【正文】 s interest. In order to obtain the improvement, the stochastic PWM method arises at the historic moment. !ts principle is the stochastic change turnon frequency causes the electrical machinery electromagnetism noise to be limited to approximately the belt white noise (in linear tiequency coordinate system, various frequencies energy distribution is even), although the noise a decibel number has not always changed, but weakens greatly take the fixed turnon frequency as the characteristic colored noise intensity. Because of this, even if in 1GBT by widespread application today has had to limit regarding the carrier frequency is paring the low frequency the situation, stochastic PWM still had its special value: On the other hand explained eliminates the machinery and the electromagnetism noise best method enhances the operating frequency blindly, the stochastic PWM technology was precisely provides an analysis, has solved this kind ofqucstion brandnew mentality.Spatial voltage vector control PWM Spatial voltage vector control PWM (SVPWM) also calls the magnetic flux sine PWM law .It take the threephase profile whole production eficet as the premise, take approaches the electrical machinery air gap the ideal circular rotary field path as the goal, has the actual magnetic flux with the inventor ditTcrent sv}itett pattern to approach the base director circle magnetic flux, by theirs parison result decided the inventor the switch, forms the PWM profile. This law embarks from the electric motor angle, regards as the inventor and the electrical machinery a whole, inscribes the polygon to approach the circle the way to carry on the control, causes the electrical machinery to obtain the peaktopeak value constant circular magnetic field(sine magnetic flux). The concrete method divides into the magnetic flux splitring type and themagnetic flux closed loop type. The magnetic flux splitring law synthesizes anequivalent voltage vector with a two nanvanishing vector sum null vector, if the sampling time enough is small, may synthesize the random voltage vector. When this law output r。 A temperature regulator must maintain the temperature of say, an oven constant despite the heat loss in the oven. A voltage regulator must also maintain must output voltage constant despite variation in the load current. For any system to provide an output, ., speed, temperature, voltage, etc, an error signal must exist under steadystate conditions.electrical brakingIn many speed control system, ., rolling mills mine winders, etc., the load has to be frequently brought to a standstill and reversed. The rate at which the speed reduces following a reduced speed demand is dcpdent on the stored crgy and the braking system used. A small speed control system (sometimes known as a velodyne) can employ mechanical braking, but this is not feasible with large speed controllers since it is difficult and costly to remove the heat generated.The various methods of electrical braking avaiable are:(I)Regenerative braking.(2) Eddy current braking(3) Dynamic braking.(4) Reverse current braking(plugging).Regenerative braking is the best method, though not necessarily the most economic. The stored energy in the load is converted into electrical energy by the work motor(acting temporarily，a gcrator) and is returned to the power supply system, The supply system thus acts as a sink into which the unwanted energy is delivered. Providing the supply system has adequate capacity, the consequent rise in terminal voltage will be small during the short periods of regeneration. In the WardLeonard method of speed control of DC motors, regenerative braking is inherent, but thyristor drives have to be arranged to invert to regenerate. Induction motor driver can regcrate if the rotar shaft is driven faster than speed of the field. The advent of lowcost variable variablefrequency supplies from rotating Thyristor inverters have brought about considerable charges in the use of induction motors in variable speed drives. Eddy current braking can be applied to any machine, simply by mounting a copper or aluminium disc on the shaft and rotating it in a magnetic field. The problem of removing the heat generated is severe in large system as the temperature of the shaft, bearing, and motor will be raised if prolonged braking is applied.In dynamic breaking, the stored energy is a resistor in the circuit. When applied to small DC machines, the armature supply is disconnected and a resistor is connected across the armature (usually by a relay, contactor, or thyristor). The field voltage is maintained, and braking is applied down to the lowest speed. Induction motors require a somewhat more plex arrangement, the stator windings being disconnected from the AC supply and reconnected to a DC supply. The electrical energy generated isthen dissipated in the rotor circuit. bynamic braking is applied to many large AC hoist system where the braking duty is Goth severe and prolonged.Any electrical motor can be brought to a standstill by suddenly reconnecting the supply to rcvcssc the direction of rotation (reverse current braking). Applied under controlled conditions, this method of braking is satisfactory for ail drivers. lts major disadvantabe is that the electrical energy consumed by the machine when braking is equal to tho stored orgy in the load. This increases the running cost significantly in large drives.Equal pulse width PWM law VVVF (Variable Voltage Variable Frequency) installs in the early time is uses PAM (Pulse Amplitude Modulation) to control, its inventor part which the technology realizes but only can output the frequency adjustable squarewave voltag。 s Guide .1987.[17]Astrom K J, Hang C C, Persson and Ho W K,Towards Intelligent PID Control. Automation, 1992, 28 (1):19[18]Ho WK, Lim and Xu Wen, Optimal gain and phase margin tuning for PID controllers Automation, 1998,vo1，34,No,8:1009