LIM Questions

Tuesday, February 4, 2003 4:42 PM
Do Linean Induction Motors propel roller coaster trains based on how much power is given to the LIM's or is there another factor that determines how much force is applied to the train? Also, if LIMs are given a certain constant amount of power, is the speed of the train limited, or will it continue to increase, as long as the LIMs are on?

For example. The LIMs on Flight of Fear ar Paramount's Kings Island are fed a certain amount of power. In this case, lets say that the launch track is endless. Will the train continue to accelerate as long as the power is fed to the LIMs, or will it reach a certain speed, and continue to stay at the speed?

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Intelligence is a God given gift: Know how to use it.

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Tuesday, February 4, 2003 4:57 PM
Dave can probably give a more precise explanation, and his site:
http://capital2.capital.edu/admin-staff/dalthoff/lim.html
is awesome.

However, I'll give it a shot. In a LIM motor, since the fins "ride the wave" of electromagnetic goodness, as long as you have enough power to keep the train on the wave, more power isn't going to help you. (Of course, more power will allow you to keep the train on a faster wave, but that will require some reprogramming). So you can't simply increase the power to increase the accel., but you could increase the power and reprogram a bit to get the desired increase in speed. This is not linear because friction eventually starts eating up a lot of your energy (at least part of the frictional force is directly proportional to your speed), so it takes more energy to increase the speed of the train.

However, in theory, with a strong enough current, strong enough fins, and an appropriately programmed computer, you could make your train go as fast as you wanted.


(here's where Dave comes in and shows me all the mistakes in my analysis).

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Be polite and ignore the idiots. - rollergator
"It was like that when I got here."
"I'll procrastinate later."

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Tuesday, February 4, 2003 5:01 PM
Maybe I should be a little more specific. I don't want to know how fast LIMs can make a coaster go, but HOW exactly they make them go in the first place.

My second question is if a set of LIMs is given a constant ( meaning not changing ) amount of power, will the train that they are propelling constantly accelerate, or will it reach a certain speed, and stay at that speed?

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Intelligence is a God given gift: Know how to use it.

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Tuesday, February 4, 2003 5:43 PM
Okay. Read Dave's site. Then you will understand:

a) How LIM's work.

b) The fact that the amount of power does not determine the speed of the train.

I guess a simple analogy is as follows. If you tie a wagon by a rope to a car (the LIM's = the rope in this analogy), as long as the rope is a certain strength, you can accelerate to a given speed. But if the car accelerates too quickly, the rope will break and you'll be left on your ass.

If you increase the strength of the rope (which in the analogy is increasing the power to the LIM's), you'll be able to accelerate faster, and your top speed will be higher (but only because of friction...in the frictionless environment, you could accelerate to infinite speed eventually), but simply increasing the strength of the rope will not make you go faster unless you change the speed of the car.


LIM's are *programmed* to create a wave of a certain speed (the speed of the car) and the train is locked into the wave by the power supplied. Thus, more power just locks the train into the wave harder.
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Be polite and ignore the idiots. - rollergator
"It was like that when I got here."
"I'll procrastinate later."
*** This post was edited by ApolloAndy 2/4/2003 10:44:48 PM ***

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Wednesday, February 5, 2003 3:15 AM

A LIM operates like a three phase motor. The LIM controller creates a sine wave of electricity that causes a magnetic wave as stated before. The magnetic wave moves from one coil to the next coil inside the LIM. The train is pulled along by this magnetic wave. To increase the speed or the wave (and the train) you have to increase the frequency of the sine wave.
Three phase motors run at some multiple or division of 3600 revolutions per minute depending on the number of poles in the motor. This is because the power coming off of the pole operates at a frequency of 60 HZ (pulses per second). 60 HZ X 60 seconds = 3600 rpm.
A special motor controller takes the incoming power and changes the frequency to control the speed of the trains. It starts out with a low frequency and increases it as the train accelerates. As the controller increases the frequency, the LIMs simply draw more current because they are working harder. The faster the motor controller increases the frequency, the more power will be drawn provided you have a source that can supply the power. You could program a controller to accelerate the train just about as fast as you wanted provided your power source could provide the current and your motors or LIMs could handle the current.
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Just waiting for May 4, 2003. :)
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Wednesday, February 5, 2003 6:38 AM
I tried to add to the discussion last night but CoasterBuzz crashed on me when I tried 'cause Jeff is still tweaking the site...

Anyway, CPIsMyHome pretty much got it. The synchronous speed of a motor is determined by power line frequency and pole spacing. On Flight of Fear, for instance, the pole spacing varies along the launch track...wider pole spacing yields lower force but higher speed. I think there are four different types of LIM used on Flight of Fear.

If you give the train enough space and operate the launch track at a particular frequency, eventually the fin motion will achieve the synchronous speed of the LIM. This is exactly what you want if you're building a monorail or a subway or a baggage handling system. But for a roller coaster, it's pointless to operate at the synchronous speed because at that speed you get no acceleration. So in practice, the coaster will not reach the synchronous speed of the launch track...it will instead accelerate for the full length of the launch.

