Resistors, 2-pole series dc motor starter

Thread Starter

wbs

Joined Jul 29, 2020
9
hi, I have self made series dc motor(single phase). I want to run it with a car battery. Wire thickness is about 1/5 inch diameter.
Armature resistance is extremely low.

How to limit huge inrush current? Can I build for example (manyally operated) 2-pole starter by myself?
I mean switch manyally to lower resistance until enough cemf.
How to find resistors? What kind specification resistors? What ohms? Are there any other simple way to prevent huge inrush current without complicated circuits or motor control card?

Thanks
 

Marley

Joined Apr 4, 2016
353
It is true that there will be a large inrush current. When the battery is first connected, the motor is at standstill and effectively stalled. However, this is not really a problem with a car battery which can supply a large current.
Once the motor comes up to speed, which should be quick, the self-generated EMF (not the same as back EMF!) produced by motor (as a generator) will oppose the battery voltage and reduce the current. If your motor is well made!
The high inrush current can be a problem with switches that control the motor. A series resistor is one solution.
To specify the resistor, decide what is the maximum allowable inrush current. Probably set by the type of the switch contacts.
Lets say 100A. With a 12V supply the resistor will be R=V/I = 0.12ohm.
This resistor will lose power when the motor is up to speed. It will lose even more power when the motor is stalled. But this should only be for a very short time.
Lets say that the running current is 5A. Then the resistor will lose 3W. The resistor needs to be able to dissipate this power.
A better solution is to use a NTC thermistor. Specified by its cold resistance. As it warms up it will reduce its resistance.
 

MaxHeadRoom

Joined Jul 18, 2013
20,911
hi, I have self made series dc motor(single phase). I want to run it with a car battery. Wire thickness is about 1/5 inch diameter.
Armature resistance is extremely low.
Is this a series wound field motor?
If so these operate in a runaway condition where the rpm is limited by friction and load etc.
If running high current, pick up a automotive starter solenoid, auto wreckers etc.
Max.
 

Thread Starter

wbs

Joined Jul 29, 2020
9
It is true that there will be a large inrush current. When the battery is first connected, the motor is at standstill and effectively stalled. However, this is not really a problem with a car battery which can supply a large current.
Once the motor comes up to speed, which should be quick, the self-generated EMF (not the same as back EMF!) produced by motor (as a generator) will oppose the battery voltage and reduce the current. If your motor is well made!
The high inrush current can be a problem with switches that control the motor. A series resistor is one solution.
To specify the resistor, decide what is the maximum allowable inrush current. Probably set by the type of the switch contacts.
Lets say 100A. With a 12V supply the resistor will be R=V/I = 0.12ohm.
This resistor will lose power when the motor is up to speed. It will lose even more power when the motor is stalled. But this should only be for a very short time.
Lets say that the running current is 5A. Then the resistor will lose 3W. The resistor needs to be able to dissipate this power.
A better solution is to use a NTC thermistor. Specified by its cold resistance. As it warms up it will reduce its resistance.
Thank you Marley very much. I am mainly doing a handwork exercise now, not knowing really anything about electronics. I designed and implemented earlier a small fpga chip based hdl- language programmed digital techique card not undersanding "real electronics" with huge number of diodes, amplifieds and so on.

So, I must ask again. I have very thick armature wire able to work with about 100 amperes continiously (doing real work) and near zero armature winding resistance initially. Are there really NTC- thermistors to take some possible 500 amperes inrush current 12v? If yes, how to find-some code needed to find? It is possible to implement inrush current limiter with resistors as traditional start motor is doing (but with complicated spring and other addititional elements. )
To my uneducated mind one would to let some sensible amount of current (50 amperes flow until self-generated EMF there and then switch resistor off current flowing freely? Resistors can 't take load of 12v 100a long time? I have very high current switches.
 

Marley

Joined Apr 4, 2016
353
Now you say you have built this motor yourself and it is a series type.
I take that to mean it has a wound field coil(s) - not permanent magnetic field - and has a wound armature with a commutator.
Running from a 12V (car battery) supply. Correct?
How much mechanical output power are you expecting from this motor?
What are you driving with it? How is it controlled (switched)?
 

