Inrush current is required to establish a magnetic field in the core steel of electric machines such as motors and transformers each time voltage is applied. The peak value of inrush current, which can be as high as 20 times rated current, will be different each time the equipment is energized due to residual magnetism in the core. While reaching a very high peak value, the inrush component in a typical motor decays within the first few cycles of the line voltage. The peak value and duration are characteristics of the motor and completely independent of the load. This inrush current must be considered when selecting circuit breakers for motor circuits, but is only a minor part of the starting transient; the locked rotor current is usually the obstacle to successfully starting the motor without adverse effects on the power system.
At rated load the rotor of an ac induction motor turns at slightly less than synchronous speed, the difference being termed slip. For example, a nominally 3,600-rpm motor may have a rated speed of 3,540 rpm; a nominally 1,800-rpm motor a rated speed of 1,760 rpm, and so on. Rotor slip is what allows the motor to develop torque, and in fact, if the load increases, tending to increase the slip, the effect is an increase in rotor torque (and current) to match the load requirement. This situation is analogous to matching a pump to a system using curves of head vs. flow. For any given load, the operating speed of the motor will be determined by the point where the motor characteristic speed-torque curve crosses the load characteristic speed-torque curve.