Loading / Rapid de-loading of DC motor

[chazemz]

HiI have a device comprising a dc motor connected to a body and connected to the motor are four rotor arms. As the rotor arms are rotated they pass a point on the body that applies load to the motor then rapidly de loads the motor. So that every quarter turn the motor is loaded and then de loaded. The device is suspended by a length of string so that there is no contact with the ground. What I am observing is that as the rotor arms accelerate and decelerate every quarter turn there is no counter rotation of the body. Am I to understand that in the de loading process that the flux density in the motor decreases rapidly so that the speed must increase at the expense of torque. This acceleration without ( or with very little ) torque may explain the lack of torque reaction observed. Hopefully someone can help explain the process better?.

 

[Baluncore]

Welcome to PF.
Is the motor connected to a fixed voltage?
What are the rotor arms connected to, blades?
Do the rotors have the same axis? vertical or horizontal?

We really need a sketch of the configuration.

 

[chazemz]

Hi thank you for your reply. Hope the sketch helps. As you can see the load is produced by the magnets being forced together and when the poles are facing the magnet in the tube fires down the tube de loading the motor. The voltage is fixed so the rotor arms are moving very slowly when the tube magnet fires down the tube so the change in angular momentum will be minimal. The repelling magnet fields in the loading process explain the absence of torque reaction for half of the interaction but the lack of it in the acceleration phase has up to now puzzled me. Hopefully you can explain this to me as I am not an expert on dc motors Since the body is not moving in the opposite direction you can turn the motor off as the rotor arms begin to accelerate and the following rotor arm magnet will collide with the body magnet imparting its momentum onto the body causing the body to move in the direction of the rotor arms. I welcome any further questions you may have.

 

[Baluncore]

This is certainly an intriguing device. It has all the ingredients found in designs for perpetual motion machines, and a motor.
What returns the sliding magnets to the outer ends of their tubes after passing that repulsive body magnet?
What is the purpose of the machine?

 

[chazemz]

Centrifugal action returns the magnets to the outer edge ( they are fleeing from the center ) as the rotor arms accelerate after each interaction and the purpose of the machine is to convert electrical energy into momentum or in this case angular momentum. Just so we are clear this is no perpetual motion device, power must be supplied to the motor to enable the rotor arms to rotate.

 

[jim hardy]

Am I to understand that in the de loading process that the flux density in the motor decreases rapidly

Is your motor of a permanent magnet or wound field construction ?
Former is for all practical purposes constant flux.
Latter has flux in proportion to field current.

 

[chazemz]

Hi The simple answer is I do not know what type of motor it is. I bought the motor off ebay about 5 years ago. If it helps it was sold as a motor for opening and closing garage doors.If you could explain the de loading events in the wound field construction type I would be very grateful and please remember my knowledge of dc motors is some what limited at the present time

 

[jim hardy]

If you could explain the de loading events in the wound field construction type I would be very grateful and please remember my knowledge of dc motors is some what limited at the present time

DC motors with brushes are described by these two equations

Counter EMF = K X Flux X RPM ,
Torque_{in ft-lbs} = (same)K X 7.04 X Flux X Armature Current
where
K is an emprical constant for an individual motor
Flux in a permanent magnet motor is a constant namely the strength of field magnet,
Flux in a wound field motor is a variable set by field current.

If it's a permanent magnet type, or wound field with constant field current , you can lump Flux and K into one constant that I call KΦ,
so the formulas become
Counter EMF = KΦ X RPM
Torque = 7.04 KΦ X I_armature

In a proper lab we measure Counter EMF by spinning the motor at known RPM's with fixed flux and measuring how much voltage it generates. Plot values at several speeds, slope of that line is KΦ.

Anyhow, to your de-loading
Assume initial condition is the motor is a constant flux type and is running steady state ;
constant speed with some value of torque for load
reducing external torque demand leaves excess torque available to accelerate the motor's rotor and whatever parts are affixed to it
so they accelerate increasing RPM
Counter EMF goes up proportionally with RPM, opposing applied voltage, causing armature current to decrease
less armature current results in less torque produced
when torque produced (7.04 KΦ X I_armature) matches external torque demand, acceleration ceases and you've reached new equilibrium speed.

