a curious ignorant ask about small dc motor starting

Thread Starter

wbs

Joined Jul 29, 2020
31
hi,
I have done earlier very small dc series motor 12 yards of about 21 gauge armature winding.
The calculated resistance was 0.264 ohms, giving with 20 v battery 76 amperes inrush current, it started with cordless drill battery nicely the armature weight was about only 1/2 lbs! Seems still odd to me because battery is not capable to provide this high current!?
I know at least a little bit about theory for example about back emf. I do not still understand how this is possible because electricity is so fast.
the warming energy 76x76x 0.26 watts did not burn insulation of course effecting very short time because back emf or how battey do not notice too high current demand and damage!?
The end result was good anyway.

Now i constucted a bigger one motor with same winding and was thinking to run it, now armature is about 2 lbs.

I tried to start it with drill battery and a thermistor (Amether ms35 3r030, 3 ohms and max 30 amperes 1200 J instantaneous failure energy). Battery and thermistor destroyed. The armature did try to start short time moving! The battery was only 1.5 ah, voltage dropped to 13.8 can't charge anymore.

I have not spare cordless drill battery any more to damage with it and do not want to use soft starter.

What I have to destroy still in the name of fun and experimentation is
-motor
- 2 pieces of 12v motor bike batteries
- a 50v li-ion battery with bms (40 a max peak current, short circuit protection)

I know it is up to me in what order I destroy my things in the name of fun, but have anybody any opinion about this.

In advange i know answer: (already in my mind) do not try to do something do not understand at all!

Is soft starter only way or give up?

thanks
 

MisterBill2

Joined Jan 23, 2018
13,722
One choice for at least experimenting is to jump backward many years to the 1940's motor starting scheme. Back then, for many motors, a series resistance was used to limit current during the starting sequence. There is quite a bit of information available in older texts.
The actual momentary current drawn by a motor is limited by more than the motor resistance. The wiring resistance and power source internal resistance also limit the current.
But for your present situation I wonder about the magnetic field used for the second motor experiment. And it does not seem at all wise to start with a maximum power application. You have demonstrated that it can result in the destruction of assets that may not be simply recovered.
I am trying to imagine why you would not want to use a soft starter for the initial testing of a new device. Soft starters are a proven method of avoiding damage and overloads in motor applications.
 

Thread Starter

wbs

Joined Jul 29, 2020
31
One choice for at least experimenting is to jump backward many years to the 1940's motor starting scheme. Back then, for many motors, a series resistance was used to limit current during the starting sequence. There is quite a bit of information available in older texts.
The actual momentary current drawn by a motor is limited by more than the motor resistance. The wiring resistance and power source internal resistance also limit the current.
But for your present situation I wonder about the magnetic field used for the second motor experiment. And it does not seem at all wise to start with a maximum power application. You have demonstrated that it can result in the destruction of assets that may not be simply recovered.
I am trying to imagine why you would not want to use a soft starter for the initial testing of a new device. Soft starters are a proven method of avoiding damage and overloads in motor applications.
Thank you very much MisterBill2,
in fact i already we're thinking about something like that (2-point starter) but i have not any idea how to implement in practice this power resistor? I have also be surprised that this is said to be possible even for big motors.
Nobody ever tells about that, i have tried seek net how this is done.
For example 1 ohm commercial 750 watt power resistors are huge in size (lot of resisting wire around ceramic core). Glass melting owens use huge amount of kanthal wire to heat owen with highly resisting wire. (Also one thing i wonder is that big resistors drop voltage so that voltage left over motor is very small).

Why not use soft starter: idea to implement old kind of motor in addition that, idea was to have handwork not learn electronics.
 

BobTPH

Joined Jun 5, 2013
6,074
Your motor will not draw 76A from the battery for two reasons.

1. The battery has internal resistance that is probably greater than the winding resistance.l in your case.

2. The coil has inductance, which means the current actually starts at zero and ramps up over time. This is different from back EMF, which happens only after the motor is spinning.
 

Thread Starter

wbs

Joined Jul 29, 2020
31
You forgot to include the output resistance of the battery.
what i did mean by calculated resistance was resistance of armature winding (stator winding was very short and thick - serial motor, left it out).

so i calculated R=0.0172x12/0.78, where 12 is wire lenght in meters and 0.78 comes from 1 millimeter diameter wire. Did i forget something important?
This was everything i did understand to be needed to calculate about resistance in this context.
 

MisterBill2

Joined Jan 23, 2018
13,722
In addition there is the resistance of the connections and the internal resistance of the battery, as well as the battery capability. So the current would not be that 76 amps.
 

MaxHeadRoom

Joined Jul 18, 2013
26,270
what i did mean by calculated resistance was resistance of armature winding (stator winding was very short and thick - serial motor, left it out).

so i calculated R=0.0172x12/0.78, where 12 is wire lenght in meters and 0.78 comes from 1 millimeter diameter wire. Did i forget something important?
This was everything i did understand to be needed to calculate about resistance in this context.
The only way to get an accurate reading of the starting resistance of a brushed motor is to lock the rotor, use an ammeter and voltmeter to measure the current at low voltage and calculate resistance, it is required to test in a few positions of the armature and take lowest reading.
 

Thread Starter

wbs

Joined Jul 29, 2020
31
The only way to get an accurate reading of the starting resistance of a brushed motor is to lock the rotor, use an ammeter and voltmeter to measure the current at low voltage and calculate resistance, it is required to test in a few positions of the armature and take lowest reading.
 

