Motor locked rotor current calculator

What is ‘locked rotor current’?

A motor at rest acts as if its rotor is locked. The locked rotor current and the starting current of a motor are one and the same. It is the starting current drawn by a motor when full nominal voltage is applied to its terminals. Here is a simple locked rotor current calculator. Enter the motor rated power, nominal voltage and NEMA code and click on calculate button.

Locked rotor current calculator

Nominal voltageV
Motor rated power
Design Code

Calculated Values

Locked rotor currentA

How to use the calculator:

The above tool requires following data for calculation:

  1. Motor Type – Single-phase or three-phase. (Mandatory)
  2. Motor rated power in HP or kW. (Mandatory)
  3. Input supply voltage. (Mandatory)
  4. NEMA code. (Mandatory)

After entering the required data, click on the “Calculate” button to find out the locked rotor current of the motor.

The appropriate selection of fuses, circuit breakers, overload relay, cables, and other switchgear is necessary to protect the motor from damages. The motor full load current forms the basis for the selection of all these equipment.

Motor FLC calculation

For single phase AC motors

For single-phase motors, when kW is known:

    \[    LRC =\frac{P_k_W \times 1341 \times (KVA/HP)}{Voltage} \]

For single-phase motors, when HP is known:

    \[    LRC  =\frac{P_h_p \times 1000 \times (KVA/HP)}{Voltage} \]

For Three phase AC motors

For three-phase motors, when kW is known:

    \[    LRC =\frac{P_k_W \times 774 \times (KVA/HP)}{Voltage} \]

For single-phase motors, when HP is known:

    \[    LRC  =\frac{P_h_p \times 577 \times (KVA/HP)}{Voltage} \]

Where,

  • Voltage: The phase-to-phase voltage for a 3-phase supply.
  • Rating: The power rating of the motor in kW.
  • KVA/HP: Maximum value corresponding to the NEMA code.
  • Efficiency (η): The efficiency of the motor.

Locked rotor KVA per Horsepower

The National Electrical Manufacturer’s Association (NEMA) has designated code letters to classify motors by the rotor kVA per horse power. Here is the tabulation of those code letters and kVA per horsepower values.

Letter designation(KVA/HP) Minimum(KVA/HP) Maximum
A13.14
B3.153.54
C3.553.99
D44.49
E4.54.99
F52.59
G2.66.29
H6.37.09
I7.17.99
K88.99
L99.99
M1011.19
N11.212.49
P12.513.99
R1415.99
S1617.99
T1819.99
U2022.39
V22.4 –
For further details refer: https://www.engineeringtoolbox.com/locked-rotor-code-d_917.html

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