Articles / Quasi-frequency control method for soft start asynchronous motors

The soft-start for asynchronous motors are cheap and reliable solution for the vast majority of mechanisms without speed regulation. The advantages of using the soft-start obvious. However, there are drawbacks. Reduced starting torque of the engine is the main obstacle to the use of soft-start device for asynchronous motors. At the "heavy starting" it is possible that the soft starter can not cope with its function. In practice one has to increase the inrush currents that always causes increased heating of the motor, and sometimes leads to failure of the soft starter or even motor. As a result of these, mechanisms have to be equipped with expensive frequency converters. Today, however, to replace the classical phase regulation in the soft starter comes a new control algorithm, which allows to combine the advantages of frequency control and cheapness soft starter. This quasi-frequency control.

What exactly is a quasi-frequency control?

Quasi-frequency control - a special algorithm for thinning of the control pulses arriving at the power thyristors, which allows to form motor current low frequency. Reducing the frequency of the current increases the torque. This method can generate a frequency of 7.14 Hz, 12.50 Hz and 16.67 Hz. As you can see, use only quasi-frequency method is impossible to disperse the engine to full speed. Gives the best result a combination of classical and quasi-frequency method.

Fig.1. Classic start                                  Fig.2. Quasi-frequency start

Figure 1 shows the current waveform (yellow line) in the classic start at zero speed. Blue line - torque sensor. For comparison, Figure 2 shows the current waveform at the quasi-frequency algorithm (frequency of 7.14 Hz) under the same conditions. Although the shapes of the currents differ greatly, the actual current is practically the same for both modes. When the quasi-frequency start, torque a lot more!

What additional benefits of using a quasi-frequency start?

At low speeds, the soft starter with a quasi-frequency control method allows some brake modes. The braking performance is dependent on the motor speed and decreases with an increase speed. At rated speed braking torque is missing, however, is at a rate of 50-60% of the nominal braking effect is quite significant. Thus, the soft starter with quasi-frequency algorithm allows to completely abandon the dynamic braking device on some mechanisms.
Algorithm quasi-frequency control allows make reverse motor at a frequency of 25 Hz without changing the phase sequence of the supply voltage.

At present the method of quasi-frequency control allows you to get the highest starting torque at the lowest current.

The enterprise MIKONT Ltd realized the possibility of combining the quasi-frequency algorithm and phase control with current limit at a given level and is used in soft starters induction motors.

The soft starter with a quasi-frequency control is implemented on the basis of the controller «PROTON».