Proper treatment includes inertia and writes differential equations of motion just like in your university dynamics course.

I'm guessing your motor is a permanent magnet type because they're so much easier to manufacture than wound field.

Look it over. Nowadays it's common to find task specific microcontrollers embedded. If it's brushless it certainly has one and all bets are off.

But i'll wager you find a simple brushed DC permanent magnet motor not much different from the one in your car's electric windows.(Caveat- last one of those i had apart was a 1998 model)

Is above headed in the right direction for you?

good luck, and keep us posted?

 

[chazemz]

Thank you for the information I was looking at a self excited series wound motor in particular at small current, speed is inversely proportional to flux etc but as I have said I have no way of knowing motor type. The speed of the motor is around 60 rpm so every quarter second the motor decelerates to a near standstill and then accelerates into the next repelling interaction slowing once more and then repeating the process as long as power is available to the motor. On the point of excess torque available is the motor accelerating to seek balance ie in the loading process the system becomes unbalanced, or is it conserving as in an ice skater bringing their arms in so that they accelerate without pushing against the ice.The puzzle here is how the rotor arms are able to accelerate without pushing on the body.

 

[jim hardy]

I was looking at a self excited series wound motor

series wound motor has field in series with armature, so flux is proportional to armature current.

is the motor accelerating to seek balance

yes, Kirchoff dictates that applied voltage must get balanced.

 

[jim hardy]

Thought experiment

were it frictionless you could turn the motor off and it should keep turning forever.

Will body oscillate as a magnet in a vane approaches then recedes from the magnet on the body ?

 

[chazemz]

Hi I take it both of these thought experiments are in relation to the device? To answer the first ,the momentum is transferred to the body that must rotate through its environment and so interact with it. The time that it is able to do so is dependent upon the density of this environment. So to put a time scale on things if in deep space a long time , if through treacle not so long. On the subject of forever it would be better to look at the formula supplied with the sketch. If flux goes to zero then speed must be infinity. Assuming there is a rate of acceleration it is reasonable to conclude that it will take an infinite amount of time for the motor to accelerate to infinity or in other words forever. The second is the word in and not on. If the magnet were ON then as it passes the body magnet the repelling magnetic interaction will push against the body magnet pushing the body backwards whilst the rotor arms will also get a push . I would have thought that the body would step backwards as the rotor arms accelerate With the magnet IN this interaction can not take place as the energy build up in the collision phase is released by the momentum of the magnet traveling down the tube and the body does not get its magnetic push backwards. On the motor I am using a ELVI code 102.567/FCE type. A person has asked for specs on this motor on the internet so when someone gives an answer I can supply this to you. If you feel the answers are vague in any way please contact and I will attempt to be more clear .

 

[jim hardy]

Where i was heading is
there's two pieces
the arm-body magnets
and the motor

if the motor tries to maintain constant speed which with constant applied voltage it will,
will it apply to the body torque that's equal and opposite to that of the first repelling then attracting magnets as vanes sweep past ?

 

[chazemz]

Hi Jim sorry I misread the questions. My sketch may be misleading, The body magnet has the north pole facing inward towards the end of the rotor arms. The magnets in the tubes are all north pole facing outward so that as the tube magnets attempt to travel across the body magnet they face each other and the body magnets are propelled down the tubes so they are no longer influenced by the body magnet. The motor is now in a no load situation and is free to accelerate. The following magnet which is at the end of the tube rotates into the body magnets repelling magnetic field and as the distance between them decreases the repelling magnetic fields build until the sudden release so there are no attracting magnets as the vanes sweep past. Apologies for any confusion caused. On the body torque in the repelling phase , the body magnet wants to push away from the oncoming rotor arm so it pushes back against the torque reaction locking the body in place.If a tube magnet sticks in the tube and misses a repelling interaction, the rotor arms will accelerate and the body will start to counter rotate when the tube magnet becomes unstuck the system goes back to its acceleration and deceleration mode with no counter rotation of the body. You are right that there are two systems here and I am reasonably certain it is what happens in the motor that is the key to this.
Regards Roger

