Start Capacitor Vs Run Capacitor: 3 Important Facts


Start capacitor and run capacitor, both are motor capacitor, both are used for different purpose in the motor operation. Construction of both the capacitor is same, let’s discuss Start Capacitor vs Run Capacitor.

Start CapacitorRun Capacitor
They are used for starting the motor. They are used for providing continuous power to the motor. 
Small duty cycle.Long duty cycle.
Higher Capacitance rating (70-120 micro Farad)Lower Capacitance rating (7-70 micro Farad)
Remain connected to the circuit for short period of time.Remain connected to the circuit for long period of time.
The start capacitor is used during the startup phase of the motor and gets disconnected from the circuit when the motor reaches its predetermined speed, which is 75% of the maximum speed of the motor. Run Capacitor stays directly linked with the auxiliary coil even after the starting capacitor is disconnected (or deactivated) from the circuitry. The run capacitor remains constantly charged while providing a controlled contiguous (or continuous) power to the motor. 

Can I use a Run Capacitor as a start capacitor?

The purpose of starting capacitor is to lag the current in a gallery winding during the starting operation of the motor, and it gets disconnected from the circuit when the router reaches its predetermined speed.

The run capacitor can be operated as a start capacitor, whereas the start cannot be implement as a Run capacitor. To start the motor or develop high torque across the motor, a high capacitance value is required to show the run capacitor array (two or more capacitors are connected in cascade) can be connected.

The capacitor value of the run capacitor is very much smaller than that of the start capacitor; a single running capacitor will not be able to start the motor as it cannot provide enough torque to the motor. There won’t be any problem (or drawback) with the run capacitor to start the motor, but the starting (or beginning) character may not be up to mark, and the motor may take a higher(or intense) starting current with lower torque.

What happens when a Run Capacitor goes bad ?

Capacitor failure can be of two types. Catastrophic failure is generally caused by the motor starting circuit being engaged for too long. The top of the starting capacitor has been blown off, and the inside has been slightly or fully ejected. The capacitor may be just raptured pressure relief blister.

The motor can display various problems if a run capacitor fails, including not starting vibrating, overheating, slow start, or motor buzzing. The motor will not have an uniform electric field that will cause the router(or root) to hesitate at irregular spots. A bad Run capacitor will cause the motor to become noisy, have high energy consumption, drop performance, overheating, etc.

What is the purpose of a Starting Capacitor ?

The start capacitor is come up with in auxiliary (or start) windings of the motor. The capacitance of a start capacitor is much elevated than that of a run capacitor.

The objective of the start capacitor is to provide enough torque to start (or energize) the motor, and it gets disconnected (or deactivated) from the circuit after the motor reaches a predetermined  (or predestined) speed. Without a start capacitor when the voltage has applied to motor, the motor will generate (or give rise to) a humming sound. The capacitance range of a start capacitor is between 70 to 120 micro Farad.

Image Credit: “Inside of a capacitor” by Razor512 is licensed under CC BY 2.0

Start capacitor increases motor starting torque and allows the motor to be cycled off and on rapidly. The start capacitor is designed in such a way that it is used just for a small time period. They can’t stay energized for longer.

How to tell AC capacitor is bad ?

AC capacitor is an integral part(or component) of an outdoor condensing unit of an air conditioner(or AC) or heat pump. AC capacitor provides sufficient power to the motor, which steers the air conditioning system.

Sign of bad AC capacitor:

  • Smoke or burning smell from the exterior air conditioner component
  • Air conditioning is not bring about cold air even after a long time of operation
  • Humming noise from the air conditioner
  • Old HVAC system
  • AC terms off on its own or on random
  • AC doesn’t start working immediately after turning on
  • High energy consumption causes high energy bills without expectation

Effect of failed AC capacitor:

  • Overheated system circuit
  • Short-circuiting in the AC (or cooling system).
  • Power Rushes or Surges.
  • Electrical discharge or Lightning strikes.
  • Intensely peak outdoor temperature

Sneha Panda

I have graduated in Applied Electronics and Instrumentation Engineering. I'm a curious-minded person. I have an interest and expertise in subjects like Transducer, Industrial Instrumentation, Electronics, etc. I love to learn about scientific researches and inventions, and I believe that my knowledge in this field will contribute to my future endeavors. LinkedIn ID- https://www.linkedin.com/in/sneha-panda-aa2403209/

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