# How do you select a transformer for a motor?

Contents

## How do I know what size transformer I need?

Calculate an example as follows. A 120-volt motor has a load amperage of 5 amps. Multiply 120 volts times 5 amps this equals 600VA now lets multiply the 125 percent start factor. Take 600 times 1.25 this equals 720VA and most transformers are sized by a factor of 25VA or 50VA.

## How do I choose a transformer?

To select the proper transformer, determine the input and output voltages, frequency and current. The faster the voltage changes, the higher the frequency. The higher the current, the more heat is produced.

## How many amps is a 25 kVA transformer good for?

Three Phase Transformer

KVA 208V 480V
20 56.6 24.1
25 69.5 30.1
30 83.4 36.1
37.5 104 45.2

## How do you calculate the power of a transformer?

Input and output power

1. Electrical power is calculated by multiplying voltage (in volts) by current (in amps).
2. If a transformer is 100% efficient, then the input power will equal the output power. …
3. V P is input (primary) voltage.
4. I p is input (primary) current.
5. V s is output (secondary) voltage.

## What is the transformer formula?

Vp=−NpΔΦΔt V p = − N p Δ Φ Δ t . This is known as the transformer equation, and it simply states that the ratio of the secondary to primary voltages in a transformer equals the ratio of the number of loops in their coils.

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## How do you choose a 3 phase transformer?

The frequency of the line supply and electrical load must be the same. A three phase transformer is selected which is designed to operate at this frequency having a primary (input) equal to the supply voltage and a secondary (output) equal to the voltage required by the load.

## What size breaker do I need for a 45 kVA transformer?

For a 45kVA transformer, 125A×1.25=156A, so use a 175A protection device.