RUN THREE PHASE MOTOR LIKE CAR ALTERNATOR - VARIABLE RPM - VARIABLE LOAD 1

[MRSSPOCK]

Hi.

Can anyone tell me is there a way that I can rig up a three phase motor in such a way that it can be used in a similar fashion to a car alternator.

The whole idea is to use the motor to absorb the output from a internal combustion engine, in a home made dynamometer set up.

I have seen a few projects on youtube where people have used a three phase motor as a generator, attached to either a petrol engine or another driving motor, but it appears that putting a heavy load onto the "generator" just stops it generating.

I intend putting quite a heavy load on it as I intend dragging a 50kW engine to virtually its stalling speed, so in that regard, the "generator / absorber / brake" would actually be being run over a range of RPM, unlike the youtube projects or other projects I have seen, where people appear to be aiming to run the generator at a specific RPM, so as to provide a stable supply. I have no need to stabilise the supply, since I just want to dump it off somehow, via some form of heating element I presume.

That being the case, I suppose unlike a car alternator, I don't really have any need to regulate the voltage output.

I don't really know very much at all about electric motors, but I was hoping that a predetermined load could be applied, so that as engine RPM increases, hopefully the load on the "generator" would increase, as the generator output increases, thereby creating its own terminal angular velocity so to speak.

Having a series of predetermined loads, (found via trial and error), might then provide various terminal angular velocities, enabling various engine RPM performance to be analysed.

Would there a be a way to do that practically or am I just being ridiculous, expecting a novice can achieve that?

I have a 75kW three phase motor at my disposal.

Thanks

 

[Skogsgurra]

This is being done all the time. I have run all sorts of dynos from little ten kW units to large MW+ brakes for Scania and Volvo Penta. The unwind reel on a winder works the same way. Braking the sheet to keep web tension up and delivers the brak energy to the grid. Trains that brake (modern electrical ones) do the same thing.

And guess what? What is a wind turbine? Yes - a brake that delivers energy to the grid. Mature technology and, as I said, used everywhere, all the time.

Connect the motor to a VFD with regeneration and just do it!

OK, you may need to read the manual. You may need to understand a little bit more about induction motors and frequency inverters. But it is doable and there is not an incredible lot of learing to do.

Probably lots of stuff on the net. GIYF.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[MRSSPOCK]

Thanks Skogsgurra.

It's encouraging you say that it's easy, but forgive my ignorance.

I have been Googling the last five hours but I can't see any real examples of such, where I might get a clue where to start.

Could you please explain what a "unwind reel" or even a "winder" is.

I had a look on Google and I can't even figure out what that is.

Or "braking the sheet to keep web tension up".

That went straight over my head too.

Thanks

 

[ScottyUK]

Google regenerative dynamometer, because that is basically what you are building. You might need the distribution system owner's agreement to connect a large regenerative load - effectively a generator - onto their network.

 

[Skogsgurra]

Sorry to confuse you. An example (only partly shown) of a winder is here:

https://encrypted-tbn1.gstatic.com/...ozjxHpbMXxA9Nv6D-Rsk95UJH_6QPbDVKdRvlPi7pSvEL

The paper wep is going from the mother reel seen to the right, taken over a few guide rolls and spread rolls and divided into narrower sheets just before the operator's knees.

The paper speed is often 1500 to 2000 m/min and the web tension (the force in the paper) needs to be controlled within a few percents or the slitters (the round knives) will not work properly and result in paper salad instead of neat rolls for the printing press.

The brake motor is usually in the 200 - 500 kW range.

But, mind you, this has nothing with a dyno to do. It just shows another application of a regenerative brake.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[itsmoked]

MRSSPOCK; The motor you have needs to be "excited" which means power has to be provided to it, so magnetic fields can be formed, so IT can then generate. That's why the yTubers have problems, they want their motor to generate without supplying it some electrical energy to support its needs.

A VFD (Variable Frequency Drive) is a device to control the speed of an induction motor - like yours. If you get the correct style it will drive your motor flawlessly and conversely let your motor generate to its heart's content, clear down to speeds below your engine's operating speed.

Now, where "regenerative" models of VFDs come in is they can drive the power they receive from the motor back into the power supply the drive is hooked to. This saves you great gobs of energy money if you care. It also can cause some difficulties if your supply can't accept that much power. If you are going to be a "dyno shop" this is the only logical way to proceed. If you aren't going to be a dyno shop then skip the "regenerative" aspect entirely as it doubles the cost of the VFD.

Instead, use a standard VFD instead. A standard VFD operates exactly the same way as a regenerative VFD except it can't send the excess back to the power supply. It shoves the excess into a capacitor bank inside itself. The capacitor bank is fine for most cases when the motor briefly 'generates' like when it's told to come-to-a-stop. In your case you would be continuously driving energy into the VFD that will fill the capacitors completely in a second or two. The result would be the VFD tripping on overvoltage and losing all control. To prevent that, standard drives allow you to hook a specific large power resistor to its two "Brake Terminals". The VFD then shunts current thru the resistor to keep the level of charge on its internal capacitor below tripping voltage. You just need to keep the resistor cool.