 

[jim hardy]

okay

it needs a diagram showing the forces
the word picture leaves me confused

i'll guess you're describing a continuous process where rotor and motor slow down as magnets approach one another and speed up as they retreat
and you are asking why it doesn't wobble back and forth when hung from a string ?
Good question, my ceiling fan visibly torques against its mount when starting .

Sum of torques = I dω/dt , I = moment of inertia

action-reaction pair , torques get applied to both rotor and body by the motor , each has its moment of inertia and that of rotor changes when magnets move
motor has its own moment of inertia too
whole assembly has its own moment of inertia
since whole thing hangs from a string like a mobile it seems natural to sum torques
but they don't look easy to quantify.

The motor i suspect is permanent magnet field which with constant voltage will try to run at constant speed
how fast it accelerates and decelerates depends on its internal design
some fast servo motors use a thin brass cylinder for their rotor because such an armature has a very small moment of inertia so can change direction quickly.
Yours i doubt is anything that exotic.

Gonna take some algebra , i think.old jim

 

[Baluncore]

Conservation of angular momentum would suggest that the body should counter rotate when the motor drives the rotor arms. If the body remained static when the rotors were driven, then you will have invented a helicopter that does not need a tail rotor.
I cannot see why this configuration is being contemplated, or why it might be built.

 

[CWatters]

+1

My guess is he hold the body stationary until the motor is up to speed then releases it. If he does than then the body only spins when released because the rotor experiences air resistance rather than because of conservation of angular momentum.

 

[CWatters]

as the rotor arms accelerate and decelerate every quarter turn there is no counter rotation of the body.

Averaged over long periods (say one revolution) there is no change in the momentum of the rotor. So over the same time period you wouldn't expect any change in the angular momentum of the body (due to conservation of angular momentum).

Over shorter periods you might expect to see the body oscillate back and forth. However if the body has much higher mass than the rotor (where is the battery?) then the motion of the body might be quite small or barely detectable. (eg If I run laps around my garden there is no obvious counter rotation of the planet).

Edit: The motor is probably operating in a constant speed mode so it will be trying to smooth out any changes in velocity caused by the magnets. So are you sure the rotor really is accelerating/decelerating?

 

[Baluncore]

I suspect that the force that pushes the magnet towards the rotor centre and so slows down the rotation during that process, also speeds up the rotor because the radius of the mass is being reduced. Once the arm has passed the fixed magnet the process is reversed.

 

[chazemz]

I totally agree with Jim that the word picture is confusing. My daughter has skype on her laptop so if one or all of you gentlemen have the same I can demonstrate the device to you in real time.I am in the UK but I am certain a mutual time or times can be arranged. I have no contact with the device I simply turn the power on and turn the power off after a few revolutions. The end result is that the body rotates in the same direction as the rotors were moving , from a stationary start. It works every time so there will be no excuses and I will demonstrate multiple times for your satisfaction. The body moves very visibly and you will be in a better position to comment if you see it in operation.
Best regards to all Roger

 

[CWatters]

Put it up on youtube?

 

[CWatters]

The end result is that the body rotates in the same direction as the rotors were moving

What happens if you reverse the direction the rotor is moving?

Replace the string with a wire as string is a twisted material.

