Voltage Unbalanced

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. 

Figure 12.3 Heating effect caused by an unbalanced voltage system...

Figure 12.4(b) Equivalent circuit diagram with an unbalanced supply voltage (at slip = 2 - S)... 

Figure 12.5 Equivalent stator currents during unbalanced voltage...

The unbalanced voltage will produce an additional rotor current at nearly twice the supply frequency. For example, for a 2% slip, i.e. a slip of 1 Hz, the negative sequence stator current, due to an unbalanced supply voltage, will induce a rotor current at a frequency of (2/- 1) = 99 Hz for a 50 Hz system. These high-frequency currents will produce significant skin effects in the rotor bars and cause high eddy current and hysteresis losses (Section 1.6.2(A-iv)). Total rotor heat may be represented by... 

Industrial platinum resistance thermometer sensors Effects of unbalanced voltages on the performance of 30 cage induction motors BS EN 60751/1996... 

Stator heat on an unbalanced voltage (a) Maximum theoretical heat... 

Unbalanced Voltages and Single Phasing Protection, M/s Minilee Controls Private Ltd. 

When the primary of a three-phase two-winding transformer, having its secondary wound for a three-phase open delta, is connected across an unbalanced supply system, a residual voltage across the open delta will appear. This is the principle on which this transformer is based (Figure 15.4(a)). As discussed in Section 21.2.2, and illustrated in Figure 21.7, the phasor sum of all the three line to ground voltages in a three-phase balanced system is zero, i.e. 

Nevertheless, unbalance is a major occurrence in either case and an unbalanced protection, in addition to short-circuit protection is imperative in all types of capacitor units. Hence it is also possible to design series protected capacitor units in larger sizes and for higher voltages, which are more economical, compared to smaller units. 

Unbalanced voltage across (he open delta. This is for grounded systems. For isolated systems, it will be three limes this... 

Consider a bus duct having a rated current of 4000 A and an unbalanced current in the middle phase of 4400 A. Determine the size of the reactor to achieve a balanced voltage system. 

Two-phase Three-wire.— Each single-phase current has its own outgoing wire. The return wire serves both. Voltage between two outside wires is 41 per cent more than that between outside and return wire. Current in return wire 41 per cent greater than in cither outside wire. An unbalanced system. 

The accurate determination of the linearity of the furnace heating rate is not an easy matter. From a plot of temperature versus time, as shown in Figure 6.15a, the heating rate can be estimated to 5% or better. To determine minute fluctuations of the furnace heating-rate curve, the curve in Figure 6.15b is used. A DTA curve is recorded with the reference chamber filled with an inert material (a-alumina, for example) while the sample chamber is empty. In this unbalanced condition, the fluctuations in the power input voltage are easily seen. This type of behavior is also seen if the AT thermocouples are unsymmetrically located in the furnace chamber (46). It should be noted that it is even more difficult to measure accurately heating rates of l°C/min or less. 

For balanced loads and for voltages above 1000 V a 3-wire source is used and hence only three bnsbars are needed. Unbalanced loads are seldom enconntered at high voltages. Typical busbar normal cnrrent ratings nsed in the oil industry are shown in Table 7.1. 

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

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