Technologies & Applications > Soft Starters
Soft Starters
Discover what AC motor soft starters can do for you
Soft starters offer many benefits, including reduced motor stress and higher reliability compared to traditional 3-phase motor starting methods, such as direct on line.
Traditional 3-phase motor starting methods
Direct-on-line (D.O.L)
A very simple and common way of starting 3-phase motors which uses only a main contactor and thermal overload.
Advantage
- Cost effective.
Disadvantage
- Very high starting current and starting torque causing stress to the motor and drive train.
- Does not lend itself to remote control as the high in-rush current of large contactors is unsuitable for electronic outputs.
Star-delta (SD)
Changes the wiring of the three motor windings from delta (230v across each winding) to star (400v across each winding). Reduces the starting current by 60% compared to a D.O.L starter. Consists of three contactors, thermal overload and a timer..
Advantage
- Reduced starting current and torque lowering mechanical stresses.
Disadvantage
- More expensive than a D.O.L starter.
- Multiple points of potential failure will prevent motor loads requiring more than 50% of the full motor rated torque from starting.
Soft Starters
Soft starters will solve the problems of traditional starting methods. Using electronically controlled thyristors, a soft starter provides fine control of the voltage applied to the motor windings and therefore the current. It also allows the duration of the voltage ramp up period to be adjusted to suit the application. This ability to limit the available motor torque during start up and the duration of the start period, removes the stress loadings experienced by the drive train and couplings using traditional methods reducing wear and premature failure. Improved reliability is achieved by the use of thyristors instead of mechanical contactors – no moving parts. This greater reliability leads to reduced maintenance and reduced peak currents providing energy savings. Soft starters also feature control inputs which are fully compatible with PLC outputs and other digital NPN or PNP outputs. These however are not all of the benefits of soft starters, most products within the RS range also offer a softstop function. The table below lists common problems a soft starter will overcome.
Choosing the right Soft starter
Choosing the right soft starter for the application is key. In most applications you can select according to the rated motor power in kW. However, in some cases you may need to use a larger soft starter than the rated motor power, due to the starting conditions, for example heavy duty starting, many repeat starts per hour.
