Big motor starting 2

[victa1288]

I want to start a 5000HP motor andthe system is too weak. I thinking of using Auto-tranformer and softstarting methods to start it. But whenever I tried it by ETAP, it always caused the bus Voltage drop to exceed the requirements. Does anybody here have better solution for this system?

 

[motorspert]

victa,
Are the transformer and motor new equipment? ie can you change the parameters? Where are your voltage drop limits?
The A/T can be tapped down to as low as 50%, however the torque will drop as the square of the voltage so the load torque needs to be low. What is the driven load and can it be started unloaded? The lower you can get the load torque during starting, the lower the voltage dip will be. Obviously a clutch or fluid coupling would be the best option but they present operational difficulties (maintenance, efficiency).

A VFD will get your motor away no matter what the load is - but again, the lower the starting torque the smaller and cheaper the VFD. You might 'only' need a 1500kW drive.

apowerengr's suggestion of a wound rotor is a good one and the resistance can be varied to get the inrush down to an acceptable level. Modern designs (eg WEG) have an elegant rotor shorting and brush lifting arrangement which means brush wear is very low.

Other alternatives that we have been involved with that might be worth a try: capacitors, capacitors and A/T together.

 

[victa1288]

Thank you for you guys's valuable inputs.

 

[DanDel]

victa1288, it seemed to me from your first post that you were having a problem modeling the autotransformer and/or softstart for your motor start in ETAP.
Any and all of the suggestions given here can be modeled in ETAP, which I suggest you try before you actually spend money on equipment changes. Good luck.

 

[timohearn]

One possibility that I did not see mentioned, is there any way to reduce the required torque of the load during the start. Depending on the type of load (pump, compressor, etc..) it may be possible to close or partially close a discharge valve to reduce the required torque, or unload compressor cylinders to reduce torque. Sometimes the motor starting problem can be solved mechanically.

Regards

 

[redha123]

There is a method not widely used : conecting capacitors during starting motor, this method is delicate because of overvoltage risk.