Star-delta motors

The electrolytes are non-corrosive and the electrodes do not corrode with time. This feature is of special significance when compared with an ordinary liquid resistance starter used commonly for slip-ring motors. Electrolytes do not deteriorate and therefore do not require replacement. The evaporated liquid can be replenished with drinking water when the level of the electrolyte falls as a result of evaporation. In Europe such starters have been used for over 15-20 years. Electrolyte switching is a costlier proposition compared to direct on-line or star/delta switching due to additional shorting contactor and timer, and the cost of electrolyte, its tank and thermostatic control etc. The cost may. 

Soft slarlers, star-delta, wound motor and A/T starters, or when the prime mover is fitted with centrifugal dutches or delayed-action fluid couplings... 

Star/delta or auto-transformer switching of a motor For motors compensated individually and having a star/ delta or auto-transformer switching, the changeover from star to delta windings or from one step of the auto-... 

Depending on the value of the resistor, the motor will, in general, draw a heavier current then when started using star-delta or auto-transformer methods. The resistors must be rated to carry the limited motor starting current for the time they are in circuit. 

Select special-purpose designs for the motor in which the winding arrangements are modified by external switching devices that are matched to the motor, e.g. star-delta motor and starter. 

Figure 5.7 Circuit diagram for an induction motor using a star-delta starter.

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. 

In Europe the main motor is usually an AC motor, using a star-delta starter. An inverter for the main motor is becoming more common, particularly with the smaller decanters. The inverter enables a soft start, and allows speed adjustment for different process requirements. Inverter motors can cause unwanted electrical interference, and harmonic wave forms, on the main electrical supply lines. These problems can be minimised by using electrical filters and the latest advanced electrical technology. 

The power required to accelerate the high inertia bowl to speed can be considerably higher than that required to rotate the bowl at speed. Thus, to start the decanter, the main drive motor need special consideration. This could be one of a number of alternatives, a soft start motor, a specially wound motor, a star-delta starter, or a motor with a fluid coupling. A patented alternative (Alfa Laval) to these methods, used on large decanters, is a dual set of motors [19], Both motors are energised for start up and one is switched off when the bowl is at speed. 

When large industrial motors have to be started, a way of reducing the excessive starting currents must be found. One method is to connect the motor to a star delta motor starter. 

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. 

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. 

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 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. 

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. 

Timers are integral parts of some of the control systems. They are used in the starter of the main motor, to switch from star to delta operation. They are used in control systems, for the sequential start-up and shut-down of ancillary equipment. Timers are used for the ageing of the polymer, and the on-time of... 

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