Author Topic: Making an induction motor go crazy fast.  (Read 5883 times)

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Offline Yansi

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Re: Making an induction motor go crazy fast.
« Reply #25 on: August 30, 2019, 06:53:52 pm »
Fan torque goes up square wit rpm.

On thw other hand no problem removing it ;)
 

Online Ian.M

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Re: Making an induction motor go crazy fast.
« Reply #26 on: August 30, 2019, 08:03:36 pm »
What about hydrogen cooling (as used by *BIG* generators)?   That should reduce the windage losses by an order of magnitude.   

Safety may be a bit of a problem, :scared: though if you can set up for CCTV monitored remote operation outside, including running an inert gas purge before opening the hydrogen valve, and again on shutdown before approaching it, and put the business end of the rig in a pit in the middle of a field to direct any blast upwards, with the hydrogen cylinder and any more delicate test equipment behind a berm nearby, a good distance from the operator's bunker, you should get away with it.  >:D
« Last Edit: August 30, 2019, 08:09:36 pm by Ian.M »
 

Offline Yansi

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Re: Making an induction motor go crazy fast.
« Reply #27 on: August 30, 2019, 08:19:46 pm »
Big generators have the hydrogen inside the hollow copper bars used as the windings. Not in the entire machine.
 

Online Gyro

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Re: Making an induction motor go crazy fast.
« Reply #28 on: August 30, 2019, 08:57:41 pm »
No, the big turbo-alternators, at least the ones old enough to be in my old electrical engineering books, run/ran pure (obviously) Hydrogen in a closed circuit to reduce turbulence losses and due to its higher specific heat capacity. There were all sorts of precautions, purging procedures, oil labyrinth shaft seals to protect against air leakage.


EDIT: Actually, on referring to the books in question (The Modern Electical Engineer, Arthur Arnold, Caxton press published 1927) they were only being run on an experimental basis, I don't know what happened after that. Hydrogen has a specific heat capacity 14 times that of air - and of course the much better windage losses due to much lower density as I mentioned.
« Last Edit: August 30, 2019, 09:08:49 pm by Gyro »
Best Regards, Chris
 

Offline Yansi

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Re: Making an induction motor go crazy fast.
« Reply #29 on: August 31, 2019, 08:06:07 am »
They run hydrogen just inside the winding. No doubt about that.

Never heard of any use of hydrogen for such dangerous purposes in any practical generator, currently being used. Having hydrogen within the whole machine would be more than impractical.
 

Offline mzzj

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Re: Making an induction motor go crazy fast.
« Reply #30 on: August 31, 2019, 08:21:09 am »

Rotor is 70mm diameter and 60mm long. Shaft is 14mm. Out of an automatic sliding door like this one -> https://youtu.be/nKdHCjAEKuI?t=61
I've recalculated for your actual rotor - at 16800 RPM that's 138 MPH, 11000 g, 1650 J

Once you've got containment, it needs more volts and more Hz.  >:D


Bog standard electric motors from most makes are rated to 4500 rpm up to IEC frame size 200. That is 50hp 300kg motor!
 

Online coppercone2

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Re: Making an induction motor go crazy fast.
« Reply #31 on: August 31, 2019, 08:22:26 am »
whats the point of speeding them up anyway, I always wanted them slow so they can do useful things?

fast motors are a nuisance for all mechanical things other then tiny drill bits, which I assume need special bearings anyway for vibration

grinders?
« Last Edit: August 31, 2019, 08:24:03 am by coppercone2 »
 

Online Gyro

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Re: Making an induction motor go crazy fast.
« Reply #32 on: August 31, 2019, 10:43:36 am »
They run hydrogen just inside the winding. No doubt about that.

Never heard of any use of hydrogen for such dangerous purposes in any practical generator, currently being used. Having hydrogen within the whole machine would be more than impractical.

