Unbalanced loads

Some equipment may be sensitive to unbalanced loading, unbalanced supply voltages or the harmonic content of the supply. 

Worker riding on load of dry wall, load unbalanced, worker fell, load fell on him 1... 

The bowl is suspended from an upper bearing and drive (electric or turbine motor) assembly through a flexible-drive spindle with a loose guide in a controlled damping assembly at the bottom. The unit finds its axis of rotation if it becomes slightly unbalanced due to process load. 

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. 

When a three-phase four-wire system feeds non-linear or single-phase loads this balance is upset and the unbalanced current flows through the neutral. The same relationship can be expressed as... 

Loads of welding sets, which have intermittent duty cycles, are mostly single phase, cause low p.f. and stress the generator windings intermittently. Unbalanced loads, such as single phase loads, distributed unevenly cause current unbalance and low p.f. 

In a G/F the three CTs will also measure the unbalanced load current, if any, in addition to G/F current. For an appropri.ite setting of the relay, therefore, it will be essential that the likely system unbalanced current be measured and the relay set in excess of this to detect a G/F, For systems feeding single-phase or unbalanced loads, prone to carrying high and widely fluctuating unbalanced neutral currents, it may be difficult to determine ihe likely amount of unbalance and provide a suitable setting for the G/F relay. In such cases use of four CTs or core-balanced CTs (if it is a four w ire system) would be more appropriate. 

In LT systems, where the neutral is groiintled, the neutral as well as the ground conductor tnay have to carry unbalanced currents up to hall the rating of the line currents due to single-phase loads. A ground conductor should also be rated for the same size as the neutral, irrespective of the setting of the relay. 

Gear elements must be multiplane and dynamically balanced. Where keys are used in couplings, half keys must be in place. The maximum allowable unbalanced force at maximum continuous speed should not exceed 10% of static weight load on the journal. The maximum allowable residual unbalance in the plane of each journal is calculated using the following relationship... 

The important point to note from this Example is that in a non-symmetrical laminate the behaviour is very complex. It can be seen that the effect of a simple uniaxial stress, or, is to produce strains and curvatures in all directions. This has relevance in a number of polymer processing situations because unbalanced cooling (for example) can result in layers which have different properties, across a moulding wall thickness. This is effectively a composite laminate structure which is likely to be non-symmetrical and complex behaviour can be expected when loading is applied. 

The size and proportion of the foundation block must be such that the resultant vertical load due to the compressor, block, and any unbalanced force falls within the base area. 

Turbulent or unbalanced media flow (i.e., aerodynamic or hydraulic instability) does not have the same quadratic impacts on the vibration profile as that of load change, but it increases the overall vibration energy. This generates a unique profile that can be used to quantify the level of instability present in the machine. The profile generated by unbalanced flow is visible at the vane or blade-pass frequency of the rotating element. In addition, the profile shows a marked increase in the random noise generated by the flow of gas or liquid through the machine. 

With the trend toward higher pressures in hydraulic systems, the loads on unbalanced pump and motor components become greater and this, coupled with the need for closer fits to contain the higher pressures, can introduce acute lubrication problems. Pumps, one of the main centers of wear, can be made smaller if they can run at higher speeds or higher pressures, but this is only possible with adequate lubrication. For this reason, a fluid with good lubrication properties is used so that hydraulics is now almost synonymous with oil hydraulics in general industrial applications. Mineral oils are inexpensive and readily obtainable while their viscosity can be matched to a particular job. 

Oxygen corrosion, occurring as General etch corrosionAmiform rate corrosion (less common form) Localized corrosion (takes several forms and common where waterside conditions are less than ideal) Rare in correctly treated boilers but can affect drum waterline and tubes. More common in idle and low-load boilers, where water chemistry is unbalanced, under high MU conditions and after poor chemical cleaning. Also in peak-load boilers, especially where deposition can occur. Localized corrosion can be very serious, causing metal failure. 

Just below critical speed Just above critical speed unbalanced load tends to deflection of spindle lags... 

Net unbalanced load to be resisted by passive pressure, (Section 7.7.1)... 

Expansion joints for free-movement systems can be designed for axial or offset movement alone, or for combined axial and offset movements (see Fig. 10-176). For offset movement alone, the end load due to pressure and weight can be transferred across the joint by tie rods or structural members (see Fig. 10-177). For axial or combined movements, anchors must be provided to absorb the unbalanced pressure load and force bellows to deflect. 

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