 

[chazemz]

You do surprise me. I would have thought that you would be eager to ask questions, and as I have said skype is in real time Youtube would enable me to conduct numerous goes and post the best one. I am trying to be as open as possible. The other two members who have commented on this thread have yet to reply, if they are unable or unwilling to skype then it is indeed an option.
In reply to your questions it does not matter which direction the rotors rotate and string is better than wire since it has less friction , a wire could be said to resist the bodies movement more. Please feel free to ask any further questions.
Regards Roger

 

[chazemz]

Forgive me I can be a little slow at times, I think I understand the twisting string and the rotating a few times.The rotor arms rotate not the body so the body movement at the end is not by building up a restoring force by means of twisting the string when the power is switched off. Good question by the way.
Regards roger

 

[CWatters]

In essence you are asking us to find a flaw in an experiment that appears to contradict conservation of angular momentum. I'm not sure if the forum owners will allow such a discussion for long. Moderators generally take the view that there are plenty of other places on the web to discuss non-mainstream concepts. Check out the forum rules...

https://www.physicsforums.com/threads/physics-forums-global-guidelines.414380/

If your thread is closed you might find someone prepared to continue discussion by PM but many take the view it's a waste of time to discuss things like perpetual motion and related topics. Sorry.

 

[Baluncore]

The other two members who have commented on this thread have yet to reply, if they are unable or unwilling to skype then it is indeed an option.

I have seen many mechanisms like the one described here. The moment that the spokes of a wheel are fitted with sliding magnets, I begin to wonder why I am wasting my time again. There has been no statement as to the reason for conducting this experiment, or what is expected to happen. It is not science, you have nothing to test or to prove. I don't believe the undefined experiment justifies the investment of time necessary to synchronise several people on skype.

In reply to your questions it does not matter which direction the rotors rotate and string is better than wire since it has less friction , a wire could be said to resist the bodies movement more.

Use a thin narrow ribbon for suspension. It is strong, does not resist rotation and any twisting can be seen.

 

[jim hardy]

i've been looking for the missing link in observations and thought process.

What I am observing is that as the rotor arms accelerate and decelerate every quarter turn there is no counter rotation of the body.

If a tube magnet sticks in the tube and misses a repelling interaction, the rotor arms will accelerate and the body will start to counter rotate when the tube magnet becomes unstuck the system goes back to its acceleration and deceleration mode with no counter rotation of the body.

Since the body is not moving in the opposite direction you can turn the motor off as the rotor arms begin to accelerate and the following rotor arm magnet will collide with the body magnet imparting its momentum onto the body causing the body to move in the direction of the rotor arms.

red part - maybe braking action of the motor more than the magnets ? Trying to conserve Angular Momentum ...

That experiment, turning off the motor seems significant.
Is the motor geared ? Its armature may have substantial MOI of its own. And we don't know if its angular momentum adds to or opposes that of rotor vanes.
Another experiment to investigate that - increase MOI of rotor by placing two magnets in each vane. That'll change ratio of MOI's rotor vanes to motor armature.

ps i don't mess with skype.

 

[Baluncore]

chazemz. Please post your “Aim of the Experiment”.
What hypothesis are you trying to test, or what are you trying to demonstrate ?
Unfortunately, without a clearly stated aim you are seen as only playing with toys.

 

[chazemz]

chazemz. Please post your “Aim of the Experiment”.
What hypothesis are you trying to test, or what are you trying to demonstrate ?
Unfortunately, without a clearly stated aim you are seen as only playing with toys.

The aim of the experiment was to see what happened, I am now trying to learn why.As for purpose you seem to have answered this with your comment on helicopter tail rotor. On this subject you need to refer to tandem rotors and coaxial rotors both of which do not need a tail rotor. I would be grateful if you could give me your thoughts on the NOTAR system with its use of the coanda effect to solve the counter rotation problem. To propose using such an unconventional way of solving the problem must have encountered a lot of initial resistance , would you not agree?.
Regards Roger

 

[chazemz]

i've been looking for the missing link in observations and thought process.



red part - maybe braking action of the motor more than the magnets ? Trying to conserve Angular Momentum ...

That experiment, turning off the motor seems significant.
Is the motor geared ? Its armature may have substantial MOI of its own. And we don't know if its angular momentum adds to or opposes that of rotor vanes.
Another experiment to investigate that - increase MOI of rotor by placing two magnets in each vane. That'll change ratio of MOI's rotor vanes to motor armature.

ps i don't mess with skype.

