Locked rotor current calculations

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Locked Rotor Current and Torque in Induction Motors

What is Locked Rotor Current?

Locked rotor current (LRC) is the maximum current drawn by an induction motor when its rotor is not rotating (locked condition). At startup, since the motor speed is zero, the slip is 1 (100%), and the motor draws its highest current.



  • This current is usually 3–8 times the full load current (FLC).
  • As the motor accelerates, the current gradually reduces until it stabilizes at the rated full load current.
  • LRC depends on the motor design, type, and supply voltage (higher supply voltage → lower current for the same power).

👉 In other words, "locked rotor current" is the current drawn if the motor is energized but prevented from turning.

Equivalent Circuit at Locked Rotor Condition

At locked rotor, each stator phase behaves like a series R–L circuit.

  • Resistance (R): Copper loss component.
  • Inductance (L): Reactance due to leakage flux.

This explains why the inrush current is much higher than the rated running current.

NEMA Code Letters for Locked Rotor Current

NEMA (National Electrical Manufacturers Association) classifies motors by assigning a code letter that represents the Locked Rotor kVA per horsepower (kVA/HP).

Example Table (Extract)

NEMA Code Letter

Locked Rotor kVA/HP Range

A

0 – 3.14

D

3.15 – 3.54

G

5.6 – 6.3

K

8.0 – 8.9

M

10.0 – 11.2

(The higher the code letter, the higher the inrush current.)

Example Calculation of Locked Rotor Current

Let’s calculate for a 30 HP motor with rated current 34.9 A at 460 V, code letter G (5.6–6.3 kVA/HP).

Formula:



Locked Rotor Torque (Starting Torque)

Locked rotor torque is the torque developed by the motor when starting from rest (zero speed).

  • High starting torque is essential for applications with heavy loads at startup (e.g., positive displacement pumps, crushers, cranes, hoists).
  • Low starting torque is acceptable for applications with low initial load (e.g., centrifugal fans, blowers, pumps).

Key Takeaways

  • Locked rotor current is 3–8 times the rated current of the motor.
  • NEMA code letters classify motors based on their locked rotor kVA/HP.
  • Locked rotor torque is a critical factor for selecting motors in heavy-duty applications.
  • Proper knowledge of LRC is essential for motor protection system design (overcurrent relays, breakers, contactors).

⚡ Pro Tip: Always check the motor’s datasheet for LRC and LRT before selecting protective devices, since under-sizing can cause nuisance trips, while over-sizing can risk damage to windings.

 

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