3-phase motor

[Tim Nott]

I THOUGHT I was being very clever when I wired the 3-phase dust extractor up to the starter switch of the combination machine, so when any motor was started/stopped so was the extractor. I did it posh by mounting a 3-phase socket on the outsid of the combi
This worked fine, but after about a year the dust extractor motor conked out.

Could these two events be related? An electrician I know mumbled somehing in French about 'back voltage' but I din't really understand him

 

[Digit]

Did the extractor have its own built in starter?

Roy.

 

[9fingers]

I reckon it is that funny French left hand drive 3 phase that has caused it.

Seriously though all you have done is put two three phase motors in parallel and used a single switch to turn them on and off.

Can't see why that might kill a motor though.

Bob

Currently in France but NOT doing house calls - the country is too damn large!

 

[Digit]

I'm curious Bob as to what would happen with one large motor wired in parallel with a small one. A large one might well drag the voltage down, thus increasing the current that the small one would draw. What do you think?

Roy.

 

[9fingers]

Roy,

I can certainly see some scenarios like that but I just can't see the currents will anything like the magnitude and duration of those during starting and so would be very surprised if they could lead to damage to one of the motors.

Bob

 

[OldWood]

Hi Tim
Can you help us a bit on this 'conked out'! ?

Have you had to motor looked at yet to see actually what has happened ? The simplest failure is that the smoke has got out - actually I suppose with modern sealed motors it doesn't actually show, but is there evidence that overheating has occurred and taken one or more of the windings out ?

I doubt that we have the complication of a star-delta starter as I guess the motor is not that big, so we'll eliminate that.

I take it that you have tried running the motor separate from the wiring configuration you already have to eliminate some oddity there.

Does the motor hum when the power is applied - loosing one phase, or one winding due to a break in the wire, would inhibit starting - I'm trying to remember if it would start with 2 phases if spun up.

Pay attention to 9Fingers Bob - you're probably already aware that he is our 3 ph motor guru.

Rob

 

[9fingers]

Pay attention to 9Fingers Bob - you're probably already aware that he is our 3 ph motor guru.

Rob

You are very kind Rob but there are other 'experts' here too and between us we try and help.

My experience is only born of years of tinkering with motors plus a healthy dollop of intuition.

Bob

 

[Digit]

I've seen problems in the past Bob, but the smallest motor I can recall causing any problems was 60 HP!
If you go down that route again Tim I would advise a timer delay on start up of the extractor and a similar delay when shutting off the saw/planer etc. This is always a good way of clearing saw dust etc still whirling around as the main machine winds down and should prevent any possible interaction between the two motors.

Roy.

 

[9fingers]

Ah! that beats my 'biggest motor experience' by a factor of 10 or more!

You had better take on the "guru" mantle for this! :lol:


I'm thinking that it would always be best to link motors in the way Tim wants to by low power control circuits between contactor coils rather than piping power between machines.


My set up uses a current transformer in the feed to all my 16amp sockets and the resultant signal drives a small relay. The contacts provide the logic inputs to the inverter control circuitry to spin up the cyclone.

All I have to remember to do is open the correct blast gate. I've not got round to automating those yet!
Bob

 

[Digit]

My edited post and your post crossed Bob. How about two hundred HP and multi stage stage start up Bob? :lol:
The starter looked complicated enough for a Moon landing!

Roy.

 

[9fingers]

I too was editing my post!!

In more detail, my cyclone controller first senses that a machine has been turned on by the current draw. Then is applies power to the inverter. After a couple of seconds once the inverter is ready, the low level on signal is sent to the inverter.
When a machine is switched off, the power is held on to the inverter and a stop signal sent. The inverter then applies the braking to the cyclone motor over 5 seconds and when this has stopped, the power is removed from the inverter.

Maybe a bit over the top but I have several lifetimes supply of relays and enjoy tinkering with control circuitry!

Sad but true.

Bob

 

[OldWood]

OK guys - if we are w****y waving.

As a student (late '60's) I got a holiday job for 2 years in a factory making plastic sheeting - I was going out with the boss's daughter (she dumped me but I got the job!).

There was a 4 bole mill superheated with steam under pressure, with each bole driven by a 100 hp dc motor with the field windings controlled by mag-amps. The dc was from an ac motor/ dc generator set up ( Ward-Leonard?). I can't remember what the hp of the ac motor was but it must have been a minimum of 400. The thing that was astonishing was that it was started direct on line - it had it's own transformer. I had on occasions to start this set up and it always gave me the willies as there was a large 270 degree ac current meter, that charged all the way round to 2,500 amps when you hit the red button.

At the other end of the factory there were several large machines that started with sliprings and water bath resistors that you wound in (?) as the motor ran up. It was all good education for an electronics engineer!

Sorry for the nostalgia

Rob

 

[JMcK]

I venture into this thread with some trepidation in the face of the expertise which is available but I note that Tim said "after about a year the dust extractor motor conked out".

If it was related to the switching setup would one not expect it to have done so more or less immediately? I am asking for my own information as a mere tyro in these matters

 

[Digit]

Maybe a bit over the top but I have several lifetimes supply of relays and enjoy tinkering with control circuitry!

Oh good! I thought it was just me! You should have seen my previous greenhouse!

If it was related to the switching setup would one not expect it to have done so more or less immediately?

Unfortunately not! An example of this is the 'life' of cartridge fuses, with nothing wrong 'downstream' they tend to rupture eventually due to the machine's repeat start up surges.
In the same way all electrical stuff fails eventually due to age. (Just like us!)

Roy.

 

[JMcK]

Thanks for that.

stuff fails eventually due to age. (Just like us!)

Don't I know it! :lol: :lol:

 

[OldWood]

In the same way all electrical stuff fails eventually due to age. (Just like us!)

Roy.

Yes to all of that Roy, but there's something wrong that a 3ph motor fails after a year.

OK we haven't been told yet what the failure mode is, but from the Reliability Bathtub Curve model, a year is bumping along the bottom which is kind of unusual - it's not an infant mortality nor an old age wear-out.

Rob

 

[Digit]

Granted Rob, but actually the reliability of electrical goods has a rather odd curve. The failure rate is initially, in hours run terms, high, it then improves.
According to WHICH, for example, white goods are more likely to fail within the first six months than in the next few years.
Tim may just have been damned unlucky.

Roy.

 

[OldWood]

Roy
I worked in electronic reliability for a while.

See this
http://en.wikipedia.org/wiki/Bathtub_curve

What you're talking about is infant mortality, useful life and wear out.

Rob

 

[Digit]

Exactly, though I confess that that is first time that I have heard it so defined.

Roy.

 

[Tim Nott]

Sorry guys, I kind of lost the thread, but thanks for all the helpfull replies.
What happend is that the DE started tripping the disjoncteur, first only whe connected to the combi, then when connected to a separate outlet. Finally it wouldn't power up at all.
I'd like to do a simpler solution which would be just to switch the DE on and off from its own switch (and own supply) mounted next to the combi start button. Actually I'd like to get rid of the DE completely and have something less stone age than the cloth bag/plastic bag combo.