At the moment I have three 20mm x 5mm N35 magnets piggy backed in each tube ( needed the depth to stop the magnets from simply turning in the tube ) I have tried four but encountered a sticking problem. I will try and source solid magnets of greater size and see what happens. I can change the load to the motor by simply adjusting the gap between body magnet and tube magnets. I have a couple of questions, does magnetic saturation have any part to play and does the rate of acceleration from the no load point of view change with a change in condition ie mass of rotors etc or change in value of excess torque?.

 

[CWatters]

Instead of buying more magnets why not try and simplify the experiment so it's easier to figure out what is going on? You can be very sure any effect is not primarily due to the magnets. Magnets alone are well understood and are very unlikely to account for any apparent breech of conservation of angular momentum.

 

[jim hardy]

does magnetic saturation have any part to play

no, for air doesn't saturate

You can be very sure any effect is not primarily due to the magnets.

rate of acceleration from the no load point of view change with a change in condition ie mass of rotors etc or change in value of excess torque?.

Excess torque? Raaazzzzz- undefined term.

Until you begin writing some equations to describe this thing you will continue just flailing.

Force between two magnets at known positions is calculable in fact for just a math model you could consider them springs

your original question was about the characteristic of a motor of unknown type, probably it's one of these PM gear motors
http://www.elvi.it/en/dc-gear-motors
whose torque you could approximate as T = m X (ω₀-w) ,
m being just a proportionality constant easily measured
ω₀ its no load speed
and ω is its actual speed
observe it becomes a regenerative brake when oversped as real DC motors do

you can weigh your parts and estimate their moments of inertia, and effect of magnet position on that of rotating vanes
then sum torques as you did in sophomore dynamics class

That's tedious work, to be done at a desk and drawing board not a keyboard

my intuitive feel is you'll find your answer is close to post #13

 

[Baluncore]

To propose using such an unconventional way of solving the problem must have encountered a lot of initial resistance , would you not agree?.

No. It has been abandoned many times since the 1930s, pretty much as long as helicopters have been killing people.

 

[chazemz]

Instead of buying more magnets why not try and simplify the experiment so it's easier to figure out what is going on? You can be very sure any effect is not primarily due to the magnets. Magnets alone are well understood and are very unlikely to account for any apparent breech of conservation of angular momentum.

I take on board your comments on the magnets but feel it is prudent to leave all options open at the present time. With regard to your comment on the experiment what do you suggest?.

 

[chazemz]

no, for air doesn't saturate
Excess torque? Raaazzzzz- undefined term.

Until you begin writing some equations to describe this thing you will continue just flailing.

Force between two magnets at known positions is calculable in fact for just a math model you could consider them springs

your original question was about the characteristic of a motor of unknown type, probably it's one of these PM gear motors
http://www.elvi.it/en/dc-gear-motors
whose torque you could approximate as T = m X (ω₀-w) ,
m being just a proportionality constant easily measured
ω₀ its no load speed
and ω is its actual speed
observe it becomes a regenerative brake when oversped as real DC motors do

you can weigh your parts and estimate their moments of inertia, and effect of magnet position on that of rotating vanes
then sum torques as you did in sophomore dynamics class

That's tedious work, to be done at a desk and drawing board not a keyboard

my intuitive feel is you'll find your answer is close to post #13

My mistake I left off the available. I was referring to your comments in post 8 where you said leaving excess torque available to accelerate the rotor. My question is does the rate of acceleration change due to certain conditions?. I am on a learning path here and I will make mistakes. I have viewed the elvi site but I feel my motor is pretty old. With respect to moment of inertia I would have thought that from the motors point of view the rotor arms moment of inertia increases when the magnets are forced together and drop when de loading occurs. Since it could be said the repelling magnetic fields provide support ,then you can say the rotor arms weight fluctuates due to position. Weight on string remains constant. I can only ask you to be the patient and appreciate your comments.