Strong
RC Motors LLC
Strong Engineering
Instructions for Building a Strong RC Motor
What you should find
in your Kit
Figure 1 BasicKit
Assembled
Disassembled
BasicKits are
normally shipped assembled and
contain:
·
Motor assembly
·
Allen wrench
Figure 2 SuperKit
SuperKits contain a BasicKit plus:
- Magnets
- Wire
- Motor
Mount
The motor mount may be temporarily mounted on the motor
assembly, if so loosen it and slide it off the bearing tube on back side of the
motor.
Disassembling the
motor:
- Remove
the washer and two nuts from the threaded end of the shaft.
- Loosen
the hub setscrew and slide the can off the threaded end of the shaft.
- The
stator and bearing tube can now be removed from the shaft. Hold the bearing tube in your fingers
and push it off the threaded end of the shaft. As this is done the rear bearing
(nearest the e-clip) may slide out of the
bearing tube and remain on the shaft.
This is OK and it can just be left on the shaft. The bearings are a slip fit in both ends
of the bearing tube, so once the bearing tube is off the shaft, watch that
they do not drop out and get lost.
- The
thrust washer and e-clip can remain on the
shaft, and the bearings, washer and nuts can be placed on the shaft for
safe keeping.
Figure 3 Disassembled Motor
L to R: Shaft with bearings and hardware, Can, O-ring, Stator on bearing tube, Prop saver screws, Allen wrench
Description and Terms
The “can” is sometimes referred to as the
”bell”. STRONG cans have a setscrew hub,
and some, like the one shown, may also serve as “prop savers”. With the machine screws inserted in the
threaded holes, the prop can be held on with the o-ring or rubber bands. This flexible mounting reduces the chance of
prop damage. The magnets are glued to
the inside rim of the can. The rim of
the can is sometimes called the flux ring, because it carries the magnetic flux
from each magnet to its neighbors and keeps the flux from “leaking” outside the
motor.
The “stator” is made of laminated metal. It has 9 arms and each arm will be wound with
a coil of magnet wire. The stator is
insulated with plastic to keep the wires insulation from being damaged. That would allow the winding to be shorted to
the stator.
Installing Magnets
- Start by washing your hands. That will minimize the chance of skin oil interfering
with the glue bond. Now mark the magnets so they can be
placed NSNSNS... in the can. The
poles are on the large (5X5) faces.
N is on one side and S on the other. You could use a compass to
find out which side is N, but the motor does not care which magnets are N
or S, just that they alternate NSN... or SNS... around the can. No two adjacent magnetic poles can be
the same. When stacked all the
matching poles point in the same direction. So one end of the stack is N and the
other is S. Start at one end of the
stack and mark the face of the end magnet with a marker. Then remove that
magnet and mark the next one. Slide
the magnets off the stack with a twisting motion. They are too strong to pull straight
off. CAUTION: As you set magnets down don't let them bang back
together. They could break! You can restack them, or lay them out
about 8 inches apart (much closer and they bang together).
- Special note for Curved Magnets: You can follow the above instructions
for marking magnets or you can use the natural “nesting” of the
magnets. Curved magnets will stack like this
(((((()))))) or sometimes this ))))))((((((. Magnets that nest together are the same
polarity. Magnets that come
together like this () or this )( are opposite polarity. So with the magnets stacked so six
magnets nest you can just install the six magnets in every other slot of
the magnet spacer. Then install magnets
from the remaining nest of six in the open slots and you will have the
desired alternating NSNSNSNSNSNS placement.
- Wipe the inside surface of the can with alcohol
or lacquer thinner to assure the glue can get a solid bond. Now you can begin inserting magnets in the can. Spacing the magnets
evenly around the can is easiest with the aid of a Strong Magnet Spacer. Insert the spacer in the can with the machine screw
handle facing out. Then insert 12
magnets flush with the edge of the can. (Caution: Do not insert the magnets too deep. If you put the magnets too far inside
the can they could cause problems later.)
Remember to alternate polarity by having every other magnet with
the mark facing in. Now remove the
spacer and apply glue. Some
builders prefer to install every other magnet. Then remove the spacer and glue 6. Then
repeat for the 6 opposite polarity magnets. That reduces the chance of putting two
like polarity magnets next to each other.
Either way, installing twelve precisely spaced magnets takes just
minutes! Thin CA is recommended to
secure the magnets, since it sets quickest. You can fill the gaps later
with thicker CA or thin epoxy, but many builders use only thin CA. Loctite’s
Black Max is reported to be great for gluing magnets.
