# Power factor correction (totally lost)

We have a 3 phase electronic motor with a 'useful' power 4kW. Its efficiency is 80% Currents on the lines are equal in all three phases, and they are lagging behind the voltages by 30 degree. We need to make a power factor correction in such a way, that the phase difference between the current and voltages becomes less than 10 degree.

And I am totally lost.

Is the efficiency the power factor? Do we have to do anything by the fact that it is lagging? (Other than it shows that we need to use a capacitor for the correction?) Why less then 10 degree and not zero?

Tried looking up videos and googling, but that only made me more confused :/

ps: Voltage is 230 Volt and 50 Hz. If the example needs this information.

• Hint: if it's a motor, it's a resistive-inductive load. The capacitive load would be the opposite of that. May 8, 2020 at 18:11

Is the efficiency the power factor?

No. The efficiency tells you how much of the electrical power coming into the motor is converted to "useful" power, mechanical power transferred to the load.

Do we have to do anything by the fact that it is lagging? (Other than it shows that we need to use a capacitor for the correction?)

No.

Why less then 10 degree and not zero?

A better question would be why not "less than X degrees but not less than 10 degrees?" If too much correction is done, an induction motor can become self excited and not coast to a stop promptly when it is shut off. Motor manufacturers often specify a maximum amount of correction. Correcting to zero degrees is never a good idea. Correcting to 10 degrees pf = 0.98 seems a bit much.

I assume this is an induction motor, not an "electronic motor." An electronically controlled motor should not have power factor correction.

Currents on the lines are equal in all three phases, and they are lagging behind the voltages by 30 degree

I read this observation as the line current lagging the line voltage by 30 degrees.

Take a look at a 3 phase phasor diagram: -

And concentrate on one of the line voltages, namely $$\V_{RY}\$$ - this voltage naturally leads the local phase voltage $$\V_{R}\$$ by 30 degrees. It's the same for the other line voltages - each line voltage leads its respective phase voltage naturally by 30 degrees.

Given that a resistive balanced load would draw a current in phase with $$\V_{R}\$$, I have to conclude that your "3 phase electronic motor" is running close to unity power factor.

We need to make a power factor correction in such a way, that the phase difference between the current and voltages becomes less than 10 degree.

I have serious doubts about that so, get someone in who can be trusted to make proper measurements and make a local recommendation.