How Many Kilowatt Generators Are Needed for a 10 HP 3-Phase Motor?

To determine the number of kilowatt generators required for a 10 HP 3-phase motor, it is essential to follow a series of steps that include converting horsepower to kilowatts, considering motor efficiency and power factor, and calculating the actual power requirement. This article will guide you through these steps, ensuring you make informed decisions for your electrical system.

Step 1: Convert Horsepower to Kilowatts

The conversion from horsepower (HP) to kilowatts (kW) is the first crucial step.

Formula: kW HP × 0.746

Calculation:

For a 10 HP motor:

kW 10 HP × 0.746 7.46 kW

Step 2: Consider Motor Efficiency and Power Factor

Motors are not 100% efficient. Therefore, it is important to account for the efficiency and power factor.

Typical Values: Motor efficiency: 0.9 Power factor: 0.8 to 0.9 (let's use 0.85 for conservative estimates)

Now, calculate the actual power requirement using these values.

Step 3: Calculate the Actual Power Requirement

The actual power requirement can be calculated with the following formula:

Formula: Actual kW kW / (Efficiency × Power Factor)

Calculation:

Plugging in the values:

Actual kW 7.46 / (0.9 × 0.85) ≈ 7.46 / 0.765 ≈ 9.75 kW

Conclusion

To operate a 10 HP 3-phase motor, you would need a generator that can provide at least 9.75 kW of power, taking into account efficiency and power factor. It is advisable to have a generator with a slightly higher capacity for reliable operation. A generator rated for at least 10-12 kW would be a good choice.

Note: The required generator size can also depend on how the motor is started and the nature of the starting load. Here are some considerations:

Starting Load Variations

Depending on the starting method and the load characteristics, different generator sizes may be needed:

Auto-transformer starter at 50% torque starting load like a centrifugal pump: A 15 kW unit may suffice. Full line starting with high starting torque: A 25 kW unit would be necessary.

NEMA Code G (1800 RPM) and Starting Current: An auto-transformer starter is the most effective method to reduce starting current, except for variable frequency drives (VFDs).

Generators can be designed with oversized generator ends to provide sufficient starting current for closely matched motors. The starting power factor for a standard motor is around 0.25, making it unnecessary to have a full-size engine. Additionally, the use of a centrifugal clutch allows the motor to reach 80% speed in seconds, starting unloaded until it reaches the required speed.