1. What is Locked Rotor Torque?

Locked rotor torque (LRT) is the torque produced by an induction motor when its rotor is stationary (locked) and full voltage is applied to the stator. It represents the starting torque capability of the motor.

2. How Does the Calculator Work?

The calculator uses the locked rotor torque equation:

Where:

• \( V \) — Applied voltage (volts)
• \( f \) — Supply frequency (Hz)
• \( R2 \) — Rotor resistance (ohms)
• \( R1 \) — Stator resistance (ohms)
• \( X1 \) — Stator reactance (ohms)
• \( X2 \) — Rotor reactance (ohms)

Explanation: The equation calculates the torque developed when the motor is at standstill, considering both resistive and reactive components of the motor impedance.

3. Importance of Locked Rotor Torque

Details: LRT is crucial for motor selection as it determines whether the motor can start the load. It must exceed the load torque at standstill for successful starting.

4. Using the Calculator

Tips: Enter all parameters in their respective units. Typical values for small motors: R1/R2 ≈ 0.1-1Ω, X1/X2 ≈ 0.5-5Ω. Voltage is line-to-line for 3-phase motors.

5. Frequently Asked Questions (FAQ)

Q1: What affects locked rotor torque?
A: LRT increases with higher voltage, lower frequency, and higher rotor resistance. It decreases with higher leakage reactance.

Q2: How does LRT compare to full-load torque?
A: Typically, LRT is 150-300% of full-load torque for standard NEMA design motors.

Q3: Why is LRT important for motor selection?
A: It determines if the motor can overcome static friction and inertia to start rotating the load.

Q4: How can LRT be increased?
A: By using motors with higher rotor resistance (e.g., deep bar or double cage rotors) or increasing applied voltage.

Q5: What safety factors should be considered?
A: Actual LRT should exceed load torque by 20-30% to account for voltage drops and load variations.