Calculate Locked Rotor Torque Definition

Locked Rotor Torque Equation:

\[ T_{lr} = \frac{3 \times V^2 \times R_r}{2 \times \pi \times f \times \left[(R_s + R_r)^2 + (X_s + X_r)^2\right]} \]

Voltage (V):

volts

Rotor Resistance (R_r):

ohms

Frequency (f):

Stator Resistance (R_s):

ohms

Stator Reactance (X_s):

ohms

Rotor Reactance (X_r):

ohms

Locked Rotor Torque (T_lr):

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1. What is Locked Rotor Torque?

Locked rotor torque (LRT) is the torque produced by an electric motor when its rotor is stationary (locked) and rated voltage is applied to the stator. It represents the starting torque available to overcome the inertia of a load.

2. How Does the Calculator Work?

The calculator uses the locked rotor torque equation:

\[ T_{lr} = \frac{3 \times V^2 \times R_r}{2 \times \pi \times f \times \left[(R_s + R_r)^2 + (X_s + X_r)^2\right]} \]

Where:

\( V \) — Applied voltage (volts)
\( R_r \) — Rotor resistance (ohms)
\( f \) — Supply frequency (Hz)
\( R_s \) — Stator resistance (ohms)
\( X_s \) — Stator reactance (ohms)
\( X_r \) — Rotor reactance (ohms)

Explanation: The equation accounts for the electrical characteristics of the motor to determine the mechanical torque produced when the rotor is stationary.

3. Importance of Locked Rotor Torque Calculation

Details: LRT is crucial for determining if a motor can start a particular load. It helps in motor selection, especially for applications with high starting torque requirements like compressors or conveyors.

4. Using the Calculator

Tips: Enter all electrical parameters in ohms, voltage in volts, and frequency in Hz. All values must be positive (except reactance which can be zero).

5. Frequently Asked Questions (FAQ)

Q1: What's a typical locked rotor torque value?
A: For standard induction motors, LRT is typically 150-300% of full-load torque, depending on motor design.

Q2: How does voltage affect locked rotor torque?
A: LRT varies with the square of voltage. A 10% voltage drop results in a 19% torque reduction.

Q3: What happens if LRT is too low?
A: The motor may not be able to start the load, potentially leading to overheating and damage.

Q4: How is this different from breakdown torque?
A: Breakdown torque is the maximum torque at near-synchronous speed, while LRT is at zero speed.

Q5: Can this be used for DC motors?
A: No, this equation is specifically for AC induction motors. DC motors have different torque characteristics.