The relationship between starting current and starting torque in a squirrel cage motor

Squirrel-cage induction motors (also known as squirrel-cage motors) are one of the most commonly used motor types in industry. When starting up, the characteristics of the squirrel cage motor are mainly determined by its starting current and starting torque.

Starting current

Starting current refers to the current flowing through the motor when the motor is just switched on and starts to rotate. Since the speed of the motor is zero at this time, there is no back EMF generated, so the starting current is often much larger than the current under rated operating conditions. For a typical squirrel cage motor, the starting current can reach 5 to 7 times the rated current.

Starting torque

Starting torque is the torque that the motor can produce at the moment of starting. This torque must be large enough to overcome static frictional forces and other initial loads, allowing the motor to start spinning. Starting torque is usually divided into "full load starting torque" and "no-load starting torque". The former refers to the torque of the motor when starting with a certain load, and the latter refers to the starting torque without a load.

Relation

There is a relationship between starting current and starting torque, but they are not directly proportional. In theory, a higher starting current usually means a larger starting torque, because an increase in current increases the strength of the magnetic field in the winding, thereby increasing the torque. However, in practical applications, too high a starting current can be a shock to the power grid and is also bad for the motor itself, as it causes an increase in temperature and may reduce the life of the motor.

In order to control the starting current and obtain sufficient starting torque, step-down starting methods are sometimes used, such as star-triangle starting or soft starters. These technologies reduce the impact on the grid by limiting the starting current while still providing enough torque to start the load.

In summary, although starting current and starting torque are related to some extent, measures often need to be taken to balance the relationship between the two in order to protect equipment and the grid.