Author Topic: Making an induction motor go crazy fast.  (Read 5886 times)

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Online CirclotronTopic starter

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Making an induction motor go crazy fast.
« on: August 29, 2019, 11:53:09 am »
Got a small 3-phase 6 pole induction motor that normally runs at 1000rpm @ 50Hz. Generated two sine waves 60 deg apart with the function generator and fed them to a pair of class D audio amplifier modules running on +/- 65VDC, then to three toroid step up transformers with 28V primaries in delta and 240V secondaries in star. One corner of delta primary is grounded. Gradually wound up the frequency and voltage until the speed would increase no further. 860Hz and 16,800rpm. Showed it who's boss!

Steady speed
https://youtu.be/nZZi1qC4d_c

Rundown to stop
https://youtu.be/SVhdQFP8hGA
« Last Edit: August 29, 2019, 11:55:34 am by Circlotron »
 
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Online Ian.M

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Re: Making an induction motor go crazy fast.
« Reply #1 on: August 29, 2019, 01:01:00 pm »
Are you trying to get a Darwin Award?

You are <expletive> lucky the motor rotor didn't grenade.  It only takes one rotor bar to bow by a couple of mm or break at one end or a bearing to catastrophically fail and the rotor will seize inside the stator, and tear the motor apart.

Assuming a 2" dia 2" long rotor, at 16800 RPM, the rotor rim speed is 100 MPH, there's over 8000 g of centripetal acceleration, and 500 J of kinetic energy.   

At the very least, a sane person would want containment round the motor that could safely withstand half the motor rotor mass hitting it at half the rim speed, and trap any high speed bouncing small parts.

Edit: corrected assumed rotor diameter - I did the math for 1" radius (and assumed a uniform steel cylinder for the K.E. calculation, so somewhat of a spherical cow).
« Last Edit: August 29, 2019, 02:14:57 pm by Ian.M »
 
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Online CirclotronTopic starter

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Re: Making an induction motor go crazy fast.
« Reply #2 on: August 29, 2019, 01:16:19 pm »
^^ Thank you for that.
I hadn't actually considered that possibility.  :phew:
That experiment is now terminated.
 

Online Ian.M

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Re: Making an induction motor go crazy fast.
« Reply #3 on: August 29, 2019, 01:47:57 pm »
No need to terminate the experiment.  Simply put the motor in a stout cardboard box with plenty of sand bags round it to a thickness of at least 10 times the rotor diameter,  and a viewport of 1/4" or thicker clear polycarbonate (don't use acrylic or polystyrene as they aren't shatter resistant) in line with the motor shaft and well anchored to the box and sand bags with gaffer tape.

Why cardboard, not steel, plastic etc.?  Well you want any fragments to rip through it and dissipate their energy in the sand, not bounce and try to take out the viewport. 
« Last Edit: August 29, 2019, 01:51:13 pm by Ian.M »
 

Offline schmitt trigger

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Re: Making an induction motor go crazy fast.
« Reply #4 on: August 29, 2019, 01:58:15 pm »
This appears like an experiment which Youtuber Photonicinduction would love to do.

Another solution would be to watch the experiment thru a remote tv camera, while you shield yourself behind concrete walls and/or LOTS of distance.

I like Ian's cardboard idea. A large water tank would also be effective in absorbing the shrapnel's energy.
 

Offline Mechatrommer

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Re: Making an induction motor go crazy fast.
« Reply #5 on: August 29, 2019, 02:29:14 pm »
looks like still under controlled, vibration, sound and stability under control. motor like this usually have thick rotor shaft and bearing. excessive vibration and noise usually give early warning indicates that further damage will follow soon. but safety precaution such as containment rig wont hurt and is wise to do for operation beyond specification. but imho its enclosure should take the explosive joule whatever happening inside quite well (from the look of it), the only concern is when the motor wanders around due to vibration transmitted to external motion, shorting and damaging whatever around it, like what the late youtuber did to a washing machine. a fun experiment and hack nonetheless to achieve more rpm out of an ac motor..
Nature: Evolution and the Illusion of Randomness (Stephen L. Talbott): Its now indisputable that... organisms “expertise” contextualizes its genome, and its nonsense to say that these powers are under the control of the genome being contextualized - Barbara McClintock
 

Offline Yansi

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Re: Making an induction motor go crazy fast.
« Reply #6 on: August 29, 2019, 02:33:44 pm »
Got a small 3-phase 6 pole induction motor that normally runs at 1000rpm @ 50Hz. Generated two sine waves 60 deg apart with the function generator and fed them to a pair of class D audio amplifier modules running on +/- 65VDC, then to three toroid step up transformers with 28V primaries in delta and 240V secondaries in star. One corner of delta primary is grounded. Gradually wound up the frequency and voltage until the speed would increase no further. 860Hz and 16,800rpm. Showed it who's boss!

