Making an induction motor go crazy fast. - Page 1
Forum section: electronics / projects

author: Circlotron
author_rank: Super Contributor
timestamp: August 29, 2019, 11:53:09 am
content: 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 speedhttps://youtu.be/nZZi1qC4d_cRundown to stophttps://youtu.be/SVhdQFP8hGA

author: Ian.M
author_rank: Super Contributor
timestamp: August 29, 2019, 01:01:00 pm
content: 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).

author: Circlotron
author_rank: Super Contributor
timestamp: August 29, 2019, 01:16:19 pm
content: ^^ Thank you for that.I hadn't actually considered that possibility.That experiment is now terminated.

author: Ian.M
author_rank: Super Contributor
timestamp: August 29, 2019, 01:47:57 pm
content: 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.

author: schmitt trigger
author_rank: Super Contributor
timestamp: August 29, 2019, 01:58:15 pm
content: 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.

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 29, 2019, 02:29:14 pm
content: 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..

author: Yansi
author_rank: Super Contributor
timestamp: August 29, 2019, 02:33:44 pm
content: Quote from: Circlotron on August 29, 2019, 11:53:09 ambut y uno 3phase is 120 dungarees, not 60?

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 29, 2019, 02:38:31 pm
content: 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.

author: Yansi
author_rank: Super Contributor
timestamp: August 29, 2019, 03:44:44 pm
content: Welcome to the world of VFDs. Since the 1980s or when...

author: max_torque
author_rank: Super Contributor
timestamp: August 29, 2019, 04:27:58 pm
content: 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..........

author: schmitt trigger
author_rank: Super Contributor
timestamp: August 29, 2019, 05:49:20 pm
content: 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.

author: Circlotron
author_rank: Super Contributor
timestamp: August 29, 2019, 09:46:25 pm
content: Quote from: Yansi on August 29, 2019, 02:33:44 pmI 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.

author: coppercone2
author_rank: Super Contributor
timestamp: August 29, 2019, 10:24:15 pm
content: Quote from: Yansi on August 29, 2019, 03:44:44 pmnothing 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$.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 ughAC motors are a massive trap IMO, and BLDC is not found in scrapfinds usuallyyou 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).

author: Circlotron
author_rank: Super Contributor
timestamp: August 30, 2019, 03:57:14 am
content: Quote from: Ian.M on August 29, 2019, 01:01:00 pmRotor is 70mm diameter and 60mm long. Shaft is 14mm. Out of an automatic sliding door like this one ->https://youtu.be/nKdHCjAEKuI?t=82

author: Miyuki
author_rank: Frequent Contributor
timestamp: August 30, 2019, 05:10:58 am
content: Induction motors are very robustI 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) / 7000rpmDont this small one have just plastic fan ?Also this one look like a compact design with internal fanBut agree some safety measures shall be applied

author: Circlotron
author_rank: Super Contributor
timestamp: August 30, 2019, 05:40:50 am
content: No plastic fan. The internal fan is part of the cast aluminium end rings on the rotor.

author: Ian.M
author_rank: Super Contributor
timestamp: August 30, 2019, 07:39:20 am
content: Quote from: Circlotron on August 30, 2019, 03:57:14 amI've recalculated for your actual rotor - at 16800 RPM that's 138 MPH, 11000 g, 1650 JOnce you've got containment, it needs more volts and more Hz.

author: Berni
author_rank: Super Contributor
timestamp: August 30, 2019, 08:27:58 am
content: 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.

author: Ian.M
author_rank: Super Contributor
timestamp: August 30, 2019, 09:03:57 am
content: 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.Quote from: Berni on August 30, 2019, 08:27:58 amYes. 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.

author: max_torque
author_rank: Super Contributor
timestamp: August 30, 2019, 12:15:14 pm
content: Generally the bearings will be well goosed before the rotor lets go!(as most induction motors have v cheap bearings in them)

author: schmitt trigger
author_rank: Super Contributor
timestamp: August 30, 2019, 02:14:06 pm
content: 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.

