Loading / Rapid de-loading of DC motor

  • Thread starter chazemz
  • Start date
  • #51
33
3
Just had to give more personal information so should be ok now. So just to go through your comments.
There is no unwrapping of string torsion, the body is in fact in front of rest position so acting against slight restoring force when power is switched off.
Three inertias returning to zero -separate motor and vanes momentum,( ie moving when body is not ), zero after collision - Body momentum after transfer collision as seen. zero only after transfer of energy to exterior interactions.
Body moves almost one rotation after power is switched off, in the direction of rotor arms and body is not counter rotating at that time.
Correct me if I am wrong, but to comply with force pairing, as the rotor arms accelerate and so acquire momentum. The body should acquire the same amount of momentum. So when the motor is switched off, these should cancel each other out. This clearly does not happen?
Regards Roger
 
  • #52
jim hardy
Science Advisor
Gold Member
2019 Award
Dearly Missed
9,839
4,868
There is no unwrapping of string torsion, the body is in fact in front of rest position so acting against slight restoring force when power is switched off.
Yep, i can see that now.
When it's switched off it makes one whole turn in direction of vane rotation.

as the rotor arms accelerate and so acquire momentum. The body should acquire the same amount of momentum.
At switch-on time it makes a half rotation then oscillates about rest position.
At switch-off time it makes a full rotation.

Playback at 1/4 speed shows vanes rebound at switchoff time , i think that reversal explains the difference in switch-on and switch-off motion.
 
  • #53
33
3
Hi Jim, the bounce back effect that you are noting is exactly what should happen and has no effect on the end result. It would be easier to look at this in terms of time and value.If the device were to be placed in deep space ( ie very low friction ) and a battery placed on the body we can conclude the following ( using simplistic values of one unit equals one revolution per second ). At turn on the torque is high so we will give the body counter rotation a value of 8, lasting for one second before the magnetic interactions take effect. We have agreed that there will be a small amount of counter rotation during the acceleration and deceleration process and we will give this a value of 1. We will mirror the time value in the video so 61 seconds. So at the switch off the body will have completed 69 revolutions counter to the direction of the rotor arms. Transfer of momentum takes place ( with bounce back) and we know from the observation that the value is far higher than the value of 1. We can afford to be conservative so give this a value of 4. The body structure will now rotate in the direction of the rotor arms through a very low friction environment. I am sure you will agree that at 10 to the minus 22 kgs per cubic metre the device would continue to rotate for a very long time. so passing the original start position with ease.
Quick question - when de loading occurs would there be any inductive kickback and if so what effect would it have?
Regards Roger
 
  • #54
jim hardy
Science Advisor
Gold Member
2019 Award
Dearly Missed
9,839
4,868
In space
first thought experiment- remove the magnets
at turn on the vanes would commence rotation in one direction and the body in the other
angular momentum remaining zero
the whole thing being a free body that's how it would remain,
https://en.wikipedia.org/wiki/Reaction_wheel
A reaction wheel (RW) is a type of flywheel used primarily by spacecraft for attitude control without using fuel for rockets or other reaction devices. They are particularly useful when the spacecraft must be rotated by very small amounts, such as keeping a telescope pointed at a star. They may also reduce the mass fraction needed for fuel. This is accomplished by equipping the spacecraft with an electric motor attached to a flywheel which, when its rotation speed is changed, causes the spacecraft to begin to counter-rotate proportionately through conservation of angular momentum.
Putting the magnets just makes it wobble as angular momentum re-apportions itself between rotating vane and platform.

Adding friction between vanes and air will make the thing approach condition of vanes stopped body counter-rotating
Adding a support to suspend it against gravity adds a restoring force that'll equilibriate against platform displacement from starting position

i still believe it's a simple summation of angular momentums of the parts plus summation of torques from air friction and support strings. Well, the algebra is not so simple else i'd have figured out how to write the equations. I have other projects occupying my alleged brain today.

there is nothing magic about turning off an electric motor by opening its supply circuit. Its inductance gives an electrical voltage transient. Torque drops smartly to zero.
If instead its supply circuit is not opened but the motor is allowed to act as a generator expending its rotational kinetic energy as volts X amps in that circuit, say by short circuiting its terminals, then there may well be a violent reversal of torque.



Time for some algebra .
 
  • #55
33
3
The device works as described on a thrust bearing, so there is friction but no restoring force. Will think about other comments in your reply and comment tomorrow.
Regards Roger
 
  • #56
Baluncore
Science Advisor
2019 Award
7,295
2,377
This is a conservation of angular momentum experiment. That will be more obvious if you hang it from a tape, then increase the mass inertia of the rotor and reduce the mass inertia of the body, so as to make them similar.
As usual, the sliding magnets are only there to add je ne sais quois and some complexity magic to confuse the situation and so disguise the prestidigitation.
 