Steady speed
https://youtu.be/nZZi1qC4d_c

Rundown to stop
https://youtu.be/SVhdQFP8hGA

but y uno 3phase is 120 dungarees, not 60?
 

Offline Mechatrommer

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Re: Making an induction motor go crazy fast.
« Reply #7 on: August 29, 2019, 02:38:31 pm »
can we just use offline (mains) ac-dc and then create 3 phase PWM out of some beefy mosfets to it? so no need boat anchor grade toroidals.
Nature: Evolution and the Illusion of Randomness (Stephen L. Talbott): Its now indisputable that... organisms “expertise” contextualizes its genome, and its nonsense to say that these powers are under the control of the genome being contextualized - Barbara McClintock
 

Offline Yansi

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Re: Making an induction motor go crazy fast.
« Reply #8 on: August 29, 2019, 03:44:44 pm »
Welcome to the world of VFDs. Since the 1980s or when...
 
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Offline max_torque

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Re: Making an induction motor go crazy fast.
« Reply #9 on: August 29, 2019, 04:27:58 pm »
The good news is that a typical AC motor is very robust to rotor failure, because the rotor is pretty much inside a massive iron and copper stator!  However, watch out for the crappy cast steel/ally "fan" on the end, as when they go bang, they can shed parts clean through the tin plate fan cover..........
 

Offline schmitt trigger

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Re: Making an induction motor go crazy fast.
« Reply #10 on: August 29, 2019, 05:49:20 pm »
You are correct, shrapnel from a disintegrating fan, is what one should be concerned with.

The key is that they have a larger diameter than the rotor itself, meaning larger forces, and they are protected by a thin steel mesh which can be easily breached.
 

Online CirclotronTopic starter

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Re: Making an induction motor go crazy fast.
« Reply #11 on: August 29, 2019, 09:46:25 pm »
but y uno 3phase is 120 dungarees, not 60?
I know it sounds wrong, but with one wire of the delta grounded you drive the second wire with a sine wave and the third wire with a sine wave 60 deg shifted. Again, it sounds wrong, I know. I had to simulate it to prove it to myself.
 

Offline coppercone2

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Re: Making an induction motor go crazy fast.
« Reply #12 on: August 29, 2019, 10:24:15 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


the only reasonable options is belt drives, because gears and shaft alignment are way out of most people leagues. At least the belt is.. kinda achievable. But still adds oodles of work and no one really does it. I want this thing to spin turns into learning how to use bearings and belts and pullies and ugh

AC motors are a massive trap IMO, and BLDC is not found in scrapfinds usually :(

you get all excited when you get a new blender that you got a motor (which will sit for 10+ years before you scrap it for copper).
« Last Edit: August 29, 2019, 10:31:55 pm by coppercone2 »
 

Online CirclotronTopic starter

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Re: Making an induction motor go crazy fast.
« Reply #13 on: August 30, 2019, 03:57:14 am »
Edit: corrected assumed rotor diameter - I did the math for 1" radius (and assumed a uniform steel cylinder for the K.E. calculation, so somewhat of a spherical cow).
Rotor is 70mm diameter and 60mm long. Shaft is 14mm. Out of an automatic sliding door like this one -> https://youtu.be/nKdHCjAEKuI?t=82
« Last Edit: August 30, 2019, 12:32:21 pm by Circlotron »
 

Offline Miyuki

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Re: Making an induction motor go crazy fast.
« Reply #14 on: August 30, 2019, 05:10:58 am »
Induction motors are very robust
I have from one manufacturer affirmed that I can use standard (50hz) rotor up to 400Hz safely and it is much bigger motors in 2kW (50Hz) > 10kW (400hz) / 7000rpm

Dont this small one have just plastic fan ?
Also this one look like a compact design with internal fan
But agree some safety measures shall be applied
 

Online CirclotronTopic starter

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Re: Making an induction motor go crazy fast.
« Reply #15 on: August 30, 2019, 05:40:50 am »
No plastic fan. The internal fan is part of the cast aluminium end rings on the rotor.
 

Online Ian.M

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Re: Making an induction motor go crazy fast.
« Reply #16 on: August 30, 2019, 07:39:20 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
 

Online Berni

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Re: Making an induction motor go crazy fast.
« Reply #17 on: August 30, 2019, 08:27:58 am »
That's one fast motor alright.

Tho i don't think this is quite as dangerous as it sounds from the hefty number of Joules in the thing. The rotors on induction motors are typically solid chunks of iron and aluminum and the fan is typically part of the same casting. So the thing is very strong. And if it does start to desegregate it will hit the strong laminated iron stator.

