# halfbakery, self boosting series motor?



## Sonikaccord (Dec 17, 2012)

I thought about this, except with brushless DC motors. I believe Lenz's law is the largest hurdle in the idea. Trying to recapture the collapsing magnetic field that usually boosts voltage in a boost converter also reverses the flux direction as it attempts to maintain the same flux density in the coil. There are more details, but this is close. I did make some notes about it, I'll see if I can find them.


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## dcb (Dec 5, 2009)

I see, so you briefly considered a 6 lead bldc for this. I had always thought the flux direction was a function of current direction though, not voltage (not implying any viability to my original suggestion here). 

I did leave out a bypass diode on the field, that is gonna hurt in buck mode as-is...


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## bigmotherwhale (Apr 15, 2011)

i have been working on a similar idea for quite a while 

you cannot use the inductive collapse to directly power another inductor 
you need a capacitor of the correct value, 

an inductor should always be used with an capacitor this way they behave more like a wave guide and efficiency is increased. 
you want a quasi resonant system, that is switched at the correct time to produce torque.
you can use diodes to recover the reverse voltage or you can use active switching, 
protection diodes on standard bridges kill any chance of proper recovery and cause reflected waves harmonics and heat generation, any recovery that does occur is killed by the fact that the supply cap is already charged, you must match the capacitor and inductor in a reverse relationship over time. 

match the value of the capacitor to the inductor / winding so that after it is discharged it is almost empty ready for a new charge cycle

you can combine the voltage and current of the supply with that of the recovery capacitor for increased field winding current.

if you get the size of the capacitor correct, there will be very little waste heat production and recovery and therefore voltage and current will be maximised. use low uf pulse discharge AC rated capacitors, i used microwave oven caps.

it is important to note that any recovered voltage cannot be put back into the supply without first using a capacitor shuttle, to change the time phase if you do (as it is almost always done) it will create transients and heat.

so far i have only done it on simple motors with one or two windings, one magnetic polarity, two phases. it does work extremely well with a custom set-up, however i don't know if it is possible to use a standard wound motor.

if you get it working correctly you will gain about 1/3rd of the energy stored in the magnetic field back to use again


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## bigmotherwhale (Apr 15, 2011)

try this simple circuit


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## dcb (Dec 5, 2009)

umm... I think upping the pack voltage would be simpler. I'm not trying to get free energy, just transform the existing distribution of energy a bit. But a capacitor would probably be helpful in a boost circuit.


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## major (Apr 4, 2008)

bigmotherwhale said:


> you cannot use the inductive collapse to directly power another inductor
> you need a capacitor of the correct value,


Hmmm, isn't that want happens with the standard series motor controller? Without a cap? Series field supplies armature during the off duty cycle period. I guess some of the inductance and decaying field is in the armature, but typically the bulk comes from the field and the resulting armature current produces torque in the positive (motoring) direction.



bigmotherwhale said:


> if you get it working correctly you will gain about 1/3rd of the energy stored in the magnetic field back to use again


Not sure exactly where your 1/3rd figure comes from. But again, with the standard series motor controller, nearly all the stored energy in the field due to the PWM is put to work, losing only I²R.

Not that I think there is any merit to the OP boost idea; no comment there


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## bigmotherwhale (Apr 15, 2011)

"Hmmm, isn't that want happens with the standard series motor controller? Without a cap? Series field supplies armature during the off duty cycle period. I guess some of the inductance and decaying field is in the armature, but typically the bulk comes from the field and the resulting armature current produces torque in the positive (motoring) direction."

the current in the stator and the armature are the same because they are connected together in series, they work in the on period only. a boost converter works in the flyback off period where the polarity has flipped over, its quite obvious that the energy stored in the field is wasted and not all of it ends up being used is because when the brushes move to the off position, the coils try and maintain equilibrium to the flowing current and the voltage sky rockets, you can see this as sparks on the brushes. if you connected a capacitor across them at the correct moment and then took it off when current stopped and turned around, it would be completely full and the inductor would be empty - no sparks.


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## dcb (Dec 5, 2009)

bigmotherwhale said:


> the current in the stator and the armature are the same because they are connected together in series, they work in the on period only.


Nope, that isn't what inductors do. Like a capacitor resists a change in voltage, an inductor resists a change in current. So with the typical flyback diode across both the motor and field (standard controller topology), when the pwm switches off, they reverse voltage polarity, but attempt to maintain the same current flow.

It is akin to a capacitor with a load, you turn off the pwm and the current in the capacitor reverses as it tries to maintain the voltage level.




bigmotherwhale said:


> its quite obvious that the energy stored in the field is wasted and not all of it ends up being used is because when the brushes move to the off position,


The brushes are not binary interfaces with the commutator, and the rotor windings are certainly coupled to the other rotor windings electrically and magnetically, so I don't really think that is "obvious", let alone accurate.

