In the three-phase asynchronous motors start-up process, we often want to have a large enough start-up torque, but the starting circuit should not be too large to meet the various mechanical properties while reducing the impact on the grid, but in practice these two requirements contradictory. For the squirrel-cage motor direct start, the starting current multiple of 4 to 7, while the starting torque multiplier only 0.9 to 1.3, start performance is not good.
Direct start, also known as full pressure start, that is, with the ordinary switch to the AC motor directly into the power grid. The larger the capacity of the power supply transformer, the smaller the system bus voltage caused by the starting current, allowing direct start-up as long as the rated voltage is not exceeded (10% to 15%).
Asynchronous motors direct start will have the following adverse effects:
(1) to reduce the grid voltage and affect other electrical equipment
(2) the motor overheating and accelerated insulation aging
(3) excessive electromagnetic force shock to the motor stator winding end deformation
Can be directly on the asynchronous motors starting current is too large how should we solve? Power greater motor starting current is very large, if multiplied by 4 to 7 times, the power grid will be a great impact. In practice, most manufacturers use the following methods to asynchronize the motor directly to start the current is too large to reduce the starting current.
(1) to reduce the power supply voltage (that is, step-down start, common, such as star delta start: start, the stator winding connected to the star, reducing the stator winding voltage; start is completed, into a triangle, with autotransformer start: Input autotransformer, reduce the stator winding voltage; start up, remove the autotransformer.)
(2) increase the set, the rotor impedance (start in the stator circuit in the string into the reactor, from the sub-pressure to reduce the voltage, reduce the starting current; start is complete, remove the reactor.)