ters
Electrical
- Nov 24, 2004
- 247
A while ago, I started a thread which many of you contributed to, but that story has not ended yet...
So the issue is that a 3.5 MW motor driving a large fan has problems starting. The starting method is the soft starter. Rotational inertia is high.
We discussed several causes, such as inadequate motor, inadequate starter, week and inadequate power system, etc. But calculations show that although things are marginal this still should work, but it does not.
Today we managed for the first time to put two identical 13/20MVA transformer in parallel. Their impedance is 10.5% at 20MVA but each transformer also has a current limiting reactor 6%. However, 20+20MVA system still would no start a 4MVA motor...
System: two 13/20 MVA transformers in parallel, + about 5 MW of additional load shared between two transformers.
Motor: asynchronous, TECO-Westinghouse 3.5MW, In = 385Amp, 6kV, 50Hz, 1485 rmp, locked rotor current 2300Amp.
Fan: Large rotational inertia. The fan has no load at the present (no air, dampers closed). The Impeller weight is 15,000 lbs, and the Impeller moment of inertia (WR^2) is 66,000 lb-ft^2.)
Starter: constant current, voltage ramp ~10 sec, current limit 470%, starting time 50 sec, after which bypass contactor closes (when then speed is about 80%).
We had some luck starting and running the motor using one 13/20 MVA dedicated transformer but now with 2 transformers in parallel it would not work. We tried three times and none of the starts was successful. Each time the soft starter was manually shut down after about 20 sec since motor was creating too much vibrations and was not accelerating any more.
Problems still could be electrical, but for a change we started to suspect that there may be some mechanical problems, perhaps related to the motor rotor axial movement. If the rotor indeed does try to move, either it may not have enough space to move to the electrical center or the shaft is moving back and forth around its electrical center for some unknown reason and hitting the trust.
The attached charts provide more info. During the voltage rump time, which was about 10 sec, motor runs smooth and with two transformers in parallel the initial voltage drop is rather modest - falls from 6.4kV to 6kV. This is good news, with one transfomer we had much larger initial voltage drops. However, the bad news is that after the voltage is ramped up high enough that the current reached the preset limit (470% In), some vibrations start.
As you can see, the current and voltage outline forms on the charts start at some point having "teeth" like a saw. Once stabilized there is about 3 - 4 “teeth”/second. Just judging by the ear, the frequency of mechanical vibrations is in the same range, meaning current spikes likely correspond with frequency of mechanical vibrations.
Any idea what might be happening?
So the issue is that a 3.5 MW motor driving a large fan has problems starting. The starting method is the soft starter. Rotational inertia is high.
We discussed several causes, such as inadequate motor, inadequate starter, week and inadequate power system, etc. But calculations show that although things are marginal this still should work, but it does not.
Today we managed for the first time to put two identical 13/20MVA transformer in parallel. Their impedance is 10.5% at 20MVA but each transformer also has a current limiting reactor 6%. However, 20+20MVA system still would no start a 4MVA motor...
System: two 13/20 MVA transformers in parallel, + about 5 MW of additional load shared between two transformers.
Motor: asynchronous, TECO-Westinghouse 3.5MW, In = 385Amp, 6kV, 50Hz, 1485 rmp, locked rotor current 2300Amp.
Fan: Large rotational inertia. The fan has no load at the present (no air, dampers closed). The Impeller weight is 15,000 lbs, and the Impeller moment of inertia (WR^2) is 66,000 lb-ft^2.)
Starter: constant current, voltage ramp ~10 sec, current limit 470%, starting time 50 sec, after which bypass contactor closes (when then speed is about 80%).
We had some luck starting and running the motor using one 13/20 MVA dedicated transformer but now with 2 transformers in parallel it would not work. We tried three times and none of the starts was successful. Each time the soft starter was manually shut down after about 20 sec since motor was creating too much vibrations and was not accelerating any more.
Problems still could be electrical, but for a change we started to suspect that there may be some mechanical problems, perhaps related to the motor rotor axial movement. If the rotor indeed does try to move, either it may not have enough space to move to the electrical center or the shaft is moving back and forth around its electrical center for some unknown reason and hitting the trust.
The attached charts provide more info. During the voltage rump time, which was about 10 sec, motor runs smooth and with two transformers in parallel the initial voltage drop is rather modest - falls from 6.4kV to 6kV. This is good news, with one transfomer we had much larger initial voltage drops. However, the bad news is that after the voltage is ramped up high enough that the current reached the preset limit (470% In), some vibrations start.
As you can see, the current and voltage outline forms on the charts start at some point having "teeth" like a saw. Once stabilized there is about 3 - 4 “teeth”/second. Just judging by the ear, the frequency of mechanical vibrations is in the same range, meaning current spikes likely correspond with frequency of mechanical vibrations.
Any idea what might be happening?
![[hammer]](/data/assets/smilies/hammer.gif "[hammer] [hammer]")
