The speed-torque characteristics of several NEMA design induction motors are shown in the graph below. Notice that the starting torque ranges from about 150% of full-load torque for the NEMA Design-B motor, to about 260% of rated torque for the NEMA Design-D motor. Notice also that the speed at full-load is several percent less for this Design-D motor. This drop in speed affects the full-load efficiency. Therefore, the higher starting torque is obtained at some loss and cost in the efficiency of the motor. However, other operating considerations (such as cyclic loading) 
may make this compromise desirable. In Revision 1 to MG1-1993, NEMA added new sections including performance standards for inverter fed motors, and specifications for Design-E motor designs.
Notice that induction motors do not run at synchronous speed (1800 rpm in this case). The difference between the synchronous speed and the running speed of an induction motor is called the "slip" and is expressed as a fraction. The "synchronous speed" is the speed (in RPM) of the air-gap magnetic flux as it rotates in the air gap around the inner surface of the motor. This is fixed by the line frequency and by the details of the stator winding. This stator winding is always a symmetric arrangement producing equal pairs of north and south magnetic poles. The sum of these is called the "number of poles" for the machine.
The synchronous speed and the slip fraction are defined below:
Squirrel-Cage Induction Motors are wound for the following synchronous speeds
