Motor Issues...

[BrainEater]

Ok.....this has nothing to do computers....but I know a couple people here that might know about this so here goes :

I was asked to help troubleshoot a power issue with a machine.

I *think* I solved it , but it brought up a couple questions.

The machine in question is a mixer.

Internally , it uses a 3 phase , 5hp motor....Externally , it's designed to use single phase.

The speed on this moter is adjustable.

Now , like a dummy , I did'nt write down the model number and look it up , so here's a couple questions.

1: Why , for a single phase connection , would you go through the messy business of phase conversion ? Is 3 phase motor control better in some way (other than the obvious) for speed control ?.........I can only assume this is a complete 3-phase controller and not some brute force 'static phase converter'.....

2: The problem with the machine is that it pops the breaker , with the present setup.The machine works on a) a generator , and b) the supplied connection for a different , but similar machine.So we can safely assume the machine actually works.....The tech from the company that built the rig , supplied what he called a "Buck booster" ....

I know what buck converters are , but I've never seen em used like this.The whole system seems wishy-washy to me.So the question : Is this a standard practice ...this 'buck-converter box thingy ?

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As far as I could tell , the actual problem was the fact that they were trying to run the rig from a GFCI protected breaker.I have equipment that doesnt like GFCI's too.....

So the bonus question :

Why don't electrically controlled motor systems like GFCI's ?



Thanks for any input everyone.

 

[Adrenaline]

What kind of motor is it?
Induction, brushed, brushless?
3 phases are used for larger motors, I think anything over 2-3 hp is almost always 3 phase.
The only thing I could think of short of doing phase conversion, is using a variable frequency drive for your 3 phase motor.

The issue with running large loads off of GFCIs is this:
The GFCI trips by sensing the current entering on the hot wire and subtracts the current leaving the neutral wire. If this difference is too large, say 30mA, it will trip. The issue comes in when you put a large starting current of a large motor on the GFCI, the inrush current for the motor is enough to trip the GFCI even though there isn't a ground fault.

 

[Mohonri]

Let me see if I get this right: it plugs into a single-phase 220V outlet, and has some sort of device to convert the power into 3-phase power? That *does* seem a bit odd, unless, as Adrenaline mentioned, the conversion also includes the speed control (VFD). 3-phase motors are generally induction type.

A "buck-boost" converter generally means a device that converts one level of DC voltage to another, either up ("boost") or down ("buck"). It's not a huge technological leap to go from a buck/boost converter to a full-on 3-phase inverter/VFD.

 

[BrainEater]

Thx for the replies.

The issue was in fact the GFCI . They have wired a new circuit without one , the machine is working fine.

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I really didn't get a good look at the controller but I suspect you are right that it's a VFD.It's the only way that setup makes any sense.

I did not know that about GFCI's...Learn something new every day.

 

[mikeblas]

The issue comes in when you put a large starting current of a large motor on the GFCI, the inrush current for the motor is enough to trip the GFCI even though there isn't a ground fault.

Why wouldn't the inrush current be returned on neutral? That is, why is the inrush current special, in that it doesn't return on neutral like regular operating current? I think the problem is that the motor is aging, and the windings are leaking a bit of current to ground. I the motor should be serviced.

Where do you live? Does The Code not require GFCI for your application?

Meanwhile, VFDs commonly perform phase conversion. The VFD is pretty complex anyway, and can implement phase conversion with only a little extra control circuity in addition to what's required for implement speed (or torque) control. This simplifies the facility wiring and provides the required feature, killing two birds with one stone.

 

[BrainEater]

I think the problem is that the motor is aging, and the windings are leaking a bit of current to ground. I the motor should be serviced.

Where do you live? Does The Code not require GFCI for your application?

The motor/machine is brand new , right outta the box.

It's a mixer/pump for spray on fireproofing , being used on a construction site , the hookup is temporary.

I have no idea about code requirements.I made my suggestion to the general , the general talked to the electrician , the electrician installed the new line.

 

[mikeblas]

As you gain experience, you'll learn that new equipment is not exempt from fault.

 

[BrainEater]

Oh I know that one already

I'll pass on your concerns to the company that owns the machine , and ask em to run that by the tech from the mixer manufacturer...

Thanks a lot for the input everyone !