Tackling infinity

Status
Not open for further replies.

Haji

Banned
Location
India
If we try to find out the run up time of, say, an induction motor, we would find that it takes an infinite time to change from its dynamic state to stable state. So we take time to reach some intermediate stage, say 99.5% of its full load speed as the time for the motor to have attained the stable state. Any thoughts on this? Thanks.
 

J.P.

Senior Member
Location
United States
Head hurt. Your overthinking it:)
Zero to operating speed is what matters. Math possibilities and real life are often not the same thing.

Kind of like the deal where it is impossible to divide to zero, so therefor it is impossible for two objects to touch.......
 

Besoeker

Senior Member
Location
UK
If we try to find out the run up time of, say, an induction motor, we would find that it takes an infinite time to change from its dynamic state to stable state. So we take time to reach some intermediate stage, say 99.5% of its full load speed as the time for the motor to have attained the stable state. Any thoughts on this? Thanks.
I don't see how you can conclude that. at rated operating speed the motor has finite (non-zero) torque. If it didn't, it couldn't drive a load.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
I don't see how you can conclude that. at rated operating speed the motor has finite (non-zero) torque. If it didn't, it couldn't drive a load.
The post only talks about the stable or full load speed. It does not say anything about reaching synchronous speed.
The theoretical question, to me anyway, seemed to be addressing how long the motor speed and current would contain a time dependent acceleration term (however small) and when it would reach a steady state of constant slip.
In theory the speed would only asymtotically approach constant slip.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Antelope Freeway, 1 mile...
Antelope Freeway, 1/2 mile...
Antelope Freeway, 1/4 mile...
Antelope Freeway, 1/8 mile...
Antelope Freeway, 1/16 mile...
Antelope Freeway, 1/32 mile...
Antelope Freeway, 1/64 mile...
Antelope Freeway, 1/128 mile...
... etc.

- Firesign Theatre, Waiting for the Electrician or Someone Like Him (Ralph Spoilsport Motors)

RIP, Peter and Phil
 

Besoeker

Senior Member
Location
UK
The post only talks about the stable or full load speed. It does not say anything about reaching synchronous speed.
Unless it's a synchronous motor, synchronous speed is somewhat irrelevant

The theoretical question, to me anyway, seemed to be addressing how long the motor speed and current would contain a time dependent acceleration term (however small) and when it would reach a steady state of constant slip.
In theory the speed would only asymtotically approach constant slip.
Again, I think that is something of an irrelevance.
Slip is dependent on actual loading. If that is not constant, then neither will be slip.
 

FionaZuppa

Senior Member
Location
AZ
Occupation
Part Time Electrician (semi retired, old) - EE retired.
the delta in say speed with t --> infinity would be infinitely small (convergence to zero). at some point quantum takes over and thus you have reached the definitive equilibrium state. its the same thing when you look at the RC time constant for charging a capacitor.
 

Besoeker

Senior Member
Location
UK
the delta in say speed with t --> infinity would be infinitely small (convergence to zero). at some point quantum takes over and thus you have reached the definitive equilibrium state. its the same thing when you look at the RC time constant for charging a capacitor.
Not really the same thing in my opinion. The RC charging (and discharging for that matter) is exponential. Equilibrium doesn't come into it.
 

FionaZuppa

Senior Member
Location
AZ
Occupation
Part Time Electrician (semi retired, old) - EE retired.
Not really the same thing in my opinion. The RC charging (and discharging for that matter) is exponential. Equilibrium doesn't come into it.

??
exponential or not they are both finite??
in a RC circuit amps will eventually converge to zero which suggests a quantum finite state. we know this to be equilibrium.
 

Besoeker

Senior Member
Location
UK
??
exponential or not they are both finite??
in a RC circuit amps will eventually converge to zero which suggests a quantum finite state. we know this to be equilibrium.
Check the equations for exponential relationships and you will see that it isn't so.
 

FionaZuppa

Senior Member
Location
AZ
Occupation
Part Time Electrician (semi retired, old) - EE retired.
Check the equations for exponential relationships and you will see that it isn't so.

how "fast" you get there doesnt matter, the function f(t), be it exponential, linear or other, exists for all times of 0≤t≤∞
 

Besoeker

Senior Member
Location
UK
Check the equations for exponential relationships and you will see that it isn't so.
A little more to add to my previous post........
Charging from both rectified AC and DC:

Capacitor%20charging%2006_zpsuzmpxani.jpg


It never gets there.

The second shows discharge times. I did this to assess safe times for working on VSDs after the removal of input power to ensure that the DC link capacitor bank voltage had decayed to a sufficiently safe level. My background is power electronics including VSDs.

Capacitor%20discharge%20times%2001_zpsilc5o5lp.jpg


Theoretically, the time to vet to zero volts is infinite.
You cannot reasonably equate that with a motor reaching an operating speed where its output torque matches load torque.
 
Last edited:

gadfly56

Senior Member
Location
New Jersey
Occupation
Professional Engineer, Fire & Life Safety
Once you get close enough to be inside the range of the normal speed changes it no longer matters.
View attachment 13802

The unexpressed assumption in "Zeno's Paradox" is that you are also reducing the time interval, "t" by the same fraction. 1/2t + 1/4t +1/8t ad infinitum. The infinite series sums to "t", therefore at "t" the arrow hits its target.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
An induction motor will never run at synchronous. Slip is what creates back emf, and that's how the motor works.
Slip is not what creates back EMF. Rotational speed creates back EMF.
Slip creates torque and an increased resistive component to the motor current. If anything slip reduces back EMF allowing more current to flow.
 

Fulthrotl

~Autocorrect is My Worst Enema.~
Antelope Freeway, 1 mile...
Antelope Freeway, 1/2 mile...
Antelope Freeway, 1/4 mile...
Antelope Freeway, 1/8 mile...
Antelope Freeway, 1/16 mile...
Antelope Freeway, 1/32 mile...
Antelope Freeway, 1/64 mile...
Antelope Freeway, 1/128 mile...
... etc.

- Firesign Theatre, Waiting for the Electrician or Someone Like Him (Ralph Spoilsport Motors)

RIP, Peter and Phil

"Hiya, friends! Ralph Spoilsport, Ralph Spoilsport Motors, the world's largest new used
and used new automobile dealership, Ralph Spoilsport Motors, here in the City of Emphysema.
Let's just look at the extras on this fabulous car! Wire-wheel spoke fenders, two-way
sneezethrough windvent, star-studded mudguards, sponge-coated edible steering column,
chrome fender dents, and factory air-conditioned air from our fully factory-equipped factory.

It's a beautiful car, friends, with doors to match! Birch's Blacklist says this automobile was stolen,
but for you, friends, the complete price, two thousand five hundred dollars, in easy monthly payments
of twenty-five dollars a week, twice a week, and never on Sundays..."
 
Status
Not open for further replies.
Top