Design Notes

DESIGN INFORMATION:

1. What is the power absorbed by a Motor load?


Power absorbed is the input power drawn by the motor. It should be noted that a motor’s rated kW or name plate rating is the output rating of the motor (also known as the shaft power).

When we are calculating the total load requirements of a system, we have to always use absorbed power, which is the actual power drawn by the motor.

Power absorbed= input power=kW rating/ efficiency
It is also represented as “ P1” in pump data sheets.

Also note that the Input current drawn by a motor= kW/(1.732 *V* pF)

Powerfactor correction

What is power factor?

Power factor is defined as the ratio of real power to total power

ie, pF=Real power/Apparent Power= kW/ kVA

or kW= kVA *power factor 

Real (active power) power does the useful work. Apparent power(reactive power) is non working power, which is mainly utilized for magnetization of cores of transformer, induction motors etc. Real or true power kW is the power available after reactive power requirement has been satisfied.

In an AC circuit, it is also defined as the cosine of the angle between voltage and current vector.

power factor triangle is given below

The power factor can get values in the range from 0 to 1.When power factor is close to One, it is considered as goof pF

Effects of low power factor

• If power factor is low, a system would draw more current for a given kW from the source, which will cause more voltage drop. By improving the pF, one can reduce voltage drop and thus increase the voltage level.
• Utilities will charge a higher cost to industrial and commercial clients having a low P.F.
  

What is power factor correction?

Power factor correction is an adjustment of the electrical circuit in order to change the power factor near 1. Mos electricity authorities requires that that power factor should be corrected to 0.9 or 0.95

How to correct power factor in an AC distribution system?

Power factor compensation can be defined as a process of correcting the lagging current by producing leading current so that the angle between voltage and current reduces. 

This is achieved by connecting a capacitor panel to the distribution system.

A capacitor panel shall consist of many capacitors connected in parallel to create the required kVAR.There will be a power factor control relay which  will sense the voltage and current of the system and then  calculate the required value of capacitors to be connected. This assures that only the needed capacitor is energized depending on the load and power factor of the network to ensure the power factor remains above a selected value

Indicative image of a APFC Panel

Power factor control relays

These are electronic relays  which sense the voltage /current from the system and give output signals to switch contactors ON to switch on required number of capacitors. Based on number of outputs given, 6 steps, 12 steps relays are available. For more precise control of pF, higher number of steps would be required.

Indicative image of a Power factor control relay

Calculating Required Capacitor kVAR

Calculating Required Capacitor kVAR to improve P.F from 0.85 to 0.90

Required Capacitor kVAR = kW (tan θ₁ – tan θ₂)

where kW is the connected load of the panel
(many electricity authorities agrees to apply a diversity factor on the kW value for sizing the capacitor value

so if the connected load is 1200 kW and diversity factor is 0.8
demand= 1200x 0.8 kW

tan θ₁ – tan θ₂ works out to be about 0.28

kVAR required= 1200 x 0.8 x 0.28=268 kVAR say 300 kVAR which is the nearest standard value.

following combination of capacitors units could be used to achieve this;

50x 5  kVAR
25x 2   kVAR

 A 10 step relay could be used to connect this capacitor bank

Rated current of a capacitor

${\displaystyle I_{N}={\frac {Q}{{\sqrt {3}}.U}}}$

where Q is the kVAR value and U is the phase-phase voltage in kV

for 300 kVAR,

In= 300/0.4*1.732= 300x1.44 =433.02 Amp 

Breaker size required= 630 Amp  (nearest standard value)