Difference between 1,2 and 3-leg controlling

I am trying to learn more about the soft starter. I have read the infotmation (don't understand all the english though...) on http://www.lmphotonics.com/sstart.htm#SoftStarters
But I dont understand the thing about controlling the voltage on 1,2 or 3 phases. The 1-leg controlling seems to have the poorest characteristics. Why would you like to use that one? Is it such a big difference in price?

"The soft starter can be designed to control one phase, reducing the torque but not the current on two phases, (SCR/Diode can not be used in this connection)"
or two phases reducing the torque but the current will not be optimally reduced or balanced, there will be negative sequence currents heating the rotor and reducing the torque per unit start current, (SCR/Diode can not be used in this connection)

Does this mean that the two phase has the same torque-capacity as with the one phase, and that the current is better reduced than with the one-phase, but not optimal?
there will be negative sequence currents heating the rotor and reducing the torque per unit start current
It sounds like the two-phase is more worse than one-phase?

The three-phase controlling seems to bee much better than the others if you read about it, sounds like you always should choose this one. Is it soo much more expensive, or why not always use it?
Where I practise they use 2-eg controlled soft starter to controll a 5.5kW motor, 400V. Which seems to create heating in the motor? Why do you think they do that (no one knows there...), is the three-phase so expensive, or maby there isn't such big problems with this if you only start it a couple of times a day?

High current with softstarter

Hello! Nice forum

I am comparing the currents in the start of a motor that controls a fan. I am going to test DOL, star-delta, softstarter and a Frequency converter. It is a 3-phase motor on 5.5kW, 400V, and takes about 7.3 ampere when it is running.

I have so long only tested DOL and the softstarter, but when I look at the current and the voltage on the softstarter I get a bit confused. It is a Siemens Sirius softstarter, model number 3RW3024-1AB14

https://mall.automation.siemens.com/GB/gues...08-&jumpto=6008

I have read that the soft starter is supposed to reduce the current and the torque. But when i look at the currents my meter shows about 50 ampere, and have a peak about 63 ampere. Thats only about 15 ampere less than the DOL-start. So it doesn't look like it is reducing the currents...? Anyone have a clue why it is like that?


And my second question; Isn't the softstarter suppose to work like in this picture?:


(from a ABB-pdf)

My current looks a bit like it:



But what about the voltage, that is supposed to look like the picture from ABB? Can anyone explain what I see in this picture . After I while the lines comes closer and closer to finally look like a regulary sinus-wawe.