21 Electric Motors When the entire rotor is located in the rotating fi eld, see Fig. 1.12c Induction in the rotor rods, the rotor rods are aff ected by forces that cause the rotor to rotate. The rotor speed (2) does not reach the speed of the rotating fi eld (1) since no currents are induced in the cage bars when it is rotating at the same speed as the fi eld. 1.3.3 Slip, Torque and Speed As described in sections 1.2.5 Poles, Synchronous Speed and Asynchronous Speed and 1.3.2 Squirrel Cage Motor, under normal circumstances the rotor speed (n n ) of asynchronous motors is slightly lower than the speed (n 0 ) of the rotating fi eld.
The diff erence between the speed of the rotating fi eld and the rotor is called slip (s) where: s = n0 – n n The slip is often expressed as a percentage of the synchronous speed and is typically between 1 and 10 percent. s = (n0 – n n ) 100 n0 The individual forces in the rotor rods combine to form the torque (T) on the motor shaft (see section 1.3.2 Squirrel Cage Motor). With a given value of force (F) and radius (r) the motor torque is: T = F r. Fig. 1.13 Torque on the motor shaft is the force (F) x radius (r) The relationship between motor torque, speed and current of asynchronous motors has a characteristic curve, shown in Fig.
1.14 Principal motor current and torque characteristics. This curve depends on the rotor slot design and the rod material.