Soft Starter

Schneider softstarter

Motors that run typical industrial applications employing conveyors, pumps, blowers, and compressors all have one common quality ,  they draw a large amount of current when they start up, perhaps six times what they draw during normal operation. This level of current can stress motor components and cause power quality issues on plant electrical systems, particularly for larger motors of 1 hp or more. That is why motors of this size frequently employ soft starting.

A soft starter is another form of reduced voltage starter for A.C. induction motors. The soft starter is similar to a primary resistance or primary reactance starter in that it is in series with the supply to the motor. (Three wire or standard connection) The current into the starter equals the current out. The soft starter employs solid state devices to control the current flow and therefore the voltage applied to the motor. In theory, soft starters can be connected in series with the line voltage applied to the motor, or can be connected inside the delta loop of a delta connected motor, controlling the voltage applied to each winding.

Soft starters are generally used with induction (asynchronous) motors. But they also may provide benefits powering synchronous motors. The reason is that many synchronous motors behave like induction motors initially when they start up. That is, there is a lag between the rotating electric field and the rotor position. There is a delay before the motor comes into a synchronous condition. As in the case of induction motors, synchronous motors can also draw large stator currents during start-up of perhaps five to eight times the full load current in the absence of soft starting.

How does a soft starter work?

Electrical soft starters temporarily reduce voltage or current input by reducing torque. Some soft starters may use solid-state devices to help control the flow of the current. They can control one to three phases, with three-phase control usually producing better results.

Most soft starters use a series of thyristors or silicon controlled rectifiers (SCRs) to reduce the voltage. In the normal OFF state, the SCRs restrict current, but in the normal ON state, the SCRs allow current. The SCRs are engaged during ramp up, and bypass contactors are pulled in after maximum speed is achieved. This helps to significantly reduce motor heating

Applications

• Speed and torque control are required only during startup (and stop if equipped with soft stop)

• Reducing large startup inrush currents associated with a large motor is required

• The mechanical system requires a gentle start to relieve torque spikes and tension associated with normal startup (for example, conveyors, belt-driven systems, gears, and so on)

• Pumps are used to eliminate pressure surges caused in piping systems when fluid changes direction rapidly

Advantages:

- Minimum possible start current

- No current steps

- No torque steps

- Good start torque characteristics

- Dealing with large motors (200kW +)

Soft start is technically the best reduced voltage starting system.

Star/Delta starting is the cheapest and most commonly employed reduced voltage starting system. However its performance characteristics are damaging

Important notice:

Power Factor Correction: Can it be used with soft starters?

Individual power factor correction capacitors can be used with soft starters, provided that they are installed on the input side of the soft starter and switched in using a dedicated contactor when the motor is running at full speed. The contactor should be AC6 rated for the motor full load current.

Connecting power factor correction capacitors to the output of a soft starter will cause equipment failure due to severe overvoltage. This overvoltage is created by resonance between the inductance of the motor and the power factor capacitance.

PFC capacitors can be sized using the following formula:

kVA (Cap) = √3 x Vline x 0.8 x motor no load current