The spacers depth in the can may be adjusted
using the machine screw “handle”. In
deep cans, as used for double stator motors, turn the machine screw in and
install a second threaded wood washer or a metal nut on the tip. Use this to adjust the magnet spacers’ depth
in the can.
- Magnet spacing can be done by eye, if a magnet spacer is not
available. Insert the first magnet,
with the marked pole showing, until the edge is flush with the can
rim. The next magnet goes directly
across from the first, with the marked pole showing (like the first). The
next two magnets go half way between the first two with the marked poles
NOT showing. You now have magnets at 12, 3, 6, and
Questions on placing magnets:
How do you keep the little magnets
from slamming together during this process?
I would think the magnets will
simply slam together while you are trying to slip them in such a tiny (the can)
space.
My
experience is that once placed on the rim of the can a magnet will stay in
position until moved to within about .020" of its neighbor. If the neighbor is glued, then even if the
free magnet jumps and contacts, a knife blade can separate them. Using the recommended placement technique you
never need to put two free (unglued) magnets in place at the same time. When placing the 1 and
Will it matter if you get epoxy on
the face (in the gap between the stator and magnet) of the magnet?
Epoxy on
the face of the magnets is a problem if it exceeds the air gap between stator
and magnets. These motors are designed
to run with an air gap of about 0.15 mm (.006 Inch). That does not leave much room for glue
between the magnets and stator tips.
Sometimes it is necessary to scrape the epoxy off the magnet faces. It pops right off the shiny magnet face, and
stays in the gaps.
Do you think that an uneven amount
of epoxy/CA will unbalance the motor enough to damage it? It would seem
difficult to get the can to balance properly, any method to balancing the can
after final gluing of the magnets?
Balancing
is not normally required. Normal amounts
of CA will not cause any noticeable balance problems. If epoxy is used just try to get it evenly
distributed.
Winding the stator
If you have
not yet selected the wire size and number of turns to be used, refer to the
section titled Selecting Wire Size and
Turns. Another good source of
information is the GoBrushless online performance data base. You may also contact Strong RC Motors or
others available on the internet.
Caution: Be careful of the wire
insulation as you
wind. The wire may appear bare, but it
is insulated. The insulation must remain
intact as you wind and be removed from the ends to make connections. Use your fingers, finger nails and a small
piece of plastic or wood as you wind.
Don’t use any metal which could damage the wire or stator insulation.
Wind the first coil of phase one.
The stator has 9 arms. Pick a
starting arm and with 2 inches of wire sticking out (toward the back if the
stator is already mounted on the bearing tube); start to wind around the base
of the first arm. Wind in the direction
that is most comfortable for you. Your
motor will not care, as long as they are all
the same. Try to keep the windings
tight and close to each other as you wind the first layer toward the tip. It is important to keep the windings as flat
as possible on the face of the stator that will go deep into the can. Windings that protrude on that face could rub
on the can. Continuing the first layer,
around 3-4 turns, stop and push the windings down tight toward the inside of
the stator, you should then be able to get a few more on the first layer. As you continue winding the second layer will
start to form at the tip, just continue laying the second layer toward the
center. When you reach the hub you
should be getting close to your target number of turns. Wind the final layer toward the tip and when
you approach your target number of turns wind back so the last turn ends near
the hub with the wire sticking out the same side of the stator you started
on. Now you want to continue this same
wire and start winding the arm three away from where you started.
Wind the second coil of phase
one. You will be skipping two arms and winding on the
third. It does not matter which way you
go as you skip two arms. Just go in the
same direction each time you skip arms.
To keep the wire from going too far toward the center, it is common
practice to take the wire around the back side of one arm and the front of the
next until you reach the arm to be wound.
Wind in the same direction and
with the same number of turns as you used on coil one.
Wind the third coil of phase
one. Again, skip two arms and wind the third (and last)
arm with this wire (phase). When you’re
done winding the wire around the third coil, bring it out the side your starting
wire is on and cut it off at about the same length.
You have
now wound the first phase of your three phase brushless motor. Just two more too go.
Wind the first coil of phase
two. Start the arm next to your first (either way is ok)
with a wire 1 ½ inches long (½ inch shorter than the first wire this is to
identify the wires). Wind the first coil
of this phase just as you did the others.
Wind the second coil of phase
two. After winding the first coil move in the same direction
as you did when winding phase one, and again weave past two arms and wind the
third.
Wind the third coil of phase
two. After winding the second coil, again weave past two
arms and wind the third. Cut the end of this wire leaving the end 1 ½ inches
long.
You have
now wound the second phase of your three phase brushless motor. Just one more too go.