Thread Starter

wbs

Joined Jul 29, 2020
31
Thanks to answers so far. Sure ordinary 20 v battery can't provide 76 amps. I had a vague quess that explanation to odd situation was that battery has slow chemical reaction so that it provides initially smaller current which was enough to start tiny motor. So, i was wrong. I have not means to measure this small resistances, for example i guess brushes are some additional resistors (small).

But is it really so that nobody do not find any other solution than soft starter?
Is really so that nobody can explain historical methods detailed implementation?

Of course MistarBill2 has comment to historical methods. These texts are not to my knowledge copied to internet and i have not books of that time. I have tried lot of google searces as " 2-point dc motor starter technical implementation".



implemenation" and lot more, no result.
 

MisterBill2

Joined Jan 23, 2018
13,722
My explanation in post#3y explains it, but not in much detail. The description of the series FIELD winding as "short and thick" points towards an inadequate stator magnetic field as a large part of the problem.. if there is not enough magnetic field then the motor current will certainly be excessive. That is a given fact, always true in every case with comutated motors. So that may be a portion of the problem. There must be enough magnetic field strength to provide the torque and produce the counter EMF that reduces the current.
Also, in a series DC motor, the resistance of the field also tends to limit the current a bit. So an insufficient field will certainly be a problem.

Soft starters using modern electronics are still available, and still very important in some high inertia applications. The last one I used was from a company named "SAFCO", but that was quite a few years ago.
I suggest a lot more study of electrical circuits and devices before building devices like motors and such. The foundation of knowledge of the principles is more important than seeing what happens without understanding what the causes are. And real books are much less likely to provide false information than the common sources on the internet.
 

Thread Starter

wbs

Joined Jul 29, 2020
31
My explanation in post#3y explains it, but not in much detail. The description of the series FIELD winding as "short and thick" points towards an inadequate stator magnetic field as a large part of the problem.. if there is not enough magnetic field then the motor current will certainly be excessive. That is a given fact, always true in every case with comutated motors. So that may be a portion of the problem. There must be enough magnetic field strength to provide the torque and produce the counter EMF that reduces the current.
Also, in a series DC motor, the resistance of the field also tends to limit the current a bit. So an insufficient field will certainly be a problem.

Soft starters using modern electronics are still available, and still very important in some high inertia applications. The last one I used was from a company named "SAFCO", but that was quite a few years ago.
I suggest a lot more study of electrical circuits and devices before building devices like motors and such. The foundation of knowledge of the principles is more important than seeing what happens without understanding what the causes are. And real books are much less likely to provide false information than the common sources on the internet.
I did write the gauge of armature winding wrong initially but correct in metric (diameter 1 millimeter )which is i guess between 20-21 gauge i guess ( calculations in metric unit were correct). The stator winding was 2 millimeter diameter about 7-8 gauge?the lenght of stator winding was 10 rounds both sides making some 3 yards together. This gives stator resistance 0.016 ohms. Perhaps i have understood wrong way serial motor construction principle: " only few turns of thick wire on stator". Can be wrong idea then ?



Perhaps i must rethink the matter.
 

MisterBill2

Joined Jan 23, 2018
13,722
For Max, consider that the TS has mentioned a field winding more than once, and commented about the serial connection of the field, I presume that it is a series wound connected field DC motor. What we do not know is anything about the rotary part, nor much, until post #12, about the field.
Based on the limited description, my evaluation is that the field should have many more turns of wire. ALL of the universal mototrs that I have examined seem to have a bit more turns in the field than on the rotor, and since the current is the same at all points in a series circuit, the wire size should be similar between rotor and stator.
And what has not been mentioned yet is how many poles on the rotor. My guess is that it is a 2-pole rotor. Not efficient nor representative of an actual practical motor.
 

Thread Starter

wbs

Joined Jul 29, 2020
31
For Max, consider that the TS has mentioned a field winding more than once, and commented about the serial connection of the field, I presume that it is a series wound connected field DC motor. What we do not know is anything about the rotary part, nor much, until post #12, about the field.
Based on the limited description, my evaluation is that the field should have many more turns of wire. ALL of the universal mototrs that I have examined seem to have a bit more turns in the field than on the rotor, and since the current is the same at all points in a series circuit, the wire size should be similar between rotor and stator.
And what has not been mentioned yet is how many poles on the rotor. My guess is that it is a 2-pole rotor. Not efficient nor representative of an actual practical motor.
 

Thread Starter

wbs

Joined Jul 29, 2020
31
This motor (to my grand son, not intended to be "practical") was intended to be very similar to historical first motors, 3 armature teeth but winding serial armature and field- not universal motor not shunt.

As i did say i have no theoretical or practical experience ( just a hobby make present to my grandson

I did open an old thrown away car started motor.

This moror had 4 field poles mine has 2, it had 8 turns of field winding every pole about 6 gauge wire, metric 14 square millimeters. (Of course i do not think to use to start a car even talking of a car starter motor.)
 

MaxHeadRoom

Joined Jul 18, 2013
26,270
This motor (to my grand son, not intended to be "practical") was intended to be very similar to historical first motors, 3 armature teeth but winding serial armature and field- not universal motor not shunt.
First motors were shunt field DC motors.;?
 

Thread Starter

wbs

Joined Jul 29, 2020
31
Thanks to add a piece of my history knowledge. I am interested and even little bit dissapointed that nobody knows about historical details of the implementation of early inrush current prevention like resistors always presented to just exist with 3-point starters, 2-point starters.

I was mainly thinking that this basic simple armature of 3 (teeth, poles, slots, coils ) dont know the best word for this design. This 3 poles armature is still, I suppose, very early starting design type. Please, correct me again if I am wrong. MisterBill2 said my motor have 2 of these armature parts (not actual motor).
 
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