Electric Motors part 2
Brian Mulder    m


Last month we covered what makes up a motor
this month we look at how to wind the coils on the motor.

Putting on the Turns
What wire and how thick?
We use single strand enamelled copper wire for our motors.  Some people like to call it magnet wire. There are other types of wire that can be used from square shaped copper strands to mutli-stranded litz wire.  As the expected power handling capability of a motor rises, the need for better wire becomes more important.

One way of handling the problem is to use parallel strands instead of a single strand of thicker wire.    The reason for this is two-fold.  Thick wire becomes troublesome to wind and there is the issue of skin effect.

Just briefly, skin effect is a term used when the electrons flowing through a copper wire tend to travel only in the outer circumference of the wire.  The centre of the wire is then just unwanted weight.

This starts happening when the switching frequency of the current rises.  The skin effect issue has been debated for some time as to whether it is a concern in our motors, since the switching frequencies in our speed controllers are not very high.   Some swear by it, and others not.  For our lower power motors though, we will not be concerned about it.

To get any suitable amount of power out of a motor, we want the lowest resistance possible.  Resistance in our windings is what we use to calculate copper losses which reduce the overall efficiency of our power system.    So the incentive is to use the thickest wire we can but too thick wire will mean too few turns and unless you are designing a very high speed motor, we will want more turns to create useable torque.

Too thin wire results in high resistance and then you cannot get enough current through the motor.   If we raised the operating voltage of our motor, Ohms Law says more current will flow through.  Yes, but the copper losses will cause heat build-up and destroy the motor.    Weve got to use the appropriate wire for the intended power.

For CD rom type motors, the range of wire used is from about 0,4mm to 0,6mm diameter.

The 0,4mm wire will obviously allow more turns, but have higher resistance than the 0,6mm wire.  The trick in this case becomes an issue of how you connect your wire ends.

There are two methods used, one called Star and the other Delta, but we will discuss this further in a little while.

As for determining the resistance of your motor, trying to measure resistances of less than 1 ohm is not easy unless you have sophisticated equipment.  Conventional digital multimeters are not up to it as they do not take the instruments leads into consideration.

To make life a little easier, you can predict the resistance of your motor by measuring the total length of wire used and then calulating the resistance using the data from the table which follows.

This way you can get an idea of what your motors resistance will be.
 
diameter
section area
resistance
section area
resistance
section area
resistance
(mm)
(mm²)
(mW /m)
(mm²)
(mW /m)
(mm²)
(mW /m)
     
2 strand
 
3 strand
 
0,30
0,071
247,57
0,141
123,79
0,212
82,52
0,35
0,096
181,89
0,192
90,95
0,289
60,83
0,40
0,126
139,26
0,251
69,63
0,377
46,42
0,45
0,159
110,03
0,318
55,02
0,477
36,68
0,50
0,196
89,13
0,393
44,56
0,589
29,71
0,55
0,238
73,66
0,475
36,83
0,713
24,55
0,60
 0,283
 61,89
 0,565
 30,95
 0,848
20,63 
0,65
 0,332
 52,74
 0,664
 26,37
 0,995
 17,58
0,70
 0,385
 45,47
 0,770
 22,74
 1,155
 15,16
0,75
 0,442
 39,61
 0,884
 19,81
 1,325
 13,20
0,80
 0,503
 34,82
 1,005
 17,41
 1,508
 11,61
0,85
 0,567
 30,84
 1,135
 15,42
 1,702
 10,28
0,90
 0,636
 27,51
 1,272
 13,75
 1,909
  9,17
0,95
 0,709
 24,69
 1,418
 12,34
 2,126
  8,23
1,00
 0,785
 22,28
 1,571
 11,14
 2,356
  7,43
1,05
 0,866
 20,21
 1,732
 10,11
 2,598
  6,74
1,10
 0,950
 18,41
 1,901
  9,21
 2,851
  6,14
1,15
 1,039
 16,85
 2,077
  8,42
 3,116
  5,62
1,20
 1,131
 15,47
 2,262
  7,74
 3,393
  5,16
1,25
 1,227
 14,26
 2,454
  7,13
 3,682
  4,75
1,30
 1,327
 13,18
 2,655
  6,59
 3,982
  4,39
1,35
 1,431
 12,23
 2,863
  6,11
 4,294
  4,08
1,40
 1,539
 11,37
 2,079
  5,68
 4,618
  3,79
1,45
 1,651
 10,60
 2,303
  5,30
 4,954
  3,53
1,50
 1,767
  9,90
 2,534
  4,95
 5,301
  3,30

Winding the Motor
Firstly, let me say that winding stators is not easy.  It takes a fair amount of practice to get it right, and can be very time consuming at first.

Winding 20 turns of thin wire onto a tooth can appear to be quite simple, but what we are trying to do is wind anything from 10 to 30 turns of suitably thick wire onto a tooth that would not really allow for so many turns.

