Power factor correction capacitor for Single Phase and FHP motors

Contact the motor manufacturer for instructions before changing any capacitors as there are many different types of motors which are subject to nasty operating characteristics whenever deviation from original design specs is initiated.

Start Capacitor :The electrolytic start capacitor helps the motor achieve the most beneficial phase angles between start and main windings for the most locked-rotor torque per locked-rotor ampere. It is disconnected from the start circuit when the motor reaches about 75% of full-load speed.

The start capacitor is designed for short-time duty. Extended application of voltage to the capacitor will cause permature failure, if not immediate destruction. Typical ratings for motor start capacitors range from 100 to 1,000 microfarad (uF) capacitance and 115 to 125 volts AC.However, special applications require 165 to 250-Vac capacitors, which are physically larger than capacitors of lower voltage rating for the same capacitance. Capacitance is a measure of how much charge a capacitor can store relative to the voltage applied.

Run Capacitor : These are constructed similarly to start capacitors, except for the electrolyte. They are designed to serve continuously in the run circuit of a capacitor-start /capacitor-run motor. They withstand higher voltages, in the range of 250 to 370 Vac.

They also have lower capacitance, usually less than 65 uF.

So capacitors are standardized, and you only use appropriately them

Good luck,

It should actually be on the motor's data plate. If not, I believe Grainger's has a rule of thumb type of chart online (I know it used to be in the deadtree catalogue. BTW, it used to be a question on the Master Electrician's test: "What is the most important part of an electric motor" (worth 60% of final grade). Answer: The data plate.

The real answer is, most FHP motors do not require PF correction as their contribution to poor PF will only be noticeable in aggregate, and many 1 phase motor designs already have capacitors in them just to make them work in the first place. So of the few designs where there are no caps (i.e. Shaded Pole or Cap Start- Induction Run where the caps are switched out completely), the sum aggregate total of lagging PF they will contribute is usually too low to be worth correcting, and if it is high enough, bulk correction is usually fine.

Sir,

About 20 years a product called a power factor controller was marketed for single phase motors. It was based on NASA studies. It was a little tricky at times to use because it measured the power factor and adjusted motor voltage to bring this very close to unity. There were times when it couldn't respond fast enough to a change in load and resulted in a motor stopping.

It may still be available on the market. The concept is valid and it may be a good alternative to installing capacitors only.

Regards--JMM

Common misconception.

That device (Google "Nola Controller") was NEVER meant to control power factor. The term "power factor controller" was based on the fact that it used power factor measurement to determine that a motor was loaded or unloaded, then phased back the voltage to the motor with SCR control in an attempt at saving energy if the motor was more than 50% unloaded. So it controlled based on power factor, but it did not control power factor. It did nothing for improving the power factor of the unloaded motor other than the fact that as overall power was reduced, the effect of poor pf was reduced with it. Power factor itself did not change under that controller's influence.

Thanks for the reply.

Let me guess, the run capacitor has a role to increase the power factor also?

In some applications, we need to use high capacity single phase motors, say 15HP. It may not be capacitor run, say induction run. In such cases, we may need to insist for exclusive capacitor for power factor improvement? But, how to assess the rating? I have the formula, kVAR rating = Load in kWx(tanØ1-tanØ2), where power factor is to be improved from cosØ1 to cosØ2. But this formula is for 3 phase. How to apply for single phase circuits? Also advice me, if I finalise a rating, how to connect the capacitor? across the line and neutral?

Pls reply.

Sir,

I started to reply, but then did a little research. Look at the information at http://Imphotonics.com/pwrfact.htm. I hope this helps some.

JMM

Should read http://www.LMPhotonics.com/pwrfact.htm

I have a question for you Sir.

I come to know that we can reduce our electical bill if we add Capasitor at home.

Im using almost 1000w of elcrical appliances and up to 3500w can you please let me know the actual value capasitor value reqired to reduce my bills. I wish to join my capasitor paralel to the Distribution board.

My email address grner@danisco.com

The role of capacitor in such a case is to increase the power factor, there by to reduce the current drawn to cope with the power running. So, by adding capacitor line loss (due to the high current) will be reduced and the electrical wiring will be more secured.

Though the power quality is increased, the energy consumption remain the same. So, you can never reduce the energy bill. Still but, the penality if any through the energy bill of the electricity supplier for the non-use of capacitor will be now relaxed.

"I come to know that we can reduce our electical bill if we add Capasitor at home."

This is a lie, do not believe it! Improving Power Factor only saves MONEY, not energy, and only if your utility assesses a penalty for having poor Power Factor. The vast majority of utilities only assess PF penalties to their larger industrial users, not residential users. it takes a special meter to even see poor PF, and that meter is more expensive than the tiny amount of revenue the utility can recover by installing and maintaining them on every residence. Even if you are assessed a penalty, improving PF on a load as tiny as 3.5kW will not likely be worth the cost of installing a capacitor.

But do not believe the lies about saving energy, it is just not true. CURRENT is reduced, but current is not energy, it is only a component of energy. When you tally all of the factors into how you are charged for energy, improving PF makes no difference to you, the user and bill payer.