Having looked at DOL starting in the last article, we will now consider Star Delta Starting (or Wye/Delta in the US). This is the first reduced voltage starting method we will look at.
Voltage reduction during star-delta starting is achieved by physically reconfiguring the motor windings as illustrated in the figure below. During starting the motor windings are connected in star configuration and this reduces the voltage across each winding 3. This also reduces the torque by a factor of three (see Motor Starting - Introduction). After a period of time the winding are reconfigured as delta and the motor runs normally.
Power and Control Circuiting
The two circuits illustrate the power and control required to enable star delta starting of a motor. The power circuit also shows the short circuit and overload protective devices.
During operation the contactor -KM1 is initially closed to put the motor in to star configuration. At the same time -KM2 is closed to apply power to the motor. At the end of the starting period, -KM1 is opened and then -KM3 is closed to place the motor wordings in a delta connection. Note: this is an open transition starting (see below).
The control circuit is more complicated than that of a DOL starter and is going to take a bit more explaining/. When the start button, -S2 is pressed, -KM1 energises (closing the power contactor) and in turn enables -KM2 to energise putting the motor into star.
After a time delay the -KM2 time delayed contact will switchover energising -KM1 putting the motor into delta (while at the same time opening the star contatator). The time delay is normally adustable and can be set on site to match the starting characteristics of the motor and mechanical load.
To avoid shorting the motor, the -KM3 closing contact (67,68) will close only after the -KM1 contactor is opened (55,56) - typically around 40 mS. The two contactors are also mechanically interlocked to prevent inadvertent shorting of the motor should something go wrong with the control wiring. To stop the motor button -S1 is pressed and all power is removed from -KM1, -KM2 and -KM3.
Open or Closed Transition Starting
At some point during starting it is necessary to change from a star connected winding to a delta connected winding. Power and control circuits can be arranged to this in one of two ways - open transition or closed transition.
In open transition the power is disconnected from the motor while the winding are reconfigured via external switching. This is the method discussed above.
Open transition starting is the the easiest to implement in terms or cost and circuitry and if the timing of the changeover is good, this method can work well. In practice though it is difficult to set the necessary timing to operate correctly and disconnection/reconnection of the supply can cause significant voltage/current transients. So much so that voltage drops and disturbances seen can be worse than if the motor were simply DOL started and for this reason closed transition is often specified..
In closed transition the power is maintained to the motor at all time. This is achieved by introducing resistors to take up the current flow during the winding changeover. A fourth contractor is required to place the resistor in circuit before opening the star contactor and then removing the resistors once the delta contactor is closed. These resistors need to be sized to carry the motor current. In addition to requiring more switching devices, the control circuit is more complicated due to the need to carry out resistor switching.
Motor Starting - Star Delta Characteristics:
Available starting current: 33%
Peak starting current: 1.3 to 2.6 In
Peak starting torque: 33%
Advantages:
Simple
Low Cost
Good Torque/Current Performance
Disadvantages:
Low Starting Torque
Break In Supply - Possible Transients
Six Terminal Motor Required
DOL and Star Delta starters are by far the most common type of motor starting employed. Electronic starting is fast catching up due to other benefits it provides (and I’ll be discussing this later in the series). In the next article I’ll start discussion on the less common starting methods , first of which will be auto-transformer reduced voltage starting.
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