Starting Three Phase Induction Motors

Starting Three Phase Induction Motors

Starting Three Phase Induction Motors

Three phase motors are powered by three phase mains power supply. The stator in a three phase motor has provisions for slots that can accommodate three phase winding. The three phase windings are arranged so that they can produce a rotating magnetic field. The rotor has parallel laminated conductors shorted at the ends by the end rings.  The conductors are made of heavy copper or aluminum bars and are skewed from the central axis. This prevents poles from locking which can cause magnetic humming or stalling of the motor. Three phase motor starting circuits control motor starting speed by limiting the voltage of the power fed to the motor. This is a paper on the star delta and autotransformer three phase motor starting circuits.

The star delta motor starting circuits operate by reconfiguring the stator windings. The starting voltage in star delta is reduced by the realignment of the windings into a star. This reduces the voltage across each winding by a factor of three as compared to direct online starting. The current flowing through each winding is 1√3 = 0.58 (58%) of the line current (Boldea & Nasar, 2010). Also, the overall torque from the motor is also reduced by a factor of three. The star delta starters are made up of three contactors, the main contactor (KM3), the star contactor (KM1), and the delta contactor (KM2).

The star delta connection has three states of operation, the off, star, open, and delta state. In the star state, the main contractor, KM3, and the star contactor, KM1, are closed. After a given duration, the star connection is opened and the delta connection is closed and the motor starts running at full throttle. This switching from star to delta state is controlled by the timer built into the starter. All contactors are open in the off state and only the main contactor is closed in the open state (Boldea & Nasar, 2010).

Figure 1: Star delta starting circuit diagram

Autotransformer motor starting circuits reduce the starting current by limiting the starting voltage across the squirrel-cage motors. Three motor terminals and leads are used in autotransformer starting circuits. During starting, the motor is connected to the tappings of the autotransformer. Contactors K2M and K1M are closed. This reduces the starting current by the square of the transformer voltage ratio. When the motor reaches 80 to 90% of its rated full speed, the transformer star contactor is opened and the transformer windings act as chokes (Keljik, 2007). The chokes reduce the motor voltage slightly below the supply voltage but the speed remains unaffected. Finally the main contactor K3M closes via the star contacts supplying full voltage to the motor.

 

Figure 2: Autotransformer motor starting circuit diagram

Star delta method of three phase motor starting has several advantages. Star delta is simple and rugged, therefore, appropriate for a wide range of applications. It is also cheaper than other voltage control techniques, has good current and torque performance, and it halves the full load operation current. This method has a number of drawbacks such as low starting torque, transients in supply, requires a six terminal motor, and has a low starting torque (Keljik, 2007). The autotransformer method of starting current control is advantageous over the star delta method because it can be used for starting loaded large motors. The main drawbacks of the autotransformer method are that it is expensive, complex, and bulky, which limits its application (Keljik, 2007).

There are two major differences between the DC an induction motor starting circuits. In DC motors, the starting control circuits limit the starting current fed to the motor while in induction motors, the control circuits limit the voltage across the motor. Also, DC motors starting circuits use eternal resistances to control the starting current while induction motors starting circuits reconfigure the motor stator windings.

Three phase induction motors use three phase current to set up a rotating magnetic field in the stator. Three phase control circuits are used to limit the starting current by controlling the induction effect of the current. This is achieved through configuration of the stator wings. Delta and autotransformer methods offer smooth induction motors starting current control.

References

Boldea, I., & Nasar, S. A. (2010). The induction machines design handbook. Boca Raton, FL:       CRC Press/Taylor & Francis.

Keljik, J. (2007). Electric motors and motor controls. Clifton Park, NY: Thomson Delmar            Learning.

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