The alternating current (AC) induction motor is often referred to as the workhorse of the industry because it offers users simple, rugged construction, easy maintenance and cost-effective pricing. As a result of these factors, more than 90 percent of motors installed worldwide are AC induction.
Despite its popularity, the AC induction motor has two limitations: It is not a constant-speed machine, and it is not inherently capable of providing variable-speed operation. Both limitations require consideration, as the quality and accuracy requirements of motor/drive applications continue to increase.
Figure 1. Cutaway of squirrel cage AC induction motor opened to show the stator and rotor construction, the shaft with bearings and the cooling fan.
Motor slip is necessary for torque generation
An AC induction motor consists of two assemblies,stator and rotor. The stator structure is composed of steel laminations shaped to form poles. Copper wire coils are wound around these poles. These primary windings are connected to a voltage source to produce a rotating magnetic field. Three-phase motors with windings spaced 120 electrical degrees apart are standard for industrial, commercial and residential use.
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The rotor is another assembly made of laminations over a steel shaft core. Radial slots around the laminations' periphery house rotor bars,cast-aluminum or copper conductors shorted at the ends and positioned parallel to the shaft. Arrangement of the rotor bars resembles a squirrel cage; hence, the term squirrel-cage induction motor. The name "induction motor" comes from the AC "induced" into the rotor via the rotating magnetic flux produced in the stator.
The interaction of currents flowing in the rotor bars and the stators' rotating magnetic field generate torque. In actual operation, rotor speed always lags the magnetic field's speed, allowing the rotor bars to cut magnetic lines of force and produce useful torque. This speed difference is called slip speed. Slip also increases with load and is necessary for torque production.
Slip depends on motor parameters
The formal definition of slip is:
S = (ns , n) x 100 percent/ns , where
ns = synchronous speed
n = actual speed
At low values, slip is directly proportional to the rotor resistance, stator voltage frequency and load torque, and inversely proportional to the second power of supply voltage. The traditional way to control wound-rotor-induction-motor speed is to increase slip by adding resistance in the rotor circuit. The slip of low-hp motors is higher than that of high-hp motors because rotor-winding resistance is greater in smaller motors.
As seen in Table 1, smaller and lower-speed motors are associated with higher relative slip. However, high-slip large motors and low-slip small motors also are available.
Table 1. Motor slip of selected aluminium and cast iron NEMA motors, with synchronous speed ranging from 3600 RPM to 900 RPM.
As one can see, full-load slip varies from less than 1 percent (in high-hp motors) to more than 5 percent (in fractional-hp motors). These variations may cause load-sharing problems when motors of different sizes are connected mechanically. At low load, the sharing is about correct; but at full load, the motor with lower slip takes a higher share of the load than the motor with higher slip.
As shown in Figure 2, rotor speed decreases in proportion to load torque. This means that rotor slip increases in the same proportion.
A = Synchronous speed
B = Rotor speed
C = Rotor slip
D = Torque
Figure 2. The speed curve of an induction motor. Slip is the difference in rotor speed relative to that of the synchronous speed. CD = AD , BD = AB.
Relatively high rotor impedance is required for good across-the-line (full voltage) starting performance (meaning high torque against low current), and low rotor impedance is necessary for low full-load speed slip and high operating efficiency. The curves in Figure 3 show how greater rotor impedance in motor B reduces the starting current and increases the starting torquebut it causes a greater slip than in standard motor A.