Thread: single to 3 phase vsd confusion.

Originally Posted by RayG

Here's a question, what do you understand by the term VA rating?, and how do you calculate it?

Regards
Ray

Ray

On the subject of electricity, Vars are even more of an intangible. I descibe vars as "wattless power." It does explain why, for example, a 3KVA generator will only run a motor of approx 2500watts.

I found this link which might help:

Power Calculator for Generators: Convert kVA to kW, kW to kVA, kW to HP

Regards
Paul

Originally Posted by Dave J

Going by what I see on the forum I do find you get over involved, being condescending to members and you like to push your point across and have the last say.

Pete F,
Read this and absorb it. It's spot on.
Chris

David I have no idea as to what you're talking about, I do not think the whole world is out to get me If you read back through this thread you'll see that the sarcasm and generous use of eye rolling smilies did not come from me. Unremarkably I don't appreciate that, nevertheless as far as I was concerned the matter was over and I had moved on, it was only when a "for the record ..." comment was posted that the whole matter was raised again. That comment did not come from me. So how in God's name you can now blame me for killing a thread is a complete mystery.

Phil, you're quite correct, and no thus far I have not had the pleasure of meeting Ray in person, but I am sure that it will indeed be a pleasure when I do. As far as any tirade, nothing could be further from the truth, I posted a lengthy explanation of what happened here as I saw it and what my background in the field may be in the hope that it would stop this going any further. Sadly these things tend to get rolled over into other threads and in my experience that is when real problems begin to emerge on boards. I believed it was best to "clear the air" as it were and get it over with, so that others may move on too. The comments leading up to it by others were unhelpful given the circumstances to say the least, and the next thing I could see was the moderators having to take action. You're absolutely right though Phil, every action does indeed generate a reaction, hence why I pointed out the initial "action" at post #26. I don't know where that came from nor the reason for it, only that it, and the generous use of the eye-rolling "smilie" was not appreciated. Other than the correction to the statement made, a correction that despite stating precisely why it is incorrect and the dangers in doing so, does not seem to be accepted, I have absolutely no desire to try to take anything away from Ray's knowledge in this field and take my hat off to the time he also spends trying to help others.

Now, if everyone has finished with their opinion of my writing style and my clearly excessive personality flaws can we move on. Anyone else ... Beuller ... Beuller ... anyone? Perhaps some may also do a bit of self-reflection of their own and consider when they may feel a well worded PM is often a better option for all concerned!

Paul, you make some extremely good points in your posts, as they were spot on. Motors are very complex. You have some good questions and I hope you get them answered to your satisfaction.

Pete

Originally Posted by Pete F

Sorry Stuart, quite correct. Obviously I was speaking metaphorically to mean those who actually know something about the topic. I take it you'd like me to remove your name from the "rest of the world" list then?

No I meant it was a little over the top that you assume the rest of the world "those who actually know something about the topic" agree with you.

Originally Posted by Pete F

I actually penned a lengthy PM to Ray to try to get to the bottom of it, however given the continuing tone of the responses simply thought "suit yourself" and deleted it.

I also wrote a lengthy reply but realised all my points are covered in this thread for anyone that would like to take the time to go back, read and understand all the posts in order.

Ok now that that is over with onto Paul,

Bearing in mind that these answers don't necessarily apply to running a 5hp on single phase, but I wasn't sure if you meant the questions to or not.

"Can that high starting draw be mitigated by the VSD?" Yes, the ramp time(on the huanyang inverter I have) can be set to 6500 seconds. How much this effects current I havent measured but it would be less than DOL starting(but if you were talking single phase thats not going to happen anyway).

"Would that preculde the option of using a VSD to run it off a 1ph supply?" No, But you would remove the star start contactor, timer etc, wire the motor in delta and let the VSD take care of the starting.

"Lastly, my impression is that VSDs are effective down to about half the normal speed (frequency) only." ummmm yes and no, it depends what you are tyring to do. Vector drive VSDs will run the motor at full torque down to and at 0 rpm. Other VSDs wont do as well. But if you only want enough power to jog the machine over slowly while setting up they may well do the job. Setting the machine up so you run the motor over speed maybe a better option if you are worried about having full Hp at all times(for 1440rpm motors).