  • #57
jim hardy
Science Advisor
Gold Member
2019 Award
Dearly Missed
9,839
4,868
One more thing to check.

I've seen more than one experimenter fooled by his test equipment.

You don't actually switch off the motor, you switch off some sort of big gray power supply looking thing .

What is it ?
What is its output characteristic when powered off?
Is it capable of absorbing current from the motor, which would let it act as a generator , effectively regenerative braking? Sure looks that way from the video.

Put a switch between the supply and the motor so you know that you're "switching it off ."


That motor being designed for garage doors may be equipped with a mechanical brake . Got a datasheet for it ?
 
  • #58
33
3
In space
first thought experiment- remove the magnets
at turn on the vanes would commence rotation in one direction and the body in the other
angular momentum remaining zero
the whole thing being a free body that's how it would remain,
https://en.wikipedia.org/wiki/Reaction_wheel


Putting the magnets just makes it wobble as angular momentum re-apportions itself between rotating vane and platform.

Adding friction between vanes and air will make the thing approach condition of vanes stopped body counter-rotating
Adding a support to suspend it against gravity adds a restoring force that'll equilibriate against platform displacement from starting position

i still believe it's a simple summation of angular momentums of the parts plus summation of torques from air friction and support strings. Well, the algebra is not so simple else i'd have figured out how to write the equations. I have other projects occupying my alleged brain today.

there is nothing magic about turning off an electric motor by opening its supply circuit. Its inductance gives an electrical voltage transient. Torque drops smartly to zero.
If instead its supply circuit is not opened but the motor is allowed to act as a generator expending its rotational kinetic energy as volts X amps in that circuit, say by short circuiting its terminals, then there may well be a violent reversal of torque.



Time for some algebra .
Without the magnets there would be no loading and de loading of the motor?

Do you mean spin and wobble or just wobble?

I would have thought that without the magnets, adding friction to the vanes would increase the counter rotation of the body?

I think my question may have been misread, I did not say turn off the motor, but what would happen in the de loading? I thought that some inductive kickback would take place even though the power is still switched on. I am probably wrong on this and will go back and look at it again.
Regards Roger
 
  • #59
33
3
This is a conservation of angular momentum experiment. That will be more obvious if you hang it from a tape, then increase the mass inertia of the rotor and reduce the mass inertia of the body, so as to make them similar.
As usual, the sliding magnets are only there to add je ne sais quois and some complexity magic to confuse the situation and so disguise the prestidigitation.
Thank you for your kind words.The sliding magnets are there for a reason. The clue is in the title of this thread.
 
  • #60
33
3
One more thing to check.

I've seen more than one experimenter fooled by his test equipment.

You don't actually switch off the motor, you switch off some sort of big gray power supply looking thing .

What is it ?
What is its output characteristic when powered off?
Is it capable of absorbing current from the motor, which would let it act as a generator , effectively regenerative braking? Sure looks that way from the video.

Put a switch between the supply and the motor so you know that you're "switching it off ."


That motor being designed for garage doors may be equipped with a mechanical brake . Got a datasheet for it ?
The big grey power supply thing is a big grey power supply.

What is its output characteristic when powered off? _ Please explain?

The motor is switched off after the rotor arms accelerate. If the power is switched off before the rotor arms have chance to accelerate they will not get any extra push. Since you have noted the magnetic bounce back, we know momentum is transferred in that way. I will fit the switch as I have not tried it before. I will inform you of the result.

Regards Roger
 
  • #61
jim hardy
Science Advisor
Gold Member
2019 Award
Dearly Missed
9,839
4,868
The motor is switched off after the rotor arms accelerate.
No, the power supply is switched off.
I do not know whether the power supply output terminals then present an open circuit to the motor or a short circuit to the motor or somewhere in between.

That is an unknown that should be nailed down because the motor will regenerative brake into the power supply if the power supply will accept current from it.

Put one of these between the power supply and motor to eliminate that unknown.
upload_2017-3-7_12-5-52.png


If anything changes as a result then that challenges one of the basic assumptions, that the motor is 'switched off' by that power supply switch. It may instead see its supply gradually tapered off in which case the deloading is not so rapid.
If nothing changes then that basic assumption is probably okay.
But one needs to know from observation not assumption.
 
  • #62
33
3
Hi Jim, fitted switch, there is no change in the observations.
Regards Roger.
 
