Soft Starter vs Star Delta: Understanding the Differences

The vast majority of the world’s industrial applications are driven by motors. Thanks to their robust and simple design and low-maintenance operation, three-phase induction motors are used in large numbers in commercial applications, trade, and industry. When switched on directly, the typical current and torque behavior of the three-phase induction motor may negatively influence the feeding supply network and the load machine during start-up. Three-phase induction motors have a high direct starting current. Depending on the motor version, this current can be 3 times to 15 times the size of the rated operating current. (A typical value is 7 to 8 times) This results in various disadvantages. Star delta starter and soft starter are used to reduce the high starting current of the electrical motor. But there are lots of differences between soft starter and star-delta starter.

Differences Between Soft Starter and Star Delta Starter

Below you can find the major differences between soft starter and star delta starter.

1. The definitions of soft starter and star delta starter are different

A soft starter controls and reduces the starting current of an induction motor electronically. The soft starter can control the starting characteristics to match the application requirements, such as acceleration and deceleration time, starting and overload current, and motor torque.

A star-delta starter (wye-delta motor starter) is a way of starting an induction motor by using 3 contactors, 1 overload relay, and 1 timer relay. It helps to reduce the inrush current when the motor starts. But using a star delta starter does not provide a soft controlled start.

2. Soft starter has a complex working principle

A star/delta start configuration comprises a six-terminal motor that is delta connected at the supply voltage. The star/delta starter employs three contactors to initially start the motor in a star connection, then after a while, reconnect the motor to the supply in a delta connection.

Soft starter operation is based on the use of a thyristor bridge (SCRs) in an anti-parallel configuration, commanded by an electronic control board to adjust the output voltage, according to the programming done earlier by the operator. Soft starter controls the power supply voltage through a power circuit. This circuit is made up of six silicon-controlled rectifiers, where, by varying their trigger angles, the effective voltage value applied to the motor can be varied.

3. Inrush current cannot be limited to star delta starters

The first reason is to limit the inrush current that a motor draws from the utility when it is first started. This is a concern because the large starting current may cause the line voltage to dip, impacting other loads that are sensitive to low voltages. There may also be a concern if the utility limits the peak current that can be drawn or charges for exceeding the limit. The second is reduced mechanical system stress. When the large inrush current occurs, there are significant magnetic forces created in the motor windings. These cause some parts of the winding to be attracted to each other and other parts repulsed. This mechanical shock can damage the winding insulation leading to early failure. The mechanical shock of the high torques produced with the large starting current can cause the failure of system elements such as the motor shaft, belting, gearbox, and drive train, and damage to the fragile product.

In the star-delta starting, the inrush current is reduced to 1/3 when compared to a direct online start. If you use soft starters the reduction of the inrush current is adjustable and more effective depending on the product model you use.

4. Torque control is an important function of a soft starter

Using a soft starter, you can adjust the torque to the exact level required, whether or not the application is loaded. By reducing the starting torque, mechanical stress on equipment is alleviated, saving on service and maintenance costs. Torque cannot be adjusted in star-delta starting.

5. Peak currents can occur in star-delta starting

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 switchover to the delta position is necessary, and this switchover 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. When used to start an extremely light load, they may limit the start current to a lower level than a soft starter. However, severe current and torque transients may still occur.

Compared with star/delta starters, soft starters are much more flexible and provide a smooth start with no risk of transients.

6. The initial cost of a star delta starter is economic

The total initial cost of a star-delta circuit may be cheaper than a soft starter. But you should be careful if the motor and star-delta components are very distant from each other. This will increase the cabling cost due to the need for six cables.

Remember:

At lower amperage, the soft starter is expensive, but as the amperage and power go up, so does the cost of a star-delta starter.

7. Soft starter can stop the motor softly

In the controlled stop, the Soft-Starter gradually reduces the output voltage until reaching a minimum value at a pre-defined time. By reducing the voltage applied to the motor, it will lose torque, which reflects an increase in the slip, causing the motor to lose speed. If the motor loses speed, so will the driven load. This type of feature is very important in applications that require a smooth stop from a mechanical perspective.

Typical applications for soft stop are:

In pumps to prevent water hammers.

in conveyor belts to prevent goods from toppling.

If you use a star-delta starter in your motor applications, the only possible way to stop the motor is to make a direct stop without any control.

8. The soft starter has a bypass function

The soft starter integrates a bypass to minimize heat generation during run time. The bypass automatically closes when the motor reaches its nominal speed. After initial start-up, running in bypass saves energy, by reducing losses and enclosure size and thus reducing cooling requirements. Star delta starters cannot be bypassed.

9. Communication options are available in many soft starters

Today, many soft starters can be connected to a PLC using a Fieldbus system such as Profibus or Modbus. Depending on the soft starter and the Fieldbus protocol it may be possible to start and stop the soft starter, see status information, and change the settings of the soft starter from the PLC. Star delta starters do not have complex communication capabilities.

10. Soft starter can be installed easily

Soft starters are easy to install and use. They do not require so much cabling and engineering during the installation. It is much faster to wire a soft starter than a star-delta starter. Besides, soft starters are smaller than star-delta starters and the difference becomes more significant the higher the amp rating gets. You can save on panel space.

11. Internal protection functions are available in a soft starter

Soft starters have several integrated protection and warning functions. Almost any type of fault can also be detected and displayed. For example, motor overload protection, locked rotor protection, motor underload protection, high current protection, phase imbalance protection, phase reversal protection, thyristor overload protection, etc. Star delta starters do not have any protection functions. The only way to protect them is to use extra protection devices like fuse switch disconnectors, molded case circuit breakers, HRC fuses.

12. Star delta starter does not generate harmonics

Harmonics can reduce the performance and reliability of other equipment connected to the grid, such as generators and circuit breakers. The solution is to install filters and screened cables but even then, the harmonic effect is not completely removed. Even though soft starter already fulfills all emission and immunity requirements they can generate harmful harmonics at the start. Star delta starters never generate harmonics.

13. Star delta starter can make almost unlimited start stops

Star delta starters have no limitation in the number of starts/hours. But soft starters have this limitation. Because every start generates heat in the thyristors. Therefore, the values should be checked from the catalogs of the manufacturers.

14. Soft starter should be mounted vertically

To have suitable cooling, the soft starter has to be mounted vertically, and in such a way that the airways are not blocked. Devices are used in the star-delta starters can be mounted in almost any position.

15. Some soft starters can start several motors in sequence

One soft starter can control two motors in sequence. However, the control and wiring are complex and expensive and any saving in soft starter cost is often outweighed by additional component and labor costs. Star delta starters can only start and stop one motor.