Response of a motor rotor to just 1/60th of a second of current

[jake jot]

Regarding what you studied, which detail do you think changes from a 1 s pulse to a 0.999 s pulse such that the former is understandable but the latter is not?

It's more like 1 s pulse vs 0.0167 sec pulse.

If the lorentz force were a thousand times stronger, then no problem believing a 1/60 sec pulse can move the rotor (without other current or the motor off except that pulse). But thanks to rotational momentum. I think the stator magnetic field and rotor got kinda synchronize because of resonance akin to tuning folks so no need for the lorentz force to be a thousand times stronger.

 

[russ_watters]

...so a 1/30th or 30 hz AC will make the electric fan turns as fast?

Turn as fast as what? As a normally powered fan? No, obviously not.

 

[Dale]

It's more like 1 s pulse vs 0.0167 sec pulse.

Ok, so do you still understand a 0.999 s pulse? Do you still not understand a 0.017 s pulse? Where is the exact cutoff and what changes at that cutoff that makes it understandable above the cutoff and not understandable below?

If you cannot identify a sharp cutoff and you cannot identify something specific that physically changes at that cutoff then maybe your assertion of understanding or not understanding is flawed.

 

[A.T.]

If the lorentz force were a thousand times stronger, then no problem believing ...

Because "thousand times" sounds big?

 

[Vanadium 50]

There is a lot of brush in this thread. @jake jot , you will get more out of this forum if you put more into it. Thus far, you have not really explained your question well enough, and dragging Tesla into it sure won't help.

(1) In the US, AC power is at 60 Hz, so you have one cycle after 1/60 of a second. If after the first cycle the motor does not move, it is in the same state at the end as the beginning of this cycle. Agree? Then it starts the second cycle in the same condition as the first cycle, and is exposed to the same current and current profile, so of course has the same outcome. Agree? That means it doesn't move after the second cycle either. The same argument can be made for all cycles - thus forcing us to conclude that motors don't ever turn.

But they do turn. So our premise - that they don't move in 1/60 of a second must be wrong.

(2) If I turn a motor by hand (i.e. without electricity) for 1/60 of a second (providing the same force - or if you prefer, torque - as the magnetic fields do when the motor is operating normally) , will it turn? If not, how does the motor know that the force is being provided by my fingers and not the fields?

 

[jake jot]

There is a lot of brush in this thread. @jake jot , you will get more out of this forum if you put more into it. Thus far, you have not really explained your question well enough, and dragging Tesla into it sure won't help.

(1) In the US, AC power is at 60 Hz, so you have one cycle after 1/60 of a second. If after the first cycle the motor does not move, it is in the same state at the end as the beginning of this cycle. Agree? Then it starts the second cycle in the same condition as the first cycle, and is exposed to the same current and current profiel, so of course has the same outcome. Agree? That means it doesn't move after the second cycle either. The same argument can be made for all cycles - thus forcing us to conclude that motors don't ever turn.

But they do turn. So our premise - that they don't move in 1/60 of a second must be wrong.

(2) If I turn a motor by hand (i.e. without electricity) for 1/60 of a second (providing the same force - or if you prefer, torque - as the magnetic fields do when the motor is operating normally) , will it turn? If not, how does the motor know that the force is being provided by my fingers and not the fields?

What? Lorentz force can really react at 1/60 sec given typical current? Hmm... do you (or anyone) know where I can get the following? Before going further with motor. I have to play with lorentz force first to get a feel of the force of it.

 

[Dale]

Lorentz force can really react at 1/60 sec given typical current?

Why wouldn’t it? What exact frequency would stop it and what physically would happen at that frequency to stop it?

 

[nsaspook]

Short current pulses are used to move DC motors all the time The process is called PWM and the short pulse is called the duty cycle. Obviously the effect of the transient pulse(s) is more complicated (inductance, resistance and EMF effects) electrically than a DC voltage/current steady state motor drive but we can calculate the equivalent rotor force for the pulse.

 

[Vanadium 50]

What? Lorentz force can really react at 1/60 sec given typical current?

Of course. That post I made with the questions you didn't answer? It explains the contradiction you get when you assume otherwise.

 

[jake jot]

Why wouldn’t it? What exact frequency would stop it and what physically would happen at that frequency to stop it?

So the Force is strong. I'll first study, reflect and even meditate on Lorentz so I could feel the force and understand how things can wield the force.

If someone can share the computation how the lorenz force can move rotor at 1/60 sec. Please do. It's indeed a strong force after all.

I'll find my old horseshoe U magnet, maybe I can do similar experiment as the one shown early with the bar bending up as it yield to the force?

 

[Vanadium 50]

If someone can share the computation how the lorenz force can move rotor at 1/60 sec. Please do.

Perhaps instead you can show us where your 1/60 second delay comes in.

And, just out of curiosity, in the UK is it still 1/60? Or is it 1/50?

 

[Dale]

If someone can share the computation how the lorenz force can move rotor at 1/60 sec. Please do.

$\tau=nIAB \sin(\theta)$. Note the lack of any limitation on duration.

This formula is valid as long as the wavelengths are much larger than the size of the loop. For 1/60 s the wavelength is about 5000 km, so we will be fine using it for any motor that has been built and any motor that will be built in the foreseeable future.

 

[jake jot]

Perhaps instead you can show us where your 1/60 second delay comes in.

