Unbalances

Low Density Gases. A fan may have to operate on low density gas because of temperature, altitude, gas composition (high water vapor content of the gas can be a cause of low density), reduced process pressure, or a combination of such causes. To develop a required pressure, the fan has to operate at a considerably higher speed than it would at atmospheric pressure, and hence it must operate much closer to top wheel speed. Bearing life is shorter, and the fan tends to vibrate more or can be overstressed more easily by a slight wheel unbalance. Abrasion of the blades from dust particles is more severe. Therefore, a sturdier fan is needed for low density gas service. 

An unsteady ain-flow unbalance that alternates between inlets can set up an alternating thrust pattern which can be very damagiag to beariags designed for low thrust load. Mechanical vibration and elastic deformation problems and diagnostic techniques for stmctural iaadequacies ia fan design are discussed ia Reference 16. 

The relationship of the dielectric constant of the cable insulation to crosstalk can be measured by testing two cables for crosstalk with the same dimension, but different insulation materials. The cable with the lower dielectric constant has less capacitance unbalance, thus resulting in lower crosstalk than the cable with the higher dielectric constant. 

Polyetherification is similar to a polycondensation process formation of high molecular weight polymer requires precise adjustment of composition to approximately 1 1 ratio of bisphenol to dihalosulfone. Trace amounts of water gready reduce the molecular weight attainable owing to side reactions that unbalance the stoichiometry (76). The reactivity of the halosulfone is in the order expected for two-step nucleophilic aromatic displacement reactions ... 

Pump vibration measurements and analysis of the ampHtude—frequency spectmm have led to the deterrnination of possible equipment ailments. Examples of frequency relationships to particular pump vibration problems are available (62). Eor example, a one-time (lx) frequency corresponding to a mnning speed usually indicates the presence of rotor unbalance, a 2 x peak may mean a bent shaft, and a - O.dx may be a symptom of an oil whip at the sleeve bearing. 

Fault to be detected Unbalance Misalignment Bent shaft Oil whirl Eccentricity Wear Faults In gears Faults In rolling element bearings... 

Running unbal- Preventive maintenance and operator checklist anced, vibration inspections due to worn bear-, effective vibration monitor/shutdown device ings or other mechanical problem such as product accumulation behind filter screen. ... 

Running unbal- Redesign feed distributor anced—vibration. different bowl speed (usually slower) due to uneven feeding effective vibration monitor/shutdown device Adjust feed rate to get uniform distribution ... 

Voltage unbalance System harmonics Voltage variation Frequency variation Ambient temperature and Altitude. 

As standard practice, all motors are designed for a balanced and virtually sinusoidal supply system, but it may not be feasible to obtain the designed supply conditions in practice. Hence, a motor is designed with a certain in-built capacity to sustain small amounts of voltage unbalances and some degree of harmonic quantities, such that the voltage waveform may still be regarded as sinusoidal. 

The performance of a motor is greatly influenced by a voltage unbalance in the supply system. It reduces its output and torque and results in a higher slip and rotor loss. This subject is covered in more detail in Section 12.2(v). For likely deratings, refer to Figure 12.1. Asystem with an unbalance of up to 1 % or so calls for no derating, whereas one having an unbalance of more than 5% is not recommended for an industrial application, because of a... 

Protection against voltage unbalance or negative phase sequence 12/291... 

Voltage unbalance (negative phase sequenceJ This causes negative sequence components and results in excessive heating of the motor windings. An unbalanced voltage may occur due to unevenly distributed single-phase loads. 

A motor is able to sustain a negative sequence voltage up to 2% for short durations, while a sustained unbalance may deteriorate its insulation and affect... 

A voltage unbalance may affect motor performance in the following ways ... 

Due to smaller output and torque, the slip would rise and add to the rotor s losses (mainly iron losses) (see Figure 12.3). The voltage unbalance can be calculated as follows ... 

Now The negalive sequence voltage is caused by an unbalance in the magnitude of voltages in the three phases, rather than in the phase angle. 

Figure 12.1 Derating in motor output due to voltage unbalance...

Note A supply system having a voltage unbalance of more than 5% is not recommended for an industrial application, which may have a number of electric motors connected on it. Rural distribution, however, is an exception due to excessive LT loads on the same network (Section 7.6), but such loads are mostly individual and not of the industrial type,... 

One can thus easily obtain the significance of the factor 6 to represent the status of the most affected winding of the motor in the event of a voltage unbalance resulting in a negative sequence current component. For more clarity, consider equations (12.4) and (12.6) to ascertain the similarity in both these equations. Since both must represent the maximum heating effect... 

Since single phasing is a condition of severe unbalance, it causes varying proportions of dangerous currents in the motor windings, as discussed below. 

Since an inter-turn fault also causes unbalance, it is protected automatically when a negative sequence protection is provided depending upon its sensitivity and the setting. 

These can be adjusted by varying the contact traverse. The mechanism s design is such that an increase or a difference in the line currents, due to voltage unbalance or single phasing, drives the mechanism towards the tripping lever. These relays operate at 100% of their setting and are therefore set at... 

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