Electric Motor Secrets

[uusedman]

jetis,

why would you use 2 strands of .7mm instead of 1 strand of 1.3 mm? Are you hooking the 2 strands to pulse at same time? can you please explain the benefit or the reason.

[Jetijs]

I used two strands of wire per coil just because this way I can use one wire as a power winding and other as the recovery. I used bifilar coils on both my motors - V2 and V3, it is just that on V2 the wire was thicker and the windings fewer. You can also use only one wire per coil, but then you will not be able to recycle the recovered energy directly to the front end capacitor. Instead you will have to use the recycled energy to charge up a battery or to light a light bulb. This is my latest circuit that I am using right now:



You can see how each pole has its own coil with two windings. The blue ones are the recovery windings.

And this is the circuit of my V1 motor:



See how we need to capture the inductive spike if we do not use bifilar coils?

[lighty]

Why do you use same thickness of wire for both the energizing coil and recovery coil? Since you need higher current for energization of actuator coils you definitely need thicker wire (look at the cross-section area) but for recovery you don't need so much current as you're set to drive higher impedance load.

So, use higher number of turns on recovery coils (with thinner wire, of course). There is no miracle in inductive collapse- there is a rapidly varying magnetic flux inducing voltage in wire. So, since normal laws of induction apply you can easily calculate ampere/turns. In other words you don't need thick wire for recovery if you're just going to charge the capacitor and not drive any low impedance load.

BTW- capacitor in recovery circuit is there in order to lengthen very short inductive collapse impulse and thus practically "convert" it into current in order for battery to be able to receive the charge. No miracle there either. The same principles were patented by Tesla and are used in snubber circuits for decades.

[Jetijs]

Ok lighty, this sounds good. But in this case can I wind the power coil at first and the recovery coil only after that? Because you know it is hard to wind a good coil if you use two strands of wire with different diameters at the same time

[lighty]

Yup, you can do that. It would be almost impossible to wind bifilar helical coil with two wires of significantly different diameters.

[Jetijs]

Ok, made some tests today. But I did not get far, because when I got the scope shot of current waveform, I noticed that there is something wrong with it. Look for yourself:



For this test I only connected one pair of the coils to power so that only one phase was working. This is on my V3 motor with 55-60 turns of 0.7mm thick wire coils. Both coils were connected in parallel configuration with a MOSFET for each coil. The waveform was taken from a shunt that is connected in series of the power cables for one coil. The motor runs very fast - about 8k RPM, but the recovery is very bad and judging from the scope shot we can see why. There is something wrong either with the circuit or somewhere else. Has anyone an idea what could cause such a current waveform?
Will test this some more.

EDIT: just wired those coils in series. No change, the current waveform looks the same. No matter what I do with the recovery coils, the input current waveform does not change.

[lighty]

It could be a number of things. Could you take scope shot of input signal to MOSFET's gate? If the signal is square than it's not the driver part and we can troubleshoot further.

Also, you significantly raised coils impedance in v3 so it could account for slower rising time.

[Jetijs]

ok, will do that tomorrow

[lighty]

Another thing- if the MOSFET's are driven slowly through linear region they will heat up significantly. Could you also check out if they are heating up significantly more than in the v2?

[Jetijs]

Ok lighty, here is the scope shot of the MOSFET gate signal:



Looks like a perfect square wave. But the thing is that I have one MOSFET driver switching two MOSFETs in parallel. Maybe the problem is there? But then again the same happens if I connect the coils in series and use only one MOSFET. And what do you mean with "driven slowly through linear region"? These MOSFETs have heat sinks on them, but nevertheless they heat up so much that it is hard to hold a finger on the heat sinks even if only 3.5A is flowing through the circuit. And that is when they are connected in parallel so only 1.75A is flowing through each MOSFET.

EDIT: I took a scope shot of the current waveform of my V2 motor with the V2 circuit. There is also something wrong with it:


I don't get it. What is happening?

EDIT2: I put the motor away and connected an ordinary bifilar coil with welding rod core to the V2 circuit. This works just fine and I get the correct current waveform and recovery up to 70%.



Could it be that when the rotor comes in alignment to with the stator, the current consumption goes down and thus the weird waveform?

[Eric]

nice work!!