Closed circuit inside the enclosure too, otherwise you wouldn't get the windage benefit...

https://en.wikipedia.org/wiki/Hydrogen-cooled_turbo_generator
https://www.power-eng.com/articles/print/volume-113/issue-6/features/hydrogen-cools-well-but-safety-is-crucial.html
https://www.quora.com/Why-hydrogen-is-used-for-generator-cooling
https://www.flownex.com/fnxdocs/casestudies/CS42_GeneratorSealOilSystem.pdf


P.S. This is for interest, not spoiling for a fight.... Or suggesting squirting Hydrogen into an induction motor!  :)
« Last Edit: August 31, 2019, 10:48:27 am by Gyro »
Best Regards, Chris
 
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Offline Yansi

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Re: Making an induction motor go crazy fast.
« Reply #33 on: August 31, 2019, 12:43:44 pm »
Probably, only in one of the largest generator of them all, as cited in the [1] source on wikipedia, yes the may have done it. But never heard of that ever done anywhere else. 

But all of the power plants in our country (that have blocks up to 1055 MW) do use only hydrogen cooled winding.  So was I also taught at university, where I have studied power distribution, generation and electric drives.

When I try to dig for this further, I can confirm that our biggest 1055MW blocks indeed do have the whole* generator cooled by hydrogen atmosphere. Interesting. We've been told lies in the school!

*not really whole, but the rotor definitely is within a hydrogen atmosphere too
 
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Offline Miyuki

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Re: Making an induction motor go crazy fast.
« Reply #34 on: September 01, 2019, 10:01:12 am »
Hi folks I have little question about induction motors

What limit maximum torque (breakdown torque) ? Is it just stator / rotor saturation ? Or have it other causes ?
 

Offline Yansi

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Re: Making an induction motor go crazy fast.
« Reply #35 on: September 01, 2019, 10:20:03 am »
Well I think that is not as easy question to answer.

The torque characteristic may depend not only on the maximum flux, but also the type of rotor winding (wirewound + slip ring, squirrel cage, doubled squirrel cage)...

 

Offline CirclotronTopic starter

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Re: Making an induction motor go crazy fast.
« Reply #36 on: September 01, 2019, 11:03:59 am »
The rotor bars have a slight amount of inductance and the rotor aluminium end rings have a certain low value of resistance. The frequency of the current induced into the rotor is the difference between the "rotation" rpm of the stator magnetism and the actual rpm of the rotor. This difference in rpm is the "slip" speed. At some particular slip speed (rotor current slip frequency) the rotor inductive reactance equals the rotor resistance and the rotor current lags rotor voltage by 45 deg(?) and everything is happy. If you load the motor too much the rotor induced slip current frequency gets too high and consequently so does the rotor reactance, compared to the rotor resistance. This means the rotor power factor gets very low and so the torque decreases as the motor is loaded more and more. Something like that.

With a slip ring motor you can add additional external resistance to the rotor so the motor can give full torque at any rpm, right down to zero. To continually run the motor like this would be very wasteful though.
« Last Edit: September 01, 2019, 11:09:38 am by Circlotron »
 

Offline Yansi

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Re: Making an induction motor go crazy fast.
« Reply #37 on: September 01, 2019, 12:13:42 pm »
I do not think that is the answer to the question.

I understand the question as what limits the maximum achievable value of breakdown torque, in an induction motor.  And the answer to that has to be very complex, as it depends on the exact geometry and its specific construction details of an individual motors. There are at least so many types and shapes of the rotor, that it is impossible to answer that all.

Maximum flux density in the machine is definitely one of the limiting factors.  F = B*I*len still counts.

 

Offline Miyuki

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Re: Making an induction motor go crazy fast.
« Reply #38 on: September 01, 2019, 12:35:02 pm »
Even slip ring motor have always same Tmax
Just curve change its shape

 

Offline mzzj

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Re: Making an induction motor go crazy fast.
« Reply #39 on: September 01, 2019, 01:35:53 pm »
https://www.landbelectric.com/download-document/85-torque-characteristics-of-nema-abcd-e-motors.html

In many cases its desirable to limit the maximum starting torque(less mechanical strain) and peak currents.
 

