Main breaker tripping on startup, with a twist

[hurk27]

I agree that since both motors are on the load side of the 255 amp MDP main then the 60 hp motor is fly wheeling enough current to help the 75 hp to start without tripping the 255 amp breaker.

But this also shows the problem with the magnetic setting, since both motors are being fed from there own 100 amp breaker and these 100 amp breakers are not tripping, it is clearly saying that the magnetic pickup setting for the 225 amp main is set too low. requiring to have to start one motor over another might be a work around but I don't see it as a fix when most likely these two motors are operated independently, so with that in mind and the fact if the 60 hp were to ever fail they wouldn't be able to use the 75 hp then the true fix would be to reset the magnetic setting to a higher point to eliminate the tripping.

 

[ronaldrc]

Hurk

I hadn't really paid that much attention to the breaker size.

Thought we where talking about something like a 1500 amp. or something.

If its just a 225 amp I say turn her up. Don't ask me why I think that.

 

[weressl]

The 250a breaker trips instantly, and resets with out cooling. These are 2 seperate air systems.

The funny thing isn't that the breaker trips due to the inrush of the full volt starter... I get that.
Nor is it that the soft start does not trip the breaker. I understand that as well (much lower initial inrush using soft starter)

The funny thing is... the breaker does not trip if the 60hp is already running. Then the 75hp slaps in and away we go! No breaker tripped.

Why would having an additional load on the line prevent the breaker poping?

Its not really a problem we are having, I am just curious and want to understand what electrical theries (sp?) may apply to this situation.:dunce:

Try to look at the mechanical end of things how the two air-systems relate to each other, if there is a closed/open damper, how does the backpressure changes when the smaller unit is already running, etc.

Normally one would start with a closed damper then open it up after the fan had accelerated.

The clue that it happens in cold weather - denser air as somebody mentioned - is further cllue that it is on the mechanical end of things.

 

[kwired]

Some of the later posts cover some of what I was thinking while reading through.

Some things to consider (some has already mentioned):

Cold conductors have less resistance than warm conductors - this would allow for more current to flow when starting cold.

If one motor is already running when you start the other one - it is loading the supply and causing a voltage drop on the supply conductors - this alone could be somewhat of a choke that will reduce what kind of current is available when starting the other motor.

If one motor is already running when you start the other one you also may have some significant voltage drop in the source itself depending on what the capacity of the source is.

Cold equipment will be more resistant to moving - grease will be stiff - I don't buy the fact that cold air will change instantaneous current when starting much - the air will not load the motor much until at least 20 - 30% of full speed is reached - by then a lot of the high current needed just to accelerate the rotor has diminished.

Along with the last thing mentioned - locked rotor current for the first few milliseconds is what is likely tripping the breaker - once rotation starts the current drops rapidly. (This is especially true if I understand correctly that the breaker trips instantly when attempting to start, if it trips sometime in the middle of acceleration then it is not as likely to be instantaneous trip feature of the breaker that is tripping.

The first motor could be contributing to keep voltage up when second motor starts.

What you have going on is probably a combination of more than one of these conditions and who knows maybe there is something else not mentioned - but is also effected by some of what was mentioned here.