Understanding VA Input for 3-Phase Motors: A Quick Guide

When tackling the intricacies of electrical systems, particularly for 5 horsepower 3-phase motors, understanding how to calculate the VA input becomes essential for anyone preparing for the HOLT Electricians Exam. Let's get to the nitty-gritty of it, shall we?

To start, you want to know what the formula looks like. Here it is:

[ I = \frac{P \times 1000}{\sqrt{3} \times V} ]

In this equation, (I) represents the current in amperes, (P) is the power in horsepower, and (V) stands for our line voltage. This might sound a bit overwhelming, but stick with me. The reason we're multiplying by 1000 is to convert horsepower to watts because we need to work in units that correspond to power more practically.

Let's say you’re using a motor that's rated for dual voltage—460/230 volts. First things first, we need to convert our 5 horsepower into watts:

[ 5 , \text{hp} \times 746 , \text{W/hp} = 3730 , \text{W} ]

This means your 5 horsepower motor is drawing about 3730 watts when it's running full blast. Now, what's next? You guessed it; we plug this into our formula for full-load current.

For instance, working with the higher voltage rating of 460 volts, you'd actually calculate:

[ I = \frac{3730}{\sqrt{3} \times 460} ]

Calculating this gives us a current, but we’re still gearing up to find the VA input. After you've worked through the math, you'll find that the current is approximately 4.67 amperes.

But wait, there's more! To find the VA input, we need to multiply the current back by the voltage—and since we're dealing with a 3-phase system, we multiply by the square root of 3 (because three phases means more complexity in calculating variables).

Let’s summarize that VA input calculation:

[ VA = \sqrt{3} \times V \times I ]

By substituting our values:

[ VA = \sqrt{3} \times 460 \times 4.67 ]

As you crunch those numbers, you’ll land on a VA input of approximately 6055 VA. This leads us to the right answer out of our choices:

• A. 4050 VA
• B. 5050 VA
• C. 6055 VA (this is it!)
• D. 7050 VA

It’s a fascinating plunge into electrical principles, right? Understanding these fundamental calculations connects you to the pulse of the electrical industry, ensuring that when you're working with motors, you can easily assess their power requirements efficiently.

So next time you see that dual-voltage rating, don’t just breeze past it. Take time to break it down. Feel comfortable with calculations like this, and you’ll be that much closer to acing the HOLT Electricians Exam—ready to tackle whatever comes your way in the world of electrical work!