Thread Starter

wbs

Joined Jul 29, 2020
9
Now you say you have built this motor yourself and it is a series type.
I take that to mean it has a wound field coil(s) - not permanent magnetic field - and has a wound armature with a commutator.
Running from a 12V (car battery) supply. Correct?
How much mechanical output power are you expecting from this motor?
What are you driving with it? How is it controlled (switched)?
Correct. I have armature laminated stack as well as stator but winding not done yet.I know that series winding will cause runaway easily but it is easy to change the plan to compound winding and keep some let's say extra artificial load always on the shaft to prevent runaway.

This is only hobby experiment not planned well. I hope some 1-2 hp perhaps is possible as to power. It is possible, I think, initially keep enough light load on the shaft to let emf develop and add load gradually and torque is increasing hopefully.
The system has no proper control only on/off.
I was thinking only this inrush issue first. The motor has not any other purpose than funny hobby experiment, I got opportunity to play with laser cutting to get lot of thin lamination sheets to a shape I wanted.
 

Marley

Joined Apr 4, 2016
353
1 - 2 hp = 750 - 1500W approx. That's similar to a car starter motor.
1200W is 100A at 12V. Good thing your wire is thick! Of course the running current will depend on the mechanical load.
OK, I understand, it's a hobby project. I would fix it down very well. Bolt it to the bench or clamp in a vise.
Stand clear and touch the wire to the battery! Not with your bare hands. Expect sparks!
Only connect for a few seconds for the first run. Sounds like good fun to me! Sort of thing I liked to experiment with when I was about 16.
 

Thread Starter

wbs

Joined Jul 29, 2020
9
1 - 2 hp = 750 - 1500W approx. That's similar to a car starter motor.
1200W is 100A at 12V. Good thing your wire is thick! Of course the running current will depend on the mechanical load.
OK, I understand, it's a hobby project. I would fix it down very well. Bolt it to the bench or clamp in a vise.
Stand clear and touch the wire to the battery! Not with your bare hands. Expect sparks!
Only connect for a few seconds for the first run. Sounds like good fun to me! Sort of thing I liked to experiment with when I was about 16.
Thanks about advices.I am more lucky than you (of course only with this special case). You did this kind of things at the age of 16 and you had other work to do at school. I am retired (75 years) and I have no other work to do. This kind of things as you for sure know from experience are very time consuming.
 

MisterBill2

Joined Jan 23, 2018
6,330
I can not imagine winding a motor with wire bigger than is used in a started motor. If you want a resistor to limit the current use a foor or two of thin steel coat-hanger wire. But beware that it will get very hot very quick. That will keep the inrush a bit lower.
What is the intended application for this motor?
 
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Thread Starter

wbs

Joined Jul 29, 2020
9
Self made resistor sounds fine. Iron resistane still only about 10 times worse than copper has.Perhaps It would take time iron heating and motor gets enough emf during that time but perhaps iron conducs too good.
I were thinking lately series set of ordinary resistors and manual lever/arm little bit similar than 3-pole starter which has an automatic one. First I tought that how to get enough high wattage resistors but if we have many resistors in series and short time loaded (starting) it will work, it is likely same as 3-pole conventional system, resistors dissipate heat enough (shot time) even not special high wattage huge resistors.

The copper is not prolem. Annealed copper bends nicely.I have done as hobby blade smith work and gates and so on in the forgeI I built (an other earlier hoppy) I have also recycler suitable copper.

To my understanding the man principe making motor is that you need necessary rounding part (armature) and space for it to spin then you hammer (not wind) all empy space full of copper (insultation not forgotten).

The motor making is only to get something to do to forget pain (3 strokes and degenerated lumbar spine, athritis...)
 

MaxHeadRoom

Joined Jul 18, 2013
20,911
To my understanding the man principe making motor is that you need necessary rounding part (armature) and space for it to spin then you hammer (not wind) all empy space full of copper (insultation not forgotten).
That description appears very odd?
Almost sounds like you are describing a Induction (AC) motor rotor?
Otherwise does not make much sense?
Max.
 

