Is There an Advantage to Start a Wound Rotor Motor?

Compared to cage motors, wire-wound rotor motors are easier to start, which is one of the reasons why wire-wound motors are preferred in different applications.

In the previous article we talked about the squirrel cage motor starting methods, in addition to direct starting, reduced voltage starting of any one way can not be achieved to reduce the starting current at the same time to increase the starting torque, that is to say, when the starting current is reduced, the starting torque is also reduced, only to a different degree of reduction only.

For wire-wound rotor motor, can be in the rotor circuit into the starting resistance, can achieve the purpose of reducing the starting current, increase the starting torque. This method is not only suitable for the motor load running state, also suitable for the production process under the state of insufficient grid capacity of the mechanical properties of the motor test, especially for the size of the repair unit is not very large more practical. For example, blocking and open-circuit voltage detection is completed, access to a resistor, the motor will be easy to start. And then the motor no-load test.

In accordance with the starting characteristics of the winding motor, soft starting device motor is in the original collector ring structure derived from the new structure, that is, the winding rotor motor is no longer used collector ring, but the use of a soft starter, commonly used soft starter has a metal resistor wire structure, but also water resistance structure. For this type of motor test should ensure that the installation of soft starting device, pre-check the motor's rotor resistance, open-circuit voltage, plugging and electrical insulation properties, because the installation of soft starting after these items will not be able to carry out again, on the analysis of the reasons for this aspect, in our original article has also been introduced.

The actual operation of the motor, according to the load should be selected to match the resistance to ensure that the starting torque to achieve the motor starting requirements, and in the actual motor no-load test process, we can choose a suitable size of the frequency-sensitive varistor, that is, to fully meet the different specifications of the motor starting requirements.

Frequency sensitive varistor is a varistor whose equivalent impedance decreases with decreasing frequency. It is used for smooth starting of wound asynchronous motors. It is a three-phase reactor with a large iron core loss, the iron core is made of multiple solid iron or steel plates stacked with a certain thickness, generally made of three columns, with a coil wound on each column, and the three-phase coils are connected into a star shape and then connected to the rotor circuit of the wound asynchronous motor. According to the structure of the frequency sensitive varistor, is actually a three-phase transformer only in the original coil, the difference lies in the core material. The equivalent impedance of the frequency sensitive varistor is equal to the sum of the excitation impedance of the transformer and the leakage impedance of the original coil.

Frequency-sensitive varistor resistance to reflect the frequency of sensitive, is that it can be used as a winding rotor motor starting element elements, coupled with its motor speed with the stabilization of the characteristics of their own disconnection, the use of more convenient.

The frequency of rotor current is equal to the product of stator rotating magnetic field frequency and motor rotation rate, the instant the motor starts, the motor rotation rate is 1, the varistor core loss eddy current is larger, so its equivalent resistance value is also larger, so as to effectively control the starting current of the motor, and at the same time improve the starting torque of the motor; with the rise of the speed of the motor, the rate of rotation is gradually becoming smaller and smaller rotor frequency, and the associated equivalent resistance is also decreasing, to achieve the purpose of self "excision". With the increase of motor speed, the rotor frequency is also becoming smaller, and the associated equivalent resistance is also decreasing, to achieve the purpose of self "excision". In addition, by using frequency-sensitive varistor, the motor can be approSimoted to get constant torque characteristics, which realizes stepless starting of the motor.