ABB Softstarter info

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Direct On Line start (DOL)

This is by far the most common starting method available on the market since it is a very compact and a very cheap starting solution. The starting equipment consists of only a main contactor and a thermal or electronic overload relay. The disadvantage with this method is that it gives the highest possible starting current. A normal value is between 6 to 8 times the rated motor current but values of up to 14 times the rated current do exist. There is also a magnetization peak that can be over 20 times the rated current since the motor is not energised from the first moment when starting.

The exact values are dependent on the design of the motor. In general, modern motors have a higher inrush current than older ones because of the lower resistance in the windings.

During a direct on line start, the starting torque is also very high, and for most applications it is higher than necessary. This will cause unnecessary high stress on driving belts, couplings and the driven application. Naturally, there are cases where this starting method works perfectly fine and there is no need to use any other starting method.

If starting DOL, the only possible way to stop the motor is to make a direct stop.

Star-delta start

A star delta starter usually consists of three contactors, an overload relay and a timer. This starting method can only be used with a motor that is delta connected during continuous run.

The basic idea behind a star-delta starter is that during the first part of the acceleration, the motor windings are connected in star, giving a reduced current. After a preset time, the connection will change to delta which will give the full current and also the full torque.

When used in delta, the voltage across each motor winding is same as the network voltage. The motor current will split between two parallel windings with the factor 1/√3 compared to the line current. If the impedance in each motor winding is Z, then the sum of impedance for the parallel windings is Z/√3.

When the motor is star connected (Y- connected) the motor windings become serial connected. The resulting impedance will then be √3*Z, resulting in an impedance which is ((√3*Z)/( Z/√3) = 3), 3 times the impedance when delta connected. As the voltage level is the same, the resulting current when Y-connected will be 1/3 of the current when delta connected. So when starting using Star-Delta start, the star connection results in a current of 33% compared to a delta connected motor.

As the main voltage is the same the motor feels a star connection as a voltage reduction, as because the voltage across each motor windings will be 1/√3 of the main voltage. This lower voltage will also result in a torque reduction. The torque will be reduced with the square of the voltage, [(1/√3)* (1/√3) ≈ 0,33] ending up being 33% of the toque available when delta connected. However, this is only a theoretical value. A more true value is 25% as there are additional losses as well as other efficiency data valid when used star connected. This works well in an unloaded or very light loaded start, but it will not be possible to start heavier applications.

A big problem with star-delta starters appears when starting for instance pumps. The motor will accelerate to about 80-85% of the rated speed before the load torque is equal to the motor torque and the acceleration ceases. To reach the rated speed, a switch over to delta position is necessary, and this switch over will very often result in high transition and current peaks. In some cases the current peak can reach a value that is even higher than for a DOL start. Also, just as with a DOL start, the only way to stop when using a star-delta starter is to make a direct stop.

Frequency converter

The frequency converter is sometimes also called VSD (Variable Speed Drive), VFD (Variable Frequency Drive) or simply Drives. The drive consists primarily of two parts, one which converts AC (50 or 60 Hz) to DC and a second part which converts the DC back to AC, but now with a variable frequency of 0-250 Hz. By controlling the frequency, the drive can control the speed of the motor.

During start, the drive increases the frequency from 0 Hz up to the network frequency (50 or 60 Hz). By gradually increasing the frequency like this, the motor can be considered running at its rated speed for that frequency. Also, since the motor can be considered running at its rated speed, the rated motor torque is available already from start and the current will be around the nominal current. Usually, the drive trips if the current reaches 1.5 times the rated current.

When using a drive to control the motor it is possible to perform a soft stop. This is especially useful when stopping pumps in order to avoid water hammering but it can also be useful for conveyor belts.

In many applications it is required to continuously regulate the speed of the motor, and a drive is then a very good solution. However, in many applications a drive is used only for starting and stopping the motor, even though there is no need for continuous speed regulation. This will create an unnecessarily expensive solution if comparing with, for instance a softstarter.

Comparing a softstarter and a drive, the drive has a much bigger physical size and requires more space. The drive is also much heavier than a softstarter making it a less desirable solution on, for instance ships where weight is important. Finally, since the drive changes the frequency and actually creates the sinus wave, a drive will cause harmonics on the network. Additional filters as well as shielded cables are used to reduce these problems but the harmonics will typically not be totally eliminated


A softstarter does not change the frequency or the speed like a drive. Instead it ramps up the voltage applied to the motor from the initial voltage to the full voltage.

Initially, the voltage to the motor is so low that it is only able to adjust the play between the gear wheels or stretching driving belts etc to avoid sudden jerks during the start. Gradually, the voltage and the torque increase so that the machinery starts to accelerate. One of the benefits with this starting method is the possibility to adjust the torque to the exact need, whether the application is loaded or not.

Using a softstarter will reduce the starting current and thereby avoid voltage drops in the network. It will also reduce the starting torque and mechanical stress on the equipment, resulting in reduced need for service and maintenance.

Just as for a drive, the softstarter can perform a soft stop, eliminating water hammering and pressure surges in pumping systems and avoiding damage to fragile material on conveyor belts.