weaker poles will be to some degree under the opposite magnet of the
opposite polarity, the other will be, to some degree under the influence of a like polarity. Read this one is acting
as a brake on the primary pole. Both the dead short and the braking phases will add current load without
helping power production. To what degree depends upon tip gap versus crown width versus armature timing.
The dead short condition a function of brush orientation, vertical or horizontal, brush face area and com
diameter. What is important there is every degree the motor is turning under the dead short scenario is a
degree it is generating nothing but heat. The higher the percentage of the primary pole as opposed to the
braking pole the higher the conversion and the lower the amp draw. One hand washes the other.
A given winding on a given blank pretty much sets resistance, inductance and reluctance at any given rpm in
stone. Timing in concert with total field flux are your tools for maximizing rpm while maintaining the poles
position under the face of the magnet. Look at that formula for generator constant for awhile. A motor of a given
base no load speed will generate a torque constant of some value that determines the conversion of amps to
torque but it does not set the number of amps the motor will draw or even the average amps the motor will
draw over the entire rpm the motor will see. This is part of the reason that on a dyno line data sheet dividing
the amps by the torque constant does not equate to the same torque the dyno is displaying.
While it is popular thinking to say the number of amps drawn is a function of winding resistance and to a
certain point that is true it is not the entire story. The motors amp draw is, in part, set by this number but this
number only reflects the maximum possible amperage it can draw...not does draw assuming a zero short
condition at the com. Yes it is entirely possible to draw more amps than the armature resistance would
indicate due to shorting. The number most motor designers use is the Terminal Resistance. This is
measured at the point the wires contact the buss bar. This makes commutation efficiency as big a priority in
motor building as the field strength and alignment.
Okay, time to start winding this down. This was several volumes of textbook type information condensed into
a few paragraphs with a huge amount of relevant information not yet broached with holes in the details. Much of
this some will already known and for others not so much. I hope it provided knowledge to those without and
expansion for those with some.
I hope it took you from looking at a single feature of a motor as it stands alone and moved you to how these
features work in concert and how involved and complicated those concerts movements can be. The more you
know the less arduous and more fun this becomes. I hope it took you from looking at the motor as potential to
keeping its potential, the difference between gross input power and net shaft power. In essence we've broken
the motor into pieces.
1.) Create a large amount of "total flux"
2.) Align them to provide the best leverage.
3.) Convert as much amperage as the design will allow by:
4.) Limit mechanical, magnetic and electrical losses, all go to heat, not work.
It's not enough to draw big amps, you have to put them to work.
I recently read a short paragraph in Austin Hughes book "Electric Motors and Drives" that was...well just on
of those moments of realizing that no matter how much you think you know...you don't. It said, paraphrased:
"On the basis of equivalent torque, the motor with the highest rpm will have the advantage of higher
efficiency".
The caveat here was "equivalent torque"...Not just a higher rpm. In other words for the same "stall torque"
the motor with the higher rpm is more efficient. Yes, it would be for the same input amp draw it is converting
more of it to torque by wasting less to other losses. Simply jacking up the timing to obtain more rpm does not
in and of itself assure a more powerful motor, in fact it can hurt. It's sort of like the correction a friend of mine
gave me once on the old saying "Watched pots don't boil", eluding to the idea it seems to take longer for it to do
so if you watch it. The accurate expression is however, as I was corrected is; "Watched pots don't boil over". Oh
man, one word has a huge impact doesn't it?
If you are lucky enough to own a motor dyno by Fantom there are two numbers on the line data that are eye
openers. Peak efficiency and average efficiency. We've already noted that 50% of the input power at peak
horsepower is lost to "copper losses" which are unavoidable but you might be surprised to note that many
motors convert as little as under 20% at peak and under 10% average of the input to useful work. Some very
good hand built ones over 35% peak and 15% plus average. There is allot of room for improvement in most
over the counter motors and even more if you build them yourself.
