There are several methods available to soft start a motor. The more traditional methods are Direct-on-line, Star/Delta, Auto transformer and Primary resistance starters. The more sophisticated methods also provide a soft stop and include Soft Starters and Variable Frequency Drives. Variable frequency drives can provide a better soft start with significantly lower starting currents and many other benefits including energy savings. However, a Soft Starter may provide a more economical solution to motor starting over the life of the motor.

Thermal protection of the soft starter is provided by a temperature sensor located on the heatsink and by calculating the temperature of the thyristor junction with sophisticated modeling of the specific devices used. The starter fans are powered by 24VDC supply and thermally controlled when the heatsink temperature exceeds 40°C and whist the motor is ramping.

The soft starter can be configured to operate in 6 wire mode. 6 wire mode may be the preferred method due to:

• Possible reduction in the size of starter required, saving on space and /or cost.
• Wiring may already be present if a start/delta type starter was previously used.
• Reduced motor cable size.

The major advantage is that the current in the SCR is 58% less than it would be for the same motor connected in 3 wire.

Soft starter is a reduced voltage controller designed for starting standard 3 Phase induction motors. The unit is solid state, using a microprocessor to control inverse parallel (back to back) pairs of SCR's. An SCR/thyrisor is a semiconductor device that latches when triggered. Once triggered it allows current to flow in one direction only and turns off at zero current.

Soft Starters use power devices in the form of thyristors or SCR's which generally have an efficiency of about 99%. Soft Starters may be configured to operate in a continuous or bypass mode. Continuous is when the SCR's are dissipating heat whist ever the motor is running. In bypass configuration the SCR's are only operating during starting and stopping, all other times the SCR's are bypassed using a bypass contactor. When the SCR's are bypassed they dissipate no heat, therefore minimising the ventilation required. The reduced heat dissipation is a common deciding factor for bypass configuration.

Soft starters are solid-state devices that protect AC electric motors from damage caused by sudden influxes of power by limiting the large initial inrush of current associated with motor start-up. They provide a gentle ramp-up to full speed and are used only at start-up (and stop if equipped). Ramping up the initial voltage to the motor produces this gradual start. Soft starters are also known as reduced voltage soft starters (RVSSs).

The starting performance of the squirrel cage induction motors using Soft starters provides valuable economics of electrical energy. Optimum benefits are gained when a motor duty involves frequent start or stop cycles but is still likely to be worth while in systems which are in continuous operation.

Soft starter: Motor starts without any jerks i.e. smoothly and gradually picks up to the synchronous speed. The life of the motor is thus increased significantly and the maintenance problems are drastically reduced.
Auto Transformer: Motor is subjected to heavy jerks during starting and a bit later because of the sudden inrush of current .This causes severe damages to the motor winding which ultimately burn off.

This is a unique feature of the Soft starter. Its effect is to prevent an undesirably sudden deceleration of the load. It is useful in various mechanical handling and conveyor systems and in many hydraulic pumping operations where a sudden removal of the drive input can induce undesirable effects, such as the fluid hammer in the pipelines.

The Flux compensated magnetic amplifier (FCMA) Soft starter essentially consists of predominantly inductive impedance in series with the motor windings to control the motor current through a potential divider.

A soft starter is any device which reduces the torque applied to the electric motor. It generally consists of solid state devices like thyristors to control the application of supply voltage to the motor. The starter works on the fact that the torque is proportional to the square of the starting current, which in turn is proportional to the applied voltage. Thus the torque and the current can be adjusted by reducing the voltage at the time of starting the motor.

Soft starter allows the output voltage decreases gradually to achieve soft stop, in order to protect the equipment. Such as the impact of the water pump, when the pump stops suddenly, the water flow inertia in the pipe will raise the pipe and valves pressure suddenly and cause pipeline damaged. Soft stop to extend parking time will solve such the impact.

Motor acceleration torque and starting time has direct relationship. Electronic soft starter can make the motor with voltage ramp start from initial voltage to full voltage at the set time (0.5 to 2408). Like it can reduce water impact if we extend the time of water pump flow from 0 to 100%, increase the pump speed variation time means increase the starting time which can be achieved by adjusting the starting time of the soft starter. Starting time should be adjusted according to the specific loads and repeated tests, in order to achieve smooth acceleration within starting time.