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 voltage | V | |
| Motor rated power | ||
| Design Code | ||
Calculated Values | ||
| Locked rotor current | A | |
How to use the calculator:
The above tool requires following data for calculation:
- Motor Type – Single-phase or three-phase. (Mandatory)
- Motor rated power in HP or kW. (Mandatory)
- Input supply voltage. (Mandatory)
- 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:
![\[ LRC =\frac{P_k_W \times 1341 \times (KVA/HP)}{Voltage} \]](https://www.electricalclassroom.com/wp-content/ql-cache/quicklatex.com-c35058807279476dcf043809e70fd092_l3.png "Rendered by QuickLaTeX.com")
For single-phase motors, when HP is known:
![\[ LRC =\frac{P_h_p \times 1000 \times (KVA/HP)}{Voltage} \]](https://www.electricalclassroom.com/wp-content/ql-cache/quicklatex.com-71b9617698c8d397f4dc5b9fa7084e7e_l3.png "Rendered by QuickLaTeX.com")
For Three phase AC motors
For three-phase motors, when kW is known:
![\[ LRC =\frac{P_k_W \times 774 \times (KVA/HP)}{Voltage} \]](https://www.electricalclassroom.com/wp-content/ql-cache/quicklatex.com-0ba912672aa74b75ac7ba05080a91004_l3.png "Rendered by QuickLaTeX.com")
For single-phase motors, when HP is known:
![\[ LRC =\frac{P_h_p \times 577 \times (KVA/HP)}{Voltage} \]](https://www.electricalclassroom.com/wp-content/ql-cache/quicklatex.com-330e48add9aa1c04fcbedd37ef3173d3_l3.png "Rendered by QuickLaTeX.com")
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 |
|---|---|---|
| A | 1 | 3.14 |
| B | 3.15 | 3.54 |
| C | 3.55 | 3.99 |
| D | 4 | 4.49 |
| E | 4.5 | 4.99 |
| F | 5 | 2.59 |
| G | 2.6 | 6.29 |
| H | 6.3 | 7.09 |
| I | 7.1 | 7.99 |
| K | 8 | 8.99 |
| L | 9 | 9.99 |
| M | 10 | 11.19 |
| N | 11.2 | 12.49 |
| P | 12.5 | 13.99 |
| R | 14 | 15.99 |
| S | 16 | 17.99 |
| T | 18 | 19.99 |
| U | 20 | 22.39 |
| V | 22.4 | – |
Other calculators:
- Synchronous speed calculator
- Induction motor slip
- Motor torque calculator
- Motor FLC calculator
- Motor Locked rotor current calculator
- Number of poles calculator
- DOL Starter Design tool
- Star-Delta Starter design tool
- HP to kW Converter
- kW to Amps Converter
- Amps to kW converter
