3ph delta star switch, delta tripping fuse

[T0RN4D0]

Hi, im new here. Looks like a pretty active forum so i'm hoping someone can help me maybe.

Here's my problem. Neighbor just bought a used circular saw with a 10kW 3ph motor on it and asked me to rewire it, because the cable from the started switch to the motor box was all messed up.

Anyways, the delta star switching is done by a simple 16pole switch. First time around i marked all the connections and just duplicated them with the new wire. It worked in star, but the fuse tripped when i switched to delta. I then searched for the switch schematics, took everything apart and wired it according to that scheme. Still, same thing. He only had a 16A fuse, so we changed it to a 25A, didn't help.

I marked where i wired everything and tripple checked it, i think its right. I think i have a problem with phase sequence somewhere, but i'm not sure where and which wires to change.

Any ideas what could be wrong, and which wires i should change to make it work? The input phase sequence (L1, L2, L3) doesn't matter, or does it in this case?

 

[THE DUKE]

Have a look here Motor Starting - Star Delta

 

[T0RN4D0]

I went through that, and everything else i could find related to my problem.
Solved it today. Took everything apart, rewired, checked _everything_, the more i looked at it, the more i was sure everything was OK.

Then it hit me, and i let it run in star for about 15 seconds before switching to delta, and it worked. There was nothing wrong with the wiring in the first place, we just switched it too early every time.

 

[Darkwood]

Im a tad confused 10kw isn't a regular size are you sure irts not 10HP?
I assume you 16pole switch is not pole but Amp?
What is the fusing device used? Is it a mcb (60898)?

If its tripping on changeover from star to delta then it could be a few things but from what you have described i suspect you fusing device is tripping on the changeover spike from star to delta because it is undersized... to give you an idea

- A mcb for a 11kw motor either DOL or star/delta would need to be 63amp (B or C) or a 25amp(d) to ensure the mcb wont trip, this applies to a standard induction motor with a normal start up duty... next size down rated mcb's may hold but tripping may occur occasionally.

This is on the assumption that you have wired it correctly and the motor is ok.

Have you tested the motor prior to energising?


Of course upgrading any fusing device means you may have to calculate if the supply cable, volts drop, zs values all comply before energising the motor but of course you will know this anyway. To note your local regulations need to be complied to and we can't give assistance on here as we are based around the British Standards of BS7671 (amd2).

 

[Darkwood]

Incoming phase sequence will only change the rotation of the motor which could be dangerous as the blade may unscrew and fly off - always be cautious to power saw up first time - either be ready on the stop button or remove the blade until certain.


What are you a student in?

You're undertaking work here you shouldn't be without a qualified Electrician / Engineer to guide you and certainly shouldn't be trying to do this off a forum ... how will you test the supply etc meets disconnection times under fault conditions etc etc this should not be attempted as a DIY project..... im not sure how health and safety work in Slovenia bt you wouldn't be allowed to do this over here without a qualified competent person over seeing you .... i doubt this is the case as you are asking us the questions.

 

[Darkwood]

Any chance of an update ?

 

[onions1066]

Check post #3 darkwood

 

[Darkwood]

How did that sneak in there lol totally missed it... 15seconds sounds very lengthy i suspect it will only have needed a small adjustment but its not unheard off. I have larger systems with run up times half that he's just stated.

 

[T0RN4D0]

Hi, thanks for the replies. I'm not sure what responses you see and when because i'm new and my posts have to be authorized by the mods.

Im sure its 10kW. It may not be common but that doesn't mean they don't exist. (Or the plate is false). I'd attach a picture but i'm not sure i can. And the switch has 16 connections, and i messed up, it's an 8 pole switch. Still struggling with the terms.

The fusing device is a MCB, 3x25A, C rated. Looks like it really should be D rated to cope with the starting surge. It works, but you have to wait for a good 10 seconds before flipping to delta. Seems "a bit" long for a small load, but it works. I don't think there's anything wrong with it and i get over 400V on the motor clamps.

I'm in middle school for energetics.

 

[Darkwood]

It just seems excessive run up time.. is the saw blade driven through gears and or V-belts or coupled direct... also what pole is the motor (speed).

 

[T0RN4D0]

Double V belt. The motor tab says 2940 rmp, if im reading it correctly, you know how they are on old motors. How much drag is a mottor like that supposed to have on its own? I'm suspecting the bearings on the table (the shaft coupling the belts and the blade) need to be changed. The whole thing stops pretty quick once shut off, and it has decent drag if you turn it by hand.

I'm thinking high drag could be causing it to pull more power when starting, but the 10kw motor is an overkill for the blade its using right now (30ish cm blade) so it still works normally.

 

[Darkwood]

If there is no obvious braking you could try drop the belts off and spin the motor pulley by hand then also try the blade un-belted i suspect the star-delta timer was correct but blade bearings have probably corroded slightly most likely the blade bearing and also suspect its probably been in storage in a cold damp area for a while letting this bearing degrade if it was already aged or damaged.

 

[Deleted member 72666]

Unless I've got this totally wrong but It sounds to me from what you have said that it's a manual star delta starter(via this 8 pole switch you speak of) and the time is operator controlled, is this correct? If so do you have any means of under voltage protection? What else other than the switch and mcb's are in the control panel and Is this setup a 'home build'?
Like DW points out 10kw is an odd size motor and seems somewhat an overkill for that size of blade. Also there sounds to be too much drag on the motor bearings/shaft which is causing excessive current to be drawn.

 

[T0RN4D0]

Yes, there is no timer as its manually controlled. There isn't any motor protection, the whole setup is a home made unit. I think it might not be a bad idea to add one, but its not up to me. It won't run without operator presence so if there's something wrong it will be shut down manually.

Heres the tag, i see you have trouble believing me its a 10kw motor. I think a 7.5 would be ideal for this, but they probably used what they could get for the right price. The small blade was on it because it was just used (recently) for trimming slabs of wood. Now its probably gonna get a 60cm blade and you need some decent power to go through long cuts without too much hassle.



I'll suggest checking out and replacing the bearings.

 

[Knobhead]

Only just spotted this thread.

Manual Y∆ used to be common, we had them on 3.3KV Ward Leonard sets. It would be common for the operator to operate the change over before the run up had finished, tripping the quarry 3.3KV header main OCB on O/C.
Some did it deliberately so they had a longer tea break.
At change over if the poles are out of synchrony the peak current can be 10X FLC as opposed to 6X for DOL.

This is for a 300HP 550V MG set. (It was my pet).

DW, your comment on direction of a machine like this is very pertinent. The blade mandrel being designed to tighten with acceleration and load. I had quite an argument over a grind stone. The fitter had installed the stone before I’d direction checked the new motor. Production management were screaming about the time to take the stone off again. It went the wrong way, but I’d stuck to my guns. Had I not, a 3Cwt 2’6” diameter stone would have launched it’s self out of the machine.
We’ve talked about production management before. This time I was invited to watch the management arse kicking party, I was usually the guest of honor!

The bottom line for anyone.
Machines like saws, grind stones, etc. Check the direction before attaching the load.

The same goes for:
Centrifugal pumps.
Fans.
Compressors.
Some gearboxes have anti run back.
The list is endless.

Check the direction first. It will be your arse in the fire when you’ve just f**ked up £XXXX of kit.