Tom Bishop, PE
Senior Technical Support Specialist
EASA
St. Louis, MO
In his paper presented at the EASA Convention 2019, Tom Bishop explores various methods for starting three-phase squirrel cage induction motors, emphasizing the importance of understanding these methods for service centers involved in motor repair and sales. The paper covers traditional across-the-line starting and modern techniques such as variable frequency drives (VFDs), detailing the principles of operation, benefits, and potential drawbacks of each method.
Bishop begins by explaining the high current draw associated with induction motor startup, which can be five to eight times the rated current. This high current can cause voltage drops and other issues in the power supply, necessitating methods to reduce starting current. The paper categorizes these methods into reduced voltage, reduced current, and reduced voltage and frequency starting techniques.
Across-the-line (direct-on-line) starting is the simplest method, involving the application of full line voltage to the motor terminals. While this method provides the highest starting torque and shortest acceleration times, it also demands the most from the electrical power system and can cause transient torque loading on the driven equipment.
Reduced voltage starting methods include autotransformers and electronic soft starters. Autotransformers use voltage taps to control power to the motor, reducing starting current and torque. Soft starters use silicon controlled rectifiers (SCRs) to reduce the voltage supplied to the motor, allowing for smoother acceleration and the ability to set peak current limits.
Reduced current starting methods, such as reactors and resistors, directly reduce motor current by adding impedance or resistance to the motor circuit. Reactors limit current changes and provide closed transition to line voltage, while resistors offer smooth acceleration by gradually increasing voltage at the motor terminals.
Part winding (PWS) starting involves using only a portion of the motor winding, increasing impedance and reducing starting current. This method is beneficial for applications requiring utility supply voltage recovery and helps prevent excessive voltage dips.
Wye-delta (Y-D) starting changes the motor winding phase connections to reduce voltage during startup. This method is commonly used for high inertia loads and provides a smooth transition from wye to delta configuration, minimizing transient current and torque surges.
Variable frequency drive (VFD) starting offers the most advanced method, allowing for precise control of motor voltage and frequency. VFDs can maintain full load torque throughout the acceleration period while keeping motor current at or below rated full load current. This method provides smooth acceleration and deceleration, making it suitable for a wide range of applications.
In summary, Tom Bishop's paper provides a comprehensive overview of squirrel cage induction motor starting methods, highlighting the advantages and drawbacks of each technique. Understanding these methods is crucial for service centers to ensure successful motor operation and optimal performance.
Key Points Covered:
- High current draw during induction motor startup
- Across-the-line (direct-on-line) starting
- Reduced voltage starting methods: autotransformers and soft starters
- Reduced current starting methods: reactors and resistors
- Part winding (PWS) starting
- Wye-delta (Y-D) starting
- Variable frequency drive (VFD) starting
Key Takeaways:
- Various methods exist to reduce starting current and torque
- Across-the-line starting provides highest torque but demands most from power system
- Autotransformers and soft starters offer smoother acceleration and control
- Reactors and resistors directly reduce motor current
- Part winding starting helps prevent excessive voltage dips
- Wye-delta starting is suitable for high inertia loads
- VFDs provide precise control and smooth acceleration/deceleration
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