Rotor critical

Appears suddenly at or above rotor critical speed when critical is below one-half operating speed. Increasing speed increases vibration amplitude, blit whirl frequency remains constant. When speed IS decreased, vibration disappears below where it first appeared. Fnction-indnced rotor whirl Encountered m bnilt-np rotors or rotors with shrink fits or rotor disassembly to inspect fits, increase shrink fits Coupling friction has been known to induce whirl... 

Vibration peaks at specific speeds peaks can usually Rotor critical speeds CO... 

Appears on gears like rotor critical speed. Vibration Torsional resonance Usually occurs only during startup... 

Consider the dissociation of an ion AM" that may either dissociate to form the fragments and M or the INC [A", M] allowing free mutual rotation and thus reorientation of the particles. Within the INC, A" and M can recombine only if they attain a well-defined mutual orientation, i.e., the system has to freeze rotational degrees of freedom. Such a configuration allowing covalent bonds to be formed is termed locked-rotor critical configuration [169-171,177] and any reac-... 

In continuous operation, rotor and windings asymptotically attain their permanent operating temperatures at different values Trotor, Tstator. For the steady state, a hotter rotor (compared with the stator windings), Trotor > T stator/ indicates a rotor critical motor, whereas hotter stator windings (compared with the rotor), Tstator > Trotor, indicate a stator critical motor. Normally, motors with a rated power exceeding 1. .. 2 kW, are rotor critical, smaller motors are stator critical. ... 

Another example is given in Fig. 6.59. A rotor critical motor (windings in thermal class F) is to be rated for T2 and T3 operation. The rotor (here, for simplification, considered as a thermal homogeneous component) passes the T3 class temperature limit (minus 5 K) after time tE3 at point 3, and somewhat later the T2 class or rotor temperature limit (+300°C minus 10 K) after time fEi at point 1. Meanwhile, the stator windings have passed the... 

Figure 6.59 Thermal behaviour of an F class rotor critical motor, stalled at tQ after steady state at rated power.

Rotor critical motors with temperature sensors in the rotor require expensive data transmission techniques, e.g. Pt 100 resistances, data convertor with frequency modulator, and brushless data transmission to a stator-fixed... 

The important critical speed for a decanter is the lowest speed at which there is significant flexible deformation of the rotor. This speed is called the first rotor critical speed. Decanters will always have a certain unbalance, both due to the handling of solids from the process and due to wear on the rotor. Operating the decanter close to, or just above, the flexible critical speed of the rotor will result in high vibration levels and very high stresses in the rotor components. The critical value of the rotor speed will therefore be an upper limit for the operating speed, and the decanter must be operated below this speed with a safe margin. 

In general the bowl strength, the first rotor critical speed, and the maximum permissible speed of the main bearings control the maximum speed at which a decanter can be operated. 

Most centiifiigal compressor is operated at speed greater than 3,DOO rpm. Fiom Figure 7a, it shows that compressor head can be increased by Increasing its operating speed. However, increase compressor operating spe is a limited by the maximum impeller stress, rotor critical speed, and surge line. [5J... 

These estimates are frequently inaccurate because of second-order effects such as rotor saturation and harmonics. If the apphcation is at all critical, the motor manufac turer should be consiilted. 

At certain speeds, rotating masses become dynamicatty unstable and cause deflection and vibration in the rotor w hich may damage the motor. The speed at w hich such instability occurs is known as critical speed and occurs at different multiples of the rated speed. The masses must therefore rotate within 20T beknv or above the critical speeds to avoid such a situation. These vibrations settle down again at higher speeds above critical and recur at the next higher critical speed. 

Nevertheless, whenever the rotor is more critical, despite a higher rotor operating temperature, rotor thermal curves are provided by the manufacturer for facilitating protection for the rotor also through the stator. 

The first or free-free rotor bending mode frequency (based on the required margin for operation below the critical speed)... 

Critical speed is when the frequency of a periodic exciting frequency applied to the rotor-bearing support system corresponds to the natural frequency of the system. 

EiTatic high-frequency vibration amplitude and possibly an audible sound. Rotor mb Labyrinth mbs generally self-comect Disc mbs due to thrust bearing failure often self-comect temporarily through wean steel on steel shrill noise during wear Rotor deflection is critical speed... 

The standards define terms used in the industry and describe the basic design of the unit. It deals with the casing, rotors and shafts, wheels and blades, combustors, seals, bearings, critical speeds, pipe connections and auxiliary piping, mounting plates, weather-proofing, and acoustical treatment. 

A/ci =Rotor 1st critical, center frequency, cycles per minute A/gn =Critical speed, nth =Thp speed... 

Critical speeds correspond to the natural frequencies of the gears and the rotor bearings support system. A determination of the critical speed is made by knowing the natural frequency of the system and the forcing function. Typical forcing functions are caused by rotor unbalance, oil filters, misalignment, and a synchronous whirl. 

To summarize the importance of the critical speed concept, one should bear in mind that it allows an identification of the operation region of the rotor-bearing system, probable mode shapes, and approximate locations of peak amplitudes. 

Critical Speed Calculations for Rotor Bearing Systems... 

Methods for calculating undamped and damped critical speeds that closely follow the works of Prohl and Lund, respectively, are listed herein. Computer programs can be developed that use the equations shown in this section to provide estimations of the critical speeds of a given rotor for a range of bearing stiffness and damping parameters. 

This speed becomes critical when the frequency of excitation is equal to one of the natural frequencies of the system. In forced vibration, the system is a function of the frequencies. These frequencies can also be multiples of rotor speed excited by frequencies other than the speed frequency such as blade passing frequencies, gear mesh frequencies, and other component frequencies. Figure 5-20 shows that for forced vibration, the critical frequency remains constant at any shaft speed. The critical speeds occur at one-half, one, and two times the rotor speed. The effect of damping in forced vibration reduces the amplitude, but it does not affect the frequency at which this phenomenon occurs. 

Some initial impulse unbalance is often required to start the whirl motion. Newkirk has suggested that the effect is caused by interfaces of joints in a rotor (shrink fits) rather than defects in rotor material. This type of whirl phenomenon occurs only at rotational speeds above the first critical. The phenomenon may disappear and then reappear at a higher speed. Some success has been achieved in reducing this type of whirl by reducing the number of separate parts, restricting the shrink fits, and providing some lockup of assembled elements. 

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