3RW4026-1BB14 - Soft starters - SIEMENS

Soft starters are used to start three-phase asynchronous motors with reduced torque and starting current.

The SIRIUS soft starter family from Siemens includes 3 variants which differ in their breadth of functionality and price.

The SIRIUS 3RW30 and 3RW40 soft starters are suitable for simple standard applications.

The SIRIUS 3RW44 soft starter is used where the requirements for functionality are high, for example the need for communication via PROFIBUS, the provision of measured values and monitoring, or where starting conditions are extremely difficult.

The SIRIUS 3RW30 and 3RW40 soft starters include two thyristors with an antiparallel circuit on two of their three phases. For each of these two phases, there is a thyristor for the positive wave part and a thyristor for the negative wave part (see the image "Control by phase angle and configuration diagram of 'a two-phase soft starter with integrated bypass contacts'). The current in the third uncontrolled phase corresponds to the sum of the currents of the controlled phases. The phase angle allows the effective value of the motor voltage to be brought within a selectable starting time from a defined starting voltage to the rated motor voltage. The motor current is proportional to the voltage applied to the motor. The starting current is thus reduced as a function of the voltage applied to the motor. The rotational torque varies according to the square of the voltage applied to the motor. The starting torque is thus reduced according to the square of the voltage applied to the motor.

Once the motor has started, the thyristors are fully on and the full line voltage is applied across the motor terminals. Since motor voltage regulation is unnecessary in operation, the thyristors are shunted by internal bypass contacts which are rated for AC1 current. This avoids the heat dissipation that would result from continuous operation of the thyristors. This reduces heating in the direct environment of the equipment. The bypass contacts are protected in operation by an integrated electronic arc extinguishing system. This prevents damage to the shunt contacts when opening in the event of a fault, for example during a brief interruption of the control voltage, in the presence of mechanical vibrations or a fault in the electromagnet or the main contact spring.

SIRIUS 3RW30 and 3RW40 soft starters are offered as an alternative to direct and star-delta starters. Here are the essential advantages:

• Soft start

• Soft stop (3RW40 only)

• Switching without interruption and without current peaks straining the network

• Easy assembly and commissioning

• Compact, space-saving size

Here are the possible applications:

• Belt conveyor

• Roller conveyor

• Compressor

• Fan

• Pump

• Hydraulic pump

• Stirrer

• Circular saw / band saw

Benefits:

Belt conveyors, handling facilities:

• smooth start

• smooth stop Centrifugal pumps, piston pumps:

• prevention of water hammer

• extension of the lifespan of pipes Agitators, mixers:

• reduction of fan starting current:

• protects gears and V-belts