Autotransformer start on 4,000 HP 3 stage air compressor, activating at full speed?
Autotransformer start on 4,000 HP 3 stage air compressor, activating at full speed?
(OP)
I'm an elecrical specialist. I've been working on MV motors, and controllers for many years. I recently had a call to a facility with mystery trips on a compressor (see title). They had some data recorded by the DCS which showed the current increased from 280 to 500 amps, at which time the machine would trip on high vibration. Sometimes it would run for weeks with no problems. The machine had all internal components replaced (very expensive) 2 weeks ago. I should point out that during the trip events there is no increase in pressure/flow, so guide vanes were ruled out. Last week it tripped again, and I was sent to check it out. The motor tests good with 2.5 KV megger, and DLRO. The only thing that I could think of electrically was; possibly the R contactor might be dropping out (R shunts out autotransformer used for starting). I'm not sure what this would do mechanically if it happened? I brought the controls up in test, and started testing voltages. On the economizer (ATC CEM) that feeds the R coil, I read 8.9 VDC (aha!). The output is spec'd out to be 20 VDC. I ordered a new one and changed it. The new one puts out 20.4 VDC. I checked the old one with a scope. It was putting out 120 VAC. That's a lot of ripple. I think that has to be it! They start the compressor, run it overnight unloaded. the next day they start to load it, and it trips on vibration. I haven't seen the trend on most recent trip, but previously current was first indication followed by vibration. Mechanics are saying case of gear box is distorted? That's hard for me to buy, after seeing what I saw? If it had mechanical issues that bad, how could it run for weeks at a time with no problems? I'm still wondering about that R contactor. Could the AC from the economizer have done something strange to the DC coil, or armature? I'm going to swap it out with the M contactor next week. If M drops out, it stops the machine. R is more problematic to drop out at full speed. Has anyone ever seen anything like this? It's definitely not your average problem. Thanks In Advance, FJ
Attached power diagram of similar installation.
Attached power diagram of similar installation.





RE: Autotransformer start on 4,000 HP 3 stage air compressor, activating at full speed?
I would however think there are at least a couple of more likely issues.
2: You have a broken rotor bar in the motor. I've seen on numerous occasions where this seems to be a "phantom" event, but in reality is is tied to adding the extra stress of loading that was not there during an unloaded acceleration, as is likely the case on a compressor started by an RVAT. There are other ways of detecting a broken rotor bar with a vibration spectrum analysis, that might be relatively simple if your vibration sensing system is capable of capturing data.
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RE: Autotransformer start on 4,000 HP 3 stage air compressor, activating at full speed?
RE: Autotransformer start on 4,000 HP 3 stage air compressor, activating at full speed?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Autotransformer start on 4,000 HP 3 stage air compressor, activating at full speed?
But I'm not tracking your statement about how re-introducing the starting transformer after already running is going to cause axial thrust to begin with. You will get a drop in voltage and thereby torque will drop at the square of that voltage drop. Is it a vertical motor where the loss of your allows it to drop off of the thrust bearings or something? Not sure I understand that.
Another though occurred to me. Are you sure this isn't perhaps the Autotransformer being left into the circuit too long and it is heating up, tripping the thermal cutout, which drops out the R contactor? This can happen when, like in a compressor, there is something like an unload valve that is sticking closed, or someone has messed with the timing (usually in a misguided attempt to solve that problem). Typically the thermal switch on the transformer core is supposed to be in the control circuit for M, not R, but someone may have inadvertently wired it incorrectly.
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Autotransformer start on 4,000 HP 3 stage air compressor, activating at full speed?
RE: Autotransformer start on 4,000 HP 3 stage air compressor, activating at full speed?
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Autotransformer start on 4,000 HP 3 stage air compressor, activating at full speed?
RE: Autotransformer start on 4,000 HP 3 stage air compressor, activating at full speed?
Voltage doesn't control the speed, frequency does. A drop in voltage will mean a drop in torque and current. The drop in torque will most likely result in a drop in speed if the motor is loaded. As the speed drops the slip frequency increases and then the current increases. It depends on the load but with full load on the motor and a drop in voltage the current generally exceeds normal full load current.
Bill
--------------------
"Why not the best?"
Jimmy Carter