hoppyist ask about brushed cordless tools dc motor secrets (impossible thin wire etc....)

Thread Starter

wbs

Joined Jul 29, 2020
17
hi, a general level question:
I made a diy brushed serial dc motor.
I used 1mm diameter wire (15m) for armature (3 coills) and cordless drill battery 20v. It is working.

Starting current is by my theorecal calculation is very big, I even was not sure that battery do not interpret the start as short circuit (used stupidly ntc thermistor) in vain.
What kind of inrush current common drill battery can hadle without short circuit reaction?
Now I am wondring how usual cordless drill motors can even to sart and work and handle high current during work even 30 amps, because high torque and efficiency can need even 20-30 amps.
But armature wires are very thin ( perhaps 0.7mm diameter). How these wires can handle 30 amps? What is wire lenght roughly? Can fan be this efficient!?

I am hoppyist and very curious about the secret behind (1) high inrush current and (2) high efficiency, torque and thin wire secret. By my understanding many cordless drill (at least old) not use control electronics PWM or battery bms.

I am ignorant hoppyist and would appreciate answer a lot!


thanks a lot in advance!
 

MrAl

Joined Jun 17, 2014
8,142
hi, a general level question:
I made a diy brushed serial dc motor.
I used 1mm diameter wire (15m) for armature (3 coills) and cordless drill battery 20v. It is working.

Starting current is by my theorecal calculation is very big, I even was not sure that battery do not interpret the start as short circuit (used stupidly ntc thermistor) in vain.
What kind of inrush current common drill battery can hadle without short circuit reaction?
Now I am wondring how usual cordless drill motors can even to sart and work and handle high current during work even 30 amps, because high torque and efficiency can need even 20-30 amps.
But armature wires are very thin ( perhaps 0.7mm diameter). How these wires can handle 30 amps? What is wire lenght roughly? Can fan be this efficient!?

I am hoppyist and very curious about the secret behind (1) high inrush current and (2) high efficiency, torque and thin wire secret. By my understanding many cordless drill (at least old) not use control electronics PWM or battery bms.

I am ignorant hoppyist and would appreciate answer a lot!


thanks a lot in advance!
What is the 'normal' rating for the size wire you are asking about?
Could it be that not all windings are being energized at the same time?
Recall that there is a LOT of inductance present with a magnetically active metal core. That means the inrush will be limited by that inductance also.

Modern motors are going to be made using rare earth magnets that will give them higher efficiency. They do use PWM though i am not sure what you are talking about there. PWM gives the speed control functionality.
 

Thread Starter

wbs

Joined Jul 29, 2020
17
What is the 'normal' rating for the size wire you are asking about?
Could it be that not all windings are being energized at the same time?
Recall that there is a LOT of inductance present with a magnetically active metal core. That means the inrush will be limited by that inductance also.

Modern motors are going to be made using rare earth magnets that will give them higher efficiency. They do use PWM though i am not sure what you are talking about there. PWM gives the speed control functionality.
Thanks a lot. Seems to me (not sure) that "secret" to me was that all coils of cordless drill motor windings are not energized at the same time. The most simple 3 coil desing,I was using(!) really enegiszes all 3 coils at the same time, this design is ment perhaps only for demonstration purposes?
Seen still often as an example. Are real motors windings such that only one (or two coils) are active simultaneously?
Even then some let's say 22 or little bit bigger gauge wire seems have huge heating at hard load (30 a) even only 1-2 yards long/ coil. Beats still me that at hard load fan must go very slow and motor not still overheats. Is single coil wire lenght something 0.5 yards? I know that car stater motors are not multiturn/coil but have thenvery thick wire.
My numbers 0.7mm diameter is betwen 21 and 22 gauge and 1mm diameter is near 18 gauge.
15m (15 meters) is 15 yards.

thanks.
 

BobTPH

Joined Jun 5, 2013
3,101
Where are you getting your figure of 30 A? I seriously doubt that a cordless drill draws anything near that, even at startup. That said, a typical lithium ion cell used in these is about 2-3 Ah capacity. Some lithium ion cells can provide 30C current which comes out to 60 to 90A. But at thirty A draw, these batteries would last only a few minutes on a charge.

Bob
 

MisterBill2

Joined Jan 23, 2018
8,441
There is a lot more to the controls of most cordless drills than just a switch. That is one large difference. Second, the efficiency of those motors is MUCH greater than a DIY motor with only 3 poles. AND those motors have many more poles and MUCH more effective magnetic arrangements. And those motors also have many more turns in their windings. So every aspect of the good motors in the good tools is far different from a DIY motor. AND those differences all matter.
 

