3 phase motor issue

[Big J]

Hi all,

 

Hoping one of you might be able to help with a fairly major issue with my new sawmilling machine. It's a frame saw with 10 vertical blades that reciprocate to produce 9 boards on one pass.

 

The issue that we have is one of lack of power. All electrical tests have been done and everything to do with the electrical supply is perfect, the machine is progressing from Star to Delta fine and everything is wired correctly (phases in correct sequence).

 

The main motor is a 15kw lump with a 29 amp maximum draw. On start up in Star, it's pulling about 10 amps. You get a momentary 50 amp spike when you first switch it on. Then, when it switches to delta, there is a spike at 19 amps and it settles down to 13 amps. This is all as it should be. However, when you introduce a log to the machine, the current draw drops to 10 amps and as the machine starts to struggle (as it always does) it drops to about 8 amps. The machine usually stalls not long after this and you have to stop start the infeed to allow the blades to regain speed.

 

My understanding of motors is that when more is asked of them (ie, the introduction of a load) then they draw more amps. This machine is rated to 29 amps and doesn't draw more than 8 amps under load, meaning that it's effectively only cutting at 4kw, rather than 15kw. The behaviour of this motor seems to have dumbfounded every specialist that I have spoken to.

 

Does anyone have any ideas? Could it be a wiring issue? An electronics issue? Or perhaps the windings in the motor are shorting?

 

Not being able to get into full productive swing with it is costing me bloody thousands in down time.

[GreenGui]

First thing I would do is stick a Mega across the windings and check the resistances are all equal or there a bouts. A simple multimeter will not do, it will not give you an indication of the insulation on the motor windings i.e how the windings are behaving with a large current load.

 

Have you checked the running current on each phase and situation the motor is running at or just the current on one phase? Also simultaneously check the voltage on each phase (between phase and neutral or earth) and make sure it stays constant.

 

I have 3 phase and we have supply network issues during the winter. When the load is high the supply voltage drops to 190 volts on each phase and one of my machines (a refrigeration compressor) refuses to start

Edited June 30, 2016 by GreenGui
additional

[se7enthdevil]

is this not a question that should be posed to the manufacturer of the device?

 

sounds like a bit of an odd one the fact that it used less leccy once you introduce a log...

[Big J]

First thing I would do is stick a Mega across the windings and check the resistances are all equal or there a bouts. A simple multimeter will not do, it will not give you an indication of the insulation on the motor windings i.e how the windings are behaving with a large current load.

 

That was one suggestion given by a 3 phase motor specialist. Test the resistance over the 6 connections. Should be less than 2 ohms and equal. If not, something is wrong. I shall do this tomorrow.

[Big J]

is this not a question that should be posed to the manufacturer of the device?

 

sounds like a bit of an odd one the fact that it used less leccy once you introduce a log...

 

It has been posed, and they are as stumped as us. I'm just trying every avenue in case someone has an epiphany.

[GreenGui]

Have you got another equivalent size machine operating on your supply Big J that proves it good? I know you said it has been tested out OK but...

 

If it Mega's out OK personally I would re-visit the supply. I agree that the current should increase, what you describe is not normal.

[cornish wood burner]

Have you checked the motor plate for what voltage it should run on?

[Robbo90]

When I put a 3 phase lathe motor into my household supply I used a company called Drives Direct who make inverters and the like. Their website has loads of stuff on it that is way over my head so it might be worth having a look on their site for ideas or info. These people are more than helpful and will talk shop with you even if there is no sale. I have no connection with this company other than being a very satisfied customer of theirs.They are based in Kirkby in Ashfield Notts. They might be able to point you in the right direction

[Mike Dempsey]

Hi Jon

C&F electrical is a local co to me in Glenrothes who are specialist industrial sparkies. They have a section who rewind/fix motors of all sizes and know what they are doing. They have the facilities to sort all sizes of motors. Its also owned by my mate Dave Cunningham who is a woodturner, and they sort all my electrical problems and issues. Dave has been doing this for over 40 years and there isn't a problem they cant fix. I don't know if he is on holiday so you might end up speaking to Brian Carty who runs the sparky side of the business. Worth a phone call on 01592 757176

[cornish wood burner]

Assuming you have a volt meter. Check each line across the delta contactor.

If you have a voltage across the contactor then one of the contacts is burnt or broken.

Often difficult to check unless the the motor is running. You probably would get a circuit through the contacts until a heavy load is applied. If the fault was in the power contactor I think you would probably notice it on startup but worth a check as a backup plan.

I have known a similar problem across the circuit breaker in the distribution board. Goes to two phases under load.