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

 

[itsmoked]

I agree with Scotty. I bet you read an "A" that wasn't there. Because synchronous motors ARE just that "synchronized".

And, you should only need a VFD capable of handling the 35kW of your engine. Wait for someone to confirm that since the motor you have will need more excitation current than a 35kW would.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

It is possible to select either synchronous or asynchronous operation mode from several modern VFDs. ABB and Siemens can do both. There are also SRM modes available in some (that new reluctance animal that is being introduced these days).

Synchronous motors with permanent magnets (PM in the designation) are getting more and more popular because they are (slightly) more efficient than asynch motors.

You should really make up your mind and check what kind of motor you are going to use before proceeding. If asynch, then just about any VFD will work. If synch - you will have to sort out the ones that can do it. Ask the salesperson. But it won't do to be vague. Must know first.

Gunnar Englund

http://www.gke.org

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

 

[MRSSPOCK]

You're A-bsolutely correct.

I added an "A" that wasn't there!

The best about it, I already know that a preceding A in Greek inverts the meaning.

A-muse literally means to NOT think - That's what I was doing when I read A-synchronous in place of synchronous!

I already have the motor, but the spec' plate has had all the details worn off, so I don't know what it is yet.

I know someone who does, but haven't got hold of him yet.

It's off a crusher.

Thanks for your patience.

 

[Skogsgurra]

Sorry not to trust you 100 percent on this one.

It is extremely unusual to see such a relatively small synchronous motor on a crusher. ARE YOU SURE that it is synchronous?

If it is, you either have slip rings, which you do not seem to have (no place for them on the NDE) or it is a PM motor, which I do not think that it is. So, the most probable is that it is an asynchronous motor.

If the nameplate is so worn that you cannot even read the speed or type of motor, then you are in deep water. If this really matters to you, and if you are not trying to get some weird entertainment out of this conversation, you should get the facts first. Then, we can give you the answers. But this kind of discussion is not at all what we (at least me) find amusing. Or, with your ethymology, it is actually what it is.

Gunnar Englund

http://www.gke.org

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

 

[MRSSPOCK]

@skogsgurra: I don't believe I said it was one or the other. I have no idea as yet, though from your comments it seems pretty certain it must be ASYNCHRONOUS.

I have to say, needing 35kW to excite the thing is beginning to put me off this solution.

Seems I can't use it unless I have a three phase installation, which makes it a bit impractical.

I'm not amused either since I learned that :-(

 

[Skogsgurra]

You have said this or that several times. Your last words (on 3 Mar 14 18:48) on this were "I read A-synchronous in place of synchronous!"

Now, the Magic: You do not need a three phase grid to run a VFD. But if you need more power than around 5 or 10 kW, then a single-phase grid may be a problem.

Gunnar Englund

http://www.gke.org

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

 

[MRSSPOCK]

Sorry, I just meant I read it in a text book, not on the motor.

 

[itsmoked]

Since you are only going to use the 'grid' to 'excite' your motor you will only need a few hundred watts from it. The engine power will need be 'disposed of' by the resistor.

The resistor will need to be very robust as previously mentioned. Water cooling will get you a long ways. You might be able to use something like a 1kW or 2kW resistor instead of a 35kW resistor by simply dropping it in a plastic pail of running freshwater.

The key is keeping a twitchy hand on the engine throttle because if the resistor opens => the internal VFD capacitor would fully charge in probably 200ms => the drive will trip (essentially disappearing electrically) => the the engine would suddenly be completely unloaded => would seriously over-speed => possibly coming apart.

Dynoing engines is seriously dangerous. Go to YouTube and see the myriad disasters. This is why they are done in "test cells" without humans being immediately present.

Dyno tuning gone bad

Keith Cress
kcress -

http://www.flaminsystems.com

 

[MRSSPOCK]

Sorry, I misinterpreted your comment,

"the motor you have will need more excitation current than a 35kW would."

I thought you meant I needed 35kW to get it excited!

Maybe I should stay clear of such a powerful 3 phase project if I don't know more fully what's going on, lest I become the smoking resistor.....

 

[itsmoked]

Like a lot of tasks your project is not very hard or complicated to people who understand the systems and their operations. Undertaking all of it with no working knowledge of even the subsystems can certainly be daunting.

Caution is the better part of valor. No one here, in this forum for professionals, will be trying to talk you into what's actually a pretty complex project fraught with various, um, shall we say, energetic problems.

Getting out-of-the-box might help you to a solution you haven't contemplated. Why do you want to dyno an oddly middle-of-the-range engine in the first place? That's like an industrial rated 4 cylinder or a steroided 2 cylinder. Automotive 4 cyls put out way more than 50hp these days. Will it be an on-going regular occurrence? Is it a critical need? Just entertainment? Compliance testing? Give us more info and we might be able to suggest alternative solutions to your situation.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

May I throw in a question?
Given the close relationship between motor torque and motor current, should the VFD be sized for maximum torque rather than HP?
I wonder what the VFD sizing should be based on at times when the motor is regenerating at greater than rated speed?

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

 

[Skogsgurra]

Then you have another set of options.
If you just run, you are in constant power territory. Maximum available braking torque then falls after you have reached rated speed (because of the constant voltage and hence falling V/Hz).
You can move that speed up from 1800 or 3600 RPM to around 3120/6240 RPM by connecting in Delta instead of Star. Or you can throw in a transmission and have any speed range you desire.

Gunnar Englund

http://www.gke.org

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

 

[mikekilroy]

You have lots of good tech data on how to do what you want. So thought I would add ballpark prices for reference.

A 35kw continuous rated regen resistor will come in at about 3' x 3' x 5' in size. I have one our guys speced wrong so know Takes up a lot of shelf space collecting dust....

Since we supply typically 10-20kw resistors with our large spindle drive systems, I would swag for you the price for a new 35kw is around $ 3000.00 for an air cooled one (no fan, no water cooling).

And for reference, a new vfd to do this is typically around $ 7,000-10,000.

http://www.KilroyWasHere<dot>com