Motor rated current v/s Full load current v/s Nominal current

The terms, motor rated current, full load current and nominal current, are very likely to confuse electrical engineers. Even though these terms are quite similar, there are slightly different from each other. Here is the clear definition of each one of them.

Definitions

Motor rated current

The current drawn by a motor at its full load, calculated using formula is known as the rated current. Motor windings are designed to carry the rated current during normal operations and slightly higher than it for shorter duration.

Try: Simple motor rated current calculator with calculation steps

Motor full load current

The full load current of a motor is the current drawn by it while operating at full load and rated voltage. It is a measured value and can also be calculated using formulae. The full load current may vary upon the applied voltage. Also, the rated full load current (FLC) is the one specified by the manufacturer while tested at ideal conditions.

Refer: AC Motors –full load current tables

Nominal current

Nominal current is the same as the rated current. It is the current drawn by the motor while delivering rated mechanical output at its shaft.

Calculation

The formulae for rated current, full load current and nominal current are the same:

For single-phase AC motors

For single-phase motors, when kW is known:

    \[    F.L.C =\frac{P_k_W \times 1000}{Voltage \times cos \phi \times \eta} \]

For single-phase motors, when HP is known:

    \[    F.L.C =\frac{P_h_p \times 746}{Voltage \times cos \phi \times \eta} \]

For Three phase AC motors

For three phase motors, when kW is known:

    \[    F.L.C =\frac{P_k_W \times 1000}{\surd3 \times Voltage \times cos \phi \times \eta} \]

For three phase motors, when HP is known:

    \[    F.L.C =\frac{P_h_p \times 746}{\surd3 \times Voltage \times cos \phi \times \eta} \]

Where,

  • Voltage: The phase-to-phase voltage for a 3-phase supply.
  • Rating: The power rating of the motor in kW.
  • Power factor (cosΦ): The rated power factor of the motor.
  • Efficiency (η): The efficiency of the motor.

Leave a Reply