Starter Issues

[Sleigher]

Hello,
I realize that this might belong in the electrical section but it doesn't look like there is a lot of activity there and we need some help.
We have an older Vilter 4512XL that is running as a high side machine. It is spun by an even older 200 HP motor.
We have nothing but problems with the starter on this machine. It is an "across the line" starter, size 5.
The motor doesn't pull anywhere near full load amps. The compressor is starting at 33% load, everything seems fine but last week the contacts welded together, again.
It is running along side a 4416 with a slightly newer 200 HP motor. We have had few problems with this package.
I'd like to swap to a reduced voltage starter. Can anyone suggest a type and or tell me what they are using for a similar set up?
It is a 480V system.

Thanks in advance,

P

[NH3LVR]

Before changing to a different type starter have you checked the input voltage on the supply side of the starter.
If the voltage falls off during startup it can cause a lot of starter issues.

[Magoo]

Hi I agree with NH3LVR, even voltage imbalance can amplify inrush current draw.
but if system has run for decades, have a close at the motor, the winding could be damp creating the high inrush current that is welding the contacts in.
Is it all three contacts that weld in.

[Sleigher]

The voltage does drop off but it drops off because of the across the line start.
What came first?
It is usually one leg that sees most of the heat. The motor has been megged and appears to be fine.
Right next to the motor with the problems is the other 200 HP machine and in the same engine room we have five screws that are running on the same incoming power.
We have found little things over the years that we attributed past failures to. Loose wire or bad crimp on the feeders (to the compressor), none of them stopped the issue.
The last time it went out it really blew the starter apart so we replaced the entire starter. It seems to run fine for a while. I don't recall the amp draw but it is nowhere near over-amping, especially if you include the service factor.
A size 5 starter can run up to 200 HP. This is 200 on a very heavy motor. My next approach would be to go up in size but for about the same money I can probably start it softly.

[Magoo]

Hi
interesting that last time it blew the contactor up, and generally just one pole failing.
Possible a broken rotor bar in motor creating imbalance, an oscilloscope could pick that up. May be a power analyser fitted and monitor the start currents and voltage balances may indicate something, you should be able to rent an analyser for the job.
magoo