Basic Operation of AC Induction Motors (2)

Speed / Torque Curves

As an AC induction motor is started, the values of resistance and reactance offered by the motor (or seen by the power source) will vary. At the instant of applying power to a stopped motor, the magnetic field is rotating much faster than the (stationary) rotor. This implies 100% slip, so R2/s is minimized. As a result, the current drawn at starting (locked rotor) conditions is quite high. Also, it is common to design rotor slots which have dramatically different impedance at high slip (say 60 Hz for starting) versus at normal running where slip is typically in the range of 0.2 - 2 Hz. This changes the values of both X2 and R2 from starting to running conditions.

As a motor accelerates to speed from a standstill, the changing impedances result in a unique characteristic developed torque and current drawn during the time of acceleration. Depending on the design of the motor, a torque / current characteristic such as one of those shown in Figure 4 would typically result. The NEMA Design B motor is considered the most "general purpose" of these characteristic shapes, with Design C and D typically used for more "difficult to start" loads. Table 2 gives some ranges of characteristics for integral HP, 1200
and 1800 RPM motors.

As can be seen from all of these speed / torque / current curves, the current drawn by an AC motor in accelerating a load up to speed can be dramatically higher than the nominal running current. At the same time, the developed torque (during acceleration) may in some cases be less than the rated full load torque. Various methods exist to control the starting current drawn by an AC motor but the torque per amp seen during (fixed frequency) starting is always much lower than at running conditions.
The nature of an AC induction motor’s acceleration to running speed is such that it can impose high stresses on both the stator and the rotor. The high current draw also stresses the upstream power system, including cabling, transformers, switchgear, etc. For this reason, there is often significant effort made to "control" AC motor starting and acceleration - both in terms of motor design as well as application.