Motor Start and Run Capacitors

Motor Start and Run Capacitors

Blog Article

Motor Start and Run Capacitors

Functions and Differences

Role in Motor Systems

1. 
   
   Start Capacitors
   
   
   
   
   • Torque Generation: In capacitor-start motors, the start capacitor creates a significant phase difference between the main and auxiliary windings, producing high starting torque.
   
   
   • Short-Term Operation: A centrifugal switch disconnects the start capacitor once the motor reaches ~75% of its rated speed to prevent overheating.
   
   
   
   


2. 
   
   Run Capacitors
   
   
   
   
   • Efficiency Optimization: Run capacitors remain engaged during operation, fine-tuning the phase shift to improve power factor, reduce energy consumption, and minimize motor vibration.
   
   
   • Continuous Performance: Used in permanent split capacitor (PSC) motors or capacitor-start/capacitor-run (CSCR) systems to maintain smooth operation.
   
   
   
   

Common Applications

• 
  
  Start Capacitors:
  
  
  
  
  • Air conditioning compressors
  
  
  • Refrigeration units
  
  
  • Well pumps
  
  
  • Heavy-duty motors requiring high starting torque
  
  
  
  


• 
  
  Run Capacitors:
  
  
  
  
  • Fans and blowers
  
  
  • HVAC systems (e.g., condenser fans)
  
  
  • Pumps in continuous operation
  
  
  • PSC motors in household appliances
  
  
  
  

Identification and Specifications

• 
  
  Labeling:
  
  
  
  
  • Start capacitors are often labeled "START" or "CBB65," while run capacitors may be marked "RUN" or "CBB60."
  
  
  • Key specs include:
    
    
    
    • Capacitance (µF) with tolerance (e.g., 250 µF ±5%)
    
    
    • Voltage rating (e.g., "370V AC" or "440V AC")
    
    
    • Temperature range (e.g., -40°C to +70°C)
    
    
    
    
  
  
  
  


• 
  
  Physical Differences:
  
  
  
  
  • Start capacitors are typically larger and may be housed in metal cans, while run capacitors are smaller and often in plastic casings.
  
  
  
  

Replacement and Maintenance

1. 
   
   Safety Precautions:
   
   
   
   
   • Disconnect power and discharge capacitors using a resistor before handling.
   
   
   • Wear insulated gloves to avoid electrical shock.
   
   
   
   


2. 
   
   Replacement Guidelines:
   
   
   
   
   • Start Capacitors: Replace with the same capacitance (±5%) and equal or higher voltage rating.
   
   
   • Run Capacitors: Match capacitance (±10%) and voltage to maintain motor efficiency.
   
   
   • CSCR Systems: Ensure both start and run capacitors are replaced according to manufacturer specs to avoid imbalance.
   
   
   
   


3. 
   
   Troubleshooting Failures:
   
   
   
   
   • Start Capacitor Failure: Motor hums but doesn’t start, or starts slowly with reduced torque.
   
   
   • Run Capacitor Failure: Motor overheats, vibrates excessively, or shows reduced efficiency during operation.
   
   
   
   

System Configurations

• 
  
  Capacitor-Start Motor:
  
  
  
  
  • Uses a start capacitor and centrifugal switch; ideal for high-torque loads (e.g., compressors).
  
  
  
  


• 
  
  Capacitor-Run Motor (PSC):
  
  
  
  
  • Relies solely on a run capacitor for both starting and running; suitable for low-torque applications (e.g., fans).
  
  
  
  


• 
  
  Capacitor-Start/Capacitor-Run Motor (CSCR):
  
  
  
  
  • Employs both start and run capacitors for high starting torque and efficient runtime (e.g., large pumps).