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3 phase motor problems

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harris

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Many years ago I converted my Turnstyler wood lathe to a speed control version by purchasing a ready made electronic kit and changing the original 1ph motor for a 3 ph one. Last week I noticed that there were loud rumblings coming from the motor/drive belt area but couldn't pin it down. There are no obvious indications such as worn bearings etc. and the motor shaft and lathe drive shaft have no end or side to side play Yesterday when I was turning, the motor started playing up. It would suddenly stop for a second - maybe two and then run up to speed again. I checked the wiring for loose connections but there are none. From that vague description can any members suggest where to look next? There are one or two motor specialists in the area, is it worthwhile taking it in for a coat of looking at or is there more that I can do myself.
Fred.
 

9fingers

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If it is running up to speed gradually, this suggests that it is doing so under the control of the inverter.
Given that inverters are pretty reliable and if they fail it is not intermittently I would suspect the control switches.
Dust is the enemy of switches so give a good blow out with compressed air.

Bob
 

CHJ

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Expanding on Bobs control switch comments.
If it has a potentiometer for variable speed control maybe suspect worn or failed track, the carbon surface may have worn away causing intermittent contact.
Does the motor remain more stable if you move the speed controller to the extremes of its travel.
Recently replaced mine for a whole new smooth experience.
 

harris

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I had another look and listen today. I took the drive belt off the motor and just ran that. Same intermittent rumbling noise from the motor which seems to occur only when the speed is fluctuating but definitely not the bearings, it runs very quietly until that happens. The motor runs up instantly and stops instantly when the output from the inverter fails and is reinstated. When the output was varying, there was a clear "crackling" (best description I can find) almost like a sparking noise from the inverter. I took it out of the wooden cupboard and checked all connections tight and the heat sink on the back was showing no signs of getting hot. I can't get the potentiometer apart to check or spray the track or check the cable connections tight, it has been sealed by the manufacturer of the kit. So, it seems as if the inverter is defective or the connections in the potentiometer are loose. I will try a new pot and then assume the inverter is knacked if that fails to cure the problem. Can anyone suggest a better course of action or prove one way or the other that the inverter is faulty?
 

9fingers

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It is just possible that vibration has caused a joint or track to fail inside the inverter. There are a few heavy components on an inverter circuit board.
Unless you know what you are doing with high voltage electronics, I would suggest involving whoever supplied the speed control kit.
If they want to charge a lot, then you could fit another inverter - there are loads on ebay these days.

Bob
 

harris

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Thanks for the input Bob. I have now found one on E bay being bid on at £75, a new one of that type (Lenze") costs about £350 so I will try and get it and give up on the current one. I can see it might turn out to be a long and costly job to diagnose and repair it.
 

harris

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Hello again to the forum. At last I have received, mounted and wired up the LENZE inverter and it works fine. I also managed to find the two terminals to provide a reverse function by studying the hand book e mailed by a helpful customer services lady.
I need one last bit of advice to make sure the set up is correct from a knowledgeable 3ph motor person on the forum. Are you still out there Bob?

The technical specifications for the type of inverter I have gives the following data :-
Power 2.2KW and Output current 8.7 / 9.5 A (dependent on the rated mains Voltage and Carrier Frequency - don't know how to check that).

My motor is 1KW so if the output current is still set to max for a 2.2 KW motor (it can be varied) will it harm my motor in the long run if I leave it at that ??
If it will and it should be reduced, by what percentage of the max should it be reduced to. Is it as simple as - 2.2 KW motor needs max output so 1KW motor needs 45% of that?

Hope that makes sense.

Fred.
 

9fingers

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I'm still here Fred!!

Hopefully your motor should be marked with a full load current rating, there may be two figures - choose the delta wired number - the higher of the two. Program this into your inverter and if there is a boost parameter setting, consider setting this to say 25%.

In the absence of a current rating then 45% scaling is better than nothing. I've got a 4pole 1.1kW motor sitting here and that is rated for 5.2amps as a guide. If you end up with nuisance tripping of the overload sensor, then upping the current by 10-15% is possibly fine butI'd not go higher without finding out why it is tripping.

HTH

Bob
 

harris

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Thanks for that advice Bob. The motor was second hand when I got it and the motor rating plate had been removed so I can't check anything. I was only told it was a 1KW job by the company that supplied it together with the now defunct inverter. Can I assume from your reply that it is unwise to leave the new inverter possibly supplying the current required to drive a 2.2 KW motor and that it must be reduced? Just as a matter of interest, what would be the effect on the motor if the current was left at too high a level? I'm always seeking more knowledge on things electrical.

Fred.
 

9fingers

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Fred, I would always advise that an inverter is fully programmed to match the load. Without it you are not going to get the full protection that the inverter gives to the motor.
There should be a section of the programming manual that tells you what to enter, normally this will include the current, maximum operating voltage, stator resistance etc
The latter is used to detect motor temperature increase as the resistance of the windings go up when hot.

I can't think of any argument not to try and get the best out of your set up. Incorrect setting of the maximum current is like having too big a fuse in circuit.

Bob
 

harris

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Thanks for all of your advice Bob. I shall now go into the inverter programme and try to match it to my motor. At last I can do some more turning.
Fred.
 

OldWood

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Fred - can I add my pennyworth to what Bob is saying - he is the guru round here for these things, but one possibility may exist on your new inverter is that it has an auto-tune facility.

My Teko has that. If it is like the Teko, the programming is done through 'Functions' (quite possible a different title though), which allow you to vary the acceleration rates, the wiring configuration (on board control or a remote system on the lathe) and so on. They are pretty confusing until you get your head around them. The auto tune on my is Function 00-06 (if remember correctly) and you set that to 1 - run the machine which does something with the motor and takes the parameters it wants for over-heat, etc., and then sets that Function back to 0.

By the way I was having nuisance trips at low revs and found I had to increase the trip out current to prevent them. This was an oddball with the controller I think as the trip out occurred a specific time after switch on at low revs and was related to what the trip out current was set to - set it high enough (5.0A instead of 3.7A) and it stopped.

Don't be afraid of asking questions - there are enough of us here who are part of the way up the learning curve on these things. I say 'part' as they are quite complex beasts and I doubt anyone knows all the ins and outs of their capabilities.

Rob
 
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