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
|Motor rated power|
|Locked rotor current||A|
How to use the calculator:
The above tool requires the 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 relays, cables, and other switchgear is necessary to protect the motor from damage. The motor full load current forms the basis for the selection of all this equipment.
Motor FLC calculation
For single-phase AC motors
For single-phase motors, when kW is known:
For single-phase motors, when HP is known:
For Three-phase AC motors
For three-phase motors, when kW is known:
For Three-phase motors, when HP is known:
- 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.
Locked rotor KVA per Horsepower
The National Electrical Manufacturer’s Association (NEMA) has designated code letters to classify motors by the rotor kVA per horsepower. Here is the tabulation of those code letters and kVA per horsepower values.
|Letter designation||(KVA/HP) Minimum||(KVA/HP) Maximum|
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