Star and delta connections

The torque produced by the motor is therefore reduced in proportion to the square of the rms value of the applied voltage. Circuits are available for both star and delta connected motors. Closed loop feedback control may be used to adjust the firing of the thyristors, thereby making accurate speed regulation possible. These systems are only used for small machines, e.g. up to 20 kW because they tend to produce many harmonic currents and voltages in the supply. 

This method is applicable to single-star or delta-connected capacitor banks. Unbalance can be detected through the use of an RVT (residual voltage transformer) (Section 15.4.3). See Figure 26.4. The theory of operation is that any unbalance, of the system or the capacitor bank, will shift the neutral and reflect as the residual voltage across the open delta and can be used for the protective scheme. The unbalance voltage across the open delta in the event of failure of a unit in any series group can be expressed by... 

The lest voltage should be 2.5 V, irrespective of star- or delta-connected units. Tlie capacitor unit should be charged with the test voltage and discharged five times within 10 minutes. Within 5 minutes after the test the unit may be subjected to voltage lest (.3) between the terminals. The... 

Changeover from star- to delta-connection takes place automatically by means of a timer. At changeover, the motor is momentarily disconnected from the supply and the delta contactor is then closed. The closure of the delta contactor can cause a further surge of current to occur. The magnitude of the surge will depend on the speed of the motor at changeover, but it can be comparable to that taken when started direct on line. 

In an HT system, either the star is not grounded or it is a delta-connected system and hence the third harmonic is mostly absent, while the content of the. second harmonic nuiy be too small to be of any significance. For this purpose, where harmonic analysis is not possible, or for a new installation where the content of harmonies is not known, it is common practice to use a series reactor of 6% of the reactive value of the capacitors installed. This will suppress most of the harmonics by making the circuit inductive, up to almost the fourth harmonic, as derived subsequently. Where, however, second harmonics are significant, the circuit may be tuned for just below the second harmonic. To arrive at a more accurate choice of filters, it is better to conduct a harmonic analysis of the system through a harmonic analyser and ascertain the actual harmonic quantities and their magnitudes present in the system, and provide a correct series or parallel filter-circuits for each harmonic. 

These transformers are three-phase and may be connected for zig-zag or star/delta connections (Section 20.9.1). The delta may also be made open type by inserting a resistor across it to help adjust the zero-sequence impedance, if required. 

The reactor systems shown in Figures 11.12 and 11.13 are very similar, one being a star-connected system and the other a delta-connected system. The star system has the advantage of... 

The wound rotor consists of a laminated cylinder of silicon steel with copper coils embedded in slots around the circumference. The windings may be connected in star or delta and the end connections brought out to slip rings mounted on the shaft. Connection by carbon brushes can then be made to an external resistance to improve starting. 

The three-phase windings of an a.c. generator may be star connected or delta connected as shown in Fig. 2.54. The important relationship between phase and line currents and voltages is also shown. The square root of 3 (Vs) is simply a constant for three-phase circuits, and has a value of 1.732. The delta connection is used for electrical power transmission because only three conductors are required. Delta connection is also used to connect the windings of most three-phase motors because the phase windings are perfectly balanced and, therefore, do not require a neutral connection. 

Three identical loads each having a resistance of 30 n and inductive reactance of 40 are connected first in star and then in delta to a 400 V three-phase supply. Calculate the phase currents and line currents for each connection. 

This is the most commonly used method for measuring power in a three-phase, three-wire system since it can be used for both balanced and unbalanced loads connected in either star or delta. The current coils are connected to any two of the lines, and the voltage coils are connected to the other line, the one without a current coil connection, as shown in Fig. 4.19. Then,... 

I Normally the external resistances are connected in star. But for larger motors, calling for high resistance grids, they may also be connected in delta, to reduce their current rating and hence the cost. Insulation is not a limiting factor usually, in case of resistance grids. 

Many of the electrical engineering textbooks that include the subject of motors in their contents describe the eqnivalent circnit of an indnction motor as a series and parallel combination of resistances and reactances, see References 1 to 8. The eqnivalent circnit nsnaUy defines the situation for one of the three phases and so care needs to be taken to ensure that the final resnlts obtained apply to the complete motor. Care is also necessary in nsing the ohmic data from mannfacturers, they may have either star winding valnes or delta winding valnes and the choice may not be obvious. The equivalent circuit of most practical use is shown in Figure 5.1 for one star connected winding, where -... 

The torque available from the main motor varies according to the method of start up. whether it is star-delta or direct-on-line. A motor connected in star produces a starting torque one third of that provided when starting direct-on-line. Direct-on-line starting torque can be two and a half times the motor s full load torque, with a starting current of six times full load current. 

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