General informations
Asynchronous motors are the most widely used motors for machines. These motors have three coil windings, which are arranged in 120° offset manner, using the 3ph-AC the coil generates a rotation field, which revolves at 50 Hz with max. 3000 U / min. This rotary field induces a voltage in the armature winding. The arising alternating field of the armature attempts to lag the rotary field; however it never achieves its RPM. There exists a slip, which is strongly independent of the load.
In most of the cases the armature consists of grooved stators. There are Alu-rods of various forms embedded in the grooving, which are further connected to the front side with Alu-rings. These rods substitute the classical armature winding. There are round rod armatures, high rod armatures and double rod armatures. The start up behavior is influenced by the form of the armature.
A disadvantage here with an asynchronous motor is that the rod current increases extremely (6-8 times rated current) in case of standstill or low RPM. This creates high stress for the mains system, so that the safeguarding arrangements of main system often released. Apart from this the existing start up torque is too high for many applications, which leads frequently to mechanical disturbances. The starts up circuits are introduced in order to avoid these extreme conditions.
Start up circuits are based on the principle of the voltage controlling. The simplest start up circuit is the Star-Delta-Switching, which works in two voltage ranges, e.g. 230 V and 400 V. However in most of the cases this rough grading is not enough. In contrast the electronic soft start up devices enable a progressive voltage control after the Phase section principle with selectable start up time and initial voltage. The minimum necessary initial torque can be set using the initial voltage on the basis of the quadratic dependency of the voltage.
The correct setting of the initial torque is an essential point for the optimal and reliable operation of soft start up motor. There are two essential loading cases:
? Acceleration of carrier bulks, like in application of fans, planar conveyors or grinding discs. Here the torque curve starts from zero and develops quadratic via the RPM till the rated torque of the machine.
The initial voltage can start at zero. The motor begins almost immediately to rotate and the current flow is continuous. A higher initial voltage for this application again causes a torque and current bounce.
? Driving the machines with fixed initial torque like for example kneading machine, sawing machine or comminuting machines. In these cases the torque curve does not start at zero.
The initial voltage to be set should be selected in such a way that the fixed torque of the machine is overcome and the motor starts up. In case of very less set torque the motor takes unnecessary current, which causes losses and later releases the motor safeguarding systems.
