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GOALS:
- Calculate starting current for a motor with its windings connected in delta.
- Calculate starting current for a motor with its windings connected in wye.
- List requirements for wye-delta starting.
- Connect a motor for wye-delta starting.
- Discuss open and closed transition starting.
FIG. 1 Stator windings are connected in delta during the starting
period.
Wye-delta starting is often used with large horsepower motors to reduce in-rush current during the starting period and to reduce starting torque. Wye-delta starting is accomplished by connecting the motor stator windings in wye or star during the starting period and then re connecting them in delta during the run period. This is sometimes called soft starting. If the stator windings of a motor are connected in delta during the starting period, the starting current will be three times the value if the windings were connected in wye. Assume that a motor is to be connected to a 480 volt three-phase power line. Also assume that the motor windings have an impedance of 0.5 ohms when the motor is first started. If the stator windings are connected in delta (FIG. 1), the voltage across each phase winding will be 480 volts because line voltage and phase volt age are the same in a delta connection. The amount of current flow in each phase winding (stator winding) can be determined with Ohm's Law.
In a delta connection, the line current is greater than the phase current by a value of the square root of 3(__ 3) or 1.732. Therefore, the amount of line current will be:
ILINE _ IPHASE _ 1.732
ILINE _ 960 _ 1.732
ILINE _ 1662.72 A
If the stator windings are connected in wye (FIG. 2), the voltage across each phase winding will be 277 volts, because in a wye connected load, the phase voltage is less than the line voltage by a factor of the square root of 3 or 1.732.
EPHASE _ 277 V
The amount of in-rush current can be determined using Ohm's Law.
In a wye connected load, the line current and phase current are the same. Therefore, the starting current has been reduced from 1662.72 amperes to 554 amperes by connecting the stator windings in wye instead of delta during the starting period.
FIG. 2 The stator windings are connected in wye during the starting
period.
FIG. 3 Standard lead numbers for single voltage motors.
FIG. 4 Standard lead numbers for dual voltage motors.
Wye-Delta Starting Requirements
There are two requirements that must be met before wye-delta starting can be used.
1. The motor must be designed for the stator windings to be connected in delta during the run period. Motors can be designed to operate with their stator windings connected in either wye or delta. The actual power requirements are the same, depending on motor horsepower. The speed of a three-phase induction motor is determined by the number of stator poles per phase and the frequency of the applied voltage. There fore, the motor will operate at the same speed regardless of which connection is used when the motor is designed.
2. All stator windings leads must be accessible.
Motors designed to operate on a single voltage commonly supply three leads labeled T1, T2, and T3 at the terminal connection box located on the motor. Dual voltage motors commonly supply nine leads labeled T1 through T9 at the terminal connection box. If a motor is designed to operate on a single voltage, six terminal leads must be provided. The numbering for these six leads is shown in FIG. 3. Notice that the lead numbers are standardized for each of the three phases.
The opposite end of terminal lead T1 is T4, the opposite end of T2 is T5, and the opposite end of T3 is T6.
If the stator windings are to be connected in delta, terminals T1 and T6 are connected together, T2 and T4 are connected, and T3 and T5 are connected. If the stator windings are to be connected in wye, T4, T5, and T6 are connected together. Motors not intended for wye-delta starting would have these connections made internally and only three leads would be supplied at the terminal connection box. A motor with a delta connected stator, for example, would have T1 and T6 connected internally and a single lead labeled T1 would be provided for connection to the power line.
Wye connected motors have T4, T5, and T6 connected internally.
Dual Voltage Connections
Motors that are intended to operate on two voltages, such as 240 or 480 volts, contain two separate windings for each phase (FIG. 4). Notice that dual volt age motors actually contain 12 T leads. Dual voltage motors not intended for wye-delta connection will have certain terminals tied internally, as shown in FIG. 5. Although all three-phase dual voltage motors actually contain 12 T leads, only terminal leads T1 through T9 are brought out to the terminal connection box for motors not intended for wye-delta starting.
If the motor is to be operated on the higher voltage, the stator leads will be connected in series, as shown in FIG. 6. If the motor is to be connected for operation on the lower voltage, the stator windings will be connected in parallel as shown in FIG. 7.
Although dual voltage motors designed for wye delta starting will supply all 12 T leads at the terminal connection box, it will be necessary to make the proper connections for high or low voltage. The connection diagrams for dual voltage motors with 12 T leads are shown in FIG. 8. Note that the diagrams do not show connection to power leads. These connections are made as part of the control circuit.
FIG. 5 Nine lead dual voltage motors have some stator windings connected
together internally.
Connecting the Stator Leads
Wye-delta starting is accomplished by connecting the stator windings in wye during the starting period and then reconnecting them in delta for normal run operation. For simplicity, it will be assumed that the motor illustrated is designed for single voltage operation and has leads T1 through T6 brought out at the terminal connection box. If a dual voltage motor is to be connected, make the proper stator winding connections for high or low voltage operation and then change T4, T5, and T6 to T10, T11, and T12 in the following connections. A basic control circuit for wye-delta starting is shown in FIG. 9. This circuit employs time delay to determine when the windings will change from wye to delta. Starting circuits that sense motor speed or mo tor current to determine when to change the stator windings from wye to delta are also common.