armature field in figure 3 runs the direction of the wires, right to left so that the armature in this position is
presenting it's field at perfect right angles to the magnet field. This is the only position that provides the
maximum amount of force in the direction of rotation. Rotate that armature a bit right or left and an angle is
created between the armature field and the magnet field. This is called the Slip Angle. The greater this angle
the less torque is being produced on that armature. The net torque is a function of the root square mean
average of all the positions the armature occupies while energized. It also says that the armature coil, to deliver
the maximum torque has to be fully energized right at that specific point. IF the motor were operating like most
motors on this planet this is little problem as it concerns armature timing. Just position the brushes so that the
coil is full at precisely this point and at that current load. There is the other side of that coin as well. Depending
upon reluctance and speed of the motor at any given time the time the coil is turned off gives that coil a certain
amount of time to discharge as far as it can which drives a voltage into the coils, the BEMF. If that voltage is
sufficiently high by the time it is time to turn the juice back on we get a big fat spark at the commutator which
erodes it. Bottom line, there is no perfect timing that can be set. There is a timing that will enhance various
portions of the rpm scale. This is a constantly moving target as frequency affected inductance an reluctance
and rpm driven "time" is all moving about. We can't solve all of the issues but we can help by using a field
magnets with radial orientation or the next best thing, segmented magnets. The more orientation we have the
fatter (area under the curve) we have, the longer the com last is a nice side benefit as well.
There is also the issue of armature Reactance associated with motor commutation timing. It is distortion in
the magnetic field(s). Sort of an "S" wave in the total field that can be partially corrected with timing. If fact in
stationary motors it is the prime goal. Again, there are issues as the amount as it is driven by field strength and
degree of slip angle which in our application is a constantly moving target. What I'm saying in these last few
paragraphs is that it is a compromise at best.
Again refer to figure 3. Note that in the drawing the crown of the pole is the same width as the arc of the
magnet? Yea, that isn't normal at all in C and D can motors. Strap motors with short, height wise, magnets may
actually have a crown wider than the magnet face. Part of the reason you don't see these arms with the degree
of timing advance we normally use.
For the forces to oppose each other at right angles, which is the goal, they have to do so with the maximum
amount of current AND the maximum amount of area. This means keeping the pole under the magnet while
the pole is at full MMF. As we are shooting at a moving target we can further say the maximum average area
during the power on cycle. Here's why;
Kt (per amp drawn) = 1352.4 / Kv
This is the relationship between the voltage generated, BEMF, and the torque produced called the
Generator Constant. The torque, in once inches per amp drawn is equal the voltage generated in millivolts. It is
inversely proportional. The lower the BEMF the higher the motor spins but the less torque it makes and
visa-versa. It is in equal proportion. That is to say if you increase the timing to reflect a 10% increase in rpm and
you've reduced the torque by a like percentage. This is true up to the point where the pole face comes out from
under the magnet face. At this point the torque drops like a stone in a well with little increase in rpm. It should
be evident then that the narrower the crown as a percentage of the magnet face the more timing the motor will
tolerate before falling into that well. One word for ya..Low dwell armatures.
So you may be asking yourself about now why run high timing? Field strength is that answer. It is the total
field that broadly defines the torque a motor will make. Say we have a motor that will develop 800 gr/cm of
torque at stall and has a no load speed of 70,000 rpm. If we increased the magnet field so that number is now
1,000 gr/cm the rpm would be reduced to 56,000 rpm. If we then advance the timing so that the rpm is again
70,000 rpm then the torque is then reduced back to 800 gr/cm. Looks like a wash, right? Maybe it is, maybe it
isn't. You have to answer two questions first. Did the amount of timing added to restore the rpm keep the pole
under the magnet? IF not then it becomes a loss as the pole pushes against air and not flux. Does this
combination provide the gearing that is inside the envelop of possibilities?
Another point to consider is time and time is equivalent in our motors to degrees of timing. Both inductance
and reluctance are frequency dependant. Higher is worse. Remember I2R? That is the static case. Our motor
is dynamic. In the dynamic state it is (I2 + H2) / R where H = Henries inductance. The faster is spins for a given
static inductance the higher the dynamic inductance the higher the BEMF the lower the forward voltage across
the winding the fewer amps it can draw.