And, just out of curiosity, in the UK is it still 1/60? Or is it 1/50?

I'm using 60 hz electricity. Maybe Tesla chose 60 hz to match his motor which is more efficient at 60 hz? I think i read this somewhere last night.

But let me clarity. Are you and Dale implying that with the motor off the rest of the time and you suddenly apply 1/60 sec pulse, the rotor will move? or not?

 

[Dale]

Are you and Dale implying that with the motor off the rest of the time and you suddenly apply 1/60 sec pulse, the rotor will move?

Of course. Why wouldn’t it?

You claim to understand why it would move with a 1 s pulse. What do you think is so fundamentally different about 1/60 s that it wouldn’t move?

You keep avoiding my questions that I ask you. The purpose of those questions is to get you to challenge your own assumptions. Please answer these.

 

[jake jot]

Of course. Why wouldn’t it?

You claim to understand why it would move with a 1 s pulse. What do you think is so fundamentally different about 1/60 s that it wouldn’t move?

You keep avoiding my questions that I ask you. The purpose of those questions is to get you to challenge your own assumptions. Please answer these.

Because with 1 sec, there is enough time to build up the force. At 1/60 sec.. it's too fast and can the force build up that fast?

Also there is no previous acceleration. So I thought the 1/60 pulse just give it a slight push.

Or imagine a very heavy singer in a concert where he drop down and the crowd use fingers to move him across the hall. One hands can't carry him but when the entire crowd use each hand, they can make him move. I thought this was the same with rotor.

Calculations and demonstration is what will settle anything. Did you see any demo that a 1/60 sec pulse can move a rotor (without any previous rotation or current to the motor or rotor)?

 

[Dale]

At 1/60 sec.. it's too fast and can the force build up that fast?

Why would it be too fast? Which specific law of physics prevents forces from acting in 1/60 s?

 

[jake jot]

Why would it be too fast? Which specific law of physics prevents forces from acting in 1/60 s?

Let's say the energy in 1/60 sec is 3 joules.. at 1 min, it's 3 x 60 = 180 joules.. so there is more energy to make it work.. why is this not true in lorentz force?

In our high school and even college education. We were only given magnets to work with, so lorentz force was not demonstrated and not part of our normal (average citizen) upbringing. Hence it's not intuitive for the masses.

 

[jake jot]

I saw this with the north pole at right side and south pole at left side of the inductor

In the case of this actual part in the amazon motor high school kit, where is the lines of force? the top copper is just the commutator, the bottom two cooper is the windings

 

[Dale]

Let's say the energy in 1/60 sec is 3 joules.. at 1 min, it's 3 x 60 = 180 joules.. so there is more energy to make it work.

Sure. There is more energy delivered in 1 s than in 1/60 s. But do you think that the energy delivered in 1/60 s is zero? If it is not zero then there must be some force and some acceleration.

 

[jake jot]

Sure. There is more energy delivered in 1 s than in 1/60 s. But do you think that the energy delivered in 1/60 s is zero? If it is not zero then there must be some force and some acceleration.

So if the rotor is quite heavy. Then the 1/60 sec pulse won't cause any movement. Whereas the 1 sec pulse can. So it depends on how heavy the rotor is to tell whether it will move by the 1/60 sec pulse?

In the normal inductor motor design commercially produced. Will the 1/60 sec pulse move the rotor? (without other rotation or current before it)?

 

[Dale]

So if the rotor is quite heavy. Then the 1/60 sec pulse won't cause any movement.

How can you possibly conclude that from what I or anyone else in this thread has said? What physical law are you invoking to make that claim in contradiction to what you have been told?

You are just making stuff up.

 

[jake jot]

How can you possibly conclude that from what I or anyone else in this thread has said? What physical law are you invoking to make that claim in contradiction to what you have been told?

You are just making stuff up.

I don't understand. Is it not you just wrote:

"Sure. There is more energy delivered in 1 s than in 1/60 s. But do you think that the energy delivered in 1/60 s is zero? If it is not zero then there must be some force and some acceleration. ".

Meaning if it is not zero it must be some force and some acceleration smaller than 1 sec.. so if the 1/60 sec pulse is smaller in value, then the rotor needs to be lighter too. What is the problem with this analysis? I'm not making up things. Just trying to understand.

 

[russ_watters]

I don't understand...

Meaning if it is not zero it must be some force and some acceleration smaller than 1 sec.. so if the 1/60 sec pulse is smaller in value, then the rotor needs to be lighter too. What is the problem with this analysis? I'm not making up things. Just trying to understand.

1/59th sec is smaller than 1 sec too.

 

[Dale]

I don't understand. Is it not you just wrote:

"Sure. There is more energy delivered in 1 s than in 1/60 s. But do you think that the energy delivered in 1/60 s is zero? If it is not zero then there must be some force and some acceleration. ".

Meaning if it is not zero it must be some force and some acceleration smaller than 1 sec.. so if the 1/60 sec pulse is smaller in value, then the rotor needs to be lighter too. What is the problem with this analysis? I'm not making up things. Just trying to understand.

OK, you need to stop with this thread then and learn Newton’s laws first. The rotor does not need to be lighter, the change in speed is simply smaller, but still non-zero.

This thread is pointless until you learn Newton’s laws. I am closing it for now. Once you have learned Newton’s laws then you can start over asking about motors. Right now you are simply not prepared with the necessary background to understand the answers that you have already been given.

Thread closed