Star-delta starting used with large horsepower motors

Star-delta Motor Starting

Star-delta starting is frequently employed with high horsepower motors to mitigate inrush current during the start-up stage and to reduce starting torque. Star-delta starting involves initially connecting the motor stator windings in a star configuration during the motor startup phase, followed by reconnection in a delta configuration during the operational phase. This is occasionally referred to as soft starting.

When the stator windings of a motor are configured in delta during the starting phase, the starting current will be three times compared to the value if the windings were arranged in star.

Assume a motor is to be connected to a 480-volt three-phase power supply. Assume that the motor windings have an impedance of 0.5 ohms at the moment the motor is initially started. In a delta connection, the voltage across each phase winding will be 480 volts, as the line voltage and phase voltage are equal.

Refer to Figure 1.

The current flow in each phase winding (stator winding) can be calculated using Ohm’s Law.

Figure 1 – Stator windings are connected in delta during the starting period

• I_phase = E_phase / Z_phase
• I_phase = 480 / 0.5
• I_phase = 960 A

In a delta connection, the line current exceeds the phase current by a factor of the square root of three (√3) or 1.732. Consequently, the value of the line current will be:

• I_line = I_phase × 1.732
• I_line = 960 × 1.732
• I_line = 1662.72 A

When the stator windings are connected in a star configuration (see Figure 2), the voltage across each phase winding will equal 277 volts, since the phase voltage in a star-connected load is less than the line voltage by a factor of the square root of 3, approximately 1.732.

Figure 2 – The stator windings are connected in star (qye) during the starting period

• E_phase = E_line / 1.732
• E_phase = 480 / 1.732
• E_phase = 277 V

The magnitude of inrush current can be calculated via Ohm’s Law.

• I_phase
  = E_phase / Z_phase
• I_phase = 277 / 0.5
• I_phase = 554 A

When a load is connected in a star configuration, the line current and the phase current are identical. As a result, the starting current during the starting period has been decreased from 1662.72 amperes to 554 amperes.

This was accomplished by connecting the stator windings in a star configuration rather than a delta configuration.

Star-Delta Starting Requirements

Before the star-delta motor starting method can be utilized, there are two prerequisites that need to be satisfied:

Requirement #1

The motor must be engineered for the stator windings to be configured in delta during operation. Motors can be engineered to run with their stator windings configured in either star or delta arrangements. The real power needs remain consistent, contingent upon motor horsepower.

Consequently, the motor will keep running at a consistent speed irrespective of the connection employed during its design.

Requirement #2

Accessibility is required for all stator winding leads. Motors engineered for operation at a single voltage typically include three terminals designated T1, T2, and T3 at the terminal connection box on the motor. Dual voltage motors typically provide nine leads designated T1 through T9 at the terminal connection box.

A motor intended for operation at a single voltage must include six terminal leads. Figure 3 illustrates the numbering for these six leads.

Figure 3 – Standard lead numbers for single voltage motors

Keep in mind that all three phases use the same lead numbers. T4 is the terminal lead that is opposite T1, T2 is the terminal lead that is opposite T5, and T3 is the terminal lead that is opposite T6. The proper way to connect the stator windings in a delta configuration is to join terminals T1 and T6, T2 and T4, and T3 and T5.

For instance, a motor that uses a delta-connected stator would have T1 and T6 wired internally and one lead designated as T1 for connecting to the power source. The internal connections of a Wye-connected motor are T4, T5, and T6.

Dual Voltage Connections

Motors designed for dual voltage operation, such as 240 or 480 volts, use two distinct windings for each phase. Refer to Figure 4.

Figure 4 – Standard lead numbers for dual voltage motors

It is important to observe that dual voltage motors are equipped with 12 T leads. Dual voltage motors that are not designed for star-delta connections will have specific terminals internally connected, as illustrated in Figure 5.

While all three-phase dual voltage motors consist of 12 T leads, only the terminal leads T1 through T9 are routed to the terminal connection box for motors that are not designed for star-delta starting.

Figure 5 – Nine lead dual voltage motors have some stator windings connected together internally

To operate the motor at a higher voltage, the stator leads must be connected in series, as illustrated in Figure 6. To connect the motor for operation at a lower voltage, the stator windings should be arranged in parallel as illustrated in Figure 7.

Figure 6 – High voltage connection for nine lead motors

Figure 7 – Low voltage connection for nine lead motors

While dual voltage motors intended for star-delta starting will provide all 12 T leads at the terminal connection box, it is essential to establish the correct connections for either high or low voltage operation.

Figure 8 illustrates the connection diagrams for dual voltage motors featuring 12 T leads.

It is important to observe that the diagrams do not illustrate the connection to power leads. The connections are established within the framework of the control circuit.

Figure 8 – Stator winding connections for dual voltage twelve lead motors

Connecting the Stator Leads

Star-delta starting involves initially connecting the stator windings in a star configuration during the startup phase, followed by a reconnection to a delta configuration for normal operational conditions. To streamline the discussion, we will consider that the depicted motor is configured for operation at a single voltage level, with terminals T1 through T6 accessible at the connection box.

To connect a dual voltage motor, ensure the correct stator winding connections are made for either high or low voltage operation. Subsequently, replace T4, T5, and T6 with T10, T11, and T12 in the specified connections.

A fundamental control circuit for star-delta starting is illustrated in Figure 9.

This circuit utilizes a time delay mechanism to ascertain the moment when the windings transition from star configuration to delta configuration. Initiating circuits that detect motor speed or motor current to ascertain the appropriate moment for transitioning the stator windings from star to delta configuration is also prevalent.

Figure 9 – Basic control circuit for a star-delta starter using time delay

Upon pressing the Start button, control relay CR is energized, resulting in the closure of all CR contacts. This promptly energizes contactors 1M and S. The configuration of the motor stator windings is currently set up in a star formation, as illustrated in Figure 10.

The 1M load contacts establish the power connection to the motor, while the S contacts create a star configuration for the stator windings. The 1M auxiliary contact provides power to the coil of timer TR. Following a predetermined time interval, the two TR timed contacts will switch their positions.

The normally closed contact opens, resulting in the disconnection of coil S, which in turn causes the S load contacts to open.

The normally open TR contact engages, activating contactor coil 2M.

Figure 10 – The stator windings are connected in star (wye) for starting

The motor stator windings are now connected in a delta arrangement, as illustrated in Figure 11.

Figure 11 – The stator windings are connected in delta for running

Keep in mind that the delta connection is made using the 2M load contacts. Figure 12 illustrates the schematic representation of the connections of all load contacts.

Figure 12 – Stator winding with all load contacts for star-delta starting

The most critical part of connecting a star-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 star to delta. It is recommended that the circuit and components be numbered to help avoid mistakes in connection.

Refer to Figure 13.

Figure 13 – Load circuit connections for star-delta starting

Closed Transition Starting

The control circuit described thus far employs open transition motor starting. This indicates that the motor has been disconnected from the power supply during the transition from star to delta configuration. This may be problematic in certain applications if the transition induces spikes in the power line during the motor’s shift from star to delta configuration.

A method that maintains the motor’s connection to the power line is referred to as closed transition starting. The start of a closed transition is achieved by incorporating an additional three-pole contactor and resistors into the circuit.

See Figure 14.

Figure 14 – Basic schematic diagram Sizes 1, 2, 3, 4, and 5 star-delta starters with closed transition starting

The added contactor, designated as 1A, energizes momentarily to connect resistors between the power line and motor when the transition is made from star 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 star. The motor would start with the stator windings connected in delta.

Overload Setting

Notice in Figure 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.

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