Soft Starter current limit

[magneticted]

Hi Guys.

Just trying to understand soft starter principles. If we want to soft start a 3Mw motor 6Kv with 60% starting torque, and 650% current, compressor drive.and we limit the starting current to 400%, what happens to the voltage, how do we calculate the corresponding voltage at this point to ensure we have enough accelerating torque to oppose the load torque. Is the 400% current limit and voltage at this point constant throughout the speed range

 

[itsmoked]

Yes the "limit" is current. That's what you bought when you bought a SS. (Current Limiting)

BTW MV SS setup can go really badly wrong. Causing more than $$$$$ damage, if you get my drift..

Usually you want a factory rep doing the setup. You don't want to explain to your boss that you messed up one little detail and that's what burnt down your MW motor, took out the starter, burned down the west wing, and killed 3 bean counters.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Marke]

Hello magneticted

As you reduce the voltage applied to the motor, you reduce the current by the same proportion, so to reduce the current from 650 to 400%, you must reduce the voltage by 40/65. - Ohms law applies.

As the current in the stator is reduced, the current in the rotor is also reduced. The torque produced is a function of the KW dissipated in the rotor, so the torque is proportional to the current squared.

If you reduce the current from 650% to 400%, then the torque is reduced by 40 x 40 / (65 x 65)= 37.8% of Full voltage Torque. At 400% current, the start torque will be 60 x 0.378 = 22.7% torque.

As the motor accelerates, the full voltage start current reduces. If the start current is set equal to 400%, then the ratio of start current to full voltage current will slowly trend towards 1 at perhaps 80% speed. This results in an increasing torque multiplier and the start torque will trend towards full voltage torque as the motor increases in speed.

I would plot out the full voltage start current and the limited start current against speed. From this, you can calculate the torque multipler curve against speed.
Using the torque multiplier/speed curve and the full voltage torque curve, you can calculate the constant current start torque curve and plot that with the load torque curve.
Provided that the constant current start torque curve is higher than the load torque across the full speed range, then the motor will start the load. If the torque curves cross, the motor will not reach full speed.

Best regards,


Mark Empson

http://www.lmphotonics.com

 

[magneticted]

Hi Mark

Thanks for your help, not sure I'm clever enough to do what you suggested. Attached is the motor current / torque curve. It shows 590% flc the motor not made yet, and the 590% could be subject to a manufacturing tolerance of 10% hence 650% ( worst case )The client is going to limit the current as stated to 400% We shown a votage at 65% which we feel is the minimum voltage to get the compressor away. What are your thoughts does it look Ok.

 

[Marke]

Hi magneticted

It is actually very simple to do provided that you have the full voltage start current and start torque curves for the motor (against speed) and the load start torque against speed.
I usually do the calculations in 10% speed steps to give a good approximation.

Best regards,

Mark Empson

http://www.lmphotonics.com