The throttle has the form of an active-inductive impedance the value of which automatically decreases when starting from the initial value to zero value due to the change in the magnitude and frequency of the rotor current. Then all the switching equipment of the electric motor rotor circuit is excluded.
This construction design is used for motors up to 30 kW. Single-phase throttles are used for larger engines and are included in each phase of the rotor. The throttle coils are a few tens of copper wire or copper bus turns the cross-section of which is selected by the motor rotor current. Spur lines are usually provided on the coils of the throttle to adjust the in-rush starting current during the electric drive setting. Any textbook on the fundamentals of the electric drive can give information about the influence of the inductive impedance inclusion in the induction motor rotor circuit.
The active-inductive throttle impedance effect allows providing the load starting torque more than the natural characteristic with a significant multiple of startup current.
Experience has shown that when installing a new crane with the use of throttles the cost of electrical equipment and electrical materials is reduced to 50%, and the amount of electrical installation work is reduced to 70%. Throttling electric drives have shown their effectiveness not only in the mechanisms of crane bridges and trolleys but also in lifting mechanisms.
At the present time, a thyristor speed control unit has been developed. It is capable of smoothly adjusting the speed from 10-15% of the rated speed to the nominal speed over the entire range of torque limited by the throttling characteristic, which, at present, only the frequency regulator can do.
On the score of reviewed above, the following conclusions can be drawn:
● a throttle with massive (in a ferromagnetic sense) iron creates an active-inductive resistance, the value of which automatically decreases with increasing motor speed at start-up;
● when the throttle is connected to the rotor circuit, the starting current decreases to a greater extent than the starting torque;
● the mechanical characteristics of the throttle drive are similar in appearance to the "excavator" characteristics, which are desirable for smooth starting of the electric motor;
● the working part of the mechanical characteristics of the throttle electric motor drive is slightly softer than the natural characteristics, which reduces the static speed of the mechanism by 3% -8%. This defect can be eliminated by shunting the rotor rings of the motor with a contactor after the start-up is completed;
● throttle electric drive with thyristor speed regulator provides all the required modes of operation of mechanisms for lifting crane mechanisms, including the implementation of rope slack selection modes and low landing speeds;
● experience has shown the efficiency of the throttle use in the crane mechanism drives, simplifying control schemes and dramatically increasing their operational reliability;
● as a result of scientific and practical research, it was possible to develop a number of throttles in various designs which are used in electric drives with engine power up to 125 kW;
● at present, a starting throttling is developed for high-voltage (6000V, 10000V) induction motors with a phase rotor from 250kW to 2000kW, used for large blowers, pumps, mills, rotating furnaces, etc. In this case, the throttle is used only to output the mechanism to static speed.
Questionnaire for the reconstruction cost calculating of a crane to a throttle electric drive
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