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# Increasing motor's RPM + Torque with stronger magnetic fields?

#### Dretron

Jun 9, 2012
101

Well Im not asking anything. Just thanking you and replying to "CDRIVE" as he stated that my question's and examples are leading to Over-unity projects... I'm saying don't judge a person based on random question and experiments. It's really rude.

I can tell you that nobody knows exactly what a magnetic field is, or an electric field is, or what gravity is. We know they exist because we see the effects of them. .

I totally agree. So many things in phyisc's are un-clear and un-completed theories. We've only understood the simplicity of nature now were trying to unveil its tricks! Which is the most complicated thing ever!

But I don't need to tell you how to study, you're already doing better than most people.
You're experimenting.

Well because of those experiment's that I've preformed I was able to sum up conclusions and produce more theories. I've been working on motor for 2 year's now and I've found the physics in that matter wonderful and amazing!

No harm in experimenting because that possibly leads to a discovery that eventually could help me and help the work right? lol.

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Jan 21, 2010
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#### CDRIVE

##### Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
Yup, knee jerk reaction. Sorry about that.

#### wannabegeek

Aug 17, 2011
133
Rpm is not proportional to torque I know that... If one increases the other decreases...

Can we discuss this for a moment ? I'm about to re-enter school and need to brush up on my mechanics.

let w = omega = angular velocity and m = a constant mass and were only considering circular motion of a DC motor:
F = ma = m dw/dt

Force here is proportional to the rate of change of the rpm.

For a DC motor I recall reading that the back EMF is linear wrt omega. The force that must be applied to make the motor turn is at least equal and opposite to that ...right ?

There's only a force when the rpm changes, and the load causes a change that eventually equalizes. Therefore this part of the quote,' If one increases the other decreases' is not true for a DC motor without gearing.

I get caught b/c my mind goes to the everyday cases where the load is quite high and there is a need to gear down, converting kinetic energy into force, to keep turning. As long as the load is not overcoming the motor, the force should be proportional to the change in kinetic energy.

Please correct me if where I'm mistaken.
wbg

#### duke37

Jan 9, 2011
5,364
If there is no change of speed, there will be no effect of inertia.
Force = Mass * Acceleration
Torque = Inertia * Rotary acceleration

If the speed is constant the torque depends on the load and the motor current depends on the torque.

#### john monks

Mar 9, 2012
685
F = ma = m dw/dt is correct.
That is if m is rotational mass if there is such a thing.
That is in an ideal motor the magnetic force applied to a rotor is equal to the rotational inertia times the instantaneous change in rotational velocity divided by the instantaneous chug in time.

What is linear wrt omega in layman's terms?

When one increases the other decreases is true for DC or AC motors if you are if you are talking about magnetic force and the rate of change in rotor speed in an ideal motor that has no load.
With a load your statement would be correct.

What do you mean by the change in kinetic energy?
If you mean at a constant rotor velocity in an ideal motor, the change magnetic force would be directly proportional to the change in kinetic energy.

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#### Dretron

Jun 9, 2012
101
Torque is only that rotational force that is need to make the object start moving. In general its like moving an object from rest, when you move an object from rest it require so much force only at the beginning. As you start to move it become lighter and easier so what you need now is speed and a lil bit of torque.

Thats what I say to myself when looking at a motor doing work on a load. It requires torque to start moving the load from rest then relies on inertia and rpm.

#### john monks

Mar 9, 2012
685
Torque is normally defined at turning force at a given moment of time. So torque can apply to inertia as you seem to indicate or to friction such as turning a bolt.
But strictly speaking torque is the angular force for a given moment in time.

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#### Dretron

Jun 9, 2012
101
For some reason(I know its wrong but maybe...), I feel when the mag. field/force increase both values increase Torque & Rpm. Because rotational motion is cause by both magnetic fields acting on each other. From the rotor and magnets.

Its like this: Mag1 (rotor) Mag2(magnet). If both Mag's are equal there is no increase.

If Mag1 is greater by applying more input = increase.
If Mag 2 is greater by applying stronger manget/force/field = increase.

Since the rotational motion of the stator is caused by the magnetic field's interactions they Rpm&Torque are caused by it... (I'm making no sense I know lol, idea's are being developed here.)

#### Dretron

Jun 9, 2012
101
For some reason(I know its wrong but maybe...), I feel when the mag. field/force increase both values increase Torque & Rpm. Because rotational motion is cause by both magnetic fields acting on each other. From the rotor and magnets.

Its like this: Mag1 (rotor) Mag2(magnet). If both Mag's are equal there is no increase.

If Mag1 is greater by applying more input = increase.
If Mag 2 is greater by applying stronger manget/force/field = increase.

Since the rotational motion of the stator is caused by the magnetic field's interactions they Rpm&Torque are caused by it... (I'm making no sense I know lol, idea's are being developed here.)

Another useful demonstration to prove my point : here.

As you can see the closer the magnetic field is to the rotor, the greater the repulsion/attraction because of the magnetic field/force strength is becoming stronger. So ,the more torque and rpm is gained. Torque is gained some what in the beginning eventually as the rpm raises up! The torque would decrease because it is not necessary. I think the same would occur even with load.

