Question

How to calculate capacitor for 3 phase motor?

Answer 1

To calculate the capacitor size for a 3-phase motor, the goal is usually to improve the power factor or provide reactive power compensation for motors running at a lagging power factor. The exact size depends on whether you want to correct the power factor to a desired value or just provide a basic level of correction.

Here’s a general approach to calculate the capacitor size for power factor correction in a 3-phase motor:

1. Power Factor Correction (PFC) Formula

The general formula for the required capacitor (in kVAR) is:

\[
Q_c = P \times (\tan(\theta_1) - \tan(\theta_2))
\]

Where:

1. Q_c = Required capacitor size in kVAR
   

1. P = Motor power in kW
   

1. θ₁ = Initial power factor angle (before correction)  
   

1. θ₂ = Desired power factor angle (after correction)
   

2. Step-by-Step Calculation

Step 1: Determine Motor Power (P)

This is the rated power of the motor, often given in kW. If it's given in horsepower (HP), convert it to kW using:

\[
P_{\text{kW}} = \text{Horsepower} \times 0.746
\]

Step 2: Find the Initial Power Factor

This is the power factor of the motor before correction. Power factor is typically around 0.7 to 0.9 for many motors running under no-load or lightly loaded conditions, but it can be lower depending on the motor’s operation.

Step 3: Choose a Desired Power Factor

For most industrial applications, the goal is to improve the power factor to something around 0.95 to 0.98. The closer to 1.0, the better the efficiency in terms of using the electrical supply.

Step 4: Use the Power Factor Angles

Power factor angle θ can be calculated as:

\[
θ = \cos^{-1}(\text{Power Factor})
\]

For the initial power factor (θ₁) and the desired power factor (θ₂), calculate the respective angles.

Step 5: Calculate the Required Capacitor Size (kVAR)

Now, apply the values into the formula for Q_c, and this will give you the required kVAR for the capacitor.

---

Example Calculation:

Let’s say:

1. Motor power = 10 kW
   

1. Initial power factor = 0.75
   

1. Desired power factor = 0.95
   

1. Initial power factor angle:

\[
θ_1 = \cos^{-1}(0.75) = 41.41^\circ
\]

2. Desired power factor angle:

\[
θ_2 = \cos^{-1}(0.95) = 18.19^\circ
\]

3. Calculate the capacitor size (Q_c):

\[
Q_c = 10 \times (\tan(41.41^\circ) - \tan(18.19^\circ))
\]
\[
Q_c = 10 \times (0.8693 - 0.3255) = 10 \times 0.5438 = 5.44 \, \text{kVAR}
\]

So, a 5.44 kVAR capacitor would be needed to correct the power factor of this 10 kW motor from 0.75 to 0.95.

---

Alternate Approach Using kVA:

You can also use an alternate formula based on the motor’s apparent power (kVA) and the power factor:

\[
Q_c = \text{kVA} \times \left( \text{tan}(\cos^{-1}(\text{Power Factor})) \right)
\]

This method is useful when the motor’s kVA rating is more easily available.

Important Notes:

1. Always check the voltage rating and the type of capacitor that matches your motor specifications.
   

1. For large motors or motors with variable loads, it’s often a good idea to consult a professional for accurate sizing to avoid over or under-correction.
   

1. Too much capacitance can lead to overcorrection, causing the system to operate at a leading power factor, which can have negative effects like voltage instability.
   

Would you like to go through another example or need help with any of the steps?