Monday, June 6, 2011

Why does a 3 phase AC induction motor have to be connected in star delta (or wye delta) configuration - What is the significance and advantage of using star and delta in motor connection

Three phase AC induction motors are known to have a very high start-up surge current. The most practical approach to reduce the high start-up inrush current is by firstly implementing a star (or wye) configuration to the motor's 6 leads wiring terminal during the start-up run of the motor. When the motor slowly picks up speed and a significant decrease in the amount of current is achieved then the motor's 6 leads wiring terminal is then reconfigured to a delta connection.

The advantage of using a star (wye) configuration over the delta configuration when the motor starts to run from standstill is on the difference in the behavior and relation of the voltage and current in the motor's internal coil winding when connected in star and in delta configuration.

There are three coil windings inside the motor, each winding has two terminals. Two terminals per winding makes up the six leads wiring terminal accessible from the motor's terminal box which is intended for external wiring purposes where a star or delta configuration is directly connected.

Each of these 3 coil windings is made capable of carrying the rated operational voltage of the motor. This means that when 220 volts is supplied to a 220 volts motor, any one of the 3 coils of the motor is able to hold the supplied 220 volts. And when a 440 volts is supplied to a 440 volts motor, any one of the 3 coils of the motor is also able to hold the supplied 440 volts.
Delta Configuration Voltage and Current Relation
Fig-1: Delta Configuration Voltage and Current Relation
In a delta configuration, the phase voltage VP is directly proportional or equal to the line voltage VL (see Fig-1 drawing above), so applying 3 phase 220 volts from line to line (VL) would also yield the same 220 volts across each coil winding (VP) when measured from end to end of each coil.
Star Configuration Voltage and Current Relation
Fig-2: Star Configuration Voltage and Current Relation
In a star or wye configuration (refer to Fig-2 drawing above), the phase voltage VP is equal to the line voltage VL divided by the square root of three. And conversely, the line voltage VL is equivalent to the phase voltage VP multiplied to the square root of three, so a motor rated at 220 volts will have 220*√3 = 381 volts line voltage capacity from line to line (VL) to contain 220 volts phase voltage (VP) across each coil winding. This means that a 220 volts motor connected in star configuration is capable of receiving 381 volts from line to line with the capability of holding 220 volts on each of the coil windings. So applying 3 phase 220 volts line voltage to a star connected 220 volts 5 horsepower motor with a full load current of 9,79 amperes (at delta) will give us less amperes from the equation [5 hp*746 watts/hp]/[√3*381 volts] = 5,65 amperes at star configuration, which is less than the 9,79 amperes in delta configuration.

To check for the accuracy of the ampere values presented in the example, we can use the equation IL = IP*√3 provided in Fig-1 above for the delta configuration. We can solve for the line current drawn by the delta configuration by applying the current of the star configuration to the equation IL = 5,65 amperes*√3, which will give us 9,79 amperes.

This proves to show that a 3 phase AC induction motor draws less current when connected in star (wye) configuration than when connected in delta configuration, which makes it ideally important to implement an interchangeable star delta motor wiring connection that initially configures the motor to a star connection when the motor is just starting to run before finally switching the motor to delta mode after achieving sufficient rotating inertia from standstill. 

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