Parametric Analysis of Starting Performance of Asynchronous Motors

The starting of an electric motor refers to the process of its rotor running from the stationary state to the rated speed. The starting performance of asynchronous motor is mainly analyzed and judged from the parameters of starting current, starting torque, starting time, starting loss, and starting process heat. Among them, starting current and starting torque are two very important performance parameters of motor products.

For the motor starting characteristics, we have talked a lot in the previous article, the ideal state is the motor can have a large starting torque, but at the same time there should be a smaller starting current and a shorter starting time, the starting process of the motor heat is small, etc. Because the current is too large, both on the grid or the motor itself will have a large impact, may lead to a large voltage drop in the grid, so that the motor or equipment connected to it can not work properly. Because the current is too large, whether on the power grid or the motor itself will have a greater impact, may lead to a large voltage drop in the grid, so that it is connected to the motor or equipment can not work properly, the motor itself because the current is too large and the winding overheating adverse effects. Especially for motors that are frequently started, their starting performance is even more important. How to achieve a smaller starting current, larger starting torque effect, we have talked about in the past, will not repeat here.

For this problem, we analyze from the start of the squirrel cage motor. When the motor starts instantaneously, the speed of the motor is 0, the differential rate is 1, the rotating magnetic field cuts the rotor windings or guide bars at synchronous speed, inducing a large electric potential and generating a large current in the rotor circuit, and the load component of the stator current in balance with it increases rapidly, and the corresponding stator current will be particularly large.

According to the equivalent circuit analysis, when the motor is running normally, the slip rate s of the asynchronous motor is very small, the rotor resistance corresponding to the electromagnetic torque is very large, the rotor current is limited from being too large, and the stator current load is balanced with the rotor current. The stator current (the vector sum of the load component and the excitation component) is also small. At the moment when the motor starts, the slip rate is 1. At this time, the rotor resistance corresponding to the electromagnetic torque is very small. Due to the skin effect, the equivalent impedance of the motor is also smaller than the rated speed, so the starting current is very large.

Now the question arises again, why the starting torque is not large, since the starting current is large, and this involves a very critical knowledge of the other parameters associated with the starting torque.

Starting torque = motor constant x main magnetic flux x starting current x power factor

It can be seen from the above formula, starting torque and the main flux, starting current and power factor are positively correlated. Motor starting, motor power factor is particularly small, although the current is large, but its active current component, that is, the product of current and power factor is not large. At the same time, due to the starting current is very large, so the leakage impedance voltage drop on the stator winding is large, the induced potential and the main magnetic flux value decreases.

When the motor uses a wound rotor, since a starting resistor can be connected in series in the rotor circuit, the following changes occur in the motor: On the one hand, while the starting current is reduced, the power factor of the motor is significantly improved; on the other hand, the starting current is reduced , the leakage impedance voltage drop of the stator winding will also be reduced, so the induced electromotive force and main magnetic flux will not be reduced much. With the combined effect of the two factors, the product of the main magnetic flux, starting current and power factor is guaranteed, and the purpose of reducing the starting current and increasing the starting torque is achieved. For cage motors, the starting current and starting torque are controlled by adjusting the rotor slot shape.