Star / Delta Motor Starter

Google "star delta starter theory"

EnB,

Have a look at

http://cr4.globalspec.com/thread/11182/Motor-Starter

You can also search the CR4 site for star delta and you will get a number of hits.

Kind Regards

Mr. W.A Snow

EnB,

The root principle is that the motor's windings draw current that is proportional to the applied voltage. "Direct-on-Line" (DOL) and "Across-the-Line" starters all apply full voltage to the windings. All other types employ different methods to reduce the apparent voltage to the windings. By doing this, the momentary inrush load while the motor is starting up is reduced to levels that will not harm the motor, the power supplier, or other equipment/users. The implementation mode for the Wye/Delta or Star/Delta starter is easily learned by studying the resources given by others who have answered your question.

--JMM

Star/Delta motor starting.

* This initial winding configuration is Star to achieve minium starting current, which is approximately 58% of the starting current with the motor connected in the Delta configuration.

* Unfortunately, the starting torque is approximately proportional to the square of the current, so you can see that even though the starting current is reduced, the starting torque is reduced even more! You have to be careful concerning the load that you are powering. The most important criterion is that the load must turn when the motor is energized. Many loads have low starting torque requirements and high running torque requirements which makes this OK.

* After the motor begins to turn, counter emf is generated in the stator windings due to the magnet flux generated by the currents circulating in the rotor windings. Most industrial motors are induction motors with no external wired connections to the rotor, but the induced rotor currents (due to transformer action from the stator) still generate a magnetic flux that rotates and cuts the stator windings to generate a counter emf.

* This counter emf will limit the current drawn by the motor. Thus, the motor windings can be reconfigured to the Delta configuration using power contactors to break the Wye and quickly connect to a Delta configuration. The maximum current drawn by the motor is then substantially less compared to starting it directly across the line in the Delta configuration.

* There are solid state alternatives. Variable speed controls will start the motor at low speed with reduced voltage and current. Then the motor speed is smoothly ramped to operating speed. The voltage across the windings is maintained proportional to the drive frequency (Volts/Hertz).

* An analysis considering economics, reliability and the capabilities of the electric power grid that powers the motors is necessary to determine what is the best approach. Obviously, the simplist and most economical configuration is an induction motor with across the line starting, if the power source can handle the starting current, assuming that variable speed operation is not needed.