Stuart

Hi All,

Let me start by apologizing to Pete if he thought I was being condescending, I admit to being startled (several times) by his assertion that ohms law didn't apply... it's the fundamental equation for all circuit analysis.. motors included...

The question is what happens if you have a lathe with a 415 V 3 phase star connected 5.5hp motor (4kw) and you connect it to a 240V 3 phase supply..

Look at the motor name plate and for 415V it will be something around 7 amps or so...

Remembering Ohms Law, Motor Z = 415/7 = 240/x, so x = 7*240/415 = 4 amps

The power will have dropped substantially. But the lathe will run, and you won't trip the circuit breakers.. For some situations that might be a suitable solution.

If you rewired the motor as delta, then you will get full power again.

Let me go back to basics, and try to describe how an AC induction motor works, (I'll probably end up confusing everybody... ) I'll try and do this without reference to field equations, and stick with just text..

The stator is wound in such a way as to produce a rotating magnetic field, the speed that this field rotates depends on the number of poles and the frequency of the AC signal... Now we put a rotor inside the stator, the rotor consists of low impedance single turn coils, which acts a bit like a transformer, (at zero rpm at least) the current in the rotor coils generates a magnetic field and the rotor turns driven by the stator fields.

If the rotor is turning at exactly the same angular velocity as the stator field, then the slip is zero, the rotor frequency is also zero, since the rotor coils aren't cutting the flux lines of the stator field, so torque is zero.

Now we apply a mechanical load to the motor shaft, the shaft slows slightly and the rotor frequency increases, (because the rotor coils are cutting the stator flux lines more) and torque increases... so the torque in an ac motor automatically increases as the load is applied..

Now what happens when the rotor is stationary, like on power up, the rotor frequency is the same as the line frequency and slip will be 100% and current will be maximum, could be 5 or 6 times full load amps, you also get a similar situation when you stall the motor and the rotor is not turning, you might see this as locked rotor amps in the data sheets..

So how do we avoid tripping the circuit breaker on power up?
If we are running on a vfd, you can program the acceleration ramp to start at lower frequency, and for VFD's that means lower voltage, so the rotor starts turning and slip is reduced, so that you avoid the big inrush.. Soft starters work much the same way.
Starting torque is also less. But you can't have everything..

On the subject of "wattless power" motor impedances are complex, (that is complex in the sense of complex numbers), there is a resistive component and a reactive component, the wattless power refers to the reactive component of the motor Z.

I managed to dig out a fair enough sort of article on Induction Motor Theory, the maths is not too heavy, and I'd like to point to the use of Ohms law throughout the circuit analysis..

http://www.etech.net.au/InductionMotorTheory.pdf

Apologies if this is coming across like a lecture, and for those who already know this stuff, perhaps you can correct any dumb screw ups I might have made.


Regards
Ray

PS A belated thanks for all the messages of support, I feel like I don't deserve it.. but it's nice to know.

Hi All,

Just in case some might think PeteF has disappeared for some other reason, I received a PM from him to the effect that he was off to Singapore.

The disagreement between Pete and myself, eventually came down to what is the definition of ohm's law.

Pete's position is that ohm's law only applies to resistance, that is V=IR, my position is that you use the same formula substituting Impedance Z for resistance R and that, of course is the correct approach and is universal practice in circuit analysis, can you still call it "ohms law"?, I know that, it's a widespread practice and a lot of people do. Right or wrong the equation is the same, whatever name you choose to call it..

So that's that finally sorted and this marks the end of the disagreement, perhaps I could have been a bit more diplomatic ... maybe next time.

When I see disagreements on other forums, it often degenerates quickly out of control into personal abuse and name calling, threads get locked and deleted, none of that sort of stuff seems to happen here (well not much), which is good to see. I guess that means the moderators have trained us well.

Regards
Ray

Phew...good on both of youse.

Originally Posted by RayG

Hi All,


So that's that finally sorted and this marks the end of the disagreement, perhaps I could have been a bit more diplomatic ... maybe next time.

When I see disagreements on other forums, it often degenerates quickly out of control into personal abuse and name calling, threads get locked and deleted, none of that sort of stuff seems to happen here (well not much), which is good to see. I guess that means the moderators have trained us well.

