# Start capicitor sizing

Discussion in 'General Electronics Chat' started by Marvin Burke, Dec 9, 2015.

1. ### Marvin Burke Thread Starter New Member

Dec 9, 2015
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I am making a 3 phase rotary converter I do not need ant advise on motor run capacitors I understand balancing the third leg. What I do need is advice on start capacitor sizing equation. I understand that I need voltage rating of 1 1/2 times of the potential voltage of the circuit or slightly higher. Now for the mf or uf rating There seem's to be a lot of vague and conflicting information out on the interweb. Two common formula's I have found are 50 uf per horsepower and 100 uf per horsepower? How important is it to choose a rating within 10% of actual requirements for the starting capacitor? Shouldn't the equation include the starting current as well as voltage? Or should the emphasis be directed more toward phase timing as in +120 degree's (+=10%) of the AC cycle waveform? In this case the application is starting a 3 phase 15 horsepower idler motor on single phase power.

Ps. I have seen Rotary converters start up slowly tanking as much as 5 seconds to come up to speed. Also have seen the come to speed instantly hitting the max rpm so hard that they jar the building. Is this hard start detrimental to the components of the motor? Does this mean that the capacitance has hit the 120 degree point of the wave cycle exactly or does it mean that there is too much capacitance starting the motor with too much current?

Thank you my name is Marvin from East Tennessee

RRITESH KAKKAR likes this.
2. ### RRITESH KAKKAR Senior Member

Jun 29, 2010
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Good wishes Marvin for your project.

Nov 4, 2013
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4. ### Marvin Burke Thread Starter New Member

Dec 9, 2015
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What is the typical jumper wire gauge size for a 15 HP motor run cap bank? 12 Ga? 10 Ga? Will I need heavier Ga for the start cap's?

5. ### tcmtech Well-Known Member

Nov 4, 2013
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150 uf per leg on a 240 volt feed would see less than 15 amps so 12 ga will be fine.

Starting circuit currents could be over 100 amps but the short duty cycle would allow for 6 ga copper to work without problems.

6. ### Marvin Burke Thread Starter New Member

Dec 9, 2015
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Ok how about contactors and circuit breakers Considering that this application shouldn't draw more than 40 amps under a full load but under start up it may surge to 120 amps. Do I need a circuit breaker to supply 100+ amps or will a 50A circuit supply adaquet current? Also contactors the same question 50A or 125A?

7. ### tcmtech Well-Known Member

Nov 4, 2013
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15 hp = 60 amp breakers and 60 amp contactors unless it's running a larger secondary three phase electrical load.

8. ### BR-549 Well-Known Member

Sep 22, 2013
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The thermal overloads allow the surge.