MaxHeadRoom

Joined Jul 18, 2013
22,841
I made a diy brushed serial dc motor.
Most DC cordless drills are NOT usually series field motors but brushed P.M. field, . IOW shunt field motors.
The series field versions are Universal motors that run on AC/DC and are principally used in corded hand tools.
Max.
 

MrAl

Joined Jun 17, 2014
8,142
Where are you getting your figure of 30 A? I seriously doubt that a cordless drill draws anything near that, even at startup. That said, a typical lithium ion cell used in these is about 2-3 Ah capacity. Some lithium ion cells can provide 30C current which comes out to 60 to 90A. But at thirty A draw, these batteries would last only a few minutes on a charge.

Bob
In a 9.6 volt nominal cordless drill using normally NiCd cells i measured 30 amps and it could go a little higher. I havent measured any others yet. I might think that as the voltage goes up the current goes down, but then again the manufacturer might be going for more power too so the current may not go down that much.

I'll have to check some of my drills see if i can look inside and see what the wire gauge is.
 

MrAl

Joined Jun 17, 2014
8,142
Thanks a lot. Seems to me (not sure) that "secret" to me was that all coils of cordless drill motor windings are not energized at the same time. The most simple 3 coil desing,I was using(!) really enegiszes all 3 coils at the same time, this design is ment perhaps only for demonstration purposes?
Seen still often as an example. Are real motors windings such that only one (or two coils) are active simultaneously?
Even then some let's say 22 or little bit bigger gauge wire seems have huge heating at hard load (30 a) even only 1-2 yards long/ coil. Beats still me that at hard load fan must go very slow and motor not still overheats. Is single coil wire lenght something 0.5 yards? I know that car stater motors are not multiturn/coil but have thenvery thick wire.
My numbers 0.7mm diameter is betwen 21 and 22 gauge and 1mm diameter is near 18 gauge.
15m (15 meters) is 15 yards.

thanks.
I was just trying to think of any ways that we could use a wire at a conservative 15 times it's rated current. But is that maybe starting current and not normal running current?
I measured 30 amps myself in a 9,6 volt drill. I had to select a high current diode for it so wanted to know the current. But i think that was stall current.
So are you measuring stall current?
Also, the cordless drills might be assumed to be run for short time periods.
But how did you measure the current?
 

shortbus

Joined Sep 30, 2009
8,468
You guys are forgetting the BEMF that is created after the armature starts to rotate. The high, from stall/stopped only lasts a very short time -
" The term back electromotive force is also commonly used to refer to the voltage that occurs in electric motors where there is relative motion between the armature and the magnetic field produced by the motor's field coils or permanent magnet field, thus also acting as a generator while running as a motor. This effect is not due to the motor's inductance, which generates a voltage in opposition to a changing current via Faraday's law, but a separate phenomenon. That is, the back-EMF is also due to inductance and Faraday's law, but occurs even when the motor current is not changing, and arises from the geometric considerations of an armature spinning in a magnetic field. " from https://en.wikipedia.org/wiki/Counter-electromotive_force
 

MrAl

Joined Jun 17, 2014
8,142
You guys are forgetting the BEMF that is created after the armature starts to rotate. The high, from stall/stopped only lasts a very short time -
" The term back electromotive force is also commonly used to refer to the voltage that occurs in electric motors where there is relative motion between the armature and the magnetic field produced by the motor's field coils or permanent magnet field, thus also acting as a generator while running as a motor. This effect is not due to the motor's inductance, which generates a voltage in opposition to a changing current via Faraday's law, but a separate phenomenon. That is, the back-EMF is also due to inductance and Faraday's law, but occurs even when the motor current is not changing, and arises from the geometric considerations of an armature spinning in a magnetic field. " from https://en.wikipedia.org/wiki/Counter-electromotive_force
I was considering an effect like that collectively when i mentioned the inductance. But it is not clear yet if the OP meant the stall characteristics or the free running characteristics. And then there is the "screw in the wall" issue, where we see the stall character coming into play repeatedly when a screw going into wood encounters more and more friction and thus get harder and harder to turn requiring more and more torque which requires more and more current.
So i guess we will have to wait and see what he says about the way he is using and measuring the drill he is talking about.

For my old NiCd 9.6v drill, i know it takes 30 amps or more during stall, and i am sure when it is free running it takes less current until it starts to encounter that extra friction from a turning screw and then the current creeps up quickly again. I never looked at the wire size though i never thought about that i guess cause i didnt use it that much or just assumed it was designed well enough to take the extra intermittent current demand.

