Field-weakened

Field weakening unit to command the field strength lor speed regulation above the base speed. 

Field weakening - to obtain speed variation above the base speed. 

Solid-State DC Drives. The controlled-thyristor rectifier and separate-field DC motor is the solid-state motor drive in greatest use. The combination provides control over at least a 10 1 speed range, plus an additional two to three times by field weakening. Depending upon the power level, the rectifier is operated directly from the AC supply lines, or via a transformer. Typical speed regulation of 2% can be accomplished with a single control system. The horsepower and speed limitations are set by the DC motor, not by the semiconductor rectifiers. The DC motor and rectifier can be combined to any required power level. 

Speed adjuslmeiU from base speed downward is obtained by armature voltage control. The armalure current and torque in this range is limited by the thyristor ratings or motor temperature rise. Control above base speed at constant horsepower is obtained by field weakening. An example system is shown in Figure. 3-17a. In this... 

Lead length, flight width, and flight clearance were 203.2, 20, and 0.20 mm, respectively, in all sections of the screw. A 38 mm diameter screw cooling hole was drilled in the shank end of the screw, and it extended 4 diameters into the feed section. The first 2.5 diameters of the screw were inside a water-cooled smooth-bore feed casing. The motor was 450 kW and had a base speed of 1750 rpm. The gearbox reduction was 17.33 1 for a maximum screw speed of 101 rpm at the base motor speed. The motor could be field weakened to provide higher screw speeds. 

With field current control, the speed of a DC motor can be increased beyond its nominal speed by field weakening. Because voltage thereby remains constant at the armature, the latter is not suitable for actual speed feedback in the field suppression range. 

The most precise torque measurement is achieved in the area of nominal speed. Here, accuracies are achieved to approx. 2.5% of nominal torque. The greater the field weakening, the more torque measurement accuracy decreases. 

But on dehydration by heating, this crystal field weakens and the crystal becomes strained. The ESR intensity thus measured may indicate the extent of this strain. The acidity (vacant orbital) can be related to the strain or disorder developed within the crystal. 

Provided the acceleration rate is reduced above rated motor speed, the peak current in the motor can be reduced approximately in proportion to the gear ratio increase. In the example shown in Figure 5, the peak current is 15% lower in the field weakened case (higher gear ratio) than the case without field weakening. When calculating peak current as a percentage of rated... 

Figure 5. Comparison of motor current for fully fluxed and field weakened operation...

Case 1 No Field Weakening Case 2 Field Weakened... 

Operation of electric machines can be divided into two areas base load range and field weakening range. In the base load range, the machine is capable of delivering maximum torque (MNom or Mwax) at aU speeds from standstill. Nominal power is then available at nominal speed UNom ... 

Field weakening A method of increasing the speed of a wound field motor by reducing field... 

As the harder stocks are generally more scorchy , because they increase in temperature more rapidly when being mixed, lower speeds are used. To get increased torque at lower speeds very often a DC motor which is slant rated using field weakening is specified. What this means is that it is possible to specify a DC electric motor so that it develops its full horsepower at 80% full speed as well as at full speed, e.g. 100 hp at 1200 rpm and 100 hp at 1500 rpm. This means that the torque up to 1200 rpm is constant, and it is reduced between 1200 and 1500 rpm. The slant rated or field weakened specification is ideal for a mixer drive, so as to have high torque at low speeds and reduced torque at high speeds. 

Table 25 shows typically worst-case torque requirements for four extruder sizes. The range of screw speed represents plastomers at the low end and LDPE at the high end. The specific torque value can be simply mul-tiphed by the maximum screw speed to determine the recommended drive size. The transition from LDPE to plastomers is the most severe. When running LDPE and plastomers on the same machine, field weakening must be used to satisfy the increased screw speed and lower torque requirements of LDPE. The processing performance of narrow MWD, branched, unimodal and bimodal metallocene polymers is illustrated in Fig. 56. 

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