For 150 hp load of machine how much kva transformer required

How to Calculate Transformer Rating for a Particular Machine Load

When selecting a transformer for a machine, you cannot directly match the HP rating of the machine with the transformer rating. Transformers are rated in kVA, while machines are usually rated in HP or kW.

To determine the right transformer size, we need to:

1. Convert HP → kW

2. Convert kW → kVA (considering power factor)

3. Add a safety margin (20%)

4. Select the next available standard transformer rating

Flowchart: Steps to Calculate Transformer Rating

Machine Rating (HP)
        ↓
Convert HP → kW  
   (kW = HP × 0.746)  
        ↓
Convert kW → kVA  
   (kVA = kW ÷ PF)  
        ↓
Apply Safety Margin  
   (kVA × 1.2)  
        ↓
Select Next Standard Transformer Rating

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Step 1: Convert HP into kW

The standard conversion is:

$$1 \, \text{HP} = 0.746 \, \text{kW}$$

For a 150 HP machine:

$$150 \times 0.746 = 112 \, \text{kW}$$

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Step 2: Convert kW into kVA

Since the transformer is rated in kVA, and machines are usually inductive loads, we must consider the power factor (PF).

$$\text{kVA} = \frac{\text{kW}}{\text{Power Factor}}$$

Assume PF = 0.8 (typical for motors and industrial loads):

$$\frac{112}{0.8} = 140 \, \text{kVA}$$

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Step 3: Apply Safety Factor (20%)

To account for jerk loads, starting currents, and occasional overloads, we add 20% extra:

$$140 \times 1.2 = 168 \, \text{kVA}$$

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Step 4: Select the Next Standard Rating

Since 168 kVA is not a standard transformer size, we select the next higher available rating, which is:

✅ 200 kVA Transformer

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Final Selection

For a 150 HP (112 kW) inductive machine, the 200 kVA transformer is suitable.

👉 This same method can be applied for any other machine rating by following the step-by-step approach.

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General Formula

For quick calculation, you can use:

$$\text{Transformer Rating (kVA)} = \frac{\text{HP} \times 0.746}{\text{Power Factor}} \times 1.2$$

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Comparison Table (HP vs Transformer Rating)

Machine Rating (HP)	Load (kW)	kVA @ PF=0.8	With 20% Safety (kVA)	Recommended Transformer Size
50 HP	37.3 kW	46.6 kVA	56 kVA	63 kVA
100 HP	74.6 kW	93.3 kVA	112 kVA	125 kVA
150 HP	112 kW	140 kVA	168 kVA	200 kVA
200 HP	149 kW	186 kVA	224 kVA	250 kVA
300 HP	224 kW	280 kVA	336 kVA	400 kVA

Typical Transformer Connection

For machine loads, usually Delta-Star transformers are preferred, as they:

• Provide a neutral point for grounding

• Help in reducing harmonics

• Improve system stability

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✅ Practical Tip: Always choose the transformer rating higher than calculated to avoid overheating and ensure long life.

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