In brief
A soft starter limits starting current and torque surge when a three-phase motor is switched on by increasing the voltage gradually.
Contents
What is Soft starter?
An induction motor switched on direct-on-line draws a multiple of its rated current during starting and produces a torque surge. A soft starter controls the motor voltage via anti-parallel thyristors using phase-angle control and ramps it up over an adjustable ramp. This reduces current peaks, the load on the supply network and the mechanical stress on couplings, belts, gearboxes and conveyed goods. Many units also offer a soft stop, for example to prevent pressure surges in pump lines.
The soft starter must be distinguished from the frequency inverter: it changes only the voltage, not the frequency, and is therefore not suitable for speed control during operation. Compared with star-delta starting, the start is stepless. After run-up, an internal or external bypass contact often bridges the thyristors to save heat losses. Key ratings are rated current, supply voltage, control voltage, number of controlled phases, starting frequency and integrated protection functions.
It is used for pumps, fans, compressors, conveyor belts and mills running at fixed speeds. Failures are often caused by overheating, excessive switching frequency or defective thyristors after a short circuit.
In practice: maintenance and procurement
For replacement or conversion, motor power and current, supply and control voltage, starting duty (normal or heavy) and starts per hour must be stated. The set parameters such as starting voltage, ramp time and current limit should be noted before replacement.
Synonyms and abbreviations: softstarter · soft start unit · motor soft starter · reduced voltage soft starter
Frequently asked questions
What is the difference between a soft starter and a frequency inverter?
The soft starter only reduces the voltage during starting and then lets the motor run at mains frequency and fixed speed. The frequency inverter changes voltage and frequency and can control the speed throughout operation. Soft starters are smaller, simpler and less expensive. Where speed adjustment is needed to save energy, the frequency inverter is the appropriate choice.
When is a soft starter worthwhile instead of star-delta starting?
When current and torque jumps that occur when switching from star to delta are to be avoided, or when a soft stop is needed, for example with pumps. A soft starter also requires only three motor cables instead of six. Star-delta circuits, on the other hand, are very simple and built without power electronics.
What information is needed to replace a soft starter?
Important are the type designation, rated current, supply voltage, control voltage, number of controlled phases, integrated bypass, interfaces and protection functions. Motor data, load type and starting frequency are also needed. The current parameter settings should be documented so that the replacement unit can be set up straight away. The ambient temperature in the control cabinet is also relevant.
Can a soft starter protect the motor?
Many units include protection functions such as electronic overload protection, phase loss or stall detection. Whether these functions may replace separate motor protection depends on the device, the requirements of the standards and the installation design. Short-circuit protection is generally provided by upstream fuses or circuit breakers, which are specified in the manufacturer’s documentation.