1200A Fuse blowing, Need to justify my conclusion.

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VinceS

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I have a 480VAC 1200A 3PH fused feeder to a 600V Class MCC. The MCC is used to feed 2, 250HP motors, which both are coupled via belts to the shaft of a common arbor. One of the motors starts via soft start. The other which starts across the line, is delayed to start via PLC timer for 2 seconds. The locked rotor current of either motor is enough to blow the 1200A main fuses if the arbor is in a bind. Total running load is well under FLA. The total load on the MCC is not much past the 2 - 250HP motor loads, 3 - 5HP motor loads and a small lighting load.

A fuse blows nearly once a week, and I might mention are expensive. I believe everything calculates in NEC spec. The main could have been bigger to prevent this issue, but I think the company just took the bigger motor upgrade.

All motors and loads have been megged, finding no shorts.

Other than a binding arbor, I can suggest no other reason why the fuse is blowing. Any thoughts would be a help thanks in advance.
 
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VinceS said:
A fuse blows nearly once a week, and I might mention are expensive. I believe everything calculates in NEC spec. The main could have been bigger to prevent this issue, but I think the company just took the bigger motor upgrade.
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NEC requirements are minimums and typically do not provide coordination.

You need to compare the measured starting profile of your system with the fuse curve.

Typically when does the fuse blow (i.e.on Monday morning start up)?
 
Vince,

It seems odd to me how these motors are started, they are fighting each other until

running speed is reached. Unless your OK with the starting, I would research that area. jmo
 
Some microprocessor starters have monitoring. This may be worth the cost to swap to, if it keeps happening. Seems like it would be better for the second motor too.
 
1200A Fuse blowing; need to justify my conclusion.

1200A Fuse blowing; need to justify my conclusion.

Hi Vince, I am new, a september 2008 member.
Since you believe that everything calculates in NEC, we discuss other part which may not be in the NEC. (because I am not fully familiar with NEC) I am in U A E. We follow IEE and Local Authority Regulation.

Since you hava a common shaft for both motors, is there any perticular reason for selecting exactly 2seconds delay for starting the second motor. Most of the soft starter have a ramp up time of 0.5 to 10 seconds; and the starting current starts from 300% of FLA. (Some manufactures specify it even 500%!).
So please verify the time taken by the VFD motor to reach the full speed of it. I am sure it will be more than 2 seconds. So if you start the second motor ( I hope the starting method is Star/Delta) within the ramping time of the first motor, the total line current may be more and weakens the fuse elements; especially if it start and stop frequently. Why cant you increase the time delay slightly more than 2 seconds accordingly?
 
It seems odd to me how these motors are started, they are fighting each other until

running speed is reached. Unless your OK with the starting, I would research that area. jmo
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My first thought.

This is where I would start.

Utilizing a high speed line disturbance analyzer to capture the inrush current.

In addition you might want to IR everything in particular the switch for thermal issues.

Also look at equipment cycling times.
 
Answers to several response follow-up questions

Answers to several response follow-up questions

1. The fuse is blowing with random occurrences. The only thing in common is the starting of the 2-250HP motors.

2. I need to rent a 3 ph current monitor with the capacity to monitoring starting current. Current version in house is toast.

3. The design preexists my involvement. I can't answer any of the why it's designed the way it is. It does seem odd to me also.

4. Delay is PLC controlled I and not tied to realtime, so I suppose the 2 seconds varies slightly every time. This actually way be worth exploring.

5. Across the line single contactor, not Star/Delta.

6. Looking into Fuses. May have switch to a cheeper version to save money.
 
VinceS said:
I have a 480VAC 1200A 3PH fused feeder to a 600V Class MCC. The MCC is used to feed 2, 250HP motors, which both are coupled via belts to the shaft of a common arbor. One of the motors starts via soft start. The other which starts across the line, is delayed to start via PLC timer for 2 seconds.
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If I understand this correctly, the motors are mechanically coupled to the same load and thus, to each other.
If the soft starter can get the driven load up to speed then the across the line starter would be connected to a motor already running at speed.
But two seconds doesn't seem like anywhere nearly long enough for the motor to get up to speed on a soft starter.
Can you set the PLC timer to a time greater than the soft starter ramp time?
 
Although adjusting the time before the second motor is switched across the line may be enough, if the soft starter supports it (or the PLC has pulse counting so a tacho could be fitted), maybe it could bring the second motor on when the first is at or very close tothe final speed.

If the arbor is locked then really that condition should be detected and the soft start drop out to prevent the fuses blowing with rotor lock current.

This also sounds a bit odd mechanically; presumably the full mechanical load is not present during the startup phase otherwise the first motor is always going to have an excess load on it, assuming that the full load is about what the motors are rated for collectively.
 
