What is starting method?
Self starting is a feature of a three-phase induction motor. A revolving magnetic field is created when the supply is linked to the stator of a three-phase induction motor, causing the rotor to rotate and the induction motor to start. The motor slip is unity at startup, and the starting current is extremely high.
The motor can be started by connecting the three-phase induction motor directly to the full voltage of the power source. The motor can also be started by reducing the voltage supplied to it. An induction motor's torque is determined by the square of the applied voltage. When a motor is started on full voltage, it produces more torque than when it is started on decreased voltage. And also, the motor draws high starting current and it has high starting torque, so to limit high starting current and torque starter is used.
Purpose of starter:
- To minimize the heavy initial current and torque.
- To prevent against overload and under voltage.
Types of Starting Method
- Direct On-line Starter
- Star-Delta Starter
- Autotransformer Starter
- Stator Resistance Starter
Direct On-line Starter
The figure shows a schematic diagram of Direct On-line (DOL) Starter. A DOL starter is made up of a coil-operated contactor with start and stop buttons. The contactor is electrified or charged when the start push button is pressed, and it simultaneously closes all three phases of the motor to the supply phases. Here, pressing the stop button will energize the coils of the contactor and hence stop the motor from disconnecting all the three phase supply.
The induction motor will turn on by connecting it to a three-phase power source directly. The motor draws a large starting current and has a low power factor in this method. Consider that in order to minimize large voltage drops that arise with the supply resulting due to the overcurrent, a DOL starter is used.
Star-Delta Starter
The figure shows a schematic diagram of Star-delta Starter. A change-over switch is linked to the six leads of the motor's stator winding. When the motor is turned on, the change-over switch is set to 'Star,' which joins the stator windings in a star configuration.
As a result, each phase receives V/3 volts, with V denoting the three-phase line voltage. This lowers the initial current. When the motor reaches 80% of rated speed, the changeover switch is set to 'Delta,' connecting the stator windings in a delta configuration. Each phase now receives full line voltage.
The main drawback of this technology or method is a significant reduction in initial torque due to lower voltage in the star connection at startup.
Autotransformer Starter
The starting voltage of the induction motor is reduced using an autotransformer in this method. When the autotransformer is in the circuit, the tapings are adjusted such that 60 to 80 percent of the line voltage is applied to the motor during startup, and then it is connected to the full line voltage after the motor has reached a sufficient speed.
The change-over switch is connected to 'Start' at the time of startup. The decreased voltage is delivered to the motor via the autotransformer. As a result, the starting current is restricted to a safe level. The change-over switch is now set to 'Run' when the motor reaches around 80% of its rated speed. The autotransformer is disconnected from the circuit, and full line voltage is applied to the motor terminals.
Stator Resistance Starter
The figure shows a schematic arrangement of Stator Resistance Starter. During the starting process, the external resistance is linked in series with each step of the stator winding. The voltage that appears across the terminals of the motor is reduced by employing the external resistance.
As a result, the initial current is low. As the motor accelerates, the initial external resistances in the stator circuit are gradually reduced in phases. The beginning resistances are completely removed when the motor reaches its rated speed, and full line voltage is given across the motor terminals.
Related Concepts
All the methods that are discussed from star-delta starter to stator resistance starter, are also used for the starting of slip ring induction motor, one additional method that is not common to both is rotor resistance start method that is used only in slip ring induction motor.
Rotor Resistance starter
The figure describes the schematic layout of a Rotor Resistance Starter, which consists of slip-rings and a rotor circuit with a star-connected variable resistance.
The variable resistance of the handle or contactor is set to the 'OFF' position at the start. This connects the highest resistance to each step of the rotor circuit. Due to the external rotor resistance, the starting current is reduced but the starting torque is enhanced.
The external resistance in the rotor circuit decreases slightly as the motor speeds. When the motor reaches its rated speed, the handle is turned to the "ON" position, which eliminates all the external resistances from the rotor circuit.
Context and Applications
- For motors up to 25 hp, star-delta starting is employed.
- DOL stater is used for small motors upto 10 KW.
- Auto-transformer is used for large motors upto 25HP.
- Bachelor of Technology in Electrical Engineering.
- Master of Technology in Electrical Engineering.
Common Mistakes
- Motor must not be overload.
- There must be no contamination in proximity of the motor.
- Do not use low resistances to start the motor.
- Always take into consideration the heating of the motor.
- Take care of the motor vibration as it leads to wear and tear of parts.
- Take the necessary precaution for starting methods of motor.
- Note the conditions to determine the starting method must be used.
Practice Problems
Q1. What are types of Starting method?
A. DOL Starter
B. Star-Delta Stater
C. Autotransformer Starter
D. All of the above.
Answer: D
Explanation: All are the starting method of three phase induction motor.
Q2. Why starter is required?
A. To limit the starting current and torque.
B. To increase the starting current and torque.
C. To stabilize the torque.
D. None
Answer: A
Explanation: When the motor is directly connected to the supply then it draws heavy current so to minimize this current starter is used.
Q3. For Slip ring induction motor which type of special method is used?
A. Autotransformer starter
B. Star-Delta starter
C. Rotor Resistance starter
D. Stator Resistance starter
Answer: C
Explanation: All the starters are used for slip ring induction motor but Rotor resistance is a special method.
Q4. Upto 25 HP which starting method is best?
A. DOL Starter
B. Star-Delta Stater
C. Autotransformer Starter
D. Stator Resistance starter
Answer: C
Explanation: It has a number of tapings so we can control the voltage so it is used for large motors.
Q5. In Slip ring induction motor the resistance is connected to .............. side.
A. Stator
B. Rotor
C. Supply
D. None
Answer: Rotor
Explanation: It is a special method to control the speed and voltage of the motor.
Want more help with your electrical engineering homework?
*Response times may vary by subject and question complexity. Median response time is 34 minutes for paid subscribers and may be longer for promotional offers.
Search. Solve. Succeed!
Study smarter access to millions of step-by step textbook solutions, our Q&A library, and AI powered Math Solver. Plus, you get 30 questions to ask an expert each month.
Electrical Machines
Reduced voltage starting circuit
Starting methods
Starting Method Homework Questions from Fellow Students
Browse our recently answered Starting Method homework questions.
Search. Solve. Succeed!
Study smarter access to millions of step-by step textbook solutions, our Q&A library, and AI powered Math Solver. Plus, you get 30 questions to ask an expert each month.