Question

How is the starting torque of an induction motor improved?

Hint:

The starting torque is directly proportional to rotor resistance ($T_{st} \propto R_2$) when $R_2$ is small. To get a high starting torque, you must temporarily increase the resistance of the rotor circuit during starting.

Answer 1

Step 1: Understanding Induction Motor Torque Equation:
The starting torque ($T_{st}$) of a three-phase induction motor is expressed as: $$T_{st} = \frac{3}{\omega_s} \cdot \frac{V^2 R_2}{R_2^2 + X_2^2}$$ Where $R_2$ is the rotor circuit resistance and $X_2$ is the standstill rotor reactance.

Step 2: Methods of Improving Starting Torque:
To achieve maximum starting torque, the rotor resistance must be made equal to the standstill rotor reactance ($R_2 = X_2$). This is achieved using two main design approaches:

• For Wound Rotor (Slip Ring) Induction Motors: We can connect adjustable external resistors in series with the rotor winding through slip rings and carbon brushes. Adding this external resistance increases the overall rotor circuit resistance ($R_2$), which greatly enhances the starting torque while simultaneously reducing the high starting current drawn from the supply. Once the motor gains speed, these resistors are gradually bypassed.
• For Squirrel Cage Induction Motors: Since external resistance cannot be added to a sealed cage rotor, we use:
  • Double-Cage Rotors: These feature a high-resistance outer cage and a low-resistance inner cage. During starting, the high rotor frequency causes skin effect, forcing the current to flow through the high-resistance outer cage to provide high starting torque.
  • Deep-Bar Rotors: These utilize deep rotor bars to achieve a similar skin effect during startup, raising the effective starting resistance of the rotor.