Just curious, are both these motors direct drive to the arbor, that is, there's no clutch/ratcheting mechanism on the second motor? Is the first motor dragging both the arbor and unpowered second motor up to speed? If so, that seems like a really odd design to me.....
 
For what it's worth, here's an unloaded 30kW motor changeover from VSD to direct on line, the VSD output having been synchronised with the supply.
There is still a transient but the magnitude and duration are both lower that they would have been for a direct on line start.
Motorchangeover01.jpg
 
take a look at the trip curve of the use you are using. you might be able to find a fuse that allows a little more time before it trips.

I think the 2 second thing is a little odd as well. Also the fact that one is on a soft start and the other across the line, although if the first motor is dragging the second motor, its already going to be moving when the contactor kicks in.
 
Besoeker said:
If I understand this correctly, the motors are mechanically coupled to the same load and thus, to each other.
If the soft starter can get the driven load up to speed then the across the line starter would be connected to a motor already running at speed.
But two seconds doesn't seem like anywhere nearly long enough for the motor to get up to speed on a soft starter.
Can you set the PLC timer to a time greater than the soft starter ramp time?
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I concurr.

This starting scheme is used a lot in the Rock Crusher industry, because at start-up, the crusher is only a centrifugal load. But in every one of these I have built, we ALWAYS used the up-to-speed contact of the soft starter (or the closing of the bypass contactor) of motor #1 to signal the closing of the X-Line contactor for motor #2. By the way there is no reason to use Y-Delta on starter #2, in fact it just adds additional risk and complexity. The motor is already spinning at full speed when the contactor closes.

The 2 second delay may have been intended to be a delay BETWEEN that up-to-speed signal from the soft starter and the next sequence, I have had several colleagues insist on that just as a "fudge factor" (despite my challenges as to the necessity), but in this case someone probably didn't understand the concept and left out the up-to-speed part of it, or reconfigured it at some later date without understanding that issue. That kind of stuff happens all the time when bit-head programmers get involved, because they often have no clue when to comes to power delivery issues.

The 2 second delay may also have been used as a method to prevent re-closing of that contactor should you have an incomplete sequence and re-start, because without it, that motor might still be in re-generation when spinning. The 2 seconds would allow the motor's field to collapse before re-closing that contactor. I have had to do that on weak power systems. But still, that should be taking place ONLY if motor #1 is at full speed.
 
Application for the arbor is a ball mill crusher.

Application for the arbor is a ball mill crusher.

I will dig into wiring between the two starters for any interlocking between the soft start contactor and the 2 second delay to start the across the line contactor and provide more info. This interlocking will reenforces my thought as to the binding arbor cause.

I will also check into the soft start function. I will suit up and see if the soft start is programmed incorrectly or its function is not bypassed. There were no errors reported on this but I haven't verified its function. It seems to me if the soft start is bypassed and the unit is starting across the line might lead to more frequent fuses blowing.

I'm glad to see this design is in use, and somewhat common.
 
Arrrgh! Ball Mill! That's a horse of a slightly different color.

The problem with doing this on a Ball Mill is that it usually does NOT start unloaded; it's a stallion (still has balls in it) when you start it.

If it used to work fine and now doesn't, more than likely someone has added shot (balls) to it at some time and upset the starting torque profile, then attempted to compensate by changing the starting sequence by reprogramming that add-in time with the PLC, bringing the second motor in sooner to get it (the shot load) over the apex of the mill. I have seen that happen time and again in processing plants. Once it was because the production mgr wanted to make his retirement bonus by increasing throughput, not caring that it might be tearing up the machinery because that would end up being his successor's problem. If someone has an archive of the original PLC program I'd be willing to bet the timing of that add-in used to be different.

You, as the electrician, may be stuck because it's doubtful that you can influence the processing decisions just to save some fuses. You might want to consider changing out those fuses with an Electronic Trip Circuit Breaker that can be programmed to allow the short term trip function to accept a longer start time. But there would be a lot more coordination to check before doing that.
 
That may explain the following...

That may explain the following...

Discussing the operation with a senior operator, I found that this problem has occurred before. Then it goes away... Your comment on the shot load makes sense, as it wares away its load is less. Occam's Razor wins again. Now I have to sell it. Hi, I'm the guy you paid to find the problem, I'm nearly 99% sure its not a electrical malfunction. Oh by the way care to sign the invoice...

By the way SMC Plus soft start functions as setup currently set for 15 sec ride then the bypass closes providing input to the PLC ( timer 2sec ) which closes the second across the line starter (via PLC). I spoke with the install EE to confirm settings.
 
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