So if you do get it spinning past critical failure speed it most likely will not explode or send shrapnel at high speed. Tho as the failed rotor grinds to an incredibly loud halt against the stator or failed bearings the rotational inertia has to go somewhere so the whole motor will likely fly off and roll across the room, this is not something you want to get in the way of.
 

Online Ian.M

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Re: Making an induction motor go crazy fast.
« Reply #18 on: August 30, 2019, 09:03:57 am »
I wonder about how close to cold flowing the squirrel cage is.  It cant be a very hard alloy as the resistance would be too high, and with over 100 N of force radially outwards on each gramme of rotor bar, if the channels the bars run in don't support the bars well enough, bad things will happen. 

Tho as the failed rotor grinds to an incredibly loud halt against the stator or failed bearings the rotational inertia has to go somewhere so the whole motor will likely fly off and roll across the room, this is not something you want to get in the way of.
Yes.  The whole motor taking off at a few thousand RPM, and pinballing off the test equipment, walls and other objects in the room could be a little too exciting for most people.  That's an expensive brown trousers moment even if you escape serious injury, and may result in your significant other banning your hobby!

Running any high speed motor larger than you can comfortably close your hand round loose on the bench is never a good idea.  It only takes a couple of minutes to bolt or strap a motor down to something wide and heavy enough to stay put (more or less) to minimise the possible excitement.
« Last Edit: August 30, 2019, 09:06:25 am by Ian.M »
 

Offline max_torque

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Re: Making an induction motor go crazy fast.
« Reply #19 on: August 30, 2019, 12:15:14 pm »
Generally the bearings will be well goosed before the rotor lets go!

(as most induction motors have v cheap bearings in them)
 

Offline schmitt trigger

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Re: Making an induction motor go crazy fast.
« Reply #20 on: August 30, 2019, 02:14:06 pm »
As I mentioned earlier, this could well be a photonicinduction-grade experiment. Which means over stressing it to destruction to determine what would fail first.

That would be a fun video to watch.....with the proper precautions in place.
 

Offline Mechatrommer

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Re: Making an induction motor go crazy fast.
« Reply #21 on: August 30, 2019, 03:19:19 pm »
usually the weakest point is the bearing, if it has sleeve inside, its the one usually go clunking first from inside on overheat or overspeed stress. the rotor shaft usually indestructible you can repurpose it as stone punchers for a while if shaped correctly. and good brand will balance the rotor from factory, you can hear minimal sound in OP video even several magnitude more rpm than designed, this indicates well balanced rotor.
Nature: Evolution and the Illusion of Randomness (Stephen L. Talbott): Its now indisputable that... organisms “expertise” contextualizes its genome, and its nonsense to say that these powers are under the control of the genome being contextualized - Barbara McClintock
 

Offline duak

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Re: Making an induction motor go crazy fast.
« Reply #22 on: August 30, 2019, 04:54:28 pm »
In some BLDCs the rotor magnets are held in place with adhesive only - there's no radial restraining ring or framework.  These have a definite RPM limit.

I help out a friend who owns a CNC machine shop.  A couple of his machines have high speed spindles that run at 12 K and 20 K RPM instead of the more common 6 K to 8 K RPM.  These are induction motors rated for 10 to 15 HP continuous and usually have chillers to keep their cool.  I've never seen what's inside and don't know how they're beefed up for high RPM.

The spindles on these machines are something else.  They have matched pairs of ball bearings about 125 mm OD with 75 mm ID to clear the tool clamp assembly.  The oil is super thin and is not much more viscous than kerosene or diesel fuel.
 

Online Berni

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Re: Making an induction motor go crazy fast.
« Reply #23 on: August 30, 2019, 06:43:03 pm »
I would certainly love to see it spun to violent destruction just to see what it is like.

But its possible that the failure ends up being less exiting in the form of burned up windings. The resistance the fan creates goes up exponentially so the amount of power needed to spin it even faster keeps going up faster and faster. At the same time core losses get bigger and bigger as frequency is raised, causing the efficiency to begin dropping. So the required power to get it going even faster might eventually be too much for the windings and cause them to overheat.

Tho catastrophic mechanical failures are a known hazard with externally exited brushed motors where loosing excitation causes them to run away and spin faster and faster until tearing themselves apart.
 

Offline jmelson

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Re: Making an induction motor go crazy fast.
« Reply #24 on: August 30, 2019, 06:50:17 pm »
^^ Thank you for that.
I hadn't actually considered that possibility.  :phew:
That experiment is now terminated.
A guy at a flight instrument test shop was killed some years ago when he accidentally connected a 12 V aircraft gyro to 24 V DC.  It exploded and pieces hit him in the neck.
So, it CAN happen.

Jon
 


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