One thing i noticed time and time again though with my Dremel is that it gets quite hot when i use it continuously with some work piece which poses some nominal load on the rotation. It gets hard to hold after a while. My older Dremel did the same thing. So maybe many of these drills are underdesigned where they assume intermittent use. I never used my portable drills for very long at one time, maybe 10 minutes on and off for some odd jobs so maybe i never pushed it enough to find out how hot it would get.
 

MisterBill2

Joined Jan 23, 2018
8,441
Mr.Al, my Dremel would get rather hot running at almost no load. That is the nature of those things. Eventually it got to a state where no amount of good epoxy could rebuild the two end bells and so I stopped using it, and picked up a cheaper brand device that is a really dumb shape, considering that a high speed grinder is never held like a regular power drill. So far that one works.
And certainly the modern ones all use PM fields and not wound fields, and that allows more power for producing torque.
 

MaxHeadRoom

Joined Jul 18, 2013
22,841
And certainly the modern ones all use PM fields and not wound fields, and that allows more power for producing torque.
The advantage with wound fields in a series motor, is you get the very high torque at low RPM together with very High RPM's top end.
You do not get both with a P.M. field motor.
 

Thread Starter

wbs

Joined Jul 29, 2020
17
There is a lot more to the controls of most cordless drills than just a switch. That is one large difference. Second, the efficiency of those motors is MUCH greater than a DIY motor with only 3 poles. AND those motors have many more poles and MUCH more effective magnetic arrangements. And those motors also have many more turns in their windings. So every aspect of the good motors in the good tools is far different from a DIY motor. AND those differences all matter.
Thanks..

My simple motors works, at least someway running 15000 rpm no load of course. I understand that we can control with fancy electroncs a lot, for example battry bms can cut too high current and motor electronics can do a the same and more, very true.
But old cordless drill has not these advantages, beats me they seems to "broke natural laws" some
fancy way . By my minor understading torque is something as counter emf multiplied by armature current and thin wires are thin wires. How muliple poles or some design idea really can help?
(I am talking brushed dc motors). I have measured wire be more thin than 22 gauge and seen lot of wire inside. When we are talking high current during heavy use motors can survive fairly long time with high current,if somebody say that 30a is not there,15a is for sure (startup current is a different thing and can be explained extremely fast birth of cemf perhaps ). Of course, there must be some precise real solution to high continious draw at least 15+ amps but what this solution is in practice? No electric control.

My interest is only to understad things not to build new fancy motor.
 

MisterBill2

Joined Jan 23, 2018
8,441
With the much greater number of poles, the powered armateur winding is in the portion of the magnetic field that provides the most torque. That is the other reason that brush motors with more poles are more efficient. That reason is not obvious at first glance. And at 15,000 RPM the centrifugal force is rather a lot and so the motor would tend to come apart if the construction were not adequate.
How have you measured the speed of your motor, I am wondering. What sort of tachometer do you use?
 

Thread Starter

wbs

Joined Jul 29, 2020
17
With the much greater number of poles, the powered armateur winding is in the portion of the magnetic field that provides the most torque. That is the other reason that brush motors with more poles are more efficient. That reason is not obvious at first glance. And at 15,000 RPM the centrifugal force is rather a lot and so the motor would tend to come apart if the construction were not adequate.
How have you measured the speed of your motor, I am wondering. What sort of tachometer do you use?
Seems make sense, still thinking about rude EbxIa formula.

I am an old blacksmith making blades and lot more, then constuction I do is strong even for bigger thing and stress. One thing making for sure restriction is my inability to balance armature properly.

When I did understand that my 18 gauge wire 15 yards is imposible for sensible torque I toughtat least to get a good noload speed., I think that 20000 rpm still is possible shortening even further heavy field winding because rpm comes from counteremf divided by field strenght (weak field good). With series motors noload armature current goes more weak an it weakens field even more (serieswinding few turns) which weakens armature current again cycle going towards runaway or balance at high speed.
But various frictions prevent runaway if suitable : fan, bearing friction, commutator friction, air friction (armature). My fan is miserable as to cooling perhaps good at preventing runaway.
T

I was mesuring rpm with a digital optical laser tachometer with reflecting tape. I do not know the producer, tachometer has some model marking as AT-6L.

This miserable cooling is a problem.If I use 7.5 yards 18 gauge wire and 21v battery using 12a INPUT wattage is 252 (output????) and wire heating watts are 48.69 w ( possible to cool I think? not with my fan). 10a has wire heating watts 34;and 7a sensible 16.66 watts.
 

