Inductance What is it what does it do? Collosus has 8uH!

OK so I will write more on this soon but I made a video and here is a brief bit of what I think I understand! Ok so inductance in the motor is what lets the PWM (chopped up pulses of power from the caps and batteries) through the fets turn into usable power in a smother wave form in the winding. So as we are stepping into new territory with impossible to control motors and controllers that cost to much and don't do what we need them to. I am starting to experiment with inductors in series with all three windings. First I did a test with my bmx and x5 with my oscilloscope to See what I could see for voltage.... I tested the controller side of the inductors I made and the motor side as well as a measurement across one inductor. The end of the video shows a full throttle burst on my dyno on the motor side followed by the controller side and shows you can see a difference for sure! I used 14 awg wire wrapped around a piece of 1.875" od pvc tube ten turns then 8 more turns over top for a multi layered inductor to make things smaller and once I see numbers I want I will wind them in a more appropriate gauge of wire. I used plastic because there is next to no loss with a air core inductor.
[youtube]bqCZKi4ovrM[/youtube]

I used the calculator found here http://www.pronine.ca/multind.htm and it basically comes in at 15uh for these inductors.

The goal with all this is to find what works and should hopefully let me run Collossus!
I did a couple validation dyno runs before I installed the inductors in series and pulled 6.4 hp back to back. Then did two runs after and pulled 5.7 hp with the inductors both times. I believe most of that is because of the retarded timing there will be a little because the wire is thin so I will try to find some thicker winding wire asap! I tried to run Buties timing bord to bring the timing back but its causing misfires for some reason I need to investigate.

OH and I did get one successful run with burties bord at 6 deg static advance and it brought the power back up to 6.11 hp so I believe I'm on the right track.

Here is a link to some info on inductors. http://www.williamson-labs.com/480_rlc-l.htm
And a video that shows how placing one next to another can have effects. [youtube]GXcxswDcUbI[/youtube]

Here is a link to a calculator biff linked me on the collossus thread. http://www.daycounter.com/Calculators/Air-Core-Inductor-Calculator.phtml One thing I was thinking is you can run them in series and parallel in series the inductance adds up and in parallel it is the average of the total divided by the number of inductors in parallel like resistors. So because of this you can have 3 small sized inductors in series and as rpm increases by pass one at a time to lower the inductance then when you only have one in the circuit and the rpm in high enough you can add the others back in one at a time in parallel to even further lower the inductance.

Arlo1,
Glad to see you take the lead on this, & now you see why I am a fan of the ABC wound motors... Its good to see some ellemental E-theory in application. & solving some poor design choices that are becoming evident in this particular motor.

(maybe It is better said that the design choices are not poor, but more our intended usage doesn't align with the original designers intent very well with regard to controllers.) I hope you can get it to work well enough to be satisfied.

I am taking a differnet path & re-designing light motors specific to our applications. I still like brushless motors. prototyping stator laminations is going to be fun!

Thud that brings me to what I have been wondering about parallel strands in a motor. So from this link http://en.wikipedia.org/wiki/Series_and_parallel_circuits I found this.

InductorsInductors follow the same law, in that the total inductance of non-coupled inductors in parallel is equal to the reciprocal of the sum of the reciprocals of their individual inductances:



.
If the inductors are situated in each other's magnetic fields, this approach is invalid due to mutual inductance. If the mutual inductance between two coils in parallel is M, the equivalent inductor is:


If L1 = L2


The sign of M depends on how the magnetic fields influence each other. For two equal tightly coupled coils the total inductance is close to that of each single coil. If the polarity of one coil is reversed so that M is negative, then the parallel inductance is nearly zero or the combination is almost non-inductive. It is assumed in the "tightly coupled" case M is very nearly equal to L. However, if the inductances are not equal and the coils are tightly coupled there can be near short circuit conditions and high circulating currents for both positive and negative values of M, which can cause problems.

More than three inductors becomes more complex and the mutual inductance of each inductor on each other inductor and their influence on each other must be considered. For three coils, there are three mutual inductances M12, M13 and M23. This is best handled by matrix methods and summing the terms of the inverse of the L matrix (3 by 3 in this case).

The pertinent equations are of the form:

Thud, I'll bet you know about http://www.protolam.com/index.html for prototype laminations. They were the source for the laminations in an LN2 cooled motor that I am familiar with.

Arlo, good work on adding inductance. You could add a core to your coil and substantially up the inductance for the low speed high load condition making the motor "softer" to the controller, then withdraw the core at speed to minimize the losses in the secondary inductors.

bigmoose said:

then withdraw the core at speed to minimize the losses in the secondary inductors.

Click to expand...

Thats is a 100% awesome idea bigmosse that will help keep size and weight down! I cant belive I didn't think of that! I guess my minds just not up to the speed of your's yet.... :wink: Time for more fish, coffee, and chocolate!

