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?

Hello,
price is one of the reasons for using different types of SS however space is also often a reason. A SS that controls only one leg is purely for torque reduction to protect the mechanical load, a two phase control is a compromise which is cheaper but also usually smaller. The three phase control is the largest and most expensive option but will provide the best operation.
Ken

Hello Elpontus,

The information provided by Kens is essentially correct. The following is simply another way of explaining the difference.

Soft Starters with one controlled phase - Designed to reduce mechanical shock during motor starting. These devices do not reduce motor starting current. Products that fall into this category are generally limited to 11kW.

Soft Starters with two controlled phases - Designed for motors up to 250kW. Products falling into this category will reduce both starting current and starting torque. Some products in this category may include motor protection functionality.

Soft Starters with three controlled phases - Designed for motors to 800kW plus. Products falling inot this category will provide lower starting currents and better control over starting torque. Most products in this category offer motor protection functionality.

The variation in products available ensure product users have the option to purchase a product which satisfies their individual needs at the best possible price.

I hope the above helps.

Regards,
GGOSS.

Hello Elpontus,

Welcome to the forum.
Good replies from both Kens and GGOSS - what would I expect??

The reduced number of control phases increases the start current for a given torque and also the power dissipated in the motor for a given load.

I would not recommend single leg controllers for anything other than small motors with a very low inertia load.
I believe that two leg controllers are OK on small motors with low inertia also. For larger motors or high inertia loads, I would strongly recommend a three leg controller, but at the end of the day, all three are available. Because the power assembly largely determines the price, the number of controlled legs strongly dictates the price.

Best regards,

Thank you, now I think I understand it better! But... what about the heating problems in the rotor with 2-leg controlled soft starter? Is this a big problem, or when does it become a problem? Is it on very big motors, or is it on motors that are often started and stopped?
Could it for example be a problem on "my" motor, 5.5kW, that starts and stops a few times a day, but not like every hour or so..

The heating problem exists with all soft starters products, particularly when the starting currents are set so low that the motor labours to accelerate the load. However it is far more serious with soft starters that provide control of 1 or 2 phase only, and that's why we tend to apply them only to machines that can accelerate to speed fairly quickly.

Hope that helps.

Regards,
GGOSS

Strictly speaking, the power dissipated in the rotor of the motor is directly related to the inertia of the driven load provided that the motor is able to spin up easily. If the start current is reduced too much, there will be additional losses in the motor.
If you control only two phases, there will be negative sequence currents in the rotor which will increase the rotor losses. In the case of the single phase control the problem will be even worse.
The two phase controller will require more start current for a given motor to develop the same toque as a three phase controller. Hence I believe that it should be avoided for applications with extended starting times (high inertia) and frequent starts.
Larger motors have a much lower surface area per KW than small motors and are therefore more sensitive to increased losses than small motors. You can get away with increased losses at start with a small machine that you can not contemplate with a large machine.

Best regards,