How do I find Capacitor rating to start / run Single phase motor

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Discussion Overview

The discussion revolves around determining the appropriate capacitor ratings for starting and running a 1.1KW single-phase motor, which is currently missing these components. Participants explore methods for calculating or estimating the required capacitor values, considering the motor's specifications and operational context.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant seeks guidance on calculating the necessary capacitor ratings for a 1.1KW single-phase motor, noting the absence of both start and run capacitors.
  • Another participant suggests contacting an industrial supplier for assistance, questioning the reason for the missing capacitors.
  • A participant from Malawi expresses difficulty in accessing industrial suppliers and requests a method to estimate capacitor ratings for trial and error.
  • Some participants propose that typical capacitor values can be similar for motors of the same horsepower and voltage, although they do not provide specific calculations.
  • One suggestion involves testing the motor with a running capacitor while leaving the starting capacitor unconnected, to check if the motor can start manually.
  • Another participant mentions that starting capacitors for 230VAC motors typically range from 16-22uF, based on their research.
  • There is a note about motors potentially having a centrifugal switch that disconnects the starting capacitor once a certain speed is reached, although this is not confirmed for the motor in question.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the exact method for calculating capacitor ratings, and multiple viewpoints regarding testing and estimating values are presented. The discussion remains unresolved regarding the best approach to determine the necessary capacitors.

Contextual Notes

Limitations include the lack of specific guidelines for calculating capacitor ratings and the potential influence of local conditions on the availability of components. The discussion also reflects uncertainty about the motor's condition and operational requirements.

sharper1968
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I am trying to refurbish a 1.1KW single phase motor (1445 RPM) but both start and run capacitors are missing.

How can I calculate what capacitors are required.

Motor is Rated 7.2 A (220 VAC)

Look forward to your help.

Thanks.
 
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Call an industrial supplier and give them the make/model of the motor. They will set you up. Do you know why these items are missing? There is usually a good reason for it. Something else is wrong with the motor and the 2 capacitors were salvaged?
 
Thanks for your reply, which unfortunately is not a lot of help in my circumstances.

I am in Malawi, Central Africa, and "industrial suppliers" are very thin on the ground. Being in Malawi is also the most likely explanation for the capacitors being missing - in Africa if you don't use it you lose it! It is of course possible that the is a problem with the motor, but I have done a few basic tests and I think it is ok. Obviously replacing the capacitors was the best place to start.

Is there not some way to calculate the likely ratings so I at least have somewhere to start with trial and error?

Many thanks.
 
I'm sure there is a way to calculate it, or a typical set of guidlines for it. However, I wouldn't know what they are. I DO know that the values will be similar for similar types of motors of a given horsepower rating and voltage. You do realize that you don't need a starting capacitor to do a few basic tests? If you have the running capacitor installed and just leave the starting capacitor out, NOT bypassed, just leave the terminals unhooked, apply power to the motor and manually spin the motor in either direction. The motor should spin up. Of course you haven't tested the starting winding, but you might look into wiring a bunch of light bulbs in parallel with each other but in series with the starting winding (in place of the capacitor). If it does start on it's own, it will not have much starting torque, but it should prove that the starting winding is good if it does spin up on its own.
 
Just a note. It's probably not the case here, but I have seen motors where the start windings (and capacitor) get disconnected form the circuit once the armature speed is high enough. The motor has a centrifugal switch on one of its shaft ends.

After some google searching; looks like for a 230vac motor, the start caps are 16-22uF.

Regard
 
dlgoff said:
Just a note. It's probably not the case here, but I have seen motors where the start windings (and capacitor) get disconnected form the circuit once the armature speed is high enough. The motor has a centrifugal switch on one of its shaft ends.

After some google searching; looks like for a 230vac motor, the start caps are 16-22uF.

Regard

If the motor has a starting capacitor as well as a running capacitor then I am sure that the starting capacitor is switched out after a certain RPM. I assumed that was a known.
 

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