Thread: Another VFD & 3 phase question

Originally Posted by Ueee

Hi guys,
Just like Simon I like to pull things apart just for a look! Can you explain for me why hitting an unpowered coil against the permanent magnets can de-magnitize them?

Hi Ewan,

I trust you are getting over your health scare.

+1 on the demagnitization question - this is news to me. Be interested to hear the answer.

Cheers

Rob

Hi Ewan,

I think you and I were in Hospital at the same time... I escaped today and made it past nurse Ratchett with only minor injuries.. I trust you are recovering ok.

I would have contributed a bit to this thread, but I think it's all been said already.

Except for the demagnetization question... years ago I used to build 3 axis machines with stepper motors, mostly for making carpet samples and a guitar making machine, early on we had lot's of trouble with stepper motors losing torque after being dissassembled, never found a satisfactory answer, but I can confirm that you lose magnetism just by dissassembling and re-assembling.. so we just stopped doing it and scrapped the ruined motors. I think a purpose built jig might solve the problem to stop things touching during assembly.

I'd be interested if anyone else has had similar experiences and what the explanation is?

Regards
Ray

Hi Ray,
I wonder if its something along the lines of dyno bike hubs and magnetic chucks?
And a Get well soon to you also.
Keep it up guys and we are going to need a pinned thread!

Stuart

Originally Posted by Ueee

Chris, thanks for the explanations of all things dc motors, especially the warning about disassembly. Just like Simon I like to pull things apart just for a look! Can you explain for me why hitting an unpowered coil against the permanent magnets can de-magnitize them?

It depends on the type of magnet material. Some motors need a "keeper" ring between rotor and stator for installation or removal of the rotor. Some other motors are safe to disassemble without a keeper ring. Trouble is, it is neaarly impossible to tell beforehand. Upmarket motors, for which a manual exists, will clearly state if a keeper ring or another special procedure is necessary. I did read somewhere that magnets with the same permeability as air do not need a keeper ring. Motors with Alnico magnets do for sure need a keeper - removing the armature of a motor with Alnico magnets will instantly degrade the magnets and halve the motor's power rating. The same happens if the keeper ring is removed from the stator before the rotor is re-inserted. Other magnets do not like the metal-to-metal contact when removing the armature.

Another problem, and I think this affects all kind of magnet materials, is they can get knocked hard by the armature when disassembled. You can de-magnetise a magnet by hitting it hard with a hammer. And some particulary brittle magnet materials can shatter. The best recommendation is: never take a PM motor apart unless you have a very good reson to risk it.

By the way, step motors are another type of DC motor. I can assure you that taking a step motor apart will render it completely useless. It will loose half of whatever torque it had after reassembling, and aas a consequence loose steps when accellerating/decellerating.

Interesting stuff Chris.

Modern high power magnets are obviously far more complex than most people realise.

Glad I didn't pull my treadmill motor apart. I had considered making up a new end plate to mount it with.

Cheers

Rob

Originally Posted by nearnexus

Interesting stuff Chris.

Glad I didn't pull my treadmill motor apart. I had considered making up a new end plate to mount it with.

Same! Usually this type of advice would have been useful to me YESTERDAY but lucky for me I have not opened any of my PM motors. It would have only been a matter of time though before curiosity got to me!

Don't clam up on us now Chris! What else have you got stored away in that grey matter worth sharing about motors??!

Simon

Well, OK after a little bit of wheeling and dealing I bought a VFD. After some declined offers and then some counter offers, I managed to get a massive $10 off! Still, better in my pocket. So the damage ended up at $149 delivered for a 2.2Kw 3HP VFD.

Can't wait to get it now.....

Simon

While everyone is talking VFDs,

I have a 4kW 2850 rpm motor on a sander that I would like to vary the speed on (i.e. down to about 50 to 75% of it's full speed).
Was thinking one of those Huanyang (3Ph to 3Ph) jobbies might do the trick.
I know a number of people have the smaller units - I also have a Huanyang 1.5 kW unit on a WW lathe, but does anyone have one of these bigger units?

Thanks

Hi Bob,

I use a 4kw Huanyang on the surface grinder spindle, works fine.

The only thing you should perhaps consider is fixed wiring, rather than a power point. ( same as a stove or large aircon )

Regards
Ray

Hi Ray,
Isnt Bob talking about a 415V 3ph to 415V 3ph VSD?

Stuart

p.s. Hope you are all better or still improving.

Originally Posted by Stustoys

Hi Ray,
Isnt Bob talking about a 415V 3ph to 415V 3ph VSD?

Stuart

Yep that's correct.

The sander already has wiring built into it all I would do is wiring the existing harness to the VSD and the hooking up VSD to motor.

Hi Bob,

Doesn't matter, Should be much the same quality as the 240V model, I'd guess.

Regards
Ray

thanks

My VFD arrived today! WOW! for a mug punter who has never seen one of these in real life they are certainly small for what they can do! There really is nothing of them. And I was worried about where I could fit it. Can just about go anywhere near the mill/lathe on the wall. I standard power lead would have been handy though!

Can't wait to have a play!

Simon