When you
wind wire three, start with 2 ½ inches of wire (½ inch longer than wire one),
and when finished with the last arm cut it off leaving the same length.
The winding
is now complete. You should now have all
arms wound with equal turns of wire, and six pairs of leads coming out the same
side of the stator.
Check for shorts.
Use a multi-meter to test for potential shorts between phases or from
any phase to the stator. (If you need
more explanation on testing for shorts send an email to don@strongrcmotors.com.) If you do not have a meter just skip this
step, but be sure to use a fuse in the battery lead to the controller when
testing.
Caution: Do not apply CA to windings.
Winding failures have been reported a few weeks after applying CA to
secure them. Apparently some CA glues
attack some wire insulation. Potting or
covering the windings with anything is not recommended. Leave them to be cooled in the breeze. If the last coil or two needs to be retained,
use a tiny spot of hot glue or epoxy, but use as little as possible.
Make Motor Connections.
The six motor leads can be connected in WYE ( aka star or “Y”) or
Delta. If you have not decided which
connection is best for your application see the section titled Connecting Windings, Star or Delta? Another good source of information is the
GoBrushless online performance data base.
You may also contact Strong RC Motors or others available on the
internet.
Figure 4 "WYE" Connection
To make a “WYE” connection, connect the starting (or ending)
ends of the three phases together. The
six leads should be on adjacent stator arms.
Starting from either end of the six, the first three wires should be
three different lengths. Remove ½ inch
of insulation from each of three wire ends.
(It can be scraped off with a knife, or removed with sand paper or
solvent.) These are the motor power
leads. A connector can be added
later. The remaining adjacent wires
should also be three different lengths.
These get twisted together and soldered after removing the insulation
down close to the coils. Solder about ¼
inch of the twist and then cut it off leaving about 1/8 inch of soldered joint
that can be folded toward the space between coils. It should be out of the way but not shorted
against a coil wire. Heat shrink tubing
can be applied to prevent a possible short.
Figure 5 DELTA Connection
To make a “Delta” connection, connect the end of each phase to the start of the next
phase. The six leads should be on
adjacent stator arms. Number them from
either end 1-6. Start with the center two
wires (3 and 4), clean off the insulation, twist them together and solder. Next, take wire 1 and twist and solder it to
wire 5. Now finish by twisting wires 2
and 6 together and soldering. These
three twisted pairs are the motor power leads.
A connector can be added later.
Installing the Winding Terminator
The
winding terminator is a small circuit board that serves as a terminal strip for
the winding connection. The idea is to
eliminate all bending and motion of the magnet wire leads coming from the
stator, and keep them as short as practical. Any flexing and motion
should take place in the multi-strand leads going to the controller. The Terminator can be installed now or after
the motor is assembled. If you are not
using a Winding Terminator skip this section.
After
the stator is wound, slide the terminator on the bearing tube and stop about
half way to the stator. Remove the insulation from the motor leads up to
about 3 mm from the stator. Now thread each of the bare winding
leads through its own small hole in the terminator. Now you can
slide the terminator closer to the stator and at the same time loop the winding
lead over the edge of the terminator and through the hole again. When the
terminator is where you want it and the winding leads are in position, you can
apply instant or epoxy glue to secure the terminator to the bearing tube.
Then apply solder to the winding wires and solder dots around the small
holes on the terminator board. Apply a generous dab of
solder. Once soldered any excess
magnet wire can be cut short. To attach
the leads from your connector or ESC, pre-solder or "tin" the leads
after removing about 2 mm of insulation. Then just place them on the dabs
of solder and touch it with a heated soldering tip. No heat shrink is
needed. The direction the wires leave the terminator might need to be
adjusted for some installations. Just apply heat and reset the wire in
the dab of solder to get it just the way you want it.
Assemble the motor
The motor
can now be reassembled by reversing the disassembly steps.
- Remove
the washer, two nuts and one bearing from the threaded end of the
shaft. That leaves the e-ring, thrust washer and one bearing.
- Slide
the stator-bearing tube assembly on the shaft and over the rear bearing.
- Slide
the front bearing on the shaft and into the front of the bearing tube.
- Slide
the can on the shaft and over the stator.
Before tightening the setscrew hub in position pull it slightly
forward, to allow a small running clearance between the rear bearing and
the thrust washer. If the can were
locked in position with the bearing tube pushing the rear bearing tight
into the e-clip, friction could go up when the
aluminum bearing tube expands as it warms up. Tighten the can hub in position with just
a few thousandths of an inch clearance at the thrust washer.
