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The general high-voltage three-phase asynchronous motor commonly used in the project is divided into two kinds: one is the squirrel-cage AC asynchronous motor, while the other is wire-wound AC asynchronous motor. Both of the two motors are not equipped with function of self-speed regulation, which, however, is achieved by the speed regulating device through controlling the motor. The speed regulation of motor actually is to control the mechanical power of the motor, and the rotating speed of the motor is in direction proportion to the mechanical power. There are two principles to change the mechanical power - electromagnetic power control and loss power control. What is controlled by the electromagnetic power is the ideal no-load rotating speed, which is also the energy-saving speed regulation with the highest efficiency. What is controlled by the loss power is the rotating speed drop increase, which is the energy-consuming speed regulation with low efficiency. The two kinds of speed regulation, with the same technical performance, are the same in principle, but are different in method. At present, there are two most advanced ways of speed regulation by electromagnetic power, including internal feedback chopped wave speed regulation and speed regulation by frequency variation. The internal feedback chopped wave speed regulation is applicable to wire-wound internal feedback asynchronous motor, which directly changes the electromagnetic power of low-voltage rotor through increase of electric potential, and meanwhile feed the slip power of the rotor shifted out back to the internal feedback winding of the stator. The special advantages of internal feedback speed regulation are to avoid the high voltage of stator and realize low-voltage control of high-voltage motor. The high-voltage components of the speed regulating control system is in operation with low voltage, and the system is equipped with separate hydraulic soft-startup device, thus forming the combined functions of soft startup, soft bypass and soft switch. The system is characterized in advantages like simple operation, extremely high safety and reliability, low cost and convenient maintenance. The speed regulation by frequency variation is applicable to squirrel-cage asynchronous motor, which changes the electromagnetic power of stator through control on stator voltage. In order to avoid damage of main magnetic flux invariableness when adjusting the voltage and prevent larger amount of loss, the frequency variation is required at the same time as an auxiliary way to make the ratio between electric potential and frequency a fixed value. The whole set of high-voltage frequency variation is under high voltage, so it needs two variables as control voltage and frequency. The system is very large and complicated, so the heavy maintenance is required. It is also very difficult to control the harmonic produced by electric network, since there is no separate soft-startup device but a frequency changer. Once the frequency changer is in fault, it is difficult to start up the high-power motor only with the switch, because in this way, there will be a heavy impact on the motor with poorer safety and reliability as well as higher cost.
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