Cereal motors and parallel universes

[kopcicle]

TL;DR

To create a 30-40V , 4-5A , method of switching two 12V 1.5A brushed DC coreless motors between series and parallel while returning to default series connection after power or control signal is removed.

Ref: https://www.physicsforums.com/threads/parallel-series-dpdt-with-mosfets-help.904631/post-5698363

It's been nearly 10 years but I'd still like to thank and acknowledge the thread starter and participants.
The purpose of the above schematic was to series or parallel two batteries to drive a heater element (R1).
What I'd like to do is replace the batteries in the schematic with DC motors.

I have this horrible habit of over complicating things. I'm hoping that's all I've done here.
Really all I'm trying to do is switch two brushed dc motors from series to parallel without resorting to something as large as a relay or as complicated as a micro controller. My weight and space limitations are near microscopic.
I'll include some curious details for reference.

https://toshiba.semicon-storage.com..._en_20250422.pdf?did=13889&prodName=SSM6N57NU

https://www.nexperia.com/product/PMPB47XP

I'll admit that it's convenient to find a package as small as 2 x 2mm.

https://precisionminidrives.com/pro...4_v0BGTPGd6F2vyNk86ZQA30I22dKcGnOdLb4uPY-bn47

This type of motor comes in many different specifications but for my case the 20V, 5A (total) is close enough.

Because someone is going to ask why:

Motors in series have full amperage, but half voltage.
Motors in parallel have full voltage, but half amperage.

So, if you had a 20V 5A system:

Each motor in series would see (a maximum of) 5A @ 10V.
Each motor in parallel would see (a maximum of) 2.5A @ 20V.

With DC motors, they create "Back EMF", essentially a voltage that has to be overcome, proportional to their RPM.

So, you start off in series, so you get maximum power (Amperage), and ramp up until the current you are putting in meets the back EMF.
Then you switch to parallel, so you can continue accelerating, albeit not quite so strongly (Higher voltage overcomes the back EMF, but the halved amperage means you don't accelerate so quickly).

You would thus end up with a torque curve something like this:
Code:

| | ___________|
| | ___/ |
| | ___/ |
|<- Series ->| ___/ |
| | ___/ |
| | ___/ |
R |___/ |
P __/ |
M _/ | |
| _/ | |
| _/ |<----------- Parallel ------------>|
| _/ | |
| / | |
|/ | |
+----------------------TIME------------------------

While the power seen by the motor (This is for one motor) looks like this:
Code:

|----5A----,
| |
| |
| |
| |
| |
| |
| '--------------AMPS-------------2.5A-
|
| ,==============VOLTS============ 20v=
| |
| |
|==== 10v====='
|
+-------------------------------------------------

Without switching, the graph speed/time graph would look like this (with switching shown in dotted line):
Code:

| ............
| .../
| .../
| .../
| .../
| .../
R ____________________________________
P __/
M _/
| _/
| _/
| _/
| /
|/
+----------------------TIME------------------------


So the question is...

Can you think of a simple circuit that would sense the back EMF and switch
from series to parallel as back EMF increased?

Huge bonus points for the switch point being adjustable with a miniature multi-turn potentiometer

Precision microdevices even put this bit of mental quicksand on the web for me.
https://www.precisionmicrodrives.com/ab-026

So, that's about it. I can probably get to the back EMF sensing on my own. it's the H-bridge as a DPDT that is kicking my ...


I found that simply winding a number of turns of of magnet wire around the outside of the the motor I could get enough of a representative voltage that tracked motor rpm. Ideal for my use. Possibly an OpAmp for gate driver.
A whole lot of words for what is essentially a sensorless tachometer.


Any thoughts you can offer would be greatly appreciated.

 

[256bits]

TL;DR: To create a 30-40V , 4-5A , method of switching two 12V 1.5A brushed DC coreless motors between series and parallel while returning to default series connection after power or control signal is removed.

Any thoughts you can offer would be greatly appreciated.

Hi.
I have a question.
What is a 'cereal' motor in the title?
Is Kellogs going to be the new next best thing after AI.

PS. My own spelling has degraded to the point where I have to look up words, is it an 'e', an 'a', a single or double 'd'. Auto correct does some funny embarrassing too.