The Necessity of Soft Starting

In engineering practice, the three-phase asynchronous motor stands as the most prevalent driver in both civil and industrial electrical equipment. When these motors are connected directly to the power supply — a method known as hard starting — they can draw inrush currents as high as 5 to 7 times their rated current. Such electrical surges can overload the supply network and associated switchgear, jeopardizing stability. Beyond electrical stress, direct starting generates high peak torque, delivering mechanical shocks that not only strain the motor itself but also degrade coupled machinery. These abrupt disturbances can also impair the performance of other devices sharing the same grid, leading to malfunctions or reduced operational efficiency.

Operating Principle of a Soft Starter

Key Motor Startup Indicators

The startup behavior of an asynchronous motor is primarily evaluated using two factors: the multiple of starting current and the multiple of starting torque. A soft starter operates by gradually altering the voltage applied to the motor during startup, thereby curtailing both the inrush current and the associated torque shock.

Limiting Inrush Current

By restricting the current surge, the soft starter mitigates unnecessary torque impacts and diminishes the electrical shock to the distribution network. This protection helps prevent false triggering of disconnect switches and contactors. For motors with frequent start-stop cycles, soft starters can effectively limit temperature rise, significantly extending service life. Among various technologies, the silicon-controlled rectifier (SCR) soft starter is the most widely used in engineering applications.

SCR Soft Starter Working Mechanism

In an SCR-based soft starter, three-phase thyristors are connected in series between the power supply and the motor. By manipulating the firing angle of the SCRs, the device controls the voltage supplied to the motor. At startup, the conduction angle increases progressively from zero, allowing terminal voltage to ramp up smoothly. This ensures the motor accelerates gradually until it reaches the torque needed for normal operation. Once the motor achieves rated speed, the SCRs are fully triggered, and a bypass contactor closes to eliminate harmonic pollution and extend SCR life. For stopping, the bypass contactor is first disengaged, then the conduction angle is steadily reduced, lowering motor voltage and speed until it comes to rest — enabling a controlled soft stop.

Selecting a Soft Starter

Common Types

Soft starters can be broadly classified into:

• Bypass Type – Once the motor reaches rated speed, a bypass contactor shunts the soft starter, minimizing SCR heat losses and boosting efficiency. This design can even be used to sequentially start multiple motors.

• Non-Bypass Type – The SCRs remain in full conduction, and the motor operates under full voltage. This is common in applications with frequent short-term duty cycles.

• Energy-Saving Type – When the motor is lightly loaded, the soft starter automatically reduces the stator voltage, lowering excitation current and improving power factor.

Sizing and Protection Considerations

Selection is based on motor rated power, current, and load characteristics. Typically, the soft starter capacity should slightly exceed the motor’s operating current. Comprehensive protection features — including phase loss, short-circuit, overload, phase sequence, overvoltage, and undervoltage safeguards — should be incorporated for robust operation.

Suitable Application Scenarios

Soft starters are ideal for squirrel-cage induction motors where speed control is unnecessary. Their typical voltage range is AC 380V to 660V, with capacities from a few kilowatts up to 800kW. They excel in pump and fan systems requiring both gentle starting and stopping. In variable-load conditions, where motors run lightly loaded for long periods with only brief heavy-load intervals, non-bypass types can offer notable energy savings.

Practical Examples of Soft Starter Use

1. Air Compressors – Transition to energy-saving mode under light load; automatically balance phase currents during voltage imbalance, reducing heat and extending lifespan.

2. Centrifugal Pumps – Control fluid acceleration and deceleration to minimize surge-induced system pulsations, lowering maintenance costs.

3. Bridge Cranes – Implement dual-slope starting for optimal acceleration control, boosting productivity while reducing damage to goods.

4. Belt Conveyors and Automated Lines – Enable smooth starting with preset low-speed operation to prevent material displacement or liquid spillage.

5. Ventilation Fans – Replace outdated electromechanical starters to reduce belt wear and mechanical stress, cutting repair costs.

6. Crushing Machines – Utilize stall and overload protection to prevent overheating damage caused by blockages or jams.

7. Choppers – Substitute for autotransformer starters to reduce grid impact and conserve energy.

8. Mixers – Employ dual-slope starting and preset acceleration to avoid mechanical faults, save energy, and eliminate the need for a variable frequency drive.