MrAl

Joined Jun 17, 2014
8,142
Seems make sense, still thinking about rude EbxIa formula.

I am an old blacksmith making blades and lot more, then constuction I do is strong even for bigger thing and stress. One thing making for sure restriction is my inability to balance armature properly.

When I did understand that my 18 gauge wire 15 yards is imposible for sensible torque I toughtat least to get a good noload speed., I think that 20000 rpm still is possible shortening even further heavy field winding because rpm comes from counteremf divided by field strenght (weak field good). With series motors noload armature current goes more weak an it weakens field even more (serieswinding few turns) which weakens armature current again cycle going towards runaway or balance at high speed.
But various frictions prevent runaway if suitable : fan, bearing friction, commutator friction, air friction (armature). My fan is miserable as to cooling perhaps good at preventing runaway.
T

I was mesuring rpm with a digital optical laser tachometer with reflecting tape. I do not know the producer, tachometer has some model marking as AT-6L.

This miserable cooling is a problem.If I use 7.5 yards 18 gauge wire and 21v battery using 12a INPUT wattage is 252 (output????) and wire heating watts are 48.69 w ( possible to cool I think? not with my fan). 10a has wire heating watts 34;and 7a sensible 16.66 watts.
A wild idea that might actually work...

Encase the entire motor in a closed chamber of distilled water or maybe some sort of oil. Use proper seals for the output shaft (similar to a combustion motor crankshaft seal). Would keep everything cool but we'd have to figure out how to protect the iron parts from rust, maybe a coating of some sort.

One of the problems of cooling with devices that have significant 'depth' to them is getting the heat from the inner most parts to the outside surface so it can dissipate. The main mechanism is thermal conduction. If we could have a liquid bath of some sort maybe we could even have it circulate inside the parts of the motor that get hot via narrow ducts. That would help get the heat out.
We'd also need a radiator to dissipate the heat. The whole thing would end up working somewhat like a water cooled computer CPU.
 

MisterBill2

Joined Jan 23, 2018
8,441
A submerged coolant brush type of motor does not seem like a good choice, and for that speed and typical motor clearances the fluid heating would be impressive.
 

MrAl

Joined Jun 17, 2014
8,142
A submerged coolant brush type of motor does not seem like a good choice, and for that speed and typical motor clearances the fluid heating would be impressive.
Well actually it would have to be a special design. Waterways for coolant to flow, etc.
Think like combustion engine cooling design.
 

Thread Starter

wbs

Joined Jul 29, 2020
17
A wild idea that might actually work...

Encase the entire motor in a closed chamber of distilled water or maybe some sort of oil. Use proper seals for the output shaft (similar to a combustion motor crankshaft seal). Would keep everything cool but we'd have to figure out how to protect the iron parts from rust, maybe a coating of some sort.

One of the problems of cooling with devices that have significant 'depth' to them is getting the heat from the inner most parts to the outside surface so it can dissipate. The main mechanism is thermal conduction. If we could have a liquid bath of some sort maybe we could even have it circulate inside the parts of the motor that get hot via narrow ducts. That would help get the heat out.
We'd also need a radiator to dissipate the heat. The whole thing would end up working somewhat like a water cooled computer CPU.
Thank you all participating my crazy project and finding wild ideas for cooling.

Luckily I am living in Finland and one winter was cold ( -22F), this would help running my motor outside.
I tested mechanical strenght of my motor spinning it's saft with ac drill. All good with 30000 rmp,
Huge tree block and very big bolts as support. I was worrying about armature vibration but armature teeth did not hit stator even balancing is fairly poor. Wires can't wly from slots because this 3 teeth armature has not real slots, also commutator slips (made of copper pipe) can't fly away I was using recycled garden hose fastener also my brushes were recycled (broken washing machine) too soft I think.

I am not satisfied even this 7.5 yards wire lenght version, output watts would be very small anyway.
My initial wild idea was a strong version, high voltage battery with bms restricting inrush current and some about 10 gauge armature wire, I have armature and stator laminated but perhaps this current motor is my last project with constructing motors. I will give this to my grandson.

My earlier crazy ideas after brain stroke were and done: make operating system kernel for brand new Itanium processor (tested with simulator) (documented www.isosika.net), make some symbian phone games, get high rating at solving chess puzzles on net and portrait painting. Only this kernel and chess results were satisfactory to me. The motor is not satisfactory but must accept that I think.
 
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