Three Phase Induction Motor Starting Methods
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Three Phase Induction Motor Starting Methods
An example of an asynchronous AC motor is a Three-phase induction motor. It is the electrical motor used to drive Mechanical loads most frequently in industries. The inrush current of the motor is the amount of current drawn by the Induction motor when it is first turned on.
Numerous issues with the supply system, such as line voltage drop, are brought on by the high starting current. The operation of other Electrical devices connected to the same line is impacted by the system voltage drop. As a result, we must modify some Induction motor starting techniques in order to lower the starting current of the motor to a range within which it can operate.
There are two types of Three-phase induction motors.
- Squirrel cage induction motor
- slip-ring induction motor
In this article, we will discuss the Three Phase Induction Motor Starting Methods.
Three Phase Induction Motor Starting Methods
Squirrel Cage Induction Motor Starting Methods
The following are the starting methods have been developed for starting the three-phase induction motor:
- Direct On Line (D.O.L.) Starter
- Auto-Transformer Starter
- Star-Delta Starter
- Variable frequency Drive(VFD)
- Soft starter
Direct On-Line Starter
This method of starting the motor is known as a direct online starter, or D.O.L. Starter, because the three-phase electric supply is fed directly to the induction motor. The circuit diagram for the direct online starter is shown in the following figure.
The starting current, which is typically 5 to 7 times the rated current, is very large in this method of motor starting because it is not reduced. Fuse or circuit breakers, contactors, and overload protection relays make up a direct online starter’s circuit. There is no provision for controlling the starting current, just like with line starters. As a result, this technique is frequently applied to induction motors with ratings under 5 kW.
Stator Resistance Starter
Reduced voltage starting techniques include stator resistance starting. The rotor draws a significant amount of current when an induction motor is started because the slip is initially equal to unity. As the motor gains speed and accelerates, the motor current decreases. The total resistance impedes the current if additional resistance is added in series with the stator resistance, which lowers the motor’s starting current.
In this technique, we link external resistance in series with each stator winding phase at the beginning. The voltage across the motor terminals decreases as a result of the voltage drop in the external resistance. Less stator current results from the lower stator voltage. As the motor accelerates, the stator resistance is now gradually removed from the stator circuit. When the motor reaches its maximum speed, all external resistances are eliminated, and the stator is supplied with the full recommended voltage across the motor terminals.
The followings are the downsides of this method.
- The power loss in the external resistances takes place in the form of heat.
- The reduced voltage causes slow acceleration of the motor.
Star-Delta Starter
Reduced voltage starting techniques include star-delta starting. The most popular way to start induction motors is with a star-delta starter. The motors that are intended to run on delta-connected stators are operated using this technique. The following figure displays the star-delta starter’s circuit diagram.
In this technique, the motor’s stator winding is connected in the star during the starting phase and in the delta during normal operation. The supply voltage drops to 57.7% of the normal voltage when the motor is started in a star connection, which reduces the starting current of the motor. The motor is changed to a delta connection once it has reached about 80% of its typical speed.
A double-through switch (nowadays contractors) is used for changing the connection from star to delta.
In star running, the torque delivering capacity is one-third of the full torque capacity of the motor.
Autotransformer Starter
Additionally, it is a form of reduced voltage starting. Autotransformer starting is the technique used to start a three-phase induction motor in order to lower the motor’s starting current. Both delta-connected and star-connected three-phase induction motors can be started using the autotransformer starter. The following figure displays the circuit diagram for the autotransformer starter.
Here, the autotransformer used is a three-phase step-down transformer having different taps to obtain different voltages. The autotransformer starter generally has three steps of voltage, 50%, 65%, and 80%. The more voltage steps are possible to get a smooth acceleration of the motor. The motor is started at a lower voltage tap and the tap is changed to higher taps with the acceleration of the motor.
This method starts the motor by applying a lower supply voltage via the autotransformer. The autotransformer is automatically disconnected from the circuit by the shorting contactor once the motor has reached a speed of about 80% of its rated speed, at which point it operates on the full supply voltage. It has every motor protection feature, including overload and no volt protection. Here, take note that the motor’s starting torque is inversely correlated with voltage. As a result, at lower voltages, the motor’s starting torque decreases, and autotransformer starters are suitable for turning on loads that require little starting torque.
The autotransformer starter has the following advantages.
- Low Power loss
- Low starting current
- Suitable up to 25 hp motor, depending upon the strength of power system
Soft Starter
The Soft starter is a type of reduced stator voltage electronic starter. The power circuit diagram of the soft starter is given below.
When the motor starts, the voltage is reduced, and as it gains speed, the voltage is raised. Consequently, start softly. From zero to the full line voltage, the voltage can be controlled. The output voltage fed to the stator is controlled by the thyristor’s fluctuating firing angle. The reliability is increased by the soft starter’s reduction of jerks on the motor and the driven equipment during starting.
Variable Frequency Drive(VFD)
The block diagram of VFD is given below.
In addition to controlling the speed of an induction motor, the VFD can be used as a motor starter. An advanced soft starter known as a VFD modulates the frequency to regulate the motor’s speed. For the PWM inverter to keep the motor’s flux constant, the voltage is also adjusted in proportion to changes in frequency.
Slip Ring Induction Motor Starting Methods
There are four methods used to start a slip ring induction motor. They are-
- Direct On-Line (D.O.L.) Starting
- Autotransformer Starting
- Stator-Resistance Starting
- Rotor-Resistance Starting
The first three methods—DOL starting, Stator-Resistance starting, and Autotransformer starting—are the same as those for the squirrel cage induction motor that we previously covered. Only slip-ring induction motors can use the rotor resistance starting technique.
Rotor Resistance Starter
Only slip-ring induction motors can use the rotor resistance starting method. Because this method involves adding external resistance to the rotor circuit, which can only be done with wound rotors. The circuit diagram for the rotor resistance starter is shown in the following figure.
A star-connected external resistor is connected in series with the motor’s rotor through slip rings and brushes in the rotor resistance starter. The entire external resistance connected to the rotor circuit significantly lowers the starting current during motor startup. As the speed rises, this resistance is gradually removed from the circuit. The external resistor is completely removed once the motor reaches its normal speed, at which point it operates as a cage rotor induction motor.
Because of the externally added resistance in the rotor circuit, the slip ring induction motor has a higher starting torque. The external resistance raises the motor’s power factor as well.