Electrical Contactors: Star Delta Switching

1 Introduction

Star-delta (wye-delta) starting continues to be used for switching on 3-phase induction motors. When this type of connection method is used, the starting current is reduced to 1/3 of the current required for direct switching.

When switching from a star (wye) circuit to a delta circuit, transients can occur in the motor, which, further intensified by an unfavorable constellation of line frequency and inductor field, lead to the development of larger current peaks than is the case if the stopped motor is directly switched into a delta connection. In an unfavorable case, the following problems could occur:

• Short-circuit devices could trip,
• The delta contactor could become welded or be subject to high contact erosion,
• The motor could be subject to extreme dynamic overloading.

A preferred circuitry configuration for star-delta (wye-delta) starters is described in this Functional Example. When the main circuit is connected in a favorable manner, it is possible to reduce the transient currents and current peaks that develop when the motor is switched over from a star (wye) connection to a delta connection

2 Function

Two different types of motor connections are often listed in star-delta (wye-delta) starter circuit diagrams: One for clockwise and one for counterclockwise motor rotation. When installing the motor, care should be taken to ensure that the transient current peaks are as small as possible. Correct wiring of the motor terminal board is essential

1.1 Description of the Functionality for Clockwise Motor Rotation

2.1.1 The Preferred Circuitry Configuration is implemented

The vector diagram illustrated below shows the voltages that develop in a motor with clockwise rotation when the motor is switched from a star (wye) to a delta connection. In this case, the motor terminals are correctly connected according to the preferred circuitry configuration, meaning that phase L1 is connected to the motor terminals U1 and V2, L2 is connected to V1 and W2, and L3 is connected to W1 and U2:

Fig. 1: Correct connection of the motor phases for clockwise rotation

Fig. 2: Vector diagram for switching from a star (wye) connection to a delta connection for clockwise motor rotation when the motor phases are correctly connected

During the idle changeover delay, the rotor lags behind the phase sequence. The magnetic field induces a damped residual voltage, which is listed here in the voltage vector diagram for phase L1: L N U 1'− . When switching over to a delta connection (Fig. 1 and Fig. 2), the stator winding carrying this residual voltage is supplied with the line voltage L1 L3 U − . Thanks to the favorable vector position of the residual voltage L N U 1'− and the line voltage L1 L3 U − , which have approximately the same values, the differential voltage ΔU is relatively small. Thus, the current peaks resulting from this voltage also remain small.

2.1.2 The Preferred Circuitry Configuration is not Implemented

The motor also rotates clockwise when the motor terminals are connected as follows:

Phase L1 is connected to motor terminals U1 and W2, L2 is connected to V1 and U2, and L3 is connected to W1 and V2.

Fig. 3: Incorrect connection of the motor phases also resulting in clockwise motor rotation

Both the lagging residual voltage and the damped residual voltage are active in the stator. When the motor is switched to a delta connection, the phase winding with the vector L N U 1'− is supplied with the line phase L1 L2 U − . However, each of these voltages has a different vectorial direction. The differential voltage ΔU is high, thus causing auto correspondingly high transient current peak. Switching from a star (wye) connection to a delta connection therefore results in the following vector diagram

2.2 Changing the Direction of Rotation from Clockwise to Counterclockwise
When the motor rotates counterclockwise it is sufficient to exchange two phases in any position. Then the same conditions would prevail as described above for clockwise rotation. In order to also keep the transient current peak as small as possible in this case when switching from a star (wye) connection to a delta connection, the wiring must be carried out as illustrated below

Refer to Section 3 for information regarding the wiring of the main current and the control current. The circuit diagrams for contact assemblies for star-delta

(wye-delta) starting with clockwise and counterclockwise motor rotation are illustrated in the preferred circuitry configuration.

2.3 Device Rating for Standard Start-up

Star (wye) contactor: Ie Motor x 0.33

Line/delta contactor: Ie Motor x 0.58

Overload relay: Ie Motor x 0.58

Comment: When two phases in the circuit are exchanged to change the direction of rotation, the connection is automatically switched from the more favorable connection to the less favorable one and vice versa.
℘ = Transient current factor = transient current peak/starting current peak
In theory, the transient current factor has a maximum value of 2.
For example, measured: Favorable connection: ℘ = 0.8
Unfavorable connection: ℘ = 1.37

3 Assembly and Wiring

3.1 Main Circuit Overview

In the following graphic, the preferred circuitry configuration for the main circuit is illustrated for a star-delta (wye-delta) connection for clockwise and counterclockwise motor rotation