Hi jetijs!

teriffic work!! allways an inspiration to me as well!
i have been stuck with working my job to save up some money for awhile
now i am back to working on my ver 3 motor it too has 2 bifillar coils
only i chose to go a more mechanical route in the switching method. but i will get back to my motor at a later time.

as for yours i think the engineering is great!
i am curious if we could compare scope shots from accross your emitter and collector of your main driving transitor. you might need to use both bands on your scope. 1 for the driving coil and 2 for the recovery coil but be carefull!! i learned the hard way that because both bands in a scope have a common ground they can short!! so you might need something like a 1 to 1 transformer to isolate 1 of the bands.

the way the picture of your current flow rose and then dipped back down looked a lot like the forth scope picture i took of my first motor across the emitter/collector found here Eric's page, Photos this is a shot i took while loading the shaft down with my fingers. notice how the vertical also dips down. normally this motor and circuit run at a stable room temp, no noticeable temp increase. but when i get that dip on the scope the tranny and resistors get pretty hot. i have learned in the case of my motor that the on time was set too long. so i shortened the on time to fix it. i am curious if something similar is going on with your motor. it seems to me that your circuit shouldn't be getting so hot in the first place. doesnt that mean that too much continuous amperage is running through your circuit? i would think that if you are only getting 17% recovery and the amount of recovery is based on the impedance of your coil as the field collapses, a rather stable amount regardless of how much your on time might be held on for to long, since the on time is based on V=IR i would think you need to dial back the on time to balance it (roughly) with your recovery amount.

hope that helps!!
Eric

[LowTechIsCool]

Just a thought

So is the problem that you are getting current to flow through the primary coil even after the mosFET is turned off?

I am wondering, if when you turn turn off the mosFET, a large voltage manifests on the primary coil and causes your protection diode to be reverse biased? This might account for the current continuing to flow after you have turned off the mosFET.

-Chris Corkum

[Jetijs]

Thanks Eric
I will think about what you said.

Chris, the strange waveform is only in the MOSFET ON time, you can clearly see where the pulse starts and where it ends. It is this ON time when the current behaves like this.

[UncleFester]

So is the problem that you are getting current to flow through the primary coil even after the mosFET is turned off?

I am wondering, if when you turn turn off the mosFET, a large voltage manifests on the primary coil and causes your protection diode to be reverse biased? This might account for the current continuing to flow after you have turned off the mosFET.

-Chris Corkum

What is the actual peak to peak on the gate signal? What is the RDSon for the mosfets you are using? I notice you are using an inverted gate signal driver. Either one of these could be a clue. These mosfets should be very cool at that current unless there is a gate, duty cycle, or low voltage issue on the gate.

[Jetijs]

UncleFester, I have not checked the gate signal and what is the RDSon?

Anyway, I did some experiments today and collected some data.
I used my V2 motor for testing. I used only one set of coils wired in series. There was only one MOSFET driving those series coils. For pulsing I used a separate optoswitch and I pulsed its LED with a signal generator. Here is the test data:

Pulsing at 70Hz 50% dutycycle square wave
1) the rotor is in alignment with the stator
current taken from power supply - 0,92A
current running through the circuit - 1,50A
recovery - 38%
The current waveform:


2) the rotor is NOT in alignment with the stator
current taken from power supply - 3,2A
current running through the circuit - 2,3A
recovery - 3%
The current waveform:


Pulsing at 275Hz 50% dutycycle square wave
1) the rotor is in alignment with the stator
current taken from power supply - 0,3A
current running through the circuit - 0,6A
recovery - 50%
The current waveform:


2) the rotor is NOT in alignment with the stator
current taken from power supply - 2,85A
current running through the circuit - 3,03A
recovery - 6%
The current waveform:



Pulsing at 2,5KHz 50% dutycycle square wave
1) the rotor is in alignment with the stator
current taken from power supply - 0,1A
current running through the circuit - 0,2A
recovery - 50%
The current waveform:


2) the rotor is NOT in alignment with the stator
current taken from power supply - 0,43A
current running through the circuit - 1,03A
recovery - 41,7%
The current waveform:


Pulsing at 11,3KHz 50% dutycycle square wave
1) the rotor is in alignment with the stator
current taken from power supply - 0,35A
current running through the circuit - 1,11A
recovery - 68,4%
The current waveform:


2) the rotor is NOT in alignment with the stator
current taken from power supply - 0,36A
current running through the circuit - 1,07A
recovery - 66,3%
The current waveform:



Pulsing at 18KHz 50% dutycycle square wave
1) the rotor is in alignment with the stator
current taken from power supply - 2,5A
current running through the circuit - 3,44A
recovery - 72%
The current waveform:


2) the rotor is NOT in alignment with the stator
current taken from power supply - 2,6A
current running through the circuit - 3,5A
recovery - 74,2%
The current waveform:



The power supply voltage was always the same - 12V. Interesting that at low frequencies, the circuit consumes way more current when the rotor is not in alignment with the stator and also the recovery is way down when the rotor is not aligned with the stator. But as soon as we increase the pulse frequency, the both currents (when rotor is and is not aligned with the stator) become almost equal and no matter is the rotor is or is not aligned with the stator, the current consumption stays almost the same ans the recovery also almost the same. At frequencies higher than 18Khz, the infrared LED of the optoswitch is staying ON all the time because it can not fully turn on and off at that speed, even if the duty cycle is reduced.
Also, the duty cycle adjustment is important because it will change the current draw and recovery percentage. I will upload a small video so you can see how the current waveforms behave if frequency, duty cycle and rotor alignment is changed.