Why the need for a smooth start of the induction motor

Now induction motors are ubiquitous due to its low cost, simple design and high reliability. Induction motors have been better engine DC almost everything except those things that required smooth speed control.

The soft starter (SCP) Just created to remove the disadvantages of asynchronous motors. In this article I will tell you about SCP more.

Why is it necessary to limit inrush currents

As it is known during starting of the motor in the first moments created significant start pulse, which is required to overcome the torque load on the shaft.

And in order to form this momentum, the engine must be quite a large amount of power, which he takes from the connected network.

And here comes the first problem, namely, undervoltage, which may adversely affect the load connected to the same network.

For example, the strong drawdown of stress can cause an increase in current consumption of the connected engines, which further squandered voltage and can cause a complete stop rotating mechanisms.

A further negative factor is large inrush currents that flow when the rotor windings is isolated by a great amount of heat energy that may damage the insulation and thus the failure of products.

A third adverse factor is the possibility of mechanical damage by a sharp jerk.

Here, in order to eliminate these negative factors, and the need to limit inrush current, to implement the so-called soft-start.

What are ways to limit inrush currents exist

So, in order to smoothly run an induction motor and to minimize the inrush current, there are two options:

1. The stator winding current is reduced. To this end, the winding is divided into three separate coils and connect on a star system. The free ends of the leads on the collectors (contact ring), which are fixed on the stem shaft.

Further connected to the collector resistor, the resistance of which the first starting point has a maximum value. As the rheostat resistance is reduced on the rotor current is increased, which means that the motor starts to spin.

Such plants are called motors with wound rotor and are actively used in cranes, trolley buses and trams.

2. By reducing the voltage and current that are transmitted on the stator. Such an embodiment is realized as follows:

- With the help of auto-transformers and dimmers.

- a special scheme based on the SCR or triac. These are the key schemes are called Devices of smooth start (SCP) Which are also referred to as soft-starters.

Note. Frequency converters also allow you to implement a smooth start of the engine, only such adjustments compensate for a sharp jump in torque inrush currents also remain as high.

Schedule changes at different shift voltages embodiments is as follows:

A change of current and torque at the same time will look like this:

conclusion

As seen smooth start induction motor has obvious advantages but one such circuit complexity leads to its appreciation.

Therefore, before embarking on the implementation of the soft starter should be calculated, and whether the benefits of the "upgrade" or better use a motor with direct current, which allows you to smoothly adjust the speed without substantial financial injections.

Thank you for your attention!