Caution: Using two set screws is not recommended. The brass hub has two threaded holes. The
second hole is only needed when using the hub as a “prop saver”. Using
two setscrews to position the can could cause the can to eventually become
loose on the shaft. The shaft needs to be pushed against the side of the
hub bore, not suspended in mid position on the tips of two screws.
Position the can axially and secure it with one setscrew. That loads the
shaft tight against the far side of the hub hole, and forces it to align
with the shaft. Only use the second threaded hole if you are using a prop
saver. And even then completely tighten one prop saver screw locating and
aligning the can. Then just tighten the second one enough to keep it from
falling out.
Hold the
bearing tube and turn the shaft or can.
You should feel free smooth cogging as the magnets move over the
stator. The rotor should almost jump
from one position to the next. This
cogging should be equal as the can rotates.
Any sticky
spots, friction or noise when the rotor turns indicates some thing is
rubbing. If you think you may have a rub
go to the section on diagnosing and fixing rubs before you begin powered
testing.
Powered Testing
Mounting the Motor
Drill a
hole slightly larger than the bearing tube (about 8 mm or “O” size drill) in
the center of a 1 by 1.5 inch piece of ½ inch plywood. Then make a saw cut into the hole from the
center of one of the short sides. Continue the cut out the far side of the hole
half way to the edge. Now, insert the
motor in the test mount and clamp in a vise.
Connections to the Motor
The
controller has three wires coming from the business end. These wires connect to the three wires on the
motor. The motor will run with the wires
connected in any order, and it will reverse rotation if you switch any two of
the connections. Honest, it’s that easy!
Bench testing a new CDROM motor
First,
a fuse is highly recommended when checking out a new motor on the bench. It can
be placed in either lead from the battery.
A 25 cent fuse could save a $40-75 brushless ESC. A 5 Amp fuse is good for all but the hottest
CDROMS.
Start
the testing without a prop. It should
start right up and run smooth with no strange noises from start up to full
throttle. Full throttle no load should
draw less than 1 amp on all but the hottest winds. If all is well, install a small prop (5-3 or
less) then work up to larger props. If you don’t have an Amp meter, frequently
check the battery, wires and motor, and shut down if anything gets too warm.
You may want to consider using an old or inexpensive battery until you gain
confidence in the new motor/prop combination. Five "D" dry cells will
give you 7.5-8.0 V, simulating a 2S LiPo.
Old NiCd batteries or even 6 or 12 V lead acid batteries can be used on
the bench.
Your
motor should start and run with any commercial brushless controller.
Share
your JOY or let me help, send an Email after the first bench test. djarmstrong@wideopenwest.com
Selecting Turns and Wire Size
Winding Turns
The number
of turns of wire on each arm of the stator will be the biggest factor in
determining the performance of the motor.
Use the charts below to select a number of turns that matches the prop
and Amperage you want to run.
The charts
below were generated using MotoCalc. All
the predictions were made using only one set of data on a 22.7 mm stator motor
with 20T “Y” winding tested at 3S voltage.
MotoCalc is a great tool and it is highly recommended if you want to
become expert at matching windings, motors, and planes. It can be downloaded from www.motocalc.com and a free 30 day trial is
offered.
UNDERSTANDING THE CHARTS
Using the
first chart as an example, look at the straight line near the lower left.
That is Current VS RPM of a 25T motor. To get different RPM's you use
different props. The whole chart is WOT. That 25T motor will turn a
4-3 prop about 11000 RPM on 1 A, or if you used a 10-6 prop it would
only turn about 4000 RPM and it would draw 5A. Now move right, skip
the 20T motor line, and look at the yellow points on the line for a 15T
motor. It would spin the 4-3 prop much faster (about 19K) and draw more
current (3.3 A), and if you put a 10-6 prop on, it would pull over 10
A! (The data point looks like it would be about 11-12 A at 5000
RPM. It is not actually shown because such a high current would
overheat this motor.) So a motor that has fewer turns is more
powerful, will spin faster and pull more current. Why not keep reducing
turns? Two reasons, you may overload your battery, or if you have a big
battery, you will over heat the motor. I hope you can see the
straight lines are motors of a given turns and the curved lines are different
prop loads. If you build a 22.7 mm single stator motor your performance should
be pretty close to what’s shone on the charts.
For example, a 20 turn motor running on 3 LiPo cells (3S) should turn a
6-3 prop about 11,000 rpm on 3.5 to 4 amps.
For the technical types-- I
know both the chart and table are not mathematically correct. They cannot
be derived from pure theory and numbers. There are details of motor and
prop design that will allow a motor to perform slightly better (or worse) than
what is shown. And, likewise, all details of wire and winding technique
that are not accounted for in the winding table. But they are very useful.