Hi Dave,
I was browsing their sight at lunch today as a matter of fact. Also searching for some silicon steel to attempt a little chemical machining on some silk-screend lamination expriments. I can find all sorts of shim stock in 1010 steel. I wonder how bad it would suck the efficancys to as compaired to the real good stuff?

Google is letting me down searching for over the counter photo-reactive masking agents. :lol:

I have like 25 diferent stators drawn in various sizes...need to focus on a single unit & see if it fits in the budget anytime soon.

Thud, First, these guys have some off the shelf laminations: http://www.rbourgeois.com/induction_motors_laminations.a234.en.html

Second, here are some photo masks that are relevant to other processes. One would have to test to see if they were acid stable. I would bet most UV cured resins are.

Here you go USA Source Photoresist, "use on any metal with most any etchant" http://www.capefearpress.com/puretch.html

China Source Photoresist http://www.green-technology.net/english/product.asp-prono=11_SP_A400.htm
PCB etch resist should work http://www.thinktink.com/stack/volumes/voli/store/catalog.htm
http://www.ikonicsimaging.com/photoresist_film.php
http://www.problast.com.au/getting_started.html
http://www.granthams.com/page2.html

As for lamination materials it is very important for the power levels that you want to go to that you use Si bearing steel or better yet Cobalt bearing. You need good magnetic properties (flux) and high resistivity to kill the eddy currents.
Here is a link to lamination material properties. Also do not forget that the laminations need an insulation coating. The thinner the better. Protlam will get your started:http://www.protolam.com/page7.html

Here is what I have hanging around on laminations from various sources:

EDIT: Whooppee! I found the reference for you that the pix below came from. Carpenter Steel Tech Center.
http://www.cartech.com/techarticles.aspx?id=1624

This is what I am talking about with respect to the advantages of SiIron: "Silicon Core Iron "A" has magnetic properties like those of electrical iron. However, it has electrical resistivity of 25 -ohm-cm, compared with 13 for electrical iron. Silicon Core Iron "A-FM", a free-machining variation of the alloy, has nearly identical magnetic properties.

Silicon Core Iron "B", with electrical resistivity of 40 -ohm-cm, has been used in applications requiring very low hysteresis loss, high permeability, low residual magnetism and freedom from magnetic aging. Silicon Core Iron "B-FM", a free-machining version, is also available.

Silicon Core Iron "C" offers the maximum electrical resistivity at 58 -ohm-cm. It also has maximum initial permeability, minimum hysteresis loss, low residual magnetism and negligible magnetic aging."

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Pardon me for asking, I know I am out of my league here.

I am getting the idea here is to start or run a heavy loaded motor with high inductance, and lower inductance as load decrease, is this correct ?

I tend to think of inductance as a shock absorber.

Since i've been on E S my interest has drivin me to understand electronics ! Yeah I want that Grapefruit Out runner motor

torqueon said:

Pardon me for asking, I know I am out of my league here.

I am getting the idea here is to start or run a heavy loaded motor with high inductance, and lower inductance as load decrease, is this correct ?

I tend to think of inductance as a shock absorber.

Since i've been on E S my interest has driving me to understand electronics ! Yeah I want that Grapefruit Out runner motor

Click to expand...

Don't worry dude IM WAY out of my league as well :wink: but I am learning as fast as I can and I love it! OK so the way I see it in my dumbed down way of wording and understanding it. The inductance in the motor IS like a shock absorber. So when you have say 100 volts at your batteries and caps and your fets are trying to pulse on and of really fast (PWM) to make a motor with a KV rating of say 100rpm/volt spin say 100 rpm you need lots of inductance. Then when the motor rpm comes closer to matching the KV that = the battery voltage say 10,000 rpm at full speed for a 100rpm/volt motor with 100 volts feeding it you need very little inductance.

Well since we are all learning together, here is a little xls file that is in a zip wrapper. It has the phase resistance, phase inductance for three of our favorite motors. It calculates the motor time constant and plots the motor phase current versus time assuming our pwm cycle was at 100% and no back EMF. I posted this file about 7 months ago, I think, in the axial motor thread. It was probably corrupted in the server crash.

So you can play with it, and add some inductance and you will see how it slows the time constant down. If you have the parameters for Colossus let me know and I will put them in the file.

View attachment MotorCurrentsV2.ZIP

torqueon said:

I tend to think of inductance as a shock absorber.

Click to expand...

That's entirely correct. If you look at the mathematical theory, a shock absorber is an exact mechanical analogy to an inductor.

Just as a shock absorber will allow the car to move up and down but resist fast movement, an inductor will allow current to vary but resist rapid changes.

bigmoose said:

Well since we are all learning together, here is a little xls file that is in a zip wrapper. It has the phase resistance, phase inductance for three of our favorite motors. It calculates the motor time constant and plots the motor phase current versus time assuming our pwm cycle was at 100% and no back EMF. I posted this file about 7 months ago, I think, in the axial motor thread. It was probably corrupted in the server crash.

