That's a lot of back EMF

[William Gaatjes]

I think that I worded my earlier post, badly.

In many senses, I know a lot about inductors, sorry.

I meant that more complicated concepts, such as talking about the changes in hysteresis of the permeability of a coil between different core materials vs temperate, and magnetic strengths, and saturating the core. Does indeed confuse me. (I may have got the terminology wrong here, as it partly confuses me).

But I'm fine with basic Inductor Physics.

Saturating a core of an inductor is easy to understand.
Please look up the BH curve.

In essence, it is like filling a bucket of water. As long as the bucket is not full, you can keep adding water. But when the bucket is full, the bucket will spill over. With inductors with cores, you can fill the core with magnetic flux, you do that by letting a current flow through the inductor. As long as the core is not saturated, the current will continue to rise just (looks like a ramp) and be opposed by the inductor + core combination. As soon as the core of the inductor is saturated, meaning it is storing all the magnetic flux it can handle, there is nothing to oppose the change in current anymore. The inductor now turns into a simple wire letting a current flow determined by the ohmic resistance of the wire.
It is because these magnetic domains in the core oppose the magnetic field change. When all these domains in the core are aligned with the magnetic field created by the current flowing through the inductor, the core is filled. Saturated, so to say.

 

[SOFTengCOMPelec]

In all honesty, i am not as smart as i seem. I did not figure it all out myself.
That was done long before i even started working.
I read a lot about all the issues with stepper motors and came across a novel datasheet from ST. Although TI also has very nice dc motordrivers like the DRV8801 that do the same, using the rectifying technique to get rid of the back emf of the motor. Very high performance motordriver (2.8A) in a tiny smt package.

If you look for this datasheet from ST electronics: L6208age14, it will explain in detail how they do it for bipolar stepper motors. It is possible and fun to program a mcu to do the same way of controlling the mosfets in a H bridge.

datasheet :
http://www.st.com/web/catalog/sense_power/FM142/CL851/SC1794/SS1554/PF63235?s_searchtype=partnumber

That's an interesting datasheet. The fact that it PWMs the driving, complicates things still further. As with all/most H-bridge drivers, it goes to significant length to ensure that the top and bottom drivers, NEVER activate at the same time.

It would be SO ironic, if the MRI scanner, had many of these, same controller chips. As any moving parts of the MRI scanner, might use stepper motors. But there are many other types of motor, so they could just as easily NOT be.

I wonder if we could make a huge stepper motor, out of an old MRI scanner ?
It would be super powerful, but take all day long, per step. So a 200 step version, would take about 6 months, to do a 360 degree spin. While probably using tons of liquid Helium/Nitrogen, to do it.

 

[William Gaatjes]

That's an interesting datasheet. The fact that it PWMs the driving, complicates things still further. As with all/most H-bridge drivers, it goes to significant length to ensure that the top and bottom drivers, NEVER activate at the same time.

It would be SO ironic, if the MRI scanner, had many of these, same controller chips. As any moving parts of the MRI scanner, might use stepper motors. But there are many other types of motor, so they could just as easily NOT be.

I wonder if we could make a huge stepper motor, out of an old MRI scanner ?
It would be super powerful, but take all day long, per step. So a 200 step version, would take about 6 months, to do a 360 degree spin. While probably using tons of liquid Helium/Nitrogen, to do it.

I think it would have a lot of torque.

 

[SOFTengCOMPelec]

Saturating a core of an inductor is easy to understand.
Please look up the BH curve.

In essence, it is like filling a bucket of water. As long as the bucket is not full, you can keep adding water. But when the bucket is full, the bucket will spill over. With inductors with cores, you can fill the core with magnetic flux, you do that by letting a current flow through the inductor. As long as the core is not saturated, the current will continue to rise just (looks like a ramp) and be opposed by the inductor + core combination. As soon as the core of the inductor is saturated, meaning it is storing all the magnetic flux it can handle, there is nothing to oppose the change in current anymore. The inductor now turns into a simple wire letting a current flow determined by the ohmic resistance of the wire.
It is because these magnetic domains in the core oppose the magnetic field change. When all these domains in the core are aligned with the magnetic field created by the current flowing through the inductor, the core is filled. Saturated, so to say.

I find it a difficult concept to understand. But your explanation, has helped, thanks!

I was NOT especially sure WHAT happens, AFTER saturation, if you try to increase the current. What you describe, is what I would have thought. But I was NOT too sure.

Ignoring datasheet specs and other limitations. In general, capacitors can be freely charged/discharged, without similar limits. At least normal/standard valued ones, seen in everyday electronics.

 

[William Gaatjes]

I find it a difficult concept to understand. But your explanation, has helped, thanks!

I was NOT especially sure WHAT happens, AFTER saturation, if you try to increase the current. What you describe, is what I would have thought. But I was NOT too sure.

Ignoring datasheet specs and other limitations. In general, capacitors can be freely charged/discharged, without similar limits. At least normal/standard valued ones, seen in everyday electronics.

It all has to do with Faraday laws.
It works both ways : A changing current through a conductor will create a magnetic field. And a magnetic field change causes a current to be induced in a conductor.

https://en.wikipedia.org/wiki/Faraday's_law_of_induction

https://en.wikipedia.org/wiki/Electromagnetic_induction

I sure would like to have a t-shirt with Michael Faraday on it.
I think i will make one myself. There are a lot of copy shops where you can email a picture and they print it on a t-shirt.

 

[SOFTengCOMPelec]

I sure would like to have a t-shirt with Michael Faraday on it.
I think i will make one myself. There are a lot of copy shops where you can email a picture and they print it on a t-shirt.

I'm wearing a t-shirt, now. So I photographed it, and put it here:

(I would NEVER joke about such stuff).

 

[William Gaatjes]

I'm wearing a t-shirt, now. So I photographed it, and put it here:

(I would NEVER joke about such stuff).

:biggrin:

Well, the part about SMPS magnetics from TI (Texas Instruments) in that thread is very nice to read. I got a lot of my understanding from that, although it has faded away a bit.
I forgot about that thread.