My experience has been that actual results are not far from those shown.
Wire Size
The general
guide line for wire size is to use the largest wire you can fit on the stator
with the turns desired. That means lower
turn motors will have lower gage (larger diameter) wire. The table below is about what most builders
consider a full stator. You may get more
or less depending on technique, actual wire diameter, and the amount of winding
space between the can and stator you are using.
So using the table below if you want to build a 15 turn 22.7 motor, use
#25 wire.
|
Wire AWG |
#22 |
#23 |
#24 |
#25 |
#26 |
#27 |
#28 |
#29 |
|
Turns on 22.7 mm Stators |
7-10 |
9-11 |
12-14 |
15-18 |
19-23 |
24-26 |
27-36 |
34-?? |
|
Turns on 20 mm Stators |
5 |
5-8 |
11 |
15 |
17 |
22-24 |
25-27 |
28-32 |
Typical
wire sizes are #22-#30 and winding turns are typically 10-30 turns per
arm. 15T of #25 might be good for a 2S
battery and 8” dia. prop. 20T of #26
might be good for 3S battery and an 8” dia prop
Connecting Windings, Star or Delta?
Most motors have
“Y” connected windings. Delta is a
"hotter" configuration. So if you take a given winding (#turns and
gage) and switch it from Y to delta you get more current and speed. On the
other hand if you want to turn a 6-3 prop 11000 Rpm on a 2s LiPo battery you
can get that with either “Y” or Delta. The Delta will use 1.73 times more turns
of smaller wire, but the power, efficiency, and amp draw will be the same. So
it really comes down to what wire you have on hand and weather you like working
with more turns of fine wire (Delta)or fewer turns of larger wire (“Y”).
CD ROM "Y"
vs. Delta wiring
Here is bench
test data on one of my CD ROM motors. It is a 14g motor rebuilt with ball
bearings and 5X5X1 N45 magnets. Windings are 25 turns of #28 wire.
Using a Kokam 1020 2S battery and a 5-3 GWS prop:
RPM V@ESCin A Watts
in
7560 7.44 0.82 6
using a 12 V 3000 mAh LA battery:
11200 11.72 1.49 17
I was disappointed. I wanted more power. I was about to rewind when I realized
reconnecting the 25T #28 windings in delta might do it. Here is what I got on
the same 2S LiPO:
10740 6.6 2.5 17
The same RPM and input power as with the 12v battery on the "Y"
connection.
I have not tested it on the 12v battery, but I expect the power input to go to
about 35 W which would be a great match for a 3S battery in a high performance
light plane.
“WYE” “DELTA”
If
this is confusing, just connect in “Y” and forget about it.
Diagnosing and resolving Rubs
Rubs can
occur when magnets contact stator tips or when coil windings rub on the “disk”
portion of the can. Magnet rubs make
noise and sometimes cause starting problems. Winding rubs can cause erratic operation,
high no load current and excessive motor heating. If operation is erratic
at all that would point to a winding rub.
The easiest
way to find out what’s rubbing is to blacken the stator tips, magnet faces, and
disk area of the can with a felt tip marker.
Then spin the motor and look for shiny spots in the marked areas or on
the winding wires closest to the can. If
several magnets rubbed the same 1or 2 stator tips that suggests sanding or
grinding the offending tip might eliminate the rub. Sometimes the stator
tips have very small high spots, and a quick rub with sand paper or a fine file
will eliminate a noisy problem. If only one magnet touched, look at
the way it is secured. Is it loose? Does it stick out farther than the
others? If 2 or more stator tips touched and three or more magnets
touched with the center ones touching more than the end ones that is a
sign of excessive rotor runout. The final clue that you have a run
out problem is if the magnets are contacting near the open edge of the
can.
Magnet rubs are resolved by assuring the rotor
run out is not excessive, and magnets fit properly in the can. If magnets are securely against the can wall
the only solution is to file or grind the offending stator tips. If this is required, be careful to only file
in the direction of the laminations. Any
loads across laminations can cause laminations to bend and separate.
Winding rubs can be resolved by several
methods. The easiest is to move the
stator farther away from the disk portion of the can. This can be done by inserting a STRONG thrust
washer on the shaft between the can and front bearing. If this causes the stator to stick out the
back of the can some efficiency will be lost but the motor will operate fine
otherwise. To get the stator back inside
the can without rubbing requires the space taken up by the windings be
reduced. That means rewinding, or
carefully squeezing the windings to flatten them. It sounds crude, but this has worked on
several motors. Using two wooden blocks
with holes in them and thick enough to protect the bearing tube, squeeze the
windings in a vise or C-clamp.