Induction Motor Starting Methods and Speed Control
This module delves into the essential techniques for starting induction motors and controlling their speed, crucial aspects for the GATE Electrical Engineering (Power Systems and Machines) exam. Understanding these methods is vital for efficient operation and application of AC machines.
Why Special Starting Methods are Needed
Direct-on-line (DOL) starting of induction motors draws a very high starting current, typically 5 to 8 times the full load current. This surge can cause voltage dips in the power supply, affecting other connected equipment, and can also damage the motor windings due to excessive heat. Therefore, reduced voltage starting methods are employed to limit this inrush current.
5 to 8 times the full load current.
Common Starting Methods
| Method | Principle | Starting Torque | Starting Current | Complexity |
|---|---|---|---|---|
| Direct-On-Line (DOL) | Full voltage applied directly to stator terminals. | High | High (5-8 pu) | Simple |
| Star-Delta Starter | Motor windings initially connected in star, then switched to delta. | Reduced (1/3 of DOL) | Reduced (1/3 of DOL) | Moderate |
| Auto-Transformer Starter | Uses an auto-transformer to reduce the voltage applied to the stator. | Proportional to square of voltage reduction | Proportional to square of voltage reduction | Complex |
| Rotor Resistance Starter (Slip Ring Motors) | Adds external resistance to the rotor circuit during starting. | Increased (can be optimized) | Reduced | Moderate |
Star-Delta Starter
This is a widely used method for squirrel cage induction motors. The motor windings are connected in a star configuration during starting, which effectively applies a reduced voltage (1/ā3 of the line voltage) to each winding. This reduces the starting current to approximately one-third of the DOL current and the starting torque to one-third of the DOL torque. After reaching a certain speed, the windings are reconfigured to a delta connection for normal operation.
Auto-Transformer Starter
An auto-transformer is used to supply a reduced voltage to the motor. By changing the tapping on the auto-transformer, the starting voltage can be varied, allowing for control over both starting current and starting torque. The starting torque is proportional to the square of the applied voltage, and the starting current is proportional to the applied voltage.
Rotor Resistance Starter
This method is applicable only to slip-ring (wound-rotor) induction motors. External resistors are connected in series with the rotor windings through slip rings. These resistors limit the starting current and also improve the starting torque. As the motor accelerates, the resistance is gradually cut out.
Induction Motor Speed Control
The speed of an induction motor is primarily determined by the frequency of the supply voltage and the number of poles. Controlling these parameters allows for efficient speed regulation. Other methods involve altering the voltage or rotor resistance.
Methods of Speed Control
Speed control of induction motors can be achieved by altering supply frequency, voltage, number of poles, or rotor resistance.
The synchronous speed (Ns) of an induction motor is given by Ns = 120f/P, where f is the supply frequency and P is the number of poles. The rotor speed (Nr) is slightly less than Ns due to slip. Therefore, changing f or P directly affects the motor's speed. Voltage control and rotor resistance control are also viable methods.
- Frequency Control: By varying the supply frequency (f) using Variable Frequency Drives (VFDs), the synchronous speed (Ns = 120f/P) is directly altered. This is the most efficient and widely used method for speed control, offering a wide speed range with good torque characteristics.
- Voltage Control: Reducing the stator voltage reduces the motor's torque (T ā VĀ²). While it can reduce speed, it also significantly reduces the motor's torque capability, making it less suitable for applications requiring high torque at low speeds.
- Pole Changing: For squirrel cage motors, the number of poles (P) can be changed by reconnecting the stator windings. This provides discrete speed steps (e.g., 4-pole and 8-pole operation).
- Rotor Resistance Control (Slip-Ring Motors): Adding external resistance to the rotor circuit increases the slip required for a given torque, thus reducing the motor speed. This method is less efficient as power is dissipated in the external resistors.
The fundamental relationship governing the synchronous speed of an induction motor is (N_s = \frac{120f}{P}), where (N_s) is the synchronous speed in RPM, (f) is the supply frequency in Hz, and (P) is the number of poles. The actual rotor speed (N_r) is always less than (N_s) due to slip. Therefore, to increase speed, one can increase the frequency (f) or decrease the number of poles (P). Conversely, to decrease speed, one can decrease (f) or increase (P). Voltage control affects torque as (T \propto V^2), and rotor resistance control increases slip, thereby reducing speed.
Text-based content
Library pages focus on text content
For GATE preparation, focus on the principles behind each starting method and how they affect starting torque and current. For speed control, understand the impact of frequency, voltage, and pole changes on motor speed and torque.
Key Takeaways for GATE
Understand the trade-offs between starting current, starting torque, and complexity for each starting method. For speed control, recognize that frequency control is the most efficient and versatile method. Be prepared to solve problems involving the formulas for synchronous speed, slip, and torque.
Frequency control (using VFDs).
Learning Resources
Provides a clear explanation of various starting methods for induction motors, including DOL, Star-Delta, and Auto-Transformer starters.
Details the different techniques used to control the speed of three-phase induction motors, covering frequency, voltage, and pole changing methods.
A comprehensive resource for AC machines, often covering topics relevant to GATE, including starting and speed control.
Explains the fundamental principles and working of common induction motor starting methods with diagrams.
An introductory document explaining the concept and operation of VFDs, a key technology for speed control.
A visual explanation of how induction motor speed is controlled, focusing on the impact of frequency and voltage.
Discusses various methods for controlling the speed of AC motors, with a focus on induction motors.
A video tutorial explaining the concepts of starting torque and starting current in induction motors.
Official syllabus for GATE Electrical Engineering, which outlines the topics covered in AC machines, including starting and speed control.
A general overview of induction motors, including their operating principles, starting, and speed control aspects.