  • #63
Baluncore
Science Advisor
2019 Award
7,295
2,377
Thank you for your kind words.The sliding magnets are there for a reason. The clue is in the title of this thread.
Why not rationalise the design by placing the fixed body magnet inside the arc of the rotor like this.

magwheel.gif
 
  • #64
jim hardy
Science Advisor
Gold Member
2019 Award
Dearly Missed
9,839
4,868
Hi Jim, fitted switch, there is no change in the observations.
Good. That's answered.
 
  • #65
jim hardy
Science Advisor
Gold Member
2019 Award
Dearly Missed
9,839
4,868
If i understand the original question it is "Why doesn't the frame wobble more at the blade passing frequency?

What I am observing is that as the rotor arms accelerate and decelerate every quarter turn there is no counter rotation of the body.
Which for me is answered by watching it, i see wobble in the frame as the blades pass.

There is nothing magic about motors.
The whole of your system is the sum of its parts. Sum your momentums .
 
  • #66
33
3
If i understand the original question it is "Why doesn't the frame wobble more at the blade passing frequency?



Which for me is answered by watching it, i see wobble in the frame as the blades pass.

There is nothing magic about motors.
The whole of your system is the sum of its parts. Sum your momentums .
The original question is the same as now. How is the body able to move in the direction of the rotor arms when the power is switched off?
This fact is easy to see but difficult to understand. The visible angular momentum is out of balance in the system, this does not mean it IS, just we do not see it. Energy in MUST equal energy out, nature sets the rules. If nature is taken out of balance it will do what ever is necessary to restore energy balance. To many people a tornado spitting out golf ball sized hail stones may seem strange,but as you are well aware it is a frightening reality.
If you are saying that it is not the motor then we go back to the other system. Would I be correct in saying that as the rotor arms slow down and the torque reaction is countered by the body magnet that this energy is transferred to the magnetic fields of the colliding magnets. One of those magnets wants to turn and as the magnets are forced closer together this desire to turn grows very rapidly. Therefore at de loading a lot of potential energy ( or stored angular momentum ) is released as the magnets de torque ( maybe unconventional term ). Since this energy has doubled up, on release something has to give. The motor has to accelerate due to the applied voltage. As this has priority you may find that the body cannot counter rotate because conservation of angular momentum would then be violated .Same as newtons cradle, two balls collide , two balls accelerate. If one ball moved after the collision KE would not balance. You cannot have it both ways here, I am of the opinion that conservation of angular momentum is the priority and force pairs must step aside so that balance ( not symmetry ) is maintained. I will think further on this and comment again, If anyone has any comments to make please feel free.

Regards Roger.
 
  • #67
jim hardy
Science Advisor
Gold Member
2019 Award
Dearly Missed
9,839
4,868
Most collisions fall somewhere between elastic and inelastic.
Momentum is conserved.
Kinetic energy divides between motion and work done on the bodies.

If you are saying that it is not the motor then we go back to the other system.
Probably your motor is key. Testing with the motor switch you added answered the question "Does regenerative braking returning power to the supply? ".

Next question - "What is the nature of torsional friction in that motor?"
My intuitive feel is the motor becomes an effective mechanical brake as soon as it's de-energized. That might be just friction in the gear train, or , since it's made to drive a leadscrew mechanism it would be logical for it to have an automatic mechanical brake to hold its garage door "where is" on power fail mid-travel.



Spin your motor by hand . You'll get a feel for its internal friction and inertia, and any mechanical brake will announce itself.



One of those magnets wants to turn and as the magnets are forced closer together this desire to turn grows very rapidly. Therefore at de loading a lot of potential energy ( or stored angular momentum ) is released as the magnets de torque ( maybe unconventional term ). Since this energy has doubled up, on release something has to give. The motor has to accelerate due to the applied voltage. As this has priority you may find that the body cannot counter rotate because conservation of angular momentum would then be violated
My advice is write the formulas and apply algebra...

“When you can measure what you are speaking about, and express it in numbers, you know something about it, when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind; it may be the beginning of knowledge, but you have scarely, in your thoughts advanced to the stage of science.”

William Thomson, 1st Baron Kelvin
I think i'll bow out of this one, too many unknown details about the mechanism.
 
  • #68
CWatters
Science Advisor
Homework Helper
Gold Member
10,528
2,294
It's not clear to me why the body doesn't spin in the opposite direction to the rotor all the time the rotor is turning (eg due to air resistance). There must be something preventing it. That something can be storing energy that causes the thing to rotate in the same direction as the rotor when it's switched off.

Edit: Even if that's not the cause its potential to be the cause needs to be eliminated. You can mount the body on two ball races and use them to carry the current from the power supply to the motor.
 