So you can play with it, and add some inductance and you will see how it slows the time constant down. If you have the parameters for Colossus let me know and I will put them in the file.

Click to expand...

What parameters do you need to know for collossus? I would love to see the chart like that. I am going to spend some more time this weekend with the scope and see what I can see I will try to see how fast the pwm is and get a idea how much inductance I need to have as a minimum for my next controller not to blow up lol.

If you need me to measure the inductance of collossus I need to know how lol.

Arlo, looking for phase resistance and inductance, then we can add it to the spreadsheet. You would pretty much need one of the lab instruments that Luke, I or TexasPyro has to measure the inductance.

rhitee05 said:

torqueon said:

I tend to think of inductance as a shock absorber.

Click to expand...

That's entirely correct. If you look at the mathematical theory, a shock absorber is an exact mechanical analogy to an inductor.

Just as a shock absorber will allow the car to move up and down but resist fast movement, an inductor will allow current to vary but resist rapid changes.

Click to expand...

What are the pros and cons of upsizing the motor to increase the inductance to be more friendly to the controller? Thanks to everybody enlightening the rest of us in this discussion.

Usually the goal is to minimize the losses in the system.

Adding additional inductance outside the motor will come with some losses in the inductor.

If the motor inductance is too low, the current will rise to the saturation point before the PWM switches off and you'll have huge resistance losses in the motor winding. Peak current will be much higher too and can cause controller failure. Adding external inductance will make it take longer to reach the saturation point and keep the peak currents lower.

You can increase the PWM frequency to prevent saturation also, but core and switching losses increase with frequency.

In a typical setup, the motor windings are the inductor. Motors windings are usually not the most efficient inductors, mainly due to the lamination material. I remember a company E-cycle or something like that which used a dedicated inductor for the PWM and coverted it to DC before feeding it to the motor. This required a second set of switching FETs just to do the commutation (brushless motor). They claimed that the losses in the second set of commutation FETs was more than made up for by improved efficiency in the inductor.

I think the real reason to add external inductance is to keep the controller from blowing up. If your add-on inductor is very low loss, you may be able to run at a lower PWM frequency and reduce switching and core losses to the point where it more than makes up for the losses in the additional inductor.

Thanks fetcher yes the inductors I am adding are to SAVE my controller. Sevcon controllers have been tested on a motor that is also hard to handle and they can't run it very well because of the low inductance. I have been given this idea by someoune else doing tests like this I will let him chime in when he feels needed. :wink: The faster PWM would be nice but I have no way of programing my 24 fet to switch faster and like you said there is losses there.
I am looking at air core inductors because with a properly sized wire they will have very low losses the only fall backs are they will take up space and will add weight to my bike.
Collossus has very low inductance which is 4 turns of 186 strands of 32 awg in one direction on the first tooth followed by 4 turns in the reverse direction on the next tooth then back to the first direction on the last of the three teeth. So AaA winding pattern wich meens the middle tooth reduces the inductance of the outer two teeth!

bigmoose said:

Arlo, looking for phase resistance and inductance, then we can add it to the spreadsheet. You would pretty much need one of the lab instruments that Luke, I or TexasPyro has to measure the inductance.

Click to expand...

So I can give you an resistance or (best test I could do was voltage drop of one phase) And we can try to est inductance with an online calculator. like this one http://detrickmartin.net/coil/ best I can tell the area of 186 strands takes up the same as 10awg so I used that for my calc. And rather then using 3 teath added together I am going to make an educated guess and say the middle tooth cancels out one of the outter teath so it is the same as just one tooth!

Now I am thinking maybe I would like to get the inductance the same as my x5304 and see it a 24 fet can run it then.

I will get and post stator mesurements of collossus later and see if I can find someone to test the inductance.

It's pretty hard to accurately calculate the inductance. An inductance meter is your best bet.
I don't think the winding reversal will cancel out the inductance.

fechter said:

It's pretty hard to accurately calculate the inductance. An inductance meter is your best bet.
I don't think the winding reversal will cancel out the inductance.

Click to expand...

I don't think the center tooth cancels out another one but I do think it lowers it for sure. For the most part I am looking for a worst case value to work from then I will build a set of inductors based on that and work from there.

Does someone what to point me to a cost effictive LCR meter?

They tend to be pretty spendy. Best to find someone who has one and borrow it.
I think the one I got at work was around $275US. BK Precision, but I forget the model. Cheap but seems to work pretty good.

On your inductors, I would suggest using some kind of core like a powdered iron torroid. To get the same inductance with an air core, it will need way more copper and have a lot of resistance losses. The motor iron will saturate before the torroid most likely, so I don't see much down side.

Another way to measure would be to look at the waveform on the phase wires during switching with an oscilloscope. You can see if the slope starts to flatten out toward the end of the cycle indicating not enough inductance.