A tricky one next. An armature is a series winding from pole to pole. Commutation places the three coils in
this state. Two in series with the third in parallel with the first two. This means that all three poles are in some
state of charge always...except...during overlap where both brushes are contacting all three leaf segments at
the same time which of course results in a dead short. The pole standing alone is the primary pole we've been
concerning ourselves with. One of the weaker poles will be to some degree under the opposite magnet of the
opposite polarity, the other will be, to some degree under the influence of a like polarity. Read this one is acting
as a brake on the primary pole. Both the dead short and the braking phases will add current load without
helping power production. To what degree depends upon tip gap versus crown width versus armature timing.
The dead short condition a function of brush orientation, vertical or horizontal, brush face area and com
diameter. What is important there is every degree the motor is turning under the dead short scenario is a
degree it is generating nothing but heat. The higher the percentage of the primary pole as opposed to the
braking pole the higher the conversion and the lower the amp draw. One hand washes the other.
A given winding on a given blank pretty much sets resistance, inductance and reluctance at any given rpm in
stone. Timing in concert with total field flux are your tools for maximizing rpm while maintaining the poles
position under the face of the magnet. Look at that formula for generator constant for awhile. A motor of a given
base no load speed will generate a torque constant of some value that determines the conversion of amps to
torque but it does not set the number of amps the motor will draw or even the average amps the motor will
draw over the entire rpm the motor will see. This is part of the reason that on a dyno line data sheet dividing
the amps by the torque constant does not equate to the same torque the dyno is displaying.
While it is popular thinking to say the number of amps drawn is a function of winding resistance and to a
certain point that is true it is not the entire story. The motors amp draw is, in part, set by this number but this
number only reflects the maximum possible amperage it can draw...not does draw assuming a zero short
condition at the com. Yes it is entirely possible to draw more amps than the armature resistance would
indicate due to shorting. The number most motor designers use is the Terminal Resistance. This is
measured at the point the wires contact the buss bar. This makes commutation efficiency as big a priority in
motor building as the field strength and alignment.
Okay, time to start winding this down. This was several volumes of textbook type information condensed into
a few paragraphs with a huge amount of relevant information not yet broached with holes in the details. Much of
this some will already known and for others not so much. I hope it provided knowledge to those without and
expansion for those with some.
I hope it took you from looking at a single feature of a motor as it stands alone and moved you to how these
features work in concert and how involved and complicated those concerts movements can be. The more you
know the less arduous and more fun this becomes. I hope it took you from looking at the motor as potential to
keeping its potential, the difference between gross input power and net shaft power. In essence we've broken
the motor into pieces.
1.) Create a large amount of "total flux"
2.) Align them to provide the best leverage.
3.) Convert as much amperage as the design will allow by:
4.) Limit mechanical, magnetic and electrical losses, all go to heat, not work.
It's not enough to draw big amps, you have to put them to work.
I recently read a short paragraph in Austin Hughes book "Electric Motors and Drives" that was...well just on
of those moments of realizing that no matter how much you think you know...you don't. It said, paraphrased:
"On the basis of equivalent torque, the motor with the highest rpm will have the advantage of higher
efficiency".
The caveat here was "equivalent torque"...Not just a higher rpm. In other words for the same "stall torque"
the motor with the higher rpm is more efficient. Yes, it would be for the same input amp draw it is converting
more of it to torque by wasting less to other losses. Simply jacking up the timing to obtain more rpm does not
in and of itself assure a more powerful motor, in fact it can hurt. It's sort of like the correction a friend of mine
gave me once on the old saying "Watched pots don't boil", eluding to the idea it seems to take longer for it to do
so if you watch it. The accurate expression is however, as I was corrected is; "Watched pots don't boil over". Oh
man, one word has a huge impact doesn't it?
If you are lucky enough to own a motor dyno by Fantom there are two numbers on the line data that are eye
openers. Peak efficiency and average efficiency. We've already noted that 50% of the input power at peak
horsepower is lost to "copper losses" which are unavoidable but you might be surprised to note that many
motors convert as little as under 20% at peak and under 10% average of the input to useful work. Some very
good hand built ones over 35% peak and 15% plus average. There is allot of room for improvement in most
over the counter motors and even more if you build them yourself.