Really I was just looking for a quick and easy for boosting voltage or drawing more power from a lower voltage pack with minimal components, I think it is kinda not happening. This would result in "field strengthening" when you are trying to get higher rpm from a lower pack.


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## bigmotherwhale (Apr 15, 2011)

"Nope, that isn't what inductors do. Like a capacitor resists a change in voltage, an inductor resists a change in current. So with the typical flyback diode across both the motor and field (standard controller topology), when the pwm switches off, they reverse voltage polarity, but attempt to maintain the same current flow"

i was saying that a boost converter uses the reversal as a power source, not the initial energising current flow as series wound motors do.


as the rotor rotates, the coil goes from being connected to being disconnected completely, when it is disconnected completely yes the voltage will reverse and rise but no current can flow, it wants to but there is no path, no energy stored in the winding will be able to flow anywhere, as the current will drop to zero there will be no transformer action between windings either, i think that it is both obvious and accurate.


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## dcb (Dec 5, 2009)

bigmotherwhale said:


> i was saying that a boost converter uses the reversal as a power source, not the initial energising current flow as series wound motors do.


What reversal are you referring to (voltage, current, "zero point")? And why do you think a boost inductor isn't akin to a series inductor?



bigmotherwhale said:


> as the rotor rotates, the coil goes from being connected to being disconnected completely,


it is an analog interface as one commutator bar increases contact area with the brush and one decreases. Thereby shifting the effective rotor poles to generate torque. A coil connects to adjacent bars, and all coils form a loop. When a brush spans bars, that segment is effectively shorted out (though it probably ramps down and up as a function of brush/bar contact area, which is a function of rotor position). Most EV motors have lots of bars and rotor coils so it is a fraction of the rotor magnetizing loop that is under the brush.



bigmotherwhale said:


> when it is disconnected completely yes the voltage will reverse and rise but no current can flow, it wants to but there is no path, i think that it is both obvious and accurate.


It is the certainty with which you propose such nonsense that is the most maddening.

edit: since I was curious myself how to deal with any residual energy, it appears to be merely a function of adjusting the field neutral point (i.e. brush timing). Which is probably painfully obvious to folks like major. Get it wrong and you get sparks.


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## major (Apr 4, 2008)

bigmotherwhale said:


> as the rotor rotates, the coil goes from being connected to being disconnected completely.....


The armature coils are never disconnected during rotation. You need to study how DC armatures are wound and how they work. Also brush up on converters, especially buck converters as used for DC motor controllers. You have some misconceptions with that.


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## Sonikaccord (Dec 17, 2012)

dcb said:


> I see, so you briefly considered a 6 lead bldc for this. I had always thought the flux direction was a function of current direction though, not voltage (not implying any viability to my original suggestion here).
> 
> I did leave out a bypass diode on the field, that is gonna hurt in buck mode as-is...


Essentially, with some minor modifications on my end of course.

The current direction is a function of the changing flux direction and vice-versa. The faster that change, the larger the induced voltage. Recovering the current from the collapsing field of the armature is a tough cookie. You'll only be recovering what's left of the armature current, like what major said.

Then once you think about the conservation of energy, how much is there really to gain in those off periods? In the buck converter, the inductor(2 for series motors) is going to be feeding the current from the collapsing field into the BEMF source, converting it into work and heat.

Chew on this one:
https://www.youtube.com/watch?v=Mj4rV0AoI-Q

The animations are nice. Ignore the overunity stuff...or not.


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## Sonikaccord (Dec 17, 2012)

Actually, I think that's more closely related to major's rotary converter project. It still may be helpful.


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## dcb (Dec 5, 2009)

Sonikaccord said:


> Chew on this one:
> [links to ufopolitics video]


Sorry I ever posed the question, where is the nuke thread button? (or did you just press it?)


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## samwichse (Jan 28, 2012)

It got pressed when bigmotherwhale entered the conversation.


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## bigmotherwhale (Apr 15, 2011)

samwichse said:


> It got pressed when bigmotherwhale entered the conversation.


I thought i could be helpful and share with you my research into collapsing field recovery and generating higher voltages using motor windings, both of which ive had very good experimental results. 

I am however definitely not clued up on mechanically brushed motors, but the fact that arching occurs on the brushes and there is a great deal of radio noise, that there must be at least some voltage rise on the coils due to increasing resistance / dropping current.


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## dcb (Dec 5, 2009)

bigmotherwhale said:


> I am however definitely not clued up on mechanically brushed motors


http://www.ece.utah.edu/eceCTools/Motors/BrushDC/MotorBrushDCCommutation.pdf

I'm not sure about the thoroughness of your non-brushed motor understanding either. But even those have some degree of switching losses. But if you look at the typical inverter layout, you will see that that each coil is basically a buck converter (as is a DC motor + controller) and on a scope all those pulses get smoothed out inductively into relatively sinusoidal currents.

If I believed you had actual success relative to the OP I would inquire further, but my instincts tell me we are just peeling the onion on stubborn misconceptions, and subsequent theories founded on them.


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