--Dave Althoff, Jr.

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Wednesday, February 5, 2003 6:42 AM
And not only that, if the train.....ah, heck, never mind. Like I know what I'm talking about? I don't know any of this technical stuff.

Dave, I'm glad you're around to explain this stuff to us [me]. I think from now on I'm gonna call you Mr. Wizzard ;)

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"The movies are weird--you actually have to think about
them when you watch them."
--Britney Spears at the Sundance Film Festival

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Monday, February 10, 2003 5:31 AM
Dave (or anyone else for that matter): Do you happen to know how temperature and/or humidity affect the efficiency of the LIM? Or any other environmental factors.

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Be polite and ignore the idiots. - rollergator
"It was like that when I got here."
"faster, cheaper, and more often" that's somebody's new sig -UpsideDawnGrrrl

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Monday, February 10, 2003 6:02 AM
That's a question for anyone else, I'm afraid... :)

--Dave Althoff, Jr.

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Monday, February 10, 2003 7:29 AM
Actually, a LIM cannot reach full synchronous speed. As an induction motor rather than a synchronous motor there has to always be some slippage, otherwise there would be no induced currrent and magnetic field in the fin. This is usually just a few percent, so it isn't far from synchronous. At true synchronous speed a LIM can not generate any thrust.

Humidity should have very little effect on an LIM except to possibly degrade insulation over a period of many years.

Heat will somewhat reduce the performance of a LIM, plus the LIMs on some coasters are critical on cooling and have overheating problems on hot days.

This leaves the question of why some launched coasters tend to be shut down in cold weather. I don't know, but I can only presume that the increased drag due to thick grease, etc. is greater than any increase in motor performance due to the cold.

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Monday, February 10, 2003 8:59 AM
I was more specifically thinking about the effect of temperature on the inductance in the fin.

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Be polite and ignore the idiots. - rollergator
"It was like that when I got here."
"faster, cheaper, and more often" that's somebody's new sig -UpsideDawnGrrrl

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Monday, February 10, 2003 10:25 AM

Jim Fisher said:
Actually, a LIM cannot reach full synchronous speed. As an induction motor rather than a synchronous motor there has to always be some slippage, otherwise there would be no induced currrent and magnetic field in the fin. This is usually just a few percent, so it isn't far from synchronous. At true synchronous speed a LIM can not generate any thrust.

Interesting., My understanding was that an induction motor (linear or otherwise) would in fact reach a velocity equal to that of the field...that is to say, a synchronous speed...but would never be in sync with the moving field...it will always lag behind by a few degrees.

On the other hand, I think we've talked about this before, something about a single-pole motor running not at 3,600 RPM, but at 3,528 RPM or something like that...I'll bet if I search CoasterBuzz I could find the thread... :)

--Dave Althoff, Jr.
Too lazy to search today...

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Tuesday, February 11, 2003 5:40 PM
In order for an induction motor to have induced currents in the rotor, (=fin) It must be experiencing a constantly changing magnetic field not moving along in step with the magnetic field. Remember that a fixed magentic field does not induce electrical currents, only a varying one.

A synchronous motor moves around in step with the magnetic field. An induction motor has to have slippage (turning slower than the field) in order to get the induced currents in the rotor.

Synchronous motors with 60 Hz power turn at 3600, 1800, 1200, and 900 RPM depending on the number of poles in the windings. 3600 RPM is 60 Hz (cycles per second) times 60 seconds per minute. This is known as a one pole motor. A 2 pole motor turns at 1800 RPM, etc.

Induction motors typically turn a little slower since there must be slippage. Standard speeds would be around 3500, 1760, 1150, and 850 RPM. These are speeds for the motors at rated horsepower. If the motors are lightly loaded they will turn slightly faster, but never at synchronous speeds.

Starting a piece of heavy machinery the motor will turn much slower until the machine is up to speed. This can be very hard on the motor if it takes a long time to get things up to speed. In that case we may use a larger motor than would ordinarily be needed to get things to speed quickly. Other options include a special very expensive motor that can take the abuse or special reduced voltage starts or a variable speed drive. (Sorry for going off on a tangent)

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Wednesday, February 12, 2003 4:11 AM
...And a modern LIM is more akin to the variable speed drive, specifically something like an AC vector drive that can deliver full torque at zero speed. Which is exactly what happens on some of the Intamin Impulse coasters.

--Dave Althoff, Jr.

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Wednesday, February 12, 2003 5:29 AM
Do the compressed air and hydraulic launches apply full force from zero speed?

When I rode Xcelerator, it felt like it took half a second for the trains to really start accelerating (maybe the cable stretching under tension?) which made the launch less exciting for me than Hypersonic's which felt like it was at full force right out of the gate.

------------------
Be polite and ignore the idiots. - rollergator
"It was like that when I got here."
"faster, cheaper, and more often" that's somebody's new sig -UpsideDawnGrrrl
*** This post was edited by ApolloAndy 2/12/2003 10:30:52 AM ***

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