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Large motor with soft starter
Case: I work in Gas plant, we have a large motor with soft starter driving propane compressor.
motor details: 3.3 KV - 1.6 MW - FLA 300 A
the problem is during starting period the starting current reach to 1200 A for a second then return to 330 A, I separate the motor from the compressor but no change on starting current, what do you think the problem is?
Answer:
A little more data is needed to understand the issue.
* If the soft starter is not under commissioning, what was the behavior before?
* Is it thyristor controlled?
Assuming that the soft starter is thyristor controlled, it works in the following way:
Set the maximum motor current you do not want to overcome; this number is normally between 200 and 300% and depends on how strong is the network (the purpose of the soft starter is to limit the voltage drop on the network during motor startup).
Start the unit, the soft starter will regulate the output voltage based on the current feedback read from current transformers located on the output. The thyristor will be commanded to let enough volts through in order to maintain the maximum current set (maybe 300%).
Once the motor start turning and accelerating the volts will increase and the current will remain 300% until full volts are reached and, at that point, the current will decrease down to what is required by the load. For example:
If you are not under commissioning, put an oscilloscope to monitor output volt and output current (ideally 3 + 3 channels) in order to investigate. If you look closely to the control circuit you may see test points to connect directly to the current feedback and therefore you will need only MV probes for the output volts. Are the current feedbacks read properly? Is the voltage balanced? If you see a discrepancy check the current feedback or the power module on that leg.
By the way, after you have soft started, does it go in bypass? If yes, and the current is balanced, the problem is in the soft starter.
motor details: 3.3 KV - 1.6 MW - FLA 300 A
the problem is during starting period the starting current reach to 1200 A for a second then return to 330 A, I separate the motor from the compressor but no change on starting current, what do you think the problem is?
Answer:
A little more data is needed to understand the issue.
* If the soft starter is not under commissioning, what was the behavior before?
* Is it thyristor controlled?
Assuming that the soft starter is thyristor controlled, it works in the following way:
Set the maximum motor current you do not want to overcome; this number is normally between 200 and 300% and depends on how strong is the network (the purpose of the soft starter is to limit the voltage drop on the network during motor startup).
Start the unit, the soft starter will regulate the output voltage based on the current feedback read from current transformers located on the output. The thyristor will be commanded to let enough volts through in order to maintain the maximum current set (maybe 300%).
Once the motor start turning and accelerating the volts will increase and the current will remain 300% until full volts are reached and, at that point, the current will decrease down to what is required by the load. For example:
- If the compressor is fully loaded and the motor is designed with no margin (just for example) the current will be 100%;
- If the compressor is uncoupled the motor current will go down to magnetizing current (30% ?).
If you are not under commissioning, put an oscilloscope to monitor output volt and output current (ideally 3 + 3 channels) in order to investigate. If you look closely to the control circuit you may see test points to connect directly to the current feedback and therefore you will need only MV probes for the output volts. Are the current feedbacks read properly? Is the voltage balanced? If you see a discrepancy check the current feedback or the power module on that leg.
By the way, after you have soft started, does it go in bypass? If yes, and the current is balanced, the problem is in the soft starter.
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