Methods of Starting Three Phase Induction Motors

Written by chris passas
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A three phase induction motor utilises three-phase alternating current electric power to supply electricity to the devices to which it connects. In the three-phase system, three circuit conductors deliver three alternating currents of the same frequency to a destination. Electric power distribution grids, large motors and large electrical loads utilise a three phase inductor system to deliver electrical power.

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Direct Online Starting

Direct online starting also known as across the line starting and full voltage starting, involves connecting each terminal of a three-phase induction motor to a separate line of a device. In this arrangement, the motor current is the same as the line current and the terminal voltage of the induction motor equals the line voltage. A disadvantage of the direct online starting method is that the stator draws a high current that can damage the windings of the three-phase induction motor. The direct online starting method can also cause a voltage drop or fluctuation that can affect devices along the line.

Wye and Delta Connections

In a wye or star connection, the windings of the induction motor connect from the supply phases to the neutral. In a delta or mesh connection, the windings connect between the supply phases. A wye connection creates higher voltage to the windings of the three-phase induction motor than a delta connection. A starter with the ability to utilise both star and delta connections, also known as a wye start delta run connection, initialises the three-phase motor using a wye connection then transfers to a delta connection when the motor reaches a set speed. A disadvantage of the star-delta connection is the reduction in voltage and the low starting torque that can have an adverse effect on devices or pumps that have a high breakaway torque. A star-delta connection is more complex than a direct online connection because it utilises a speed switch and timers. The induction motor requires a shock to start and another shock when the star-delta connection transfers from wye to delta. The wye-delta method is often convenient for partial acceleration.

Series Reactor Connection

A reactor in series with the terminals of the motor decreases the terminal voltage of the induction motor, decreasing the initial current. The impedance decreases as the induction motor accelerates until a bypass method makes the motor run at full speed and full voltage.

Variable Frequency Drive

A variable-frequency drive starts a three-phase induction motor at a frequency low enough to initialise a full-rated torque without an inrush of current. The low frequency increases the torque because it increases the impedance of the rotor circuit with slip frequency.

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