Regards
Ray

I'm sure that we are all pleased to see it all put behind us, It behoves us all to be temperate and moderate in our choice of words, it can be much easier to misinterpret the written word as we have none of the cues present in spoken language to guide us in determining the intent of the writer. Some too can be more sensitive to perceived slights, when maybe none were intended, so we need to be extra careful to avoid statements which can have other interpretations, or where the ''force'' of a word is difficult to know, because some will take the best interpretation while others see it the worst possible light.

To return to the OPs issue, that of running 4Kw motor off single phase, with a V.F.D. I imagine that it could be done, that is it could be done without too much trouble and extra expense, providing that there are no complicating factors, such as 2 speed motors, and for an out of the box solution, it will be the easiest and probably cheapest to implement. However if there are complicating factors like 2 speed motors, or the need to switch motors off downstream from the VFD, there may be an argument for a Rotary Phase Converter.
These can be home brewed quite cheaply if you are a good scrounger, and have good metal bashing skills and facilities. As the RPC is generally started first without a load, inrush currents are less, and could be reduced further by utilising a (small fractional H.P, single phase) pony motor to get the RPC up to speed, and then starting your 3 phase motor from that. Commercial off the shelf solutions are out there for RPCs, but they are pretty expensive. For those without an electrical background of any sort, RayG has posted links on this and other threads which have good info to help get you up to speed, and I would recommend Practical Machinist forums which have a sub forum dedicated to this topic, and 2 useful books are Three-Phase Conversion, by Graham Astbury, No.47 in The Workshop Practice Series (available from The Book Depository for under eleven dollars AU and post free in about 5 days) and The A.R.R.L. Handbook , for Amateur Radio Operators - this tome is updated annually and a 20year old copy will be just as informative as the latest edition, but instead of costing over $100.00, you can pick up a second hand copy from the likes of Amazon or Abe Books for about $6.00 and about the same for postage. Besides the electrical theory which is covered in reasonably simple plain language without too much complex mathematics, there is a good section on rewinding transformers for anyone wanting to generate their own 415 V 3 phase. Just remember to get your theory checked by a suitably qualified person, before you build it and then get your handiwork checked after you build but before you power it up, if you are working outside your qualifications. Good luck whichever way you decide to go,
Rob.

Originally Posted by Ropetangler

To return to the OPs issue, that of running 4Kw motor off single phase, with a V.F.D. I imagine that it could be done

Remember he only has a 15amp supply.

Stuart

Thanks for pointing that out Stuart, I had read it but it had slipped my feeble old brain. With a 15A supply there is perhaps a chance it could be run, but it certainly won't give anything like full power, and even to start it would be problematical. For hard to start loads as I understand it you would want something like a 100 A supply for a DOL starter, but for light load starts, you would get away with much less, particularly if a soft start is an option.
Rob.

gave up on the idea of running a 4kw motor off single phase.
would a 2.4 kw 3 phase lathe be o.k to run off a VSD with a 15 amp power supply.

if it is,any recommendations for a reliable make-model vsd ?

Originally Posted by lather

gave up on the idea of running a 4kw motor off single phase.
would a 2.4 kw 3 phase lathe be o.k to run off a VSD with a 15 amp power supply.

if it is,any recommendations for a reliable make-model vsd ?

eBay Australia: Buy new & used fashion, electronics & home d

Looks like no worries to me, that Huanyang I've linked suggests it will pull 13A to run a 3kw motor. Their quoted current draws in the listing sounds about right, from memory my 2.2kw Huanyang on the mill pulls around 7-8 amps under full load (about 4A just spinning the spindle over without a cut) running a 1.5kw motor. And as far as a reliable make, there are a lot of guys on here running these Huanyangs (because they're so bloody cheap!), and I don't think anyone has managed to let the smoke out of one yet?

And even if you do let the smoke out of one, you can probably buy another to replace it, and still be ahead over buying a 'known' brand! Of course, if you're planning to use the VSD to actually vary the speed of the lathe significantly, you might want one of them fancy fandangled vector drives to preserve torque at low speeds, but I get the impression you simply want the VSD for its 'phase converter' abilities...

Thanks, will look into it.
won't be needing to reduce the speed with the vsd, the slowest speed on the lathe is 50 rpm.

Lather
Haven't you got a new lathe yet,
I bet your getting machining withdrawals going without a lathe for this long. Any longer and you may need to spend money on counseling, LOL

Looking forward to seeing what lathe you get.

Dave