Rotor design

IEC 600.14-12 has recommended four rotor designs, i.e. N. H and NY. HY, to define starting performance for DOL and 17A startings respectively. They are along similar lines, ones to those in NEMA MG-I. They define minimum torques, though the manufacturer can produce better ones. 

NEMA, in its publication MG-1 for induction Motors, has prescribed four rotor designs. A, B, C, and D, covering almost all sizes of LT motors, to possess a prescribed minimum 77, Tp and pull-up torques. These torques are generally as drawn in Figure 2..3 to meet all normal industrial, agricultural or domestic needs. (Refer to the said publication or lEC 60034-12 for values of these torques. lEC 60034-12 has also provided similar stipulations.)... 

Manufacturers, depending upon market needs, may adopt all or a few such designs or even have their own designs, still conforming to such stipulations. Special applications may, however, call for a custom-built motor as noted later. As a standard practice all HT motors are custom-built for each application and no rotor designs are prescribed for these. 

With a view to achieve yet more standardization in motor design, NEMA Standard MG-1 has also recommended the maximum locked rotor current of single-speed three-phase motors for the various rotor designs A, B, C, and D, for various recommended torque values. These have been derived for a 415 V a.c. system and are shown in Table 2.2. 

Table 2.2 Recommended maximum locked rotor currents for various rotor designs...

HP Approx, maximum Rotor design locked rotor current ... 

Note For such duties, the starting heat is kept as low as possible by suitable rotor design to eliminate the effect of frequent starts and stops. The margin for starting heat and braking heat should be taken into account if these are considerable. The manufacturer is a better guide for suggestions here. 

The rotor design, its cooling system or the motor size itself may have to be changed substantially for motors to be installed in fire hazard environments to limit their temperature rise in adverse operating conditions within safe limits (Table 7,6). 

Motor speed-torque curve NEMA rotor designs Special designs of rotors Effect of starting current on torque Load torque or opposing torque Selection of motors Time of start-up and its effect on motor performance Thermal withstand time... 

Robust rotor design is needed to handle unbalanee eaused by iee deposits. 

The maximum permissible vibration, in mils (0.001 in.), during shop test at rated speed shall be equal to the square root of 20,000 divided by the sum of rated speed plus meeh-anieal and eleetrieal runout for the overhung rotor design. Only the aetual total measured runout may be subtraeted from the unfiltered peak-to-peak amplitude measured during testing to attain the shaft vibration. The meehanieal-plus-eleetrieal runout subtraeted from the unfiltered peak-to-peak amplitude shall not exeeed 0.5 mils regardless of total runout. 

Figure 8-11 shows the effleieney variation with the tip speed ratio. This eurve also shows the runaway speed. Runaway speed is aehieved when turbine torque falls to zero at blade speeds higher than the design speed. If failure oeeurs above the tip speed, the rotor ean be defined as a fail-safe rotor design. 

Evenson, R.S., Mason, B., Frederick, D.V., St. Onge, and Alain, G. Development and Field Application of a Single Rotor Design Dry Gas Seal, Proceedings of the 24th Turbomachinery Symposium, Texas A M University p. 107 1995. 

Figure 8 Internal mixer rotor designs (a) tangential rotor, (b) intermeshing rotor.

FIGURE 35.19 High distributive mixing (HDM) rotor design (tangential) of Pomini SpA. 

In the case of the intermeshing rotor machine the majority of the mixing action takes place between the rotors with the shearing action being created by the rotor design. Unlike the tangential mixers where the rotors revolve at different speeds, the rotors in intermeshing machines run at the same speed. 

If the value of VERTICAL.TUBE.ROTORS is found to be true (or "yes") then clause 2 of the rule is evaluated. The references to "fact" in clause 2 cause the system to refer to a table that contains the facts for particular rotors. References to the facts ROTOR.DESIGN, TUBE.VOLUME, and K.FACTOR are applications of particular constraints to the rotors. For example, two constraints are that the rotor must have a tube volume greater than 1 mL and a k factor less than 50. Clause 3 further pares the set of rotors on the basis of k factor by taking only the best rotor and any rotor with a k factor within 50% of the k factor of the best rotor. 

Figure 4. A rule that selects rotors to minimize the run time in a plasmid DNA separation. The rule examines a set of rotors called USERS.MATCHED.ROTORS, selecting those rotors that satisfy criteria based on the rotor design, tube volume, and k factor.

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