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 30, 2019, 03:19:19 pm
content: 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.

author: duak
author_rank: Super Contributor
timestamp: August 30, 2019, 04:54:28 pm
content: 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.

author: Berni
author_rank: Super Contributor
timestamp: August 30, 2019, 06:43:03 pm
content: 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.

author: jmelson
author_rank: Super Contributor
timestamp: August 30, 2019, 06:50:17 pm
content: Quote from: Circlotron on August 29, 2019, 01:16:19 pmA 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

author: Yansi
author_rank: Super Contributor
timestamp: August 30, 2019, 06:53:52 pm
content: Fan torque goes up square wit rpm.On thw other hand no problem removing it

author: Ian.M
author_rank: Super Contributor
timestamp: August 30, 2019, 08:03:36 pm
content: 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,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.

author: Yansi
author_rank: Super Contributor
timestamp: August 30, 2019, 08:19:46 pm
content: Big generators have the hydrogen inside the hollow copper bars used as the windings. Not in the entire machine.

author: Gyro
author_rank: Super Contributor
timestamp: August 30, 2019, 08:57:41 pm
content: 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.

author: Yansi
author_rank: Super Contributor
timestamp: August 31, 2019, 08:06:07 am
content: 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.

author: mzzj
author_rank: Super Contributor
timestamp: August 31, 2019, 08:21:09 am
content: Quote from: Ian.M on August 30, 2019, 07:39:20 amBog standard electric motors from most makes are rated to 4500 rpm up to IEC frame size 200. That is 50hp 300kg motor!

author: coppercone2
author_rank: Super Contributor
timestamp: August 31, 2019, 08:22:26 am
content: 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 vibrationgrinders?

author: Gyro
author_rank: Super Contributor
timestamp: August 31, 2019, 10:43:36 am
content: Quote from: Yansi on August 31, 2019, 08:06:07 amClosed circuit inside the enclosure too, otherwise you wouldn't get the windage benefit...https://en.wikipedia.org/wiki/Hydrogen-cooled_turbo_generatorhttps://www.power-eng.com/articles/print/volume-113/issue-6/features/hydrogen-cools-well-but-safety-is-crucial.htmlhttps://www.quora.com/Why-hydrogen-is-used-for-generator-coolinghttps://www.flownex.com/fnxdocs/casestudies/CS42_GeneratorSealOilSystem.pdfP.S. This is for interest, not spoiling for a fight.... Or suggesting squirting Hydrogen into an induction motor!

author: Yansi
author_rank: Super Contributor
timestamp: August 31, 2019, 12:43:44 pm
content: Probably, only in one of the largest generator of them all, as cited in the  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

author: Miyuki
author_rank: Frequent Contributor
timestamp: September 01, 2019, 10:01:12 am
content: Hi folks I have little question about induction motorsWhat limit maximum torque (breakdown torque) ? Is it just stator / rotor saturation ? Or have it other causes ?

author: Yansi
author_rank: Super Contributor
timestamp: September 01, 2019, 10:20:03 am
content: 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)...

author: Circlotron
author_rank: Super Contributor
timestamp: September 01, 2019, 11:03:59 am
content: 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.

author: Yansi
author_rank: Super Contributor
timestamp: September 01, 2019, 12:13:42 pm
content: 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.

author: Miyuki
author_rank: Frequent Contributor
timestamp: September 01, 2019, 12:35:02 pm
content: Even slip ring motor have always same TmaxJust curve change its shape

author: mzzj
author_rank: Super Contributor
timestamp: September 01, 2019, 01:35:53 pm
content: https://www.landbelectric.com/download-document/85-torque-characteristics-of-nema-abcd-e-motors.htmlIn many cases its desirable to limit the maximum starting torque(less mechanical strain) and peak currents.

author: max_torque
author_rank: Super Contributor
timestamp: September 01, 2019, 01:44:41 pm
content: 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........

author: Berni
author_rank: Super Contributor
timestamp: September 02, 2019, 05:15:38 am
content: Quote from: max_torque on September 01, 2019, 01:44:41 pmDang 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)

author: Circlotron
author_rank: Super Contributor
timestamp: September 02, 2019, 06:18:36 am
content: ^^ 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.

author: dom0
author_rank: Super Contributor
timestamp: September 02, 2019, 02:28:01 pm
content: Quote from: coppercone2 on August 29, 2019, 10:24:15 pmWelcome to Europe, where you can just plug it in.

author: dom0
author_rank: Super Contributor
timestamp: September 02, 2019, 02:33:12 pm
content: Quote from: Yansi on August 30, 2019, 06:53:52 pmFan is internal and is part of the dynamic balancing -> remove fan, bye bearings.

author: amyk
author_rank: Super Contributor
timestamp: September 03, 2019, 11:38:32 am
content: 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...)

author: Yansi
author_rank: Super Contributor
timestamp: September 03, 2019, 11:44:52 am
content: 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.

author: Circlotron
author_rank: Super Contributor
timestamp: September 03, 2019, 11:51:39 am
content: Yep, a sliding door motor.See page 3 -->https://www.kanonkapi.com/wp-content/uploads/2018/08/MANUSA-BRAVO-2.pdf

author: amyk
author_rank: Super Contributor
timestamp: September 05, 2019, 02:03:53 am
content: Yes, I'm just saying that such speeds are not unusual for induction motors of that size and construction.