The trick is to mount the stator in some kind of jig to hold it and then using both hands, wind on the turns using a fair amount of pull to keep the windings compact.  This is extremely important, as the first layer of windings must support the next one or two layers.  If the windings are too loose, the following layer will have the wire getting in between the windings of the first layer.  This creates a total mess and you will never get the maximum number of windings onto a stator tooth like this.

A jumbled cross over of wires is simply no good.  A properly wound tooth will have tightly packed layers of windings that should look nice and neat.  And I say it again . . . it takes a lot of practice!

The 9 Pole Stator Winding Scheme
There are numerous different configurations of magnet numbers versus winding options available, some working better than others.  My own personal opinion is that there is only one way to wind your coils on a 9 pole stator.

I think a lot of alternative schemes came about due to people making a hash of the magnet configurations, or trying to make some stripped rotor with its original magnets work with a stator not designed for such.  But thats just my opinion.  My suggestion is to simply stick with the stock standing winding scheme presented below.   It is the basis of all winding schemes and the easiest to physically wind without getting yourself into a muddle . . . and as simple as it may look, you always get into a bit of a muddle, especially keeping track of how many turns you have put on the tooth.

Now if you were able to figure out what is going on from the drawing, thats great.  I find a sketch easy to understand.  A lot of motor builders however do not.   They seem unable to interpret what I call a simple drawing.

A while back I posted a query on how to terminate my windings for an LRK motor (we will get to LRK motors a little later) on RC groups.  I uploaded photos and drawings illustrating my problem . . . that eventually dragged into about 20 pages of debating on what I considered correct and others said was wrong.

At the end of the day, it became apparent that some people prefer to wind motors based on text data and others, like me, prefer a drawing or photographs.   I say a picture says a thousand words, while a line of text says little and, more importantly, opens itself up to all sorts of misinterpretation.

Those that were arguing with me appeared incapable of understanding a simple sketch and preferred to read text in lieu of a drawing.  I on the other hand, was misinterpreting ambiguous text.

So then, how do we explain the drawing in a simple text format?

If you hold the stator in such a way that you look at it edge-on ( you can only see the ends of about 4 or 5 teeth as opposed to all 9 when viewed from the top), a piece of wire that is wound clockwise around the tooth will be noted as A direction.    If wound anti-clockwise, that tooth will be regarded as been wound a direction.   Make sense to you?

Not at first!  But after talking to Brian, I sorted out that the letter itself identifies the wire, while whether it is capital or lower case identifies the direction of wind
 the capital letter means clockwise and the
 small letter means anticlockwise   thus
 A = the A wire wound clockwise, while
 b = the B wire wound anticlockwise.

Looking at the sketch, you will notice that we used three wires that is one wire per motor phase.   We are going to call them A, B and C wires.   Therefore, all teeth wound with piece of wire A will be identified as an A or a depending on the direction of the winding.   That same is said for B and C wires.

So, looking at the drawing, we have wound all teeth in the same direction, one after each other which equals the text equivalent of the ABCABCABC winding scheme.  Nine letters, one letter for each tooth.

So if I gave you a winding scheme of AbCaBCABc, would you know how to wind it?

Think so 1st tooth, A wire, clockwise,
     2nd tooth, B wire, anticlockwise,
     3rd tooth, C wire, clockwise,
     4th tooth, A wire from end of
         1st tooth, anticlockwise . . .

Well done!  Our Editor isnt just a pretty face
           I dont believe I even thought that!!!

To continue . . .
  5th tooth, B wire from end of 2nd tooth, clockwise,
  6th tooth, C wire from end of 3rd tooth, clockwise,
  7th tooth, A wire from end of 4th tooth, clockwise, exiting at END,
  8th tooth, B wire from end of 5th tooth, clockwise, exiting at END,
  9th tooth, C wire from end of 6th tooth, anticlockwise, exiting at END.

This descriptive format is used as opposed to drawings or diagrams that are more time consuming to create.   Nevertheless, drawings in my view create less confusion.

Such a pity that many people cannot read a simple drawing.  Interesting how our minds work.

Hookups
Okay we have wound the stator and should now have 6 wires hanging out.  Three of the wires will be your starting ends  and three will be your finish ends.  Aah . . . but you are no longer sure which ends are what . . . not so?  In that case, you might find it easier to mark the wires in some format prior to starting the winding sequence.  You could use pieces of tape with text written on them or colour code them whatever tickles your fancy.

So we have our six wires, but only three connections to a speed controller.  Now prior to starting to wind the motor, we should have decided on what configuration, or what purpose our motor was to be designed for.

There are two configurations in which a motor can be wired up.   One is call Star or Y and the other, Delta.

Each configuration offers slightly different properties and affects how the motor will perform.   It would seem though, that motor builders still have not really decided which is the better option.

The diagram below shows the physical winding scheme, although the three wires are shown electrically below hence the name delta.  Even then the A wire should really be shown in three windings, one on each tooth . . .


 
 
 
 
 
 
 
 
 
 
 

The electrical scheme of the star wiring is a little more obvious.

