[OT-CNC]

Hi,

I have 2 DMM DYN3 drives with 750W motors. One drive consistently trips a breaker when powering up the contactors. The breakers are new (Eaton FAZ-C20/3) 20A, C curve. Do I have the wrong trip value? Should I try 20A D curve high inrush current? I have been told standard should be fine. Is something else going on perhaps? The drives are currently hooked up to 3 phase, 240v AC high leg delta, which is going through 2 buck transformers to reduce voltage to around 222 V AC to keep it below the 230v max. There is no load on the motors yet as I'm still mounting things on the lathe. Input power seems good. Also I'm using a line filter for the logic power connection and no, I'm not powering logic with the high leg
The breaker does not trip if I power the contactor then flip the breaker to on.
What do you recommend?

[Al_The_Man]

There probably is a high inrush current if the drives have a large DC power supply built in, if they do the Capacitors initially appear as a short or very high load at switch on.
It may pay to go with time delay or inrush current rated.
Al.

[RCaffin]

We have been here before. High-leg delta is just BAD news. Adding bucking transformers just makes it worse. It might work for older 3-phase motors, but for modern power and electronics systems it is just a disaster waiting to happen. Seems that it didn't wait here.

Cheers

[OT-CNC]

RCaffin, I'm not sure what you mean. There is no issue with the incoming power. I only provided that info to help source the proper breaker. I'd like to know what inrush current curve people are using on these drives.
No issue what so ever with high leg if wired properly. Can you explain to me why bucking transformers are a problem?

[RCaffin]

When you are hooked up direct to a genuine 3-phase grid you gat power with a very low source impedance. That means it is very reliable in voltage and current.
Once you start putting transformers into the circuit, things can get very messy. Current on one phase can affect the voltage on another phase. All sorts of werd problems follow.

The USA is the only country in the world to use high-leg delta, and only in some older parts of the grid at that. Basically, it was a way of cutting high-tension cable costs, and it a real bodgy kludge. You should ask the grid supplier to replace it with a real modern 3-phase supply - at their cost.

Yeah, sounds like an inrush current problem, but with something else aggravating it. What happens if you swap the drivers? Does the fault move, or not?

Cheers
Roger

[OT-CNC]

Unfortunately changing the power grid in my part of town is out of the question so I'm stuck with what I have for the time being.

Good idea on swapping the drives. I will try that. I did notice that it only happens on the first power up.

[RCaffin]

Unfortunately changing the power grid in my part of town is out of the question

Why?
Oh, sure, they will tell you it could not be done because it would cost too much. That is not a reason; it s an EXCUSE.

Call their bluff. Go political, and complain to the local Chamber of Commerce (or whatever) that the crappy supply is endangering your business and the employment it represents. Make noises! Hassle! Eventually they will buckle and upgrade just to get you out of their hair.

Cheers
Roger

[OT-CNC]

I found the problem. I swapped the drives and the same breaker tripped. I realized that the way I'm test powering things is wrong. I have the contactors wired to 24v dc power supply which I turn on by powering on a 120v transformer. It takes a split second to get up to voltage and that causes the contactors to close slow and tripping the breaker. I added a switch in line of the 24v now all is good. Standard 20A C curve equipment breaker is working fine for now.

[OT-CNC]

Ok, just a quick update. I changed the breakers out to 20A D curve as I was still getting occasional trips on power up with the C breakers. High inrush "D"seems a better choice.