Offline max_torque

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Re: Making an induction motor go crazy fast.
« Reply #40 on: September 01, 2019, 01:44:41 pm »
The motor i helped develop for the Mclaren P1 and then used in FormulaE runs at over 20,000 rpm and has an internally water cooled rotor, where the water/glycol mix is pumped into and out of the rotor spining at that speed!  The sealing in particular gave me most of the gray hairs on my head i now have........
 

Offline Berni

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Re: Making an induction motor go crazy fast.
« Reply #41 on: September 02, 2019, 05:15:38 am »
The motor i helped develop for the Mclaren P1 and then used in FormulaE runs at over 20,000 rpm and has an internally water cooled rotor, where the water/glycol mix is pumped into and out of the rotor spining at that speed!  The sealing in particular gave me most of the gray hairs on my head i now have........

Dang i had no idea they would watercool the rotor. So the rotor shaft is hollow and the stuff is pumped in on one end and flows out the other? Indeed sounds like a sealing nightmare at those speeds even tho i have no clue about mechanical engineering.

Oh and in terms of overloading the windings before the motor gets to a fast enough speed to self destruct i just remembered that you can increase the input voltage at higher frequencies and induction motors usually like running with as much voltage as possible as long as they don't saturate. So you could likely safely double or triple the input voltage before getting into any insulation breakdown problems. This lets you put more power into it before the windings overheat(since that's purely current related)
 

Offline CirclotronTopic starter

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Re: Making an induction motor go crazy fast.
« Reply #42 on: September 02, 2019, 06:18:36 am »
^^ Yep, dead right. Similar to raising the frequency and voltage with an iron cored transformer to get more throughput. Maximum transformer current is still the same, as is maximum motor torque, but you could double or triple the voltage proportionally with frequency same as you can double or triple the rpm, leading to double or tripe the watts output of both. One limiting factor would be losses in the iron core. But it's fun to see just how far you can go without causing damage.
« Last Edit: September 02, 2019, 06:28:46 am by Circlotron »
 

Offline dom0

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Re: Making an induction motor go crazy fast.
« Reply #43 on: September 02, 2019, 02:28:01 pm »
Welcome to the world of VFDs. Since the 1980s or when...

nothing ruins your day more then 'i need a 200$ vfd to use this totally cool motor I spent 3 hours removing from a washing machine I found on the street so I can do a weekend project'.

I hate them. Thought you were in the clear ? no way, now its time to figure out which expensive 200$ mystery box won't blow up for your shoe shiner you can buy for 25$.  :P

Welcome to Europe, where you can just plug it in.
,
 

Offline dom0

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Re: Making an induction motor go crazy fast.
« Reply #44 on: September 02, 2019, 02:33:12 pm »
Fan torque goes up square wit rpm.

On thw other hand no problem removing it ;)

Fan is internal and is part of the dynamic balancing -> remove fan, bye bearings.
,
 

Offline amyk

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Re: Making an induction motor go crazy fast.
« Reply #45 on: September 03, 2019, 11:38:32 am »
That looks like the type of motor used in washing machines, which see over 10kRPM in normal service (belt-drive drum with at least 10:1 reduction, 1000RPM+ spin cycle, you do the maths...)
 

Offline Yansi

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Re: Making an induction motor go crazy fast.
« Reply #46 on: September 03, 2019, 11:44:52 am »
Didn't the OP author say explicitly, it is an induction motor from a door slammer? What would you need 10k RPM for there?

It is more than likely a standard 50/60Hz motor, likely even a multiple pole-pair type, to get torque and lower speed.

//EDIT: 6pole motor (sub-1000 RPM), as per the OP.
« Last Edit: September 03, 2019, 11:47:35 am by Yansi »
 

Offline CirclotronTopic starter

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Re: Making an induction motor go crazy fast.
« Reply #47 on: September 03, 2019, 11:51:39 am »
 

Offline amyk

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Re: Making an induction motor go crazy fast.
« Reply #48 on: September 05, 2019, 02:03:53 am »
Yes, I'm just saying that such speeds are not unusual for induction motors of that size and construction.
 


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