Typically, a Delta hookup is chosen when you want to swing a propeller at high revs and a Star hookup is used for low revving motors and big propellers.  This will become a little clearer when you look at the RPM/Volt numbers.

If you look at the Delta connection and apply power to any two terminals (eg points & ), all coils will have current flowing through them.   To illustrate how current is divided into the coils, lets say one phase equals 1 ohm of resistance. Therefore, you have a phase of 1 ohm (A) in parallel with the other two phases (B & C in series) that would total 2 ohms. Using Ohms law, we calculate that 2/3 of the current flowing into the motor will pass through phase A and the remaining 1/3 through phases B and C.  Total resistance of motor windings seen by the controller will be 0,66 ohms

If we connected the same coils in a Star configuration, you will only have current flowing in any two phases at any given moment.    The current flowing through these two phases will be equal and the total resistance will be 2 ohms.

If we applied a voltage of say 10 Volts  to our speed controller, you could get around 15 Amps flowing in the delta configuration, and a mere 5 Amps into the Star configuration.  Needless to say, the Delta configuration would be capable of  delivering a lot more power in this case.

But it does not end here.  You might be able to get more current into the motor, but the torque required to swing a large propeller might be lacking.  Yes, you could throw ample power at the motor and make that big propeller sing, but possibly at the cost of poor efficiency.

As an example, I collected a few hard drive stators to build a motor.  I wanted a reasonable amount of power out of the motor in order to fly a 72 in Piper Cub.  In order to get enough current into the motor, I tried using some 0,6mm diameter wire I had lying around.  After some struggle, I found I was only able to get about 10 or 11 turns onto each tooth.

As a starting point,  I tried a Star hookup, as I wanted a high torque motor.  On 3 Lipoly cells, I was only able to get a little over 10 Amps into the motor using a suitable prop for good efficiency.   Marginal power . . . but I wanted some in reserve!

The motor was reconfigured for a Delta hookup, which resulted in a lot more power being available.  Enough power to fly the plane, but also enough power to ruin the motor!

So what do we do now?

Well, the simplest option here would be to select a suitable voltage to power the motor. The star connection could have the number of Lipoly cells increased to 4.  This sees a good increase in power.  For the Delta connection, we do the opposite and reduce the number of cells used.

RPM per Volt
Remember in last months article, we referred to back-emf, being the voltage induced in the winding when a motor spins.  Well this parameter rears its head once more.  How you wind a motor will determine how fast it will spin and what battery pack voltage we will need to get a certain level of performance.

If you take a motor with no load attached,  and apply full throttle at say 6 volts, the motor will spin up to a maximum RPM.  If we are able to measure this RPM and divide it by the pack voltage, we will arrive at a figure called RPM per Volt.  We could then determine how fast the motor will spin for any applied voltage.

So say our motor spins at 8000 RPM at 6 Volts.

8000 / 6 = 1333 RPM per Volt

Therefore, for a pack voltage of 10 Volts, the motor will spin to 13 330 RPM.

This figure helps us determine what our motor is capable of, and whether it is suited to a particular application.

If we require a motor for a ducted fan model, then we need a motor that has a high RPM/V value.  A ducted fan rotor is designed to move air at high velocity, and can only do this when spinning at high RPM.

For a 3D type aircraft, we would need a motor that can provide a fair amount of torque in order to swing the large propeller required to generate a good amount of static thrust.  This would typically be a motor designed for a low RPM/V.

What we must remember though, as you load the motor, the predicted RPM figure will drop off.

Getting back to Delta and Star connections, there is a relationship between the two configurations and the RPM/Volt value.  If you wire up the motor for a Star connection and measure the RPM, you can calculate what the RPM/V will be for the Delta connection and vise versa.

Converting from Star to Delta hookup, the RPM/Volt value is multiplied by 1,73.

From Delta to Star hookup, multiply the RMP/Volt value by 0,578.

Basically, you are left with a method of manipulating the motor performance by simply changing the wiring scheme.  Some motor builders have gone so far as to route all six wires from the motor to a connection block that would allow them to alter the wiring scheme should they wish it.

So, how do you determine / calculate the RPM per volt value before winding a motor?

For given stator sizes and stack thickness, there are some programmes that will calculate the number of turns required to achieve certain target figures.  Most of the time though, we just wind on as many turns as we can and measure the motor parameters afterwards.  Using the measured figures, you can then work out what you need to do to achieve a specific goal.  It is pretty much experimentation and acquiring a feel for what is going on.

The magnet strength and configuration  also contribute to the RPM /volt value.

As a guideline, here are some basic points to consider

The higher the number of turns on a tooth, the greater the magnetic field produced for a given current.  Stronger magnet field results in more torque and lowers the RPM/Volt number.

For higher RPM values, fewer turns are required but results in less torque being available.  The lost torque can be recovered though by pushing more current through the motor to increase the field strength.

Thicker stator packs help reduce the RPM/Volt values while thinner ones do the opposite.
*     *     *
Next month we will look at magnets again and the LRK motor.

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