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A commonly used means of reducing inrush currents without the need of external devices is star-delta motor starting (sometimes called wye-delta starting). ill. 1 shows a typical star-delta starter.
Star-delta motors are similar in construction to standard squirrel cage motors. However, in star-delta motors, both ends of each of the three windings are brought out to the terminals. If the starter used has the required number of properly wired contacts, the motor can be started in star and run in delta.
The motor must be wound in such a manner that it will run with its stator windings connected in delta. The leads of all of the windings must be brought out to the motor terminals for their proper connection in the field.
The primary applications of star-delta motors are for driving centrifugal chillers of large, central air-conditioning units for loads such as fans, blowers, pumps, or centrifuges, and for situations where a reduced starting torque is necessary. Star-delta motors also may be used where a reduced starting current is required.
Because all of the stator winding is used and there are no limiting devices such as resistors or autotransformers, star-delta motors are widely used on loads having high inertia and a long acceleration period.
The speed of a star-delta, squirrel cage induction motor depends on the number of poles of the motor and the supply line frequency (hertz). Because both of these values are constant, the motor will run at approximately the same speed for either the star or delta connection. The in rush and line current are less when the motor is connected in star than when it's connected in delta. The winding current is less than the line current when the motor is connected in delta.
That is, the inrush and line current in the star connection are one-third the values of these quantities in the delta connection. Starting on the higher current delta connection would bump the voltage and create line disturbances.
Three overload relays are furnished on star-delta starters. These relays are wired so that they carry the motor winding current, ill. 2. This means that the relay units must be selected on the basis of the winding current, not the delta-connected full-load cur rent. If the motor nameplate indicates only the delta-connected full-load current, divide this value by 1.73 to obtain the winding cur rent, which is used as the basis for selecting the motor winding protection. A schematic showing the entire connection of the motor stator winding for a star-delta starter is shown in ill. 3.
Open transition starting for a star-delta starter is shown in ill. 4. As indicated in the line diagram on the right, the automatic transfer from star to delta is accomplished by a pneumatic timer. The timer is operated by the movement of the armature of one of the contactors. Operating the push-button station start button energizes the contactor S. The main contacts of contactor S connect three of the motor leads together (T4, T5, and T6) to form a star. At about the same time, the normally open, control contact (S) of the same contactor energizes another contactor (1M) and maintains itself.
Because the pneumatic timer is attached to contactor (1M), the motor is connected to the line in star and the timing period is started. When the timing period is complete, the first contactor (S) is de-energized. As a result, normally closed interlock S is closed, and contactor 2M is energized to connect the motor in delta. The motor then runs in the delta-connected configuration.
This start-run scheme is called open transition because there is a moment in which the motor circuit's open between the opening of the power contacts (S) and the closing of the contacts (2M).
One advantage of this starting method is that it does not require accessory voltage reducing equipment. A star-delta starter has the disadvantage of open circuit transition, but it does give a larger starting torque per ampere of line current than a part winding starter.
ill. 5 shows a modification of ill. 4. In ill. 5, resistors maintain continuity to the motor to avoid the difficulties associated with the open circuit form of transition between start and run.
With closed transition starting, the transfer from the star to delta connections is made without disconnecting the motor from the line. When the transfer from star to delta is made in open transition starting, the starter momentarily disconnects the motor and then reconnects it in delta. Although an open transition is satisfactory in many cases, some installations may require closed transition starting to prevent power line disturbances.
A size 6 closed transition starter is shown in ill. 6.Closed transition starting is achieved by adding a three-pole contactor and three resistors to the starter circuit. The connections are made as shown in the closed transition schematic diagram, ill. 5.The contactor is energized only during the transition from star to delta. It keeps the motor connected to the power source through the resistors during the transition period, ill. 7. There is a reduction in the incremental current surge, which results from the transition.
The balance of the operating sequence of the closed transition starter is similar to that of the open transition star-delta motor starter.
A single method of reduced current starting may not achieve the desired results because the motor starting requirements are so involved, the restrictions so stringent, and the needs are so conflicting. It may be necessary to use a combination of starting methods before satisfactory performance is realized. For special installations, it may be necessary to design a starting system to fit the particular conditions.
1. Indicate the correct terminal markings for a star-delta motor on ill. 8.
2. What is the principal reason for using star-delta motors?
3. In ill. 5,which contactor closes the transition?
4. The closed transition contactor is energized only on transfer from star to delta. How is this accomplished?
5. If a delta-connected, six-lead motor nameplate reads "Full Load Current 170 Amperes," on what current rating should the overload relay setting or the selection of the heater elements be based?
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