When the Start button is pressed, control relay CR energizes, causing all CR contacts to close. This immediately energizes contactors 1M and S. The motor stator windings are now connected in wye, as shown in FIG. 10. The 1M load contacts connect power to the motor, and the S contacts form a wye connection for the stator windings.
The 1M auxiliary contact supplies power to the coil of timer TR. After a preset time delay, the two TR timed contacts change position. The normally closed contact opens and disconnects coil S, causing the S load contacts to open. The normally open TR contact closes and energizes contactor coil 2M. The motor stator windings are now connected in delta, FIG. 11.
Note that the 2M load contacts are used to make the delta connection. A diagram showing the connection of all load contacts is shown in FIG. 12.
The most critical part of connecting a wye-delta starter is making the actual load connections to the motor. An improper connection generally results in the motor stopping and reversing direction when transition is made from wye to delta. It is recommended that the circuit and components be numbered to help avoid mis takes in connection (FIG. 13).
FIG. 6 High voltage connection for nine lead motors.
Closed Transition Starting
The control circuit discussed so far uses open transition starting. This means that the motor is disconnected from the power line during the transition from wye to delta. This may be objectionable in some applications if the transition causes spikes on the power line when the motor changes from wye to delta. Another method that does not disconnect the motor from the power line is called closed transition starting. Closed transition starting is accomplished by adding another three pole contactor and resistors to the circuit (FIG. 14).
The added contactor, designated as 1A, energizes momentarily to connect resistors between the power line and motor when the transition is made from wye to delta. Also note that an on-delay timer (TR2) with a delay of 1 second has been added to the control circuit.
The purpose of this timer is to prevent a contact race between contactors S and 2M when power is first applied to the circuit. Without timer TR2, it would be possible for contactor 2M to energize before contactor S.
This would prevent the motor from being connected in wye. The motor would start with the stator windings connected in delta.
FIG. 7 Low voltage connection for nine lead motors.
FIG. 8 Stator winding connections for dual voltage twelve lead motors.
FIG. 9 Basic control circuit for a wye-delta starter using time delay.
FIG. 10 The stator windings are connected in wye for starting.
FIG. 11 The stator windings are connected in delta for running.
FIG. 12 Stator winding with all load contacts for wye-delta starting.
FIG. 13 Load circuit connections for wye-delta starting.
FIG. 14 Basic schematic diagram. Sizes 1, 2, 3, 4, and 5 wye-delta
starters with closed transition starting.
Overload Setting
Notice in FIG. 12 that the overload heaters are connected in the phase windings of the delta, not the line. For this reason, the overload heater rating must be reduced from the full load current rating on the motor nameplate. In a delta connection, the phase current will be less than the line current by a factor of the square root of 3, or 1.732. Assume, for example, that the nameplate indicates a full load current of 165 amperes.
If the motor stator windings are connected in delta, the current flow in each phase would be 95.3 amperes (165/1.732). The overload heater size should be based on a current of 95.3 amperes, not 165 amperes.
QUIZ
1. Name two requirements that must be met before a motor can be used for wye-delta starting.
2. The stator windings of a 2300 volt motor have an impedance of 6 ohms when the motor is first started. What would be the in-rush current if the stator windings were connected in delta?
3. What would be the amount of in-rush current if the motor described in question #2 had the stator windings connected in wye?
4. Refer to the circuit shown in FIG. 9. Assume that timer TR is set for a delay of 10 seconds. When the Start button is pressed, the motor starts with its windings connected in wye. After a period of one minute, the motor has not changed from wye to delta. Which of the following could cause this condition?
a. TR timer coil is open.
b. S contactor coil is open.
c. 1M starter coil is open.
d. The control transformer fuse is blown.
5. Refer to the circuit shown in FIG. 9. Assume that timer TR is set for a delay of 10 seconds. When the Start button is pressed, the motor does not start. After a delay of 10 seconds, the motor suddenly starts with its stator windings connected in delta. Which of the following could cause this problem?
a. TR timer coil is open.
b. 2M contactor coil is open.
c. S contactor coil is open.
d. 1M starter coil is open.
6. Refer to the circuit shown in FIG. 9. What is the purpose of the normally closed 2M and S contacts in the schematic?
7. The motor nameplate of a wye-delta starter motor has a full load current of 287 amperes. What current rating should be used to determine the proper overload heater size?
8. Refer to the circuit shown in FIG. 14.
When the motor changes from wye to delta, what causes contactor coil S to de-energize and open S contacts?
9. Refer to the circuit shown in FIG. 14. What is the purpose of timer TR2?
10. Refer to the circuit shown in FIG. 14. When the Start button is pressed, the control transformer fuse blows immediately. Which of the following could not cause this problem?
a. Control Relay coil CR is shorted.
b. Starter coil 1M is shorted.
c. Contactor coil S is shorted.
d. Contactor coil 2M is shorted.