How to choose the right soft starter?

Electronic soft starters are today still more applied, and the scope is partly to reduce the start current and partly to reduce the mechanical stress occurring in transmissions and machines during start. However, it is important to choose the right soft starter for each application.

Construction of soft starters
Soft starters are constructed in several different ways, dependent on which applications they are to be applied for. They may be constructed for regulation of the motor in one, two or three phases with time-controlled start ramp or current-controlled start.

The 1-phased
types are primarily applied in applications for which a reduction of the mechanical influence at start of for example transmissions or other types of machines is wanted. This type of soft starters does not reduce the start current very much; it will typically be about 85% of the nominal start current.
The 2-phased
regulated soft starters can be applied for the majority of industrial applications. The start current is typically about 50-65% of the nominal start current In the non-regulated phase the current will during start be about 15% higher compared with the two other regulated phases.
In applications for start of motors with large moments of inertia and long start times, this soft starter will not be optimal, as the phase unbalance may imply that the thermal overload protection will cut out (differential release).

The 3-phased
regulated types fit for all types of applications. The start current will typically be about 35-50% of the nominal start current. This type is well suited for both general motor applications, but also where there is a need for start of motors with large moments of inertia and long start times, for example in connection with start of fans, flywheels etc. Please be aware of the thermal load degree, see underfig.1&2.


The soft starter and its applications
As mentioned, the soft starter is designed for the various applications in which it is an advantage to apply them. Fig. 1 describes various application fields and load classes: Light start 10A, standard start 10, heavy start 20 or very heavy start 30, and the advantages to be obtained by applying soft starters.
In the table the typical, maximally occurring start current that might occur during the start of the motor is stated.
As it appears from the table, most applications are categorized in trip class 10, that is the release characteristic that thermal overload relays and circuit breakers are normally delivered with. Please be aware that very few applications fall into the category “very light start”, trip class10A.

Thermal overload protection
Motor installations shall be overload protected. Most often this is done by use of contactors and/or fuses, or by using circuit breakers with short-circuit protection. It is important that the soft starter is protected by the thermal release characteristic, which is standard in class 10 (see fig. 2).
In fig. 2 the time for coherent overload grades can be seen. For example an overload of 8 times for 3 seconds at class 10 is allowable. It is important that the soft starter is thus dimensioned that it can resist the intermittent overloads that might occur during start, for example if the motor is blocked.

Short-circuit protection
All motor installations shall be short-circuit protected, as mentioned either by use of fuses or by using circuit breakers. The short-circuit protection can be executed according to the short-circuit coordination Type 1 or Type 2. Type 1 protects the installation, but not the connected components. Type 2 protects both the installation and components.
The size of the protection to be chosen can be seen in the technical data for the soft For example will 1800 A2s correspond to it that the soft starter can be protected by a 16 A D 0 fuse (gL/gG). (The short-circuit current shall be able to exceed 120A in order to protect in a proper way).
Under the same conditions it can be mentioned that a circuit breaker offers sufficient short-circuit protection acc. to Type 2, which is preferable.

Ambient temperature
The soft starter shall fulfil the specifications described in the product norm for soft starters EN60947-4-2. Here all technical specifications are to be mentioned at an ambient temperature of 40oC. Soft starters have a power dissipation corresponding to about 1W/A/regulated phase. This means that for a 3-phased regulated soft starter with a load of 10 A, a power dissipation of about 30 W will occur. In some cases it may be an advantage to mount anelectro mechanic contactor as by-pass of the soft starter. By this, the power dissipation is reduced substantially, and thus a fan for cooling of the panel should be superfluous.

Some soft starters are provided with a built-in by-pass contactor.
A feature of these soft starters is that most often they do not allow large intermittent loads and that the number of starts per hour is limited. Furthermore, it is most often difficult to get a short-circuit protection acc. to short-circuit coordination Type 2.

When you choose soft starters it is important to take the following into consideration:
• Load class
• Thermal overload protection
• Number of starts per hour
• Short-circuit protection
• Environment

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