#### john monks

Mar 9, 2012
685
I see your confusion.
What you are looking at with your motor is a rotor with a lot of friction coming from the bearings and the brushes.
Now if you had no magnets at all your rotor would not move because of the lack of torque.
So your counter EMF would be very low because your rotor would need to approach infinite speed for the counter EMF to reach the power supply voltage, but you have no torque.
When the magnets are screwed close together you can overcome the required torque to reach terminal speed, but when further apart you get less torque.
With the magnets placed many inches apart and if you had no friction the motor would run very very fast because that speed would be required to generate the counter EMF.

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#### Dretron

Jun 9, 2012
101
I see your confusion.
What you are looking at with your motor is a rotor with a lot of friction coming from the bearings and the brushes.
Now if you had no magnets at all your rotor would not move because of the lack of torque.
So your counter EMF would be very low because your rotor would need to approach infinite speed for the counter EMF to reach the power supply voltage, but you have no torque.
When the magnets are screwed close together you can overcome the required torque to reach terminal speed, but when further apart you get less torque.
With the magnets placed many inches apart and if you had no friction the motor would run very very fast because that speed would be required to generate the counter EMF.

That also supports my idea . Torque is the result of the magnetic field/force so as RPM. Once the magnetic field is increased/strengthened so as torque and the rotor's speed(RPM). And yea friction is a MAJOR problem in that demonstration if reduced will significantly increase... And its not my video personally its someones else's that I picked up from YouTube.

#### Dretron

Jun 9, 2012
101
Its just forces interacting with each other to produce motion and transfers energy and is able to do work. In a cause of a motor, if one force increase(mag1) the output would be higher, if the other force also increases (mag2), again total output increases.

#### Dretron

Jun 9, 2012
101
The executive summary: Less magnetic field means less back EMF, means more current drawn at the same voltage means more power drawn means motor spins faster.

Bob

"Exactly, or viceversa. The stronger the mag.field the greater the C-EMF the less current is drawn from the power source and he motor spins faster"

Is what I said before. Do you agree on that statement Bob?

#### Dretron

Jun 9, 2012
101
So in general magnetic strength will then increase magnetic force on the rotor = output increase. Does anyone oppose?

#### john monks

Mar 9, 2012
685
If there is lots of friction the the more magnetic field there is the more torque there is on the rotor.
Now if you had no friction then the less magnetic field there is the faster the rotor will spin.
This seems counter intuitive but this can be proven experimentally.
So your last statement is true if you have lost of friction.
If you have little or no friction your statement is wrong.

#### Dretron

Jun 9, 2012
101
If there is lots of friction the the more magnetic field there is the more torque there is on the rotor.
Now if you had no friction then the less magnetic field there is the faster the rotor will spin.
This seems counter intuitive but this can be proven experimentally.
So your last statement is true if you have lost of friction.
If you have little or no friction your statement is wrong.

Its simple really, if there is a lot of friction that means the output is greater then before! That is a good thing to find. I will experiment soon hopefully.

Just wanted to make sure the physics of the matter is right.

#### john monks

Mar 9, 2012
685
I don't think you got it.
The more friction the more magnetic torque is being applied to the rotor and the lower the rotor speed.
When I refer to the magnetic torque I am referring to a force internal to the motor.
The torque that comes as a result of an external load to the shaft is another story and things get complicated quick.
If a motor is perfect, that is no friction and has no load, then as you decrease the field winding the speed increases. This results from C-EMF.
The same motor with a heavy load the speed will decrease as you decrease the magnetic field because as you say less magnetic force.

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#### Dretron

Jun 9, 2012
101
I don't think you got it.
The more friction the more magnetic torque is being applied to the rotor and the lower the rotor speed.
When I refer to the magnetic torque I am referring to a force internal to the motor.
The torque that comes as a result of an external load to the shaft is another story and things get complicated quick.
If a motor is perfect, that is no friction and has no load, then as you decrease the field winding the speed increases. This results from C-EMF.
The same motor with a heavy load the speed will decrease as you decrease the magnetic field because as you say less magnetic force.

Ok, I'm getting confused so I'd like to clear a few things up.

Now, I know magnetic force is responsable for both RPM & Torque isn't it? In general the rotational motion of a rotor is caused by a magnetic force. The higher the magnetic force the greater the output. Based on magnetic force's law which states: F = IL x B,

If the magnetic field is a higher quantity the force would also be higher.

I'll get back to friction as soon as I clear out this point.

#### john monks

Mar 9, 2012
685
Not necessarily so.
A stronger magnetic force causes more C-EMF to occur.
The more C-EMF causes a reduction in speed.

Let's put it this way.
In an ideal motor if you drive it with one volt you will get X rpm's at the saft.
Now you take the same motor with no voltage source and spin the shaft at X rpm's you will get one volt at the terminals.
If increase the magnetic field by two you will now get two volts on the terminals.
So now if you drive that motor with one volt the shaft rotation will only be 1/2 X.

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