Step-by-step Calculation of Starting Torque in Synchronous and Asynchronous Motors

Starting torque is an important parameter in the operation of electric motors. It determines the motor’s ability to overcome initial inertia and start rotating under load. This article explains the step-by-step calculation methods for starting torque in both synchronous and asynchronous motors.

Starting Torque in Synchronous Motors

The starting torque in synchronous motors is generally low because these motors do not develop torque at standstill without additional mechanisms. However, when starting with damper windings or other methods, the torque can be estimated using specific formulas.

The approximate starting torque (T_start) can be calculated using:

T_start ≈ (V_ph × I_start × sin δ) / ω_m

Where:

• V_ph = Phase voltage
• I_start = Starting current
• δ = Power factor angle at starting
• ω_m = Mechanical angular velocity

Starting Torque in Asynchronous (Induction) Motors

The starting torque in induction motors is typically higher than in synchronous motors. It depends on the rotor current and the magnetic flux.

The common formula for starting torque (T_start) is:

T_start = (3 × V² × R₂) / (s_start × (R₂² + (X₂)²))

Where:

• V = Applied voltage
• R₂ = Rotor resistance
• X₂ = Rotor reactance
• s_start = Starting slip

Calculation Steps

To calculate starting torque, follow these steps:

• Determine the motor parameters: voltage, resistance, reactance, and slip.
• Calculate the rotor current at starting conditions.
• Apply the appropriate formula based on motor type.
• Compute the torque using the values obtained.

Accurate calculation requires detailed motor parameters and operating conditions.