  • #69
33
3
Hi, Thanks for your comments. I am really busy at the moment, I have to take my younger daughter to York medical school tomorrow for a offer holders day and on Sunday I am picking up my elder daughter from university as it is Easter break. I will be in a better position to reply to your comments on Monday or maybe Tuesday. Have a good weekend.
Regards Roger.
 
  • #70
jim hardy
Science Advisor
Gold Member
2019 Award
Dearly Missed
9,839
4,868
It's not clear to me why the body doesn't spin in the opposite direction to the rotor all the time the rotor is turning (eg due to air resistance). There must be something preventing it. That something can be storing energy that causes the thing to rotate in the same direction as the rotor when it's switched off.
The wires?
WhirlygigWires.jpg


When power is applied it swings through about 3/4 turn , looks like it's hunting about an equilibrium position .
I estimated period at around fourteen seconds, 1/14 hz.

Pick a name for starting position -
i call initial at t=0 6 o'clock because the flat vertical face points away from the wall behind
When motor starts it swings CCW to perhaps 2 o'clock,
then CW to perhaps 11 o'clock
then CCW to 3 o'clock
then CW to 9 o'clock
then CCW to 4 o'clock
then CW to 7 or 8 o'clock
then CCW to 5 o'clock
then CW to about 6:30 o'clock and gets switched off.
Where would it settle?

Wobble at vane frequency maybe (4hz?) is evident.

How it comes to be called 'reactionless' i do not see.
 
  • #71
33
3
It's not clear to me why the body doesn't spin in the opposite direction to the rotor all the time the rotor is turning (eg due to air resistance). There must be something preventing it. That something can be storing energy that causes the thing to rotate in the same direction as the rotor when it's switched off.

Edit: Even if that's not the cause its potential to be the cause needs to be eliminated. You can mount the body on two ball races and use them to carry the current from the power supply to the motor.
In the collision phase the repelling magnetic fields will stop the body from counter rotating. After the collision when the rotor arms accelerate I believe it is unclear to everyone why the body does not counter rotate. Some people do not like the term ' I do not know'; it takes them out of their comfort zone. The term 'I do not know' for me means there is something to learn and that is what I am trying to do. I am no magician and what you are observing is not magic. All I am at the moment is the boy in the crowd shouting “the king is wearing no clothes!”. The amount of energy in the rotor arms at exchange is clear to see . The lack of counter rotation is equally clear. In reply to your comments on the bearings, it may be a good idea for you to view the video of a device called dean drive. This device would not work when suspended from a string but appeared to work when in contact with the ground. Therefore any contact with the ground is a big no no. I have said that I have placed the device on a thrust bearing to observe the device with no restoring force. The end result is the same the body moves in the same direction as the rotor arms. At the moment I am looking whether there is any time lag in the magnetic fields of the motor. All options remain open for now.
 
  • #72
33
3
The wires?
View attachment 114675

When power is applied it swings through about 3/4 turn , looks like it's hunting about an equilibrium position .
I estimated period at around fourteen seconds, 1/14 hz.

Pick a name for starting position -
i call initial at t=0 6 o'clock because the flat vertical face points away from the wall behind
When motor starts it swings CCW to perhaps 2 o'clock,
then CW to perhaps 11 o'clock
then CCW to 3 o'clock
then CW to 9 o'clock
then CCW to 4 o'clock
then CW to 7 or 8 o'clock
then CCW to 5 o'clock
then CW to about 6:30 o'clock and gets switched off.
Where would it settle?

Wobble at vane frequency maybe (4hz?) is evident.

How it comes to be called 'reactionless' i do not see.
The wires exert a restoring force in either direction.

I have no idea what you are describing with the clock analogy. Please clarify.

The term 'reactionless' means without reaction. An object without weight is said to be weightless. Since the action is the acceleration of the rotor arms and there is no reaction (i.e. counter-rotation of the body) then is reasonable to use the term reactionless. You seem to like formula, so therefore
F + -F > 0
 
  • #73
jim hardy
Science Advisor
Gold Member
2019 Award
Dearly Missed
9,839
4,868
  • #74
jim hardy
Science Advisor
Gold Member
2019 Award
Dearly Missed
9,839
4,868
I have no idea what you are describing with the clock analogy. Please clarify.
angular displacement from rest in units of hour markings on a clock face rather than degrees
 

Related Threads for: Loading / Rapid de-loading of DC motor

  • Last Post
Replies
1
Views
5K
  • Last Post
Replies
2
Views
653
  • Last Post
Replies
8
Views
2K
  • Last Post
Replies
9
Views
1K
  • Last Post
Replies
16
Views
3K
Replies
4
Views
2K
  • Last Post
Replies
14
Views
3K
  • Last Post
Replies
4
Views
2K
Top