# Siemens: Calculating the amp rating for a 3-phase induction motor

Calculating the amp rating for a 3-phase induction motor is an important task for engineers involved in designing and operating electrical systems. An accurate calculation of the amp rating helps in selecting the appropriate protection devices and ensuring safe and reliable operation of the motor. In this article, we will discuss the steps involved in calculating the amp rating for a 3-phase induction motor.

## Definition of a 3-phase induction motor

It is a type of motor that operates on three-phase AC power. The motor consists of a stator (stationary part) and a rotor (rotating part). The stator contains the winding that produces a rotating magnetic field, while the rotor carries the current induced by the magnetic field and rotates as a result. The power input to the motor is provided through three-phase power supply, and the motor produces mechanical output in the form of rotational motion.

## Requirements for the calculation

To calculate the amp rating for a 3-phase induction motor, we need to consider two main factors: the motor’s power rating and the supply voltage.

The power rating of a motor indicates the amount of electrical power that it can convert into mechanical power. The unit of power is Watts (W), kilowatts (kW) or horsepower (HP).

The supply voltage is the voltage at which the motor is connected to the power supply. The unit of voltage is Volts (V).

## The formula for calculating the amp rating

Amp Rating = (Power Rating × 746) / (√3 × Supply Voltage × Power Factor)
Where Power Factor is the ratio of the actual power (Watts) to the apparent power (VA) supplied to the motor. It represents the efficiency of the motor in converting electrical power into mechanical power. The value of the power factor ranges from 0 to 1, and a higher value indicates a more efficient motor.

## A Practical Example

Let’s take an example to understand how to use this formula. Suppose we have a 3-phase induction motor with a power rating of 15 kW and is connected to a power supply of 415 V. The power factor of the motor is 0.85. We can calculate the amp rating of the motor as follows:
Amp Rating = (15,000 × 1000) / (√3 × 415 × 0.85) = 24.98 A
Therefore, the amp rating of the motor is 24.98 A.

## Considerations for the calculation

It is important to note that the above formula assumes that the motor operates at its rated power and efficiency. In reality, the actual current drawn by the motor may vary depending on the load and operating conditions. Therefore, it is essential to consider the starting current and the running current of the motor to ensure proper selection of protection devices.
The starting current of a 3-phase induction motor is typically 5 to 7 times higher than the rated current. This is because the motor requires a higher current to overcome the inertia of the rotor and start rotating. The high starting current can cause voltage drops and disturbances in the power supply. To prevent this, the motor is equipped with protection devices such as circuit breakers and fuses that can trip when the current exceeds a certain limit.
The running current of a 3-phase induction motor depends on the load and the operating conditions. It is typically 70% to 80% of the rated current. If the current drawn by the motor exceeds the rated current for an extended period, it can cause overheating of the motor and damage to the winding insulation. Therefore, it is essential to monitor the current drawn by the motor and ensure that it does not exceed the rated current for extended periods.
In addition to the above factors, the ambient temperature and altitude can also affect the current rating of a 3-phase induction motor.

In the world of electrical motors, it is important to note the term “full load amperes” (abbreviated to FLA), which refers to the amount of current (in amps) that an electrical motor draws when driving its rated load at its rated speed and voltage.
For instance, it will take 2 A to drive a 1/2 HP induction motor that is rated for 230 VAC. The below table is a useful reference for determining the FLA at various voltages for motors of various sizes and can be used to determine load calculations during electrical installations.