Squirrel cage induction motors (also known as cage asynchronous motors) are widely used in industry and daily life due to their advantages of simple structure, reliable operation and low cost. However, the characteristic of squirrel-cage induction motors of having a small starting torque is often a limiting factor in their application. The following explains this phenomenon and explores possible solutions from the principle. 

A. What is a squirrel cage induction motor? 

The rotor of a squirrel cage induction motor consists of conductive strips and a shorting ring, resembling a rat cage, hence the name. Its operating principle is based on electromagnetic induction. When the motor is connected to a three-phase AC power supply, a rotating magnetic field is generated in the stator windings. This rotating magnetic field cuts the conductors in the rotor (the rattle cage bars), thus generating an induced electromotive force in the rotor conductors. Since the rotor conductors are short-circuited by the end rings, the induced electromotive force produces induced currents in the rotor conductors. These rotor conductors with currents are subjected to forces in the magnetic field, and according to Lorentz's law, the combined force of these forces drives the rotor. 

B. Reasons for low starting torque 

1. Zero rotor speed: At the moment of motor starting, the rotor is at rest and its speed is zero. At the same time, the rotating magnetic field generated by the stator cuts the rotor conductor at maximum speed, resulting in a very high frequency of induced electromotive force and induced current. Since the phase difference between the current and voltage is very large at startup, this causes the starting torque of the motor to be severely affected. 

2. Role of rotor resistance and inductance: The rotor conductor is actually a combination of resistance and inductance. At motor startup, due to the zero speed of the rotor, the frequency of the induced electromotive force is very high, resulting in a significant effect of inductance and resistance on the current. This effect further reduces the starting torque. 

3. Influence of phase difference: The induced current in the rotor conductor lags behind the induced electromotive force. This phase difference results in the motor not being able to fully utilize the force of the magnetic field during start-up, thus reducing the starting torque. 

4. Low power factor: At motor start-up, the power factor is very low as the rotor power factor angle is close to 90 degrees. This means that the motor cannot efficiently utilize the electrical power input to produce torque at startup. 

C. How to improve starting torque? 

1. Optimize the rotor design:Change the electrical parameters of the rotor, e.g., using a double squirrel cage or deep slot rotor design. This method increases the rotor impedance during starting, boosting the induced current and thus increasing the starting torque. 

2. Increase the external starting equipment: Use soft starter or frequency converter, by controlling the starting current and frequency, gradually increase the speed of the motor, to solve the problem of insufficient starting torque. 

3. using rotor series resistance starting: in the starting stage to the rotor circuit in series resistance, increase the starting rotor impedance to improve the starting torque, but it should be noted that this method is only applicable to wire-wound induction motors.

 4. Changing the supply conditions: Increasing the stator supply voltage at the initial stage increases the stator magnetic field strength, thus increasing the starting torque. However, this method needs to be careful in order to avoid excessive starting current impacting on the motor and the grid.

 D. Considerations for Practical Applications 

Although low starting torque is a weakness of squirrel cage induction motors, they can be made to perform well in most application scenarios through reasonable design and control techniques. When selecting a motor, the actual load characteristics should be used to determine whether special attention needs to be paid to the starting performance and, if necessary, to select a motor with a special starting function or matching control equipment. 

Summarizing 

The low starting torque of squirrel cage induction motors is the result of the combined effect of their physical structure and operating principle. Through optimized design and the use of auxiliary equipment, this deficiency can be significantly improved, making them more adaptable to complex industrial and life scenarios. Understanding the principles behind this phenomenon is important for the rational selection and use of motors.