Basic Operation of AC Induction Motors (1)

Terminology and Equivalent Circuits

Before trying to understand the operation of AC induction motors on adjustable-frequency power (variable-speed), it will be useful to briefly review the basic fixed-frequency (constant speed) operation of AC induction motors. The fundamental electromagnetic components are the stator and rotor.
Examples of typical laminations which comprise the basic magnetic path in the stator and rotor are shown in Figure 1. In the most common configuration, the stator has three interconnected phase windings, and the rotor winding is a set of short circuited bars known as a “squirrel cage.” A wound stator and an aluminum die cast (squirrel cage) rotor are seen in Figure 2.

With balanced three phase voltages applied to the windings of the stator, balanced currents flow in the three interconnected phase windings. These currents produce a magnetic field which can be thought of as “rotating” within the stator at a speed given by Equation 1.

N1 = 120 x f /P (1)
N1 = rotational speed of stator magnetic field in RPM (synchronous speed)
f = frequency of the stator current in Hz
P = number of motor magnetic poles   For various numbers of motors poles, Table 1 shows the synchronous speeds based on 60 Hz and 50 Hz frequencies.
The natural tendency is for the rotor to “follow” the rotating magnetic field, and at no-load the rotor will turn at a speed virtually equal to Nl. Any difference in the rotational speed of the magnetic field and that of the rotor will result in a voltage being induced in the rotor squirrel cage winding. The resultant rotor current interacts with the magnetic field to produce torque. The difference in rotor mechanical speed versus magnetic field rotational speed is what is known as “slip.”
The equivalent circuit for an AC induction motor can help visualize some of the motor characteristics. Figure 3 shows a typical equivalent circuit for AC induction motors. The variable resistor “R2/s”
represents the way slip causes increased current and corresponding increased torque. The greater the slip, the lower this value of resistance, and the more current is going to flow in this branch of the circuit. When the slip is virtually zero at a “no-load” condition, this resistor is seen to be a very high value. As a result, the current can be thought of as all going through the “XM” or magnetizing branch of the circuit.