Strong RC Motors LLC
Instructions for Building a Strong RC Motor
What you should find in your Kit
Figure 1 BasicKit
BasicKits are normally shipped assembled and contain:
· Motor assembly
· Allen wrench
Figure 2 SuperKit
SuperKits contain a BasicKit plus:
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:
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.
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.
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 magnets for example the 12 and are glued down and the can be glued down before the is placed. It takes a little judgment to set the 12-1 gap with out seeing the 1-2 and 2-3 gaps, but it can be done. And, if you miss the first time you can pop it off and try again. I am usually able to put the 1 and magnets in at the same time, some jumping and re-separation goes on but then you can see all three gaps before applying the glue.
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 firstname.lastname@example.org.) 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.
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.
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
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
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. email@example.com
Selecting Turns and Wire Size
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.
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.
Turns on 22.7 mm Stators
Turns on 20 mm Stators
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.
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.