Function and Design Differences

1. Start Capacitors
   
   
   
   • Purpose: Provide a temporary phase shift to generate high starting torque, engaged only during startup.
   
   
   • Characteristics:
     
     
     
     • High capacitance (50–300 μF) for initial torque boost.
     
     
     • Non-polarized, often oval-shaped with a canister design.
     
     
     • Short-duty cycle rating (only energized for 10–30 seconds per start).
     
     
     
     
   
   
   
   


2. Run Capacitors
   
   
   
   • Purpose: Maintain the phase shift during normal operation, improving efficiency and reducing motor heating.
   
   
   • Characteristics:
     
     
     
     • Lower capacitance (5–50 μF) for continuous use.
     
     
     • Polarized or non-polarized, typically round with a plastic casing.
     
     
     • Long-duty cycle rating (energized throughout motor operation).
     
     
     
     
   
   
   
   

Circuit Configuration and Operation

1. 
   
   Capacitor-Start Capacitor-Run (CSCR) Motor
   
   
   
   
   • Wiring Diagram:
     
     
     

     
     

     
     

     
     

     
     

     
     

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     Line Voltage ----+---- Main Winding ----+  
     
                     |                      |  
     
                     +---- Start Capacitor ---+---- Centrifugal Switch ----+  
     
                     |                      |                           |  
     
                     +---- Run Capacitor ----+---------------------------+---- Auxiliary Winding  
     
     

     
     

     
     

     
     

     
     

     
     
   
   
   • Startup Phase:
     
     
     
     • Start capacitor and run capacitor work together to create a 90° phase shift, generating high starting torque.
     
     
     • Centrifugal switch disconnects the start capacitor at ~75% rated speed (e.g., 2,800 RPM for a 3,600 RPM motor).
     
     
     
     
   
   
   • Run Phase:
     
     
     
     • Only the run capacitor remains in the circuit, maintaining torque and efficiency.
     
     
     
     
   
   
   
   


2. 
   
   Permanent Split-Capacitor (PSC) Motor
   
   
   
   
   • Simplified Wiring:
     
     
     

     
     

     
     

     
     

     
     

     
     

     plaintext

     
     

     
     

     
     

     
     

     
     

     
     

     
     

     Line Voltage ----+---- Main Winding ----+  
     
                     |                      |  
     
                     +---- Run Capacitor ----+---- Auxiliary Winding  
     
     

     
     

     
     

     
     

     
     

     
     
   
   
   • Operation:
     
     
     
     • No start capacitor; run capacitor provides continuous phase shift, suitable for low-torque applications (e.g., fans, pumps).
     
     
     
     
   
   
   
   

Capacitor Types and Specifications

• Start Capacitors:
  
  
  
  • Technology: Electrolytic or dry-film capacitors, often filled with oil for heat dissipation.
  
  
  • Voltage Rating: 250V AC (for 120V systems) or 370V AC (for 240V systems).
  
  
  
  


• Run Capacitors:
  
  
  
  • Technology: Polypropylene film capacitors for stability and long life.
  
  
  • Voltage Rating: 370V AC or 440V AC to withstand continuous operation.
  
  
  
  


• Key Ratings:
  
  
  
  • Capacitance tolerance: ±5% for run capacitors, ±10% for start capacitors.
  
  
  • Temperature coefficient: -40°C to +70°C for run capacitors, -20°C to +60°C for start capacitors.
  
  
  
  

Selection and Sizing Guidelines

1. Start Capacitor Sizing:
   
   
   
   • Based on motor horsepower (HP) and voltage:
     
     
     
     • 1 HP, 120V: 150–200 μF
     
     
     • 3 HP, 240V: 80–120 μF
     
     
     
     
   
   
   • Formula (approximate):(C_{text{start}} (mutext{F}) = frac{3,000 times text{HP}}{text{Voltage}})
   
   
   
   


2. Run Capacitor Sizing:
   
   
   
   • Based on motor full-load current (FLC) and power factor:
     
     
     
     • 1 HP, 120V: 10–15 μF
     
     
     • 5 HP, 240V: 25–35 μF
     
     
     
     
   
   
   • Formula (approximate):(C_{text{run}} (mutext{F}) = frac{1,000 times text{FLC}}{text{Voltage} times 2 pi f}) (where (f = 60 , text{Hz}) for North American systems)
   
   
   
   

Signs of Faulty Capacitors

• Start Capacitor Failure:
  
  
  
  • Motor fails to start or takes longer to reach rated speed.
  
  
  • Overheating or buzzing noise during startup.
  
  
  
  


• Run Capacitor Failure:
  
  
  
  • Motor runs hot or with reduced torque.
  
  
  • Erratic speed or increased current draw (measured with an ammeter).
  
  
  
  


• Common Issues:
  
  
  
  • Swollen or leaking capacitor bodies (indicates internal failure).
  
  
  • Open circuit or short circuit (test with a capacitance meter).
  
  
  
  

Replacement and Maintenance

1. Safety Precautions:
   
   
   
   • Discharge capacitors before handling by shorting terminals with a resistor (100–1,000 ohms).
   
   
   • Ensure power is disconnected and verify zero voltage with a tester.
   
   
   
   


2. Replacement Steps:
   
   
   
   • Note wiring connections (color codes or terminal labels) before removing the old capacitor.
   
   
   • Install the new capacitor with the same capacitance and voltage rating (or within ±10% for run capacitors).
   
   
   
   


3. Maintenance Tips:
   
   
   
   • Inspect capacitors annually for physical damage or electrolyte leakage.
   
   
   • Replace start capacitors every 5–7 years (due to short-duty cycle stress).
   
   
   
   

Impact on Starter Motor Systems

• Reduced Starter Strain:
  
  
  
  • Properly sized capacitors minimize the load on the starter motor by reducing cranking time and current draw.
  
  
  
  


• Compatibility Considerations:
  
  
  
  • In capacitor-start motors, a failed start capacitor can overstress the starter motor, leading to brush wear or gear damage.
  
  
  
  


• Efficiency Optimization:
  
  
  
  • Run capacitors improve power factor, reducing energy consumption and heat generation in the starter motor and associated wiring.
  
  
  
  

Industry Standards and Applications

• NEMA Design Classes:
  
  
  
  • Class B motors (standard efficiency) often use CSCR configurations, while Class F motors (high efficiency) prefer PSC designs.
  
  
  
  


• Common Applications:
  
  
  
  • Start Capacitors: Compressors, pumps, and other high-torque loads.
  
  
  • Run Capacitors: Fans, blowers, and continuous-duty motors.
  
  
  
  

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