Star-Delta, soft-start, VFD… How to make the smooth start of e-motor?

Different methods of soft-start

There are several methods to soft-start electrical motors. In this series of articles, I will describe the most popular solutions. First, the most important question is, why we need soft starters? In the traditional method of starting an e-motor, full voltage, current, and torque are applied immediately when main contacts get closed. The problem starts when the motor is a bigger consumer. High starting current very often is 6 to 7 times higher than rated. Additionally starting torque gives additional vibration to equipment which e-motor runs. To eliminate these problems, we can use different kinds of soft starters.

First and most common solutions. Star-delta

First used by years, is a star-delta starter. This is the most commonly used method for the starting of 3 phase induction e-motors. The reason to use is to reduce voltage during starting big motors. Physically reconfigures of the motor windings during starting allows reducing a torque factor, starting current by 3. So E-motor starts in star mode and reconfigures to delta connection. After reconfiguration motor runs normally.

The soft starter is manufactured from a power and control circuit. The function of the power circuit is to deliver power to the electric motor. The power circuit contains three contactors, a timer, and thermal overload. Contactors are smaller than 1 traditional contactor for direct connection. The function of the control circuit is control of the power circuit. The control circuit is built from the start/stop control, contactors control, and timer. This is changeover transient.

Power Circuit for star/delta soft-start

The power circuit is that simple that I think, it doesn’t need a more detailed explanation. Control of delta star can be realized in two different methods. Can be controlled with open transition or with closed transition.

Pictures show power and control circuits with the open transition. What is important in this type of connection, is the pause between star and delta contactors. This function is done by a timer. It is important to break during switchover will be not too long. During the switchover period, the motor has to run free. While this happens, it will work as a generator so it will generate its own voltage. In the moment of connection to supply again, the generated voltage can add or subtract with power supply voltage. For a few milliseconds will happen voltage spikes and surges. The name of this physical phenomenon is transients.

The control circuit for star/delta soft-start

To start the motor with star-delta starter control voltage have to pass contacts of the overload relay. When the stop button is not in used voltage is going to the start button. Activation of the start button, transfer voltage to coil of timer relay, and simultaneously voltage pass normally closed contacts of timer relay by timer aux. Contact 1 to energize the star coil. Activation of star coil closes star auxiliary contact and the main coil is energized, main aux contact gets closed. After the timer finishes counting time, the timer aux contact 1 is getting to open and aux timer contact 2 gets closed. Star coil lost voltage and delta coil get energized. The motor is operating in full load from this moment. The motor will stop automatically in case of overload or can be stopped by a pushbutton.

Closed transition star/delta

Circuit with closed transition looks and works a little bit differently. It requires 4 contactors and sets of resistors. This technique is used to reduce the magnitude of the change over transients and the system design looks like below.

The biggest advantages of star-delta connection are that this is simple, efficient(it draws 2-time starting current), it is cheap compared to other methods. But this is not a perfect solution. Low starting torque, break-in supply (transients), high transmission, and current peaks. This solution will be still in use for a long time, but there are already better solutions, which have more smooth control and eliminates problems which are in the star-delta connection. But more about that, in the next part of soft starters.


  • Mateusz Białas

    Researcher of Autonomous and Remotely Controlled ships. Since 2014 in Maritime Industry. Experience gained at Ro-Ro, Ferry, Container, and Heavy Lifts, design of Navy Ships.

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2 years ago

Well explained. Good article for both electrical engineering students and practicing engineers.

Chit Chit Tone
Chit Chit Tone
2 years ago

Very good, Thank you I accept

Baraka Mubeni
Baraka Mubeni
1 year ago

Thanks for your explanation but in a control diagram is start button and KM4 Normally Opened electrically both connected or connected direct to contact KT-Normally Closed of Timer?

10 months ago

Please tell me how to choose Resistor for Closed Transition Star/Delta Starter. R1-3 . Thanks

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