So the VFD will provide the motor's needed excitation while allowing you to adjust the load the generator sees, and dumping the received energy into a big dc resistor. Solves all your electrical dydno issues.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

Keith, a a 50kW engine will need a a 50kW resistor, probably more ("dragging a 50kW engine to virtually its stalling speed"). Such a resistor will probably cost about the same, or more, than the upgrade from uncontrolled to controlled mains rectifier with regeneration. It will also be problematic to cool it if the test shall be going on for more than a few minutes. Especially in the summer. Cooling is a lot more expensive than heating.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[itsmoked]

There is that. It certainly depends on the test profile. I have lots of bar resistors I just toss into a pail of water. That works well for a few multi-second loads, not so well for anything longer, unless I add a running hose to the pail.

If one just wants to know the horsepower an engine can put out you only need a few seconds per test. If one wants to test the engine and its subsystems, like its cooling system, then you'd surely want the regen.

I suspect you could find non-regen VFDs all day on Ebay. Probably not so with regens.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

Of course, you could skip the resistors altogether and use the water as a braking resistor. That is being done even today. Saw huge ones to start ore crushers in Kiruna iron ore mine not so long time ago.

That worked quite well since the current was AC. For DC, you will get gassing and depending on what you add to increase conductivity (must do, pure water doesn't work so well) you will start producing H2 and NaOH (if you use salt) or H2 and O2 (I think) if you use sulphuric acid. Huge amounts, that is very prone to explode if you don't keep them separate.

I vote for a regen VFD.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[itsmoked]

Wouldn't you get chlorine gas out of the saltwater? eeeuu.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[crshears]

No free Cl2 to the best of my knowledge when it's used on AC...

A plant I worked in where decoupled 200 MW generators could be run as sychronous condensers had a 'pony' six-pole-set wound rotor induction motor connected to a five-pole-set generator for run-up purposes [coupled through the isolated phase bus before the units were synchronized]. The wound-rotor motor current was controlled using a salt-water resistor; even after performing two run-ups I never smelled a whiff of chlorine from the box.

CR

 

[itsmoked]

Yeah, agreed with AC but was thinking DC. It's interesting even on the AC that the ions seem to take so long to form that they can't. I'd think the reaction would be way faster than 8ms or so.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

I've boiled off hundreds of gallons of water per day doing run in loading on a gen set for a week. I used plastic barrels and truck leaf springs as electrodes. Never a smell of anything. Just lots of steam and water vapour. On start up a used a clamp meter and shoveled in salt until I had the current that I wanted.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Yes, Keith. Chlorine if DC. Can be vented away. The NaOH stays in the water and gets quite nasty.

This brings up an entirely different topic, that of electrolysis in AC situations. The 8 ms mentioned by Keith (or 8.333..) causes electrolysis products to be lifted off the surface during one half-cycle and then deposited back during the next half-cycle. So, not much net electrolysis.

But make the liquid (water, mostly) stream and you get an entirely different situation. I had sludge pumps, driven by VFDs, whose shaft diameters were reduced to half (and then broke) by leakage currents from the VFD. The general wisdom said that there shouldn't be any electrolysis because of the AC. But the streaming water did it. BTW, they were made from titanium just to prevent this. Didn't help.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[MRSSPOCK]

Thanks for all your replies.

@ itsmoked : Especially the very clear explanation. Thanks

Now it starts to make some sense.

This might seems a simple question, but can a VFD work with either an induction motor or an asynchronous motor?

I suppose I still need a VFD that is rated to deal with a 75kW motor, even although it isn't supplying the motor as such, but rather dumping the generated power?

Could you give me an example where such a big heat dump resistor might be employed?

I mean, where could I buy such a thing.

This will just be a hobby dyno, some the VFD regenerative option would really be a bit over the top in this instance.

Thank

 

[ScottyUK]

Cressall Resistors make loadbanks in the multi-megawatt range. 75kW is a baby. Mosebach are another manufacturer.

 

[Skogsgurra]

Your question "can a VFD work with either an induction motor or an asynchronous motor?" has a very simple answer: Yes.

The reason is that an induction motor IS an asynchronous motor - it is usually called an ASynchrouous Induction Motor, sometimes with the acronym ASIM.


Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[MRSSPOCK]

Thankyou for your replies.

Forgive my ignorance since I'm learning this stuff from a high school book.

In that book it says that the asynchronous motor is synchronized with the input frequency, whereas an induction motor isn't i.e. due to the slip component I presume, i.e. the RPM never equalling the number of poles x the input frequency directly, but differing slightly.

The book I'm referring to is from the stone age, so is it possible that back then there was no such thing as ASIM?

I suppose what I'm asking is, is there such a thing as an induction motor which is ASIM, or was that only true many years ago?

Thanks

 

[MRSSPOCK]

sorry: correction, last post should have read..

I suppose what I'm asking is, is there such a thing as an induction motor which is NOT ASIM, or was that only true many years ago?

Thanks

 

[ScottyUK]

Are you mis-reading asynchronous for synchronous? The speed of the latter is determined by supply frequency. There are a few different types of asynchronous motors other than the squirrel-cage induction motor although they are all pretty much obsolete or niche market nowadays. For example you will still find wound-rotor induction motors in service where their specific characteristics are required, but they are not commonplace.