Question

How does slip define the speed difference between the rotor and synchronous speed?

Answer 1

Slip, in the context of electric motors, defines the speed difference between the rotor (the rotating part of the motor) and the synchronous speed (the theoretical speed of the rotating magnetic field produced by the stator). The synchronous speed is determined by the frequency of the power supply and the number of poles in the motor.

Mathematically, slip (S) is defined as:


=

sync
−

rotor

sync
×
100
%
S=
N
sync
    ​

N
sync
    ​

−N
rotor
    ​

    ​

×100%

Where:


sync
N
sync
    ​

 is the synchronous speed in revolutions per minute (RPM).

rotor
N
rotor
    ​

 is the actual speed of the rotor in revolutions per minute (RPM).

The slip is typically expressed as a percentage. When the motor is operating under normal conditions, the slip is always positive, indicating that the rotor speed is slightly slower than the synchronous speed. This slip is necessary for the motor to generate torque and perform useful work. If there were no slip, the rotor would rotate at the same speed as the rotating magnetic field produced by the stator, and no relative motion would exist to induce the necessary electromagnetic interactions for the motor to produce torque.

Slip is an important parameter in motor analysis and design. By controlling the amount of slip, engineers can regulate the motor's speed and torque characteristics for specific applications. Motors with higher slip are often used in applications where variable speed and higher starting torque are required, such as in industrial machinery and appliances. On the other hand, motors with lower slip are often used in applications where precise speed control and synchronization are important, such as in clocks and timing devices.