Thread Starter

wbs

Joined Jul 29, 2020
9
That description appears very odd?
Almost sounds like you are describing a Induction (AC) motor rotor?
Otherwise does not make much sense?
Max.
That description appears very odd?
Almost sounds like you are describing a Induction (AC) motor rotor?
Otherwise does not make much sense?
Max.
You are of course right because you know already about motors.
I do not take ever a new challenge if I already know something about it, what would be the point then, no fun with it! ( still about 200 amps with a car starter dc motor need fairly big copper bars called wires or am I wrong? )

I am pretty sure I never will finalize this project.
(there is a saying: if I know I die to morrow I plant a apple tree today).
It is my style to choose a new hobby which I have zero level knowledge.
After the first stroke:
painting portraits, blade smithing, solving tactical chess puzzles on net ( all with some above zero level results, sometimes even satisfactory). Sorry btw about my "english". I
Haven't even spell checker on ipad I write.
 

MaxHeadRoom

Joined Jul 18, 2013
20,911
The older automotive starters had a wound series field, both armature and field are wound with insulated copper windings.
I believe the copper 'bars' you are referring to sounds more like the conductors embedded in a AC induction motor (squirrel cage) rotor.
Modern ones have aluminum bars or coils cast into the rotor, these are not insulated.
Max.
 
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MisterBill2

Joined Jan 23, 2018
6,330
The older automotive starters had a wound series field, both armature and field are wound with insulated copper windings.
I believe the copper 'bars' you are referring to sounds more like the conductors embedded in a induction motor (squirrel cage) rotor.
Modern ones have aluminum bars or coils cast into the rotor, these are not insulated.
Max.
I have disassembled starter motors and there was one that had the rotor windings composed of separate bars. It looked like they had been brazed to the round wires that connected them on the end. It was a slightly larger diameter motor, no clue what it was for originally.
 

MaxHeadRoom

Joined Jul 18, 2013
20,911
I have disassembled starter motors and there was one that had the rotor windings composed of separate bars. It looked like they had been brazed to the round wires that connected them on the end. It was a slightly larger diameter motor, no clue what it was for originally.
Sounds like a induction motor to me, i.e.AC version.
Early squirrel cage format was just this.Copper!
Max.
 

Tonyr1084

Joined Sep 24, 2015
4,711
One place I worked they had a test rig with hundreds of these resistors in series/parallel. We're talking megawattage. Naval Power Systems. I'd imagine you can make something smaller than what they had but still attain the high wattage, low resistance they did with a similar series/parallel setup. You can change the number of series plates to attain a specific resistance then put them in parallel to cut that resistance back down. The more you add in series the higher the resistance. The more you parallel, the lower the resistance. And the more you add - the higher the wattage you create. They also used a fan to cool this megawatt resistor.
1596377807645.png
 

Thread Starter

wbs

Joined Jul 29, 2020
9
One place I worked they had a test rig with hundreds of these resistors in series/parallel. We're talking megawattage. Naval Power Systems. I'd imagine you can make something smaller than what they had but still attain the high wattage, low resistance they did with a similar series/parallel setup. You can change the number of series plates to attain a specific resistance then put them in parallel to cut that resistance back down. The more you add in series the higher the resistance. The more you parallel, the lower the resistance. And the more you add - the higher the wattage you create. They also used a fan to cool this megawatt resistor.
View attachment 213829
Megawatage! Wow! In principle I understand something about what you say. Too much for me anyway, I guess. A car sarter motor has 12volts and inrush current even 600amps making modest 7200w. At my level perhaps I try wet wood as resistor, it is a very poor conductor or try this steel wire.
Still I think that in a way or an other I must to get to know how the miracle has done at the practical level with usual car starter motors. I have seen pictures of moving arm going from high to low resistance end. Hard to think that this is only a theoretical picture or that they use very many series/parallel chains. Must be a more simple